5. review of domestic and regional longline catch and effort...

STRIPED MARLIN: BIOLOGY AND FISHERIES

5. Review of domestic and regional longline catch and effort data

Don Bromhead

5.1 Introduction 5.1.1 Regional Fishery structure

The following chapter describes and analyses commercial catch data for striped marlin, as pertains to the waters adjacent to Australia and New Zealand, namely the Eastern Indian Ocean and the Southwest Pacific Ocean. For the purposes of this report, the region is considered to comprise four main fisheries. These are the Southern and Western Tuna and Billfish Fishery (SWTBF), the Eastern Tuna and Billfish Fishery (ETBF), the New Zealand Longline Fishery, and the high seas fishery (which comprises a number of national fleets, but here will focus predominantly on Japan, whose consistent recording and fishing methods make interpretation of catch trends more reliable than for other high seas fleets).

5.1.2 A brief history of regional fisheries

The Australian Commonwealth managed tuna and billfish sector is managed as two fisheries, the Eastern Tuna and Billfish Fishery (ETBF) and the Southern and Western Tuna and Billfish Fishery (SWTBF) (Figure 5.1) These fisheries operated on a relatively small scale in the late 1980s, but underwent a rapid expansion in both effort (hooks set) and area covered, throughout the 1990s (Figure 5.2). At the same time, increased total national catches of striped marlin were also recorded, particularly on the east coast in the late 1990s.

Figure 5.1 – Areas within the AFZ managed under the Eastern Tuna and Billfish Fishery (ETBF) and the Southern and Western Tuna and Billfish Fishery (SWTBF). These fisheries extend outside the AFZ also. (Map: D.Barratt, BRS, 2002)

83


Figure 5.2 – Maps of five degree reported effort for Australian domestic fleet in 1990 and 2001, showing dramatic increase in effort and range of fishing over the past decade. In the SWTBF, the West Australian government imposed a ban on the commercial landing of striped marlin in 1999. Thus the majority of “catch” post 1999, is actually recorded as discarded catch. Whether this ban impacted on gear setting methods and subsequent catch rates of striped marlin (based on discard numbers) is not known.

New Zealand’s domestic surface longline fishery has also increased fishing effort during the late 1990s. With striped marlin being seasonally abundant in New Zealand’s waters, catches by foreign and domestic longliners had been historically significant. However, New Zealand banned the domestic and foreign retention of striped marlin in 1987 (Francis et al. 2000).

Japan has had a high seas fleet exploiting the waters of the Eastern Indian ocean and the South-west Pacific Ocean since the late 1950s and until relatively recently, these fleets fished in the EEZs of both Australia (stopped fishing in EEZ in 1997) and New Zealand (stopped fishingin EEZ in 1994/5).

5.1.3 Importance of striped marlin in domestic commercial catches

Traditionally, striped marlin has not been viewed as a target species in tuna and billfish fisheries, either domestic or international. However, it is generally acknowledged that opportunistic targeting can occur in high abundance time-area strata in international fisheries (e.g. Ward, 1996). Whether there is any evidence for this in Australia’s domestic fisheries needs to be investigated due to its implications for domestic fisheries management. In recent years, striped marlin have made up 8-9% (by weight) of total annual Australian longline

84


catches (all species), having increased from between 1-5% in the early 1990s. Whether this increase in proportional catch weight results simply from increased abundance, or increased targeting, or both, will be discussed in later sections. Striped marlin is highly valued on the Japanese sashimi markets, fetching prices comparable to sashimi tuna such as bigeye. Consequently, a domestic export market has developed for striped marlin, which may have encouraged some degree of targeting (Section 5.7). Some aspects of this market will be described in section 5.6.

5.1.4 Need for detailed analyses of striped marlin catch and effort data

In New Zealand in the late 1980s and early 1990s, recreational fishing groups expressed concern over Japanese longline exploitation of the striped marlin resource, which had traditionally been used by the domestic recreational game fishery. In part due to this reason, and in part to allow the domestic longline fishery to develop, Japanese vessels ceased fishing in the New Zealand EEZ in 1995. Some evidence has been presented claiming that a subsequent recovery in NZ gamefishing catches of striped marlin resulted directly from the cessation of foreign longline fishing in the NZ EEZ and a ban on commercial landings of billfish (Holdsworth et al. 2003). This case study will be examined in more detail later. Landing of striped marlin by commercial vessels has also been banned by the Western Australian government, after concerns were expressed by the recreational fishery, which believed that the billfish fishery was suffering under weight of commercial exploitation. Now, there is concern from recreational gamefishing organisations on the east coast over the potential impact of longlining on their access to the striped marlin resource. The expansion in effort and coverage by the commercial fleet, and a possible switch in targeting by recreational gamefishers, may have led to increased interaction between the sectors. This situation needs to be investigated so that the relative impacts of the two sectors upon each other and upon the striped marlin resource can be evaluated and the appropriate management measures determined and put in place. This chapter outlines basic trends in catch and effort with relation to striped marlin in waters adjacent to Australia and New Zealand.

5.1.5 Information required for management of striped marlin

In order to facilitate appropriate fisheries management strategies for this species, information is required on:

1. Long-term trends in catch and standardised and nominal CPUE for striped marlin caught off the east and west coasts, along with trends in size distribution.

2. Identification of core catch regions and how these may shift over various time-area strata.

3. The status of striped marlin as a byproduct species and any evidence for direct targeting.

4. The types of gear and setting methods that might result in higher catch rates for striped marlin.

5. The degree of discarding of striped marlin

6. Mortality associated with both recreational and commercial fisheries.

7. Trends in relative abundance of striped marlin in waters adjacent to Australia

8. Potential interactions between the longline and recreational fishing sectors catching striped marlin (analyses of CPUE by time area sectors).

9. The effect of foreign longline exclusion on availability of striped marlin to domestic fisheries.

This chapter seeks to provide information in relation to points 1-6 (dealing with catch and effort trends and information), while chapter 8 will deal with point 7 (abundance), and chapter 9 will deal with points 8-9 (fishery interactions).

85


The following analyses of regional fisheries catch and effort trends will look firstly at foreign (Japanese) distant water fleets catch and effort records, as these have the longest fishery “history” in waters adjacent to Australia and New Zealand. Other distant water fleets operating in the region are discussed in an earlier chapter (Chapter 3). This will be followed by detailed analyses of striped marlin catches by Australian commercial fisheries, and a summarised description of New Zealand commercial catches of striped marlin.

5.2 Data sets The following catch and effort descriptions utilise datasets drawn from a number of sources. These include:

• Australian domestic longline logbook data;

• Japanese longline 5ºx5º by month catch and effort data pertaining to striped marlin;

• New Zealand domestic longline logbook data summaries;

• East Coast Size Monitoring Program data (pertaining to individual processed weights for striped marlin caught off the east coast);

• Commonwealth Industrial and Scientific Research Organisation (CSIRO) observer data; and

• Australian Fisheries Management Authority (AFMA) observer data (for domestic and Japanese vessels).

These data are used in the current chapter to describe basic trends in catch, effort, and nominal catch per unit effort by the main longline fleets operating in the southwest Pacific and the eastern Indian Ocean. Standardised catch rates, which are used as indices of abundance, are drawn from analyses described in Chapter 8.

5.2.1 Australian data

An Australian surface-fishery logbook was first introduced in the early 1960s, but the historical data are patchy because collection was discontinuous. A Commonwealth logbook for domestic longliners was introduced in 1986. Again, return rates were poor until 1995, when AFMA enforced returns as a condition of fishing permits, maintained a monthly audit and supported the program with regular field liaison. There has not been a consistent at-sea-observer program, nor independent verification of reported catches against landings data. Consequently, the quality of Australian logbook data remains uncertain. Inconsistencies in reporting emphasise the need to verify data (Caton, 2002). An observer program has been scheduled to commence in July 2003 in the Australian pelagic longline fisheries. Various smaller observer programs have existed over time and data relating to observed catches of striped marlin was collated.

The Australian longline fleets use logbooks to take detailed records of catch and the methods and means used to catch tuna and other species (Campbell et al 2002). The data recorded on these logbooks is sometimes verified by observers placed on the boats, who take their own record of catch and effort statistics which can then be compared to the vessels log for verification. These data are used to determine catch per unit effort for the various species caught. However, given that observer coverage of the longline fleet has been extremely limited (<0.1%), and that which has occurred has been sporadic and research project oriented, it is impossible to determine the accuracy of the data in Australian longline logbooks in most cases (Campbell et al 2002). This applies to longline data from both the ETBF and SWTBF. The commercial catch of billfish by methods other than longlining (e.g. trolling, handline etc) is unknown but considered likely to be small. The ideal catch verification system is some way off but likely to involve some combination of logbook, at-sea observers, port sampling and vessel monitoring systems.

86


5.2.2 Japanese data

Catch data pertaining to Japanese longline catches of billfish in the AFZ prior to their exclusion in 1997, is based on data recorded in Japanese logbooks and held by the Australian Fisheries Management Authority (AFMA). While the logbook coverage of Japanese AFZ longliners was supported by some deployment of Australian observers, who verified catch-reporting practices and collected biological and fisheries data, the accuracy of billfish catch data is not known (Caton, 2002). Problems associated with misidentification and non-reporting are likely to play a role. Furthermore, these logbooks only record retained catch and not discards, which could be as high as 30% according to Campbell et. al. (1996). Data pertaining to catch and effort by Japanese longliners in the eastern Indian Ocean and southwest Pacific beyond the AFZ, were kindly provided by the National Research Institute of Far Seas Fisheries.

5.2.3 New Zealand data

Billfish, including striped marlin, are commonly caught by NZ licensed longliners. From 1987 onwards, foreign licensed vessels were required to release all billfish caught in the NZ EEZ (Francis et al., 2000) while domestic boats could retain only swordfish and were required to return other billfish, regardless of life status. From 1989 onwards there were three vessel types fishing within the EEZ, these being foreign licensed, foreign chartered and domestic licensed longliners. Foreign licensed vessels were excluded from NZ waters from 1994/5 onwards. Tuna longliners submit catch and effort information (including bycatch information) to the Ministry of Fisheries on Tuna Longlining Catch Effort Returns (TLCERs) and Catch Effort Landing Returns (CELRs). It is likely that these would underestimate the bycatch of certain species which might tend to be discarded at sea (Francis et al, 2000). An observer program implemented by the Ministry of Fisheries provided more reliable data in regard to bycatch, with observers recording data relating to total catches, numbers, sizes, and sexes for the different species. While observer coverage has been relatively low (annual coverage ranging between 3-8% between 1994 and 1998), and not always at a consistent level between years, it still provided an independent measure of catch and effort for striped marlin (and other bycatch species) (Francis et al, 2000). Francis et al. (2000) note that species identification during early years of the observer program was somewhat unreliable. Striped marlin are also occasionally caught in NZ purse seine sets (1.8% of sets; Bailey et. al, 1996).

5.3 Japanese longline catch and effort trends 5.3.1 Annual catch and effort trends

Japanese longline fleets have operated in the SWPO since the late 1950s. During this time, Japanese annual effort in the SWPO has fluctuated significantly (Figure 5.3), but generally remained above 10 million hooks. It peaked a number of occasions at 38.5 (1971), 50.9 (1981), 42.8 (1989) and 39.1 (1995) million hooks, with lower levels in between these periods. Peaks in total catches of striped marlin (by number) tend to coincide with peaks in annual effort. Catch rates for striped marlin fluctuated considerably over the past 30 years, however, some general long-term patterns are evident. Averaged annual catch rates (CPUE) declined through the 1970s, remained reasonably constant throughout the 1980s (perhaps declining further but to a lesser degree), and then increased marginally from 1992 through to 1998, before declining again more recently, to the lowest levels on record. It is worth noting that striped marlin catch rates are very similar in the EAFZ and the wider SWPO for most of the time period, excepting two sharp increases in catch rate within the EAFZ in the early 1980s and in the mid 1990s.

Throughout this entire period, the effort distribution for the Japanese longline fleet in the SWPO has varied considerably with latitude (Figure 5.4). Most of the effort in this region has been concentrated south of 25ºS, and north of 15ºS. Between 20ºS and 25ºS, effort has been comparatively low, however catches of striped marlin

87


0

5000

10000

15000

20000

25000

1970 1975 1980 1985 1990 1995 2000

Cat

ch (N

o. fi

sh) SWPO

EAFZ

A.

010

2030

4050

60

1970 1975 1980 1985 1990 1995 2000

Effo

rt (m

illio

n ho

oks) SWPO

EAFZB.

0.00

0.20

0.400.60

0.80

1.00

1.20

1970 1975 1980 1985 1990 1995 2000Year

CPU

E (F

ish/

1000

ho

oks)

SWPOEAFZ

C.

Figure 5.3 – Historic trends in Japanese longline: a) effort, b) catches, and c) catch per unit effort (CPUE) for striped marlin; for Japanese fleets operating in the southwest Pacific Ocean (SWPO) and inside the eastern Australian Fishing Zone (EAFZ). This SWPO region has been defined by the latitudes 0º-50ºS and longitudes 140ºE-165ºW to encompass waters east of Australia and surrounding New Zealand. (Data Source: National Research Institute of Far Seas Fisheries, 2002; AFZIS, 2002)

have were very high during the 1970s and early 1980s (peaking ~ 8000 marlin), resulting in very high catch rates in this region during that period (Figure 5.4). However catch rates, which reached over 4 fish per 1000 hooks in 1973, declined in this region from the mid-1970s onwards such that in recent years they have been less that one quarter those achieved in the early 1970s. Japanese catch rates for striped marlin caught between 25ºS-30ºS show some decline after the early 1970s, and brief increases in catch rate in 1989 and 1997 were followed by further declines. Catch rates between latitudes 30ºS-35ºS also declined after the mid-1970s, but show some recovery in the 1990s. Virtually no striped marlin have been caught by the Japanese longline fleet south of 40ºS in the SWPO.

5.3.2 Annual catch and effort trends

Catches of striped marlin in the SWPO by Japanese fleets during the period 1970-2000 varied between quarters as the fleets distribution shifted according to the species being targeted and

88


Figure 5.4 – Annual fishing effort (hooks x 100), catch and CPUE (fish/1000 hooks) for striped marlin taken by the Japanese longline fleet operating in 5 degree latitude strips within the southwest Pacific Ocean between 1970-2001. (Source: National Research Institute of Far Seas Fisheries, 2002)

89


the movements of these species. In the first quarter, large catches of marlin were taken in an area stretching from off the central and southeastern coast of mainland Australia, across to the waters off the North Island of New Zealand (Figure 5.5). This is during a period of relatively low effort in the SWPO, and CPUEs are very high during this quarter across the entire stretch from eastern Australia to northern New Zealand. The high catch area contracts and shifts south into the Tasman Sea in the second quarter, during a period of intensified fishing effort, both in temperate and tropical waters (Figure 5.5a,b). Catch rates however diminish in the Tasman and adjacent waters, while increasing to the north east of New Zealand. The high catch area progressively intensifies (ie; catches increase) and shifts northward towards the south Coral Sea progressively through the third and fourth quarters. Longline effort in the third quarter is still high off central and southeastern Australia, and off the North Island, but diminishes in the 4th quarter as boats move northward or out of the zone. The 3rd quarter is characterised by a band of moderate CPUEs stretching from the southern Coral Sea across to the waters north of New Zealand. However, striped marlin catch rates increase dramatically in the fourth quarter throughout this zone and in an expanded area to the south (Tasman Sea).

In the west, Japanese catches of striped marlin had been comparatively small throughout the first two quarters, but increased considerably starting in the 3rd quarter, peaking in the 4thquarter off the northwest coast of Western Australia (Figure 5.5a). Compared to the SWPO, relatively little longline effort was expended by Japan off the coast of Western Australia, excepting a region to the far south west of the continent in the 3rd quarter. Effort in the high catch rate area off the northwest shelf was typically highest in the 1st and 4th quarters (Figure 5.5b).

It is worth noting that Japanese longliners have consistently operated on an annual basis throughout large areas of the SWPO and EIO over the past 30 years (Figure 5.5d). The areas most consistently targeted include the Tasman Sea during the 1st to 3rd quarters, the Coral Sea during the 1st, 3rd and 4th quarters, and off the southwest and northwest coasts of Western Australia.

In addition to examining mean quarterly changes in catch and effort distribution, it is useful to consider historic quarterly catch and effort trends as the seasonal distribution of fishing effort has changed to some degree over time (Figure 5.6). Effort expended in the SWPO during the fourth and first quarters has declined to very low levels after peaking in the early 1980s at over 10 million hooks in each quarter. Before 1985, the largest catches of striped marlin were recorded in the 4th quarter. Catches during this quarter have declined to near zero (following the decline in effort) in the period since. In contrast, catches were relatively high in the 3rd quarter, from 1980 onwards, and 2nd quarter catches of striped marlin increased considerably during the 1990s. It is worth noting that catches across all quarters were extremely low in 2000, which appears to reflect both a general decrease in effort, as well as a decrease in catch rates.

It is important to note that while 2nd and 3rd quarter catch rates have remained at a relatively steady and low rate for the past 30 years, 1st quarter catch rates increased dramatically in the late 1990s, after having been in decline since the early 1970s. A similar trend is evident for 4th quarter catch rates, which while also declining over this period, have done so in a more variable manner. It is worth remembering that these data reflect nominal catch rates, and as such don’t take into account variability associated with changes in targeting and fishing methods.

5.3.3 Japanese catch and effort in the EAFZ

Prior to November 1997, Japanese longline vessels had access to fishing grounds within the Australian EEZ. Their presence in this zone preceded the development of a domestic longline fleet, and the data from these years can be examined to look at the catches and catch rates of commercial operations prior to the current domestic logbook data set. The increasing difference in effort between the SWPO and the EAFZ during the 1990s is due in part to the increasing restraints placed on Japanese longline activity within the EAFZ (Figure 5.3). The

90


Figure 5.5a – Mean quarterly catches of striped marlin by the Japanese longline fishery in the waters adjacent to Australia and New Zealand, during the period 1970-1999. (Source: National Research Institute of Far Seas Fisheries, Japan, 2002)

91


Figure 5.5b – Mean quarterly effort for the Japanese longline fishery in the waters adjacent to Australia and New Zealand, during the period 1970-1999. (Source: National Research Institute of Far Seas Fisheries, Japan, 2002)

92


Figure 5.5c – Mean quarterly catch rates of striped marlin taken by the Japanese longline fishery in the waters adjacent to Australia and New Zealand, during the period 1970-1999. (Source: National Research Institute of Far Seas Fisheries, Japan, 2002)

93


Figure 5.5d – Mean number of years fished per quarter by the Japanese longline fishery in the waters adjacent to Australia and New Zealand, during the period 1970-1999. (Source: National Research Institute of Far Seas Fisheries, Japan, 2002)

94


0

50000

100000

150000

200000

19

Effo

rt (H

ooks

x 1

00)

1st Quarter2nd Quarter3rd Quarter

02000400060008000

100001200014000

1

Cat

ch (n

umbe

r of f

ish)

0

0.05

0.1

0.15

0.2

0.25

1

CPU

E (F

ish/

100

hook

s)

Figure 5.6 – JaWest Pacific OInstitute of Fa area closures ohowever givennoted that Japof striped marof pre-spawninhave been targthe 4th quarterdomestic sens

While Japanesand 1990s, andthe region alsoon Japanese lodevelopment ocurrent catch l

A

70 1975 1980 1985 1990 1995 2000

4th Quarter

970 1975 1980 1985 1990 1995 2000

9

r

al

(i

e

e

C

B

70 1975 1980 1985 1990 1995 2000Year

panese longline effort (a), catch (b) and catch per unit effort (c) in the South-cean, by quarter during the period 1970 – 2001 (Source: National Research Seas Fisheries, Japan, 2002)

ff northern Queensland are unlikely to have contributed to differences in catch, that this was not a high abundance region for striped marlin. It should also be nese logbooks only recorded retained catch, so it is uncertain what proportion in might have been discarded. Given its value as sashimi (particularly for flesh g marlin) discarding may have been relatively low. Striped marlin appear to eted to a limited extent, in particular in the region over the Lord Howe Rise in Ward, 1996). At the same time though, Japanese fleets were aware of the

tivities concerning marlin catches (Findlay et al 2003).

fishing activity was relatively high in the southwest pacific during the 1980s as other distant water fleets built up operations in this region, other nations in started to develop domestic longline fishing fleets, and to impose restrictions ngline activities in their own EEZs. The following section discusses the f the Australian and New Zealand longline fleets, and analyses historic and vels of striped marlin within these fleets.

95


5.4 Australian domestic catch and effort trends 5.4.1 Introduction

Both Australian Commonwealth managed longline fisheries underwent rapid expansion in effort during the 1990s. This expansion of the fishery in area and over time has meant that different regions within the fishery have differing exploitation histories. The following section will examine domestic catch and effort trends on two scales: by fishery, and by fine-scale (5 degree) sub regions within each fishery.

5.4.2 Fishery catch and effort trends

The ETBF expansion started earlier than the SWTBF expansion, increasing from 0.98 million hooks in 1987 to over 11 million hooks in 2001 and 2002 (Figure 5.7). Annual catches of striped marlin in this fishery fluctuated between about 100-800 marlin up to 1994, then increased to 3248 in 1997, and then jumped to 7820 the next year. Catches stayed above 7000 over the next three years, peaking at just under 10000 marlin in 2001, before dropping below 7000 in 2002. In line with the increased catches, catch rates also reached all time highs during the late 1990s off the east coast, remaining in the range of 0.8-1.0 fish per thousand hooks until 2002 when they dropped back to 0.6 marlin/1000 hooks. Prior to 1996, mean nominal catch rates were variable, ranging between 0.06 and 0.69 fish/1000 hooks through the late 1980s and early 1990s. Based on analyses of standardised catch rates presented in Chapter 8, it appears likely that the increased catch rates in the ETBF during the mid-late 1990s was at least in part due to increased abundance or availability of striped marlin at this time. Other factors may also have played a role however. See Chapter 8 for a full discussion of this issue.

Effort expanded rapidly in the SWTBF from 1998 onwards, having previously fluctuated between 50 thousand and 0.5 million hooks per year. Effort peaked at 6.2 million hooks in 2001. However, while increased effort coincided with increased catches of striped marlin in this fishery, catches still remained at a much lower level than those taken in the ETBF. Note that post-1999 catches comprise mostly “discard” catches, due to the ban on landing of striped marlin in WA implemented from that year onwards. It is unknown if boats might land striped marlin caught off the West Coast in other state ports. Catch rates for striped marlin have shown two peaks in the past 15 years in the SWTBF, with peaks of 1.17 fish/1000 hooks (1989) and 0.69 (1997) contrasting against most years with catch rates less than 0.2.

Quarterly catch rates in the ETBF over the past 15 years have varied between years (Figure 5.8). Fourth quarter catch rates have tended to be highly variable, but account for the highest quarterly catch rates on record, having been higher than 1.0 (and up to almost 1.8 fish/1000 hooks) in 6 of the past 15 years. 1st quarter catch rates have regularly risen above 0.4 fish/1000 hooks, but never been higher than 1.0. Prior to 1996, catch rates in the 2nd and 3rd quarters were generally very low (<0.4), however during the following 4 years, catch rates in all quarters increased substantially. In the SWTBF, quarterly catch rates have shown considerable variation between years. In the early years, 2nd quarter catch rates were highest, but more recently catch rates have been higher in 1st quarter. Due to the landing ban, there is a degree of uncertainty over accuracy and meaningfulness of SWTBF data pertaining to striped marlin, which is primarily composed of discards in recent years.

5.4.3 Fine-scale (5 degree) trends in catch and effort

In the following section, the catch and effort trends in the east and west coast fisheries described in the previous section are broken down and described by 5 degree squares (to be called “sectors” from this point on) of latitude and longitude, on an annual basis. Only those 5-degree sectors in which striped marlin catches have been recorded in more than half the years in which the sector was fished have been analysed here. On the west coast there are five such sectors (named sectors 1 – 5). Sectors 6 to 21 cover the east coast fishery (Figure 5.9, 5.10, 5.11).

96


0

2000000

4000000

6000000

8000000

10000000

12000000

1985 1988 1991 1994 1997 2000Year

ETBFSWTBFNZ

A.

0

2000

4000

6000

8000

10000

12000

1985 1988 1991 1994 1997 2000Year

0

100

200

300

400

500

600ETBF(no)SWTBF(no)NZ(no)ETBF (mt)SWTBF(mt)

B.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1985 1988 1991 1994 1997 2000Year

C.

0

0.3

0.6

0.9

1.2

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

Year

Nominal CPUE

Standardised CPUE

D.

Figure 5.7 – A) Effort; B) Catch (numbers and processed weight in metric tonnes); and C) nominal catch per unit effort, for striped marlin caught in the domestic (Source: AFZIS, 2002) and NZ surface longline fisheries (Source; NIWA, 2002). D) Comparison of nominal and standardised catch rates 1990-2001 calculated for the ETBF (see chapter 8).

97


00.20.40.60.8

11.21.41.61.8

2

1985 1987 1989 1991 1993 1995 1997 1999 2001

Year

Quarter 1Quarter 2Quarter 3Quarter 4

A.

00.2

0.40.60.8

1

1.21.41.6

1.82

1986 1988 1990 1992 1994 1996 1998 2000 2002

Year

B.

Figure 5.8 – Catch per unit effort (fish/1000 hooks) for striped marlin taken in the a) ETBF and b) SWTBF during the period 1986-2002. (Data Source: AFZIS, 2002)

In the SWTBF, little effort has been expended in sector 1 and 2 (off the northwest coast), and subsequently catch numbers are fairly low. However, these sectors are characterised by high CPUEs particularly (mostly from catches taken in the 4th quarter). In contrast, sectors 3 to 5 which include the waters off the central-west and south west coastline, have shown a pattern of increasing effort, particularly in the second half of the 1990s, with this increase being more marked as latitude increases. Catches of striped marlin are higher than in sectors 1 and 2, virtue of increased effort, but catch rates are consistently lower than occur further north. Catch rate calculations may be confounded to some degree by the current ban on landing striped marlin in Western Australia.

In the ETBF, waters off the far northeast coastline (sectors 6 - 8) have been characterised by relatively low to moderate levels of longline effort, and predominantly low catches and

98


nominal catch rates for striped marlin. However, further south between 20ºS and 40ºS, longline effort levels in near shore sectors (9-12,15,20) increased considerably through the mid to late 1990s. It should be noted that these trends are based on reported catch and effort and will be underestimates to some degree in the earlier years when logbook reporting was lower. Reported catches of striped marlin increased markedly, particularly off the central eastern coast (sectors 11 and 12) to between 1500 – 3000 marlin per year per sector. Catch rates show different trends in the different sectors, but in central coastal waters (sectors 11,15) appear to be relatively steady over this period, but off the southeast coast (sector 20) show increasing catch rates since the mid 1990s. Offshore waters, particularly between 20ºS – 30ºS, show generally higher catch rates for striped marlin than the inshore waters, but there is some indication of a decline in these since about 1999. However, the level of actual catch taken from the furthest offshore sectors (Sectors 13,14,17,18) has been very low.

Regions within the ETBF which have consistently high commercial catches and high catch rates, will later be considered in conjunction with recreational catch data to determine core catch regions (Chapter 7) for interactions analyses. Trends in nominal and standardised catch rates in these regions will be examined in more detail in those sections.

5.4.4 Quarterly trends in domestic catch, effort and catch rates

Spatial trends in mean quarterly effort during the period 1990-2002 are described in Figure 5.12a. In the west, effort is concentrated off the central west and southwest coastline for most of the year, however there is a shift in effort further northwards in the 1st and 2nd quarters. The 1st and 4th quarters are also characterised by increased effort further off the shelf between 25ºS and 30ºS. Most striped marlin catches are taken off the northwest coastline in the 4th quarter, with some catches taken further south in the first and second quarters (Figure 5.12b). Nominal catch rates peak in the 3rd and 4th quarters off the north west coast between 15ºS and 25ºS (Figure 5.12c).

In the ETBF, effort is spread through waters along the length of the coast in the first quarter, contracts southwards to the central and south eastern waters in the second and third quarters, before shifting northwards again in the fourth quarter (Figure 5.12a), during which there is also a greater concentration of effort off the central-eastern coastline between longitudes 155-160ºE . Significant catches of striped marlin occur along the central-eastern and southeastern waters between longitudes 150ºE-155ºE in the first quarter, and predominantly in the southeast in the second quarter (Figure 5.12b). A similar pattern is apparent for distribution of catch rates, which are moderate in these time area strata (Figure 5.12c). However, the zone of high catches (and moderate catch rates) shifts north to waters off the central eastern coastline between latitudes 20ºS and 25ºS in the 3rd and 4th quarters. Catch rates are particularly high in the south Coral Sea and waters off the central eastern coast during the 4th quarter.

Catch and effort data presented so far have not included data on trends in sizes of striped marlin being caught. Individual size data is not recorded in domestic longline logbooks, only bulk catch (total numbers and total weight per fishing operation), yet such information is important for fisheries assessment. The following section will look at trends in size data collected for the east coast size-monitoring program.

99


A.

B.

Effo

rt (h

ooks

x m

illio

n)

Figure 5.9 – Annual domestic longline effort by 5 degree square; a) Reference grid for each 5 degree square, b) annual longline effort since 1987, with graph number corresponding to data for the same numbered grid square in the map.

100


A.

B.

Figure 5.10 – Domestic longline catch by 5 degree square per year; a) Reference grid for each 5 degree square, b) Annual longline catch since 1987, with graph number corresponding to data for the same numbered grid square in the map.

101


A.

B.

CPU

E (m

arlin

per

100

0 ho

oks)

Figure 5.11 – Domestic Longline catch per unit effort by 5 degree square per year; a) Reference grid for each 5 degree square, b) annual longline catch per unit effort since 1987 with graph number corresponding to data for the same numbered grid square in the map.

102


1st Quarter 2nd Quarter

3rd Quarter 4th Quarter

Figure 5.12a – Mean quarterly longline effort for the Australian domestic longline fleet operating during the period 1990-2002. (Source: AFZIS, 2002)

103




Figure 5.12b – Mean quarterly longline catch of striped marlin taken by Australian longline fleet during the period 1990-2002. (Source; AFZIS, 2002)

104




.

Figure 5.12c – Mean quarterly longline CPUE (marlin per 1000 hooks) for striped marlin taken by Australian longline fleet during the period 1990-2002. (Source; AFZIS, 2002)

105


5.5 Size trends 5.5.1 Introduction

The Eastern Tuna and Billfish Fishery Size Monitoring Program (SMP) is a data collecting program undertaken by WW Fisheries, under contract from AFMA, with data being reported to and held by CSIRO. WWF visits east coast fish processors to collect catch by size data for target species and some important byproduct species, including striped marlin, caught in the ETBF. This data has been collected since August 1997. The program concentrates on collecting data from the major processors and opportunistically gathers data from smaller operators when possible. Upwards of 80% of total catch data of main species may be collected as individual sizes, and given the widespread collection of data from processors located from Cairns to the South Coast, creates a meaningful coverage of the available data (Williams, 1999).

5.5.2 Striped marlin data

Striped marlin size data (weights only) is consistently collected where possible, and is recognised by the program as an increasingly important byproduct species (Williams, 2000). In the following data descriptions, all processed weights have been scaled up by a factor of 1.25, to account for weights lost by “trunking” (which is how virtually all striped marlin are processed on the east coast) and are presented as estimated whole weights. The scaling factor used here was calculated based on a limited number of records of processed (trunked) and whole weights recorded by observers on Japanese longliners operating in Australian waters. However, one of the tasks of observers to be placed on Australian longliners in future should be to determine an appropriate scaling factor based on domestic data for this species. Also note that only some operators databases indicate fish destination (export/domestic), so exports may also be underestimated. Since August 1997, the SMP has collected weight data on 13454 striped marlin, amounting to 200-250 mt processed weight per year. Most of the processed marlin data has been sampled during the 4th quarters from processors north of Sydney, but the data also indicate a smaller peak in marlin processing numbers in March and April of most years, from processors south of Sydney (Figure 5.13). Logbooks indicate that 30879 marlin have been landed during this period off the east coast, totalling 1954 mt, hence, coverage of the program for striped marlin appears to be around 43.6% of the actual catch.

5.5.3 Exports and local market prices

Many of the processors records indicate fish which have been processed for exporting (e.g. to the sashimi markets in Japan). Although the SMP coverage is just under 45% for striped marlin landed on the east coast, these records indicate when and potentially what proportion of striped marlin is exported rather than sold locally. Not all processors earmark export fish, so the figures here are likely to be conservative. Of the 13454 marlin recorded by the SMP, 1841 were exported (Figure 5.13). Of these, 1435 (78%) were processed in the southern region (south of Sydney) and predominantly during the months February to May. In fact, if SMP data are indicative of overall trends in proportions of marlin exported, it would appear that most of the marlin (70-90%) brought to southern processors during these months are subsequently exported. The percentage reported as being exported during other months, for example in the 3rd and fourth quarters when total catches (and processing numbers) are much higher and occur predominantly in the northern region, is much lower (<30%). It is likely that the variation in proportion of marlin exported will be linked to the flesh quality of the fish, which varies seasonally depending on whether they are feeding (good quality flesh, particularly in larger individuals) or just post spawning (poor quality flesh). It is important that economic estimates of striped marlin value to the fishery take into account its export value, in addition to value to local market. The Australian Bureau of Agricultural and Resource Economics (ABARE) compiles GVP estimates for species caught in the tuna and billfish fisheries. GVP estimates calculated for striped marlin attempt to take into account the

106


0

200

400

600

800

1000

1200

1997 1998 1999 2000 2001Year

0102030405060708090100

Number of marlin% sampled

A.

0

50

100

150

200

250

1997 1998 1999 2000 2001Year

0102030405060708090100

Number

% Exported

B.

0

5000

10000

15000

20000

25000

30000

1997 1998 1999 2000 2001Year

0

3

6

9

12

15Weight (kg)Price ($)

C.

Figure 5.13 –The Size Monitoring Program Data for striped marlin caught and processed on the east coast of Australia between August 1997 and July 2001 can be categorised by a) Total number of marlin for which processing data was collected, and number for which data was collected as a percentage of the total number of marlin landed by east coast longliners. b) Number of marlin recorded by processors as being exported, and percentage of marlin exported (relative to total number for which data was collected); c) Total monthly amount sold (weight in kilograms) and mean price of striped marlin sold at the Sydney fish market (Source; SMP reports 1997-2001; Sydney Fish Market Online database, 2002; AFZIS, 2002).

107


value (beach price to the fishermen) of both domestic market and export bound striped marlin catches. In the 2001/2 financial year, ABARE estimated a GVP for striped marlin in the eastern tuna and billfish fishery of $7.9 million. The export value of this catch would be greater given the packaging and transport costs associated with exporting (David Galeano, ABARE; personal communication). Local markets, specifically the Sydney Fish Market, also record weights and prices of marlin sold. Weight data indicate peaks in supply and sales in the 4th quarter of each year, and a smaller peak in the March-May months. Predictably, increased supply results in reduced prices, although other factors are also causing price variation. Mean Sydney Fish Market prices for striped marlin have fluctuated around $6/kg since 2001.

5.5.4 Weight frequencies and mean weights analyses

Over the period of the SMP (1997-2001), size data for striped marlin has been collected from 8 different processors, with the total number of processors contributing data each year ranging from 4 to 8. Northern processors (processors north of and including Sydney) are the largest recorded processors of marlin in Australia (or at least supply the greatest amount of data to the SMP), with peaks in their total processed weight occurring in the 4th quarter of each year. However, the peaks in processing of striped marlin that occur in the second quarter (Figure 5.13) are due to increased levels reported by southern processors (processors located south of Sydney). Hence the amount of striped marlin processed varies by time and by region. The data time series for these processors are continuous and will reflect the amount of catch being brought in by the ETBF boats that operate in their area. Striped marlin brought in to be processed in the southern region during the second quarter predominantly have a considerably higher mean weight per marlin (mean ranging between 80-110kg) than those being processed to the north. Mean estimated weight of marlin brought into northern processors fluctuates mostly in the range of 60-85 kg, with significant reduction in mean weight in the first quarter of each year (Figure 5.14). This may reflect the reported southerly movement of larger striped marlin at this time, while juvenile or smaller marlin will tend to stay in warmer northerly waters. The overall mean estimated size for all processed marlin appears not to have declined over the period for which this data has been collected, and if anything shows a slight increasing trend. It is worth noting however that mean sizes were lower in the first half of 2001 compared to the same period in 1999 and 2000.

30405060708090

100110120

1997 1998 1999 2000 2001Year

CombinedNorthSouthLinear (Combined)Linear (South)Linear (North)

Figure 5.14 – Mean estimated weight for striped marlin brought in to southern and northern processors each month during the period 1997-2001. The dashed line represents a trend line fit for the combined mean weights over the time period. Estimated weights are scaled up from processed weights using a scaling factor of 1.25 to account for weight lost due to trunking. (Source: SMP 2002)

108


0

100

200

300

400

500

600

700

2.5 22.5 42.5 62.5 82.5 102.5 122.5 142.5 162.5 182.5 202.5 222.5

Weight (5kg interval classes)

1997/1998

1998/1999

1999/2000

2000/2001

Figure 5.15 – Estimated weight frequencies for each of the four “financial years” for striped marlin weight data collected under the Size Monitoring Program. (Source: SMP, 2002) The trend of increasing mean weight is evident when weight frequency distributions for each of the financial year periods for which data was collected are compared, with the “modal” weight class increasing over this time (Figure 5.15).

5.5.5 Analyses of weight frequency data by quarter

Due to the apparent differences in the mean size of striped marlin being caught and processed in the northern and southern regions, the following weight frequency analyses of catches over the sampling period will be analysed on a regional basis.

Northern region: There are few clear multicohort trends apparent from the weight frequency plots of striped marlin caught and processed in the northern region (Figure 5.16). There is some evidence for occurrence of smaller size classes (20-55kg) being caught in the northern fishery during the second and third quarter of some years. The smaller size class contributes less (in relative terms) to catches brought to processors in the fourth quarter (Note: longliners may discard smaller marlin during fourth quarter to concentrate on larger pre-spawning condition adults, or it may represent a lowered availability for smaller marlin, or simply that numbers of small marlin a swamped by incoming large adults). During the third and fourth quarter the relative proportion of large 100kg+ marlin being brought into the processors increases to over 4% (compared to 0.2% in the first quarter), and the overall mean size has increased. The range of sizes in the first quarter tends to be narrower when compared to the 4th, and shifted to a lower mean size, for the northern region.

Southern region: Far fewer striped marlin are reported by southern processors on an annual basis than occurs in the northern region, hence the weight frequency data set for the southern region is relatively small, and consequently it is more difficult to determine the significance of apparent trends in the data. With this in mind, the following trends were noted in the southern region weight frequency data. Firstly, capture (or retention) and processing of smaller marlin (<50kg) is extremely rare in the southern fishery, with none brought to processors in the first quarter of any year and very few in any other quarter. In total, less than 30 striped marlin of less than 50kg have been processed by southern operators between 1997 and 2001. The percentage of total marlin catch processed which comprises 100+kg marlin increases from 1.1% (1st quarter) to 8.8% (3rd quarter) and is higher than that for northern processors, in all quarters of the year. It is noteworthy that although much fewer marlin are brought in to southern processors in the third and fourth quarters, there are still captures of very large marlin (100+kg) during this period, when such fish might be expected to be in the northern region spawning grounds. Whether these represent early returnees from

109


1997 1998 1999 2n=541

Quarter 1 Quarter 2 Quarter 3 Quarter 4

n=46 n=370

n=158n=1 n=271

n=134 n=479 n=799

n=657 n=1652n=2232

Figure 5.16 – Estimated weight frequency distribution of striped marlin brought to northern processors during the period of 1997 to 2001 (Source: SMP, 2002)

110

000 2001n=449 n=747

n=208 n=173

n=869 n=2

n=2013

(north of and including Sydney) for each quarter


1997 1998 1999 20 Quarter 1 Quarter 2 Quarter 3 Quarter 4

n=40 n=1 n=14

n=22 n=0 n=405

n=29 n=7 n=82

n=3 n=27 n=25

Figure 5.17 – Estimated weight frequency distribution of striped marlin brought to southern prothe period of 1997 to 2001 (Source: SMP, 2002)

00 2001n=16 n=94

n=320 n=281

n=17

n=52

cessors (south of Sydney) for each quarter during

111


spawning, or fish that spawned at different times, or fish which never ventured to the spawning grounds is unknown.

It should be noted that the southern region is defined on the basis of only two processors (compared to 5 in the central-north), and likely takes in marlin caught in a much smaller area than that encompassed by the northern region. Furthermore, targeting and retention of striped marlin may vary between the two regions.

5.5.6 Discussion

Variations in the mean processing weights in the northern and southern regions and over the course of the year will likely reflect the movements of marlin of different maturities (and sizes) to different areas. For example it is apparent that there is a post spawning influx of larger marlin into the southerly feeding grounds off the south coast in the second quarter. Evidence suggests that the smaller marlin predominantly prefer the warmer more northerly waters. However, another factor that effects the interpretation of size data from processors is that it doesn’t provide any information on discarding and possible changes in sizes being discarded in different seasons. For example, in the northern region during the 3rd and 4th quarters when there is an influx of mature spawning size marlin into the waters off southern Queensland, longliners may be more inclined to discard smaller marlin, creating a bias in the size distribution of marlin brought to processors that does not indicate the availability of marlin of different sizes. This needs to be investigated further.

5.6 Observer data 5.6.1 Introduction

Government or scientific observers have in the past been deployed on Japanese longliners operating inside the eastern Australian AFZ between latitudes 10ºS and 39ºS (Campbell et al, 1996), and on domestic longliners trialing seabird catch mitigation devices between latitudes 30ºS-38ºS off the east coast. In the course of these observer programs, data has been collected pertaining to catches of striped marlin, including information on life status and sizes caught.

5.6.2 Life status

One of the factors fisheries managers consider when looking at the effect of a fishery upon a fish stock or upon another fishery is the life status of the species upon capture. It is important to determine total mortality and to know the proportion of released fish that will survive to either contribute to the population growth or to be accessed by a second “prioritised” fishery. Would a ban on landing actually reduce impact on population levels (i.e., help recovery of population) or increase access by a second fishery? If a species tended to be predominantly dead or near death when brought to the boat, a ban on retention may have little effect upon the species recovery and the fishery managers might instead wish to implement measures that reduce the likelihood of the species being caught at all (e.g., gear modification or time area closures in high catch regions). Off the east coast of Australia, two different observer programs have provided data which describe trends in life status of longline caught striped marlin. The CSIRO observer program that operated on Japanese longliners in the EAFZ demonstrated that 45% of longline caught striped marlin were dead when hauled on deck, and another 25% were either sluggish or injured (Campbell, 2000).

The AMFA funded TAP chute trial employed observers who, as part of their duties, also recorded life status of striped marlin. This data set is smaller, but shows different trends in life status of striped marlin caught on Australian longliners. Just over 26% of striped marlin were dead upon being hauled on deck, while a further 21% were classified as either alive and sluggish or barely alive. A number of factors may affect the life status of a marlin when brought on deck by longlining, including time hooked in water, water temperature (which can effect metabolism and rate at which energy is expended) and other factors.

112


50 633.
A
0

10

20

30

40

Alive vigorous Alive sluggish Alive injured Alive (No details) DeadLife-status

Perc

enta

ge o

f mar

lin c

augh

t

247201

152 162

60. 63
B
0

10

20

30

40

50

Alive andvigorous

Alive andsluggish

Alive - just Dead anddamaged

Dead andflexible

Dead in rigour

Life status

23 17

8 6 2

Figure 5.18 – Life status of striped marlin caught by a) Japanese longliners in the Eastern AFZ, b) Australian longliners off the east coast, during TAP chute trials. (Source: Campbell, 2000; AFMA, 2003).

The Secretariat of the Pacific Community (SPC) has also reviewed data on the life status of longline caught striped marlin which had been collected by the National Marine Fisheries Service (NMFS) in Honolulu, and by the Fara Seas Fisheries Research Laboratory in Japan (Bailey et al, 1996). NMFS data from observers on US domestic longliners indicated that 45.5% of marlin were dead upon hauling, while on Japanese longliners, 23.3% of marlin were dead when hauled on deck (Bailey et al, 1996). Observer data from New Zealand’s surface longline fishery indicated that just over 30% of striped marlin were dead when hauled on deck in this fishery (Francis et al, 2000). A longline-based post release survival study undertaken for blue marlin indicated 77.8% post-release survival (for marlin alive at release) (Kerstetter et al. 2003).

5.7 Status as non-target species Striped marlin is still currently considered a byproduct species in Australias’ tuna and billfish fisheries. Traditionally, Australian longliners have targeted yellowfin, bigeye and broadbill swordfish. However, an increase in striped marlin catches and catch rates during the late 1990s, combined with its high market price in Japan, to which a substantial proportion of marlin is exported, led to questions over whether this species has been deliberately targeted, and to what degree.

Determining whether an operation has been geared to specifically target a species is extremely difficult. This is particularly so given that certain gear characteristics can be used to target more than one species. For example, a catch rate standardisation described in Chapter 8 demonstrated that shallow set hooks and live bait usage significantly increases striped marlin

113


catch rates by domestic longliners. However, this type of gear configuration may also be used to target other species, such as yellowfin tuna.

Australian longline logbooks (excluding the earliest version, the ALO2) provide a box beside the catch columns for 7 of the species (yellowfin, bigeye, swordfish, albacore, southern bluefin, striped marlin and rays bream) which can be ticked by a fishing captain when they have deliberately targeted a species. However, there has evidently been considerable confusion over the purpose of these tick-boxes.

This confusion has been two-fold. In many cases, fishers have ticked the box when a particular species has been caught, regardless of whether it was deliberately targeted or not. In other cases, most boxes have been ticked to indicate which species a fisher would retain if caught.

A survey of longline logbook database revealed that striped marlin had been listed as a target species on 8450 operations. The majority of these records (6433 records or 76.1%) listed at least 2 other species as being targets at the same time. Another 19.8% (1674) of these records listed only one other species as a target, with the majority of these being yellowfin tuna, a species which shares similar habitat preferences to striped marlin. If species listed as being targeted were truly being targeted (and not listed simply because they were caught), one would expect a high probability of at least 1 capture per operation. However, one might also expect a number of operations where the species was listed as a target but not caught. In the case of targeting operations where striped marlin and yellowfin were both were listed as the target, 20 records listing striped marlin as the second target species recorded no retained marlin catch at all of this species. However, all these operations recorded discarded striped marlin. Are fishermen misinterpreting the logbook and only listing caught/discarded species as targets? If so, the logbook needs to be amended to make this specification clearer.

Another, but still problematic, method for attempting to identify whether a species is being targeted is to look for the occurrence of high catch time-area strata in which very little catch is taken of species other than the one being suspected of being targeted. An example of such an occurrence for striped marlin occurred in 2002. Over a 30 day period, in a small 4º area off the east coast, 80 000 kg of striped marlin was taken by longliners. This was greater than 4.5 times the total catch of all other target species combined (yellowfin, bigeye, albacore and swordfish), taken in the same time area strata. Such targeting is likely to be opportunistic in nature, with fishers taking advantage of a high abundance region and period for this species. Incentive to target striped marlin may be even greater if other target species are in very low abundance at the time. It should also be kept in mind that this species commands prices on the Japanese sashimi market that are comparable to those for bigeye tuna, hence a clear financial motivation for taking this species when it is abundant does exist. In some time-area strata, processor records indicate between 70-90% of marlin are exported to Japan. It seems unlikely that over such a period, boats were heading out trying to target only species other than striped marlin. This is not the only instance of striped marlin being taken in large quantities in the absence of significant catches of other target species. It seems highly likely that targeting of striped marlin has occurred and is likely to occur in future.

5.8 Discarding Very few striped marlin are discarded in the Eastern Tuna and Billfish Fishery. According to longline logbook data maintained by the AFMA, reported discarding rates for this species were low in the early stages of the fishery (1985-1988) then fluctuated around 7-10% until 1993 when overall reported catch was low (<400 fish per year). Between 1994 and 1998 discarding rates were very low (~1-2%) but have since increased slightly in the last few years. The average discarding rate over the past 5 years is only 2.6%, which is comparable to those of target species such as yellowfin tuna and swordfish (~1-2% each) and less than that for bigeye (~3.5%). Recent observer data obtained from the TAP chute trial off the east coast indicated that of 98 observed striped marlin captures only 3 marlin were discarded.

114


5.9 New Zealand surface longline catch and effort trends 5.9.1 Introduction

Longliners have operated in New Zealand waters since the early 1950s, when Japanese (and later Taiwanese and Korean) high seas vessels started exploiting the fisheries resources in this area (Francis et. al, 2000). There was no formal licensing for these vessels until 1979. Both foreign chartered and domestic longliners entered the fishery in the 1988-9 season, while foreign licensed vessels ceased fishing in NZ EEZ in 1995 (Francis et. al, 2000). The main target species of this fishery are southern bluefin tuna (62% of sets) and bigeye tuna (28%). Other species such as albacore, yellowfin, northern bluefin tuna, and broadbill swordfish have also been occasionally targeted (Francis et. al, 2000). Numerous other bycatch species are also taken. However, since 1987, all foreign licensed vessels have been required to release any billfish caught within the EEZ, while domestic fishers must release all billfish excepting broadbill swordfish. This regulation was enforced due to concern expressed by recreational fishers, that commercial boats taking striped marlin were causing a reduction in recreational catch rates (See Chapter 7 – interactions analyses for further discussion of this issue).

Data presented in this section was obtained from National Institute of Water and Atmospheric Research (NIWA), who manage the catch and effort database for the Ministry of Fisheries. It spans the period 1991-2001. It pertains to surface longlining only, north of 40ºS (few marlin are caught south of this latitude). Data was supplied summed by one degree latitudinal and longitudinal squares and was sourced from the Tuna Longlining Catch Effort Returns (TCLERs). Catch from another database (Catch Effort and Landing Returns – CELRs) was not included but is minor. Given that striped marlin are not targeted in New Zealand, CPUE data can only be determined on a non-target catch basis. Furthermore the extent of reporting of catch of striped marlin in this fishery are uncertain and likely to be lower than actual catch rates, hence estimates of catch rates based on commercial data are likely to be an underestimate. Observer data is thought to be much more reliable, and the outcomes of an analyses of observer data is presented in the following section.

5.9.2 Trends in catch and effort

Foreign effort declined in the New Zealand surface pelagic longline fishery as foreign vessels were gradually excluded during the early 90s and then were excluded completely from the EEZ in 1995. At the same time, effort by the domestic fleet was increasing, and this is particularly apparent from 1997 onwards (Figure 5.19). Effort had peaked at just over 8 million hooks in 2001 (Figure 5.7). The reported catch of striped marlin is comparatively low compared to that off the east coast of Australia throughout the 1990s, but peaked at just over 2000 marlin 1999. However, the cautions regarding the accuracy of the reported catch need be kept in mind. From 1993-4 there was increased voluntary reporting of striped marlin catch and releases and from 1996-7 the Ministry of Fisheries required all marlin catch releases be reported. Francis et al. (2000) compared observed catches with reported catches and found evidence to suggest that reported catch and catch rates were considerably below the observed levels. Observed catch rates were double that for reported catch rates in the main catch season (February to March). While effort is relatively consistent in all quarters, catches of striped marlin (and catch rates) decline in nearshore areas in the second and third quarters. High catch rates areas shift east to the Tasman Sea in the second quarter while nearshore catch rates are lower in second and third quarters.

5.9 Summary and discussion This chapter has detailed catch and effort trends for Australian and New Zealand domestic fleets and the Japanese distant water longline fleets operating in waters adjacent to Australia and New Zealand and catching striped marlin. In relation to historical trends, four main events are important to note. Firstly, Japan has fished throughout the EIO and SWPO since the

115


Catch Effort CPUE (fish/1000 hooks)

Q1 Q1 Q1

Q2 Q2 Q2

Q3 Q3 Q3

Q4 Q4 Q4

Catch0.00 - 4.50

4.51 - 16.50

16.51 - 35.44

35.45 - 56.71

56.72 - 242.60

Effort200.00 - 14500.00

14500.01 - 43865.55

43865.56 - 177910.17

177910.18 - 390852.58

390852.59 - 1050663.75

CPUE0.0000 - 0.0453

0.0454 - 0.2283

0.2284 - 0.5587

0.5588 - 0.8551

0.8552 - 1.9608 Figure 5.19 – Mean quarterly catch, effort and catch per unit effort (CPUE: marlin/1000 hooks) for New Zealand’s tuna longliners catching striped marlin, during 1991-2002.

1960s, including within Australian and New Zealand EEZs. They were excluded from NZ waters in 1995 and Australian waters in 1997. Prior to this, effort in the SWPO ranged between 25-45 million hooks per year, but by 2000 had declined to less than 20 million hooks. Similarly, catch of striped marlin had fluctuated around 10 000 fish per year, with peak years in 1982 (>20 000) and 1989 (>15 000) but 1999 and 2000 saw the lowest catches (<5000 marlin) and catch rates in this region.

Both Australian and New Zealand domestic fleets have expanded in range and level of effort during the 1990s. SWTBF effort reached just over 6 million hooks in 2000 and 2001 (before dropping to 5.6 million in 2002) while ETBF effort had passed 11 million by 2002. A landing

116


ban in WA in 1999 has meant that striped marlin are discarded by longliners and there is uncertainty over the accuracy of reported discard numbers in that fishery. In the ETBF, striped marlin catch and catch rates have increased rapidly since 1994 and catch has been greater than 8000 fish (~ 700 mt whole weight) per year between 1999-2001, peaking at 9838 in 2001 marlin before dropping to 6715 in 2002. New Zealand longliners have not been allowed to retain striped marlin since 1987, and prior to 1996/7 when compulsory discard reporting was enforced, discard data are considered unreliable. Reported discards have been generally less than 1000 per year in this fishery. Observer data suggest current reporting may still underestimate true catch levels and catch rates (Francis et al, 2000).

The reason why striped marlin nominal catch rates increased during the mid-late 1990s in the ETBF is as yet unknown. A number of possible explanations can be put forward, including:

1) Exclusion of the Japanese longliners from the EAFZ. It is interesting to note that the years in which the highest catch rates were recorded in the ETBF (1999-2001) correspond to the period for which Japanese effort, catches, and catch rates for striped marlin in the SWPO were at an all time low. However, large catch and high catch rates were recorded in 1998 by Japan, when ETBF catch rates were also very high. Furthermore, Japanese catch rates were increasing over the four years prior to their exclusion from the AFZ. Potential interaction between Japanese and Australian and New Zealand domestic fleets is discussed further in Chapter 7.

2) An increase in the availability or abundance of striped marlin in the SWPO. This could occur through movement of marlin from other areas in the Pacific (due perhaps to environmental/oceanographic phenomena) or through good recruitment in this region. The south Coral Sea and waters extending east of this are spawning grounds for striped marlin but too little is known of stock structure and regional fidelity to determine if increased abundance occurred, and if so, was due to local or ocean wide population factors. The standardised catch rate presented in Chapter 8 supported the “increased abundance” theory, however this represented only a preliminary exploratory analysis.

3) Increased targeting, fishing efficiency and/or expansion of the fishery into high abundance waters.

These and other possible mechanisms are analysed and discussed in Chapter 8, which presents a standardised catch rate as an index of abundance both at whole fishery and sub-regional scales, utilising domestic catch and effort data and remote sensed environmental data.

The ETBF differs from the SWTBF and the New Zealand domestic fisheries in that striped marlin can be retained, landed and sold. Given that it is a valuable species in Japanese sashimi markets, as well as in domestic markets, there exists obvious financial incentive to do this. But determining whether this species is being targeted is highly problematic. Whether or not striped marlin is considered a target species has important implications for the way the species is managed within the fishery. Certainly the levels of total catch in recent years have approached those of official target species such as bigeye tuna, and the economic gain associated with this catch is in the order of millions of dollars (~$7.9 million in 2001/2002). These factors, along with the fact that fishermen regularly list it as a target species in logbooks, its very low reported discarding rate (comparable to other target species, and much lower than most other byproduct species), and that in some time-area strata, significant and sustained catches are taken in the absence of significant catch of any other target species, all suggest that the status of this species as byproduct should be carefully reviewed.

The following chapter will review all collected data from the recreational fishery (both Australian and New Zealand) for striped marlin, and draw comparison to trends in catch, effort, distribution and size of marlin as inferred from current analyses of commercial data. This will facilitate analyses of fisheries interactions to be presented in Chapter 7.

117


118

5. review of domestic and regional longline catch and effort...

Documents