ENDANGERED SPECIES RESEARCHEndang Species Res

Vol. 23: 177185, 2014doi: 10.3354/esr00567

Published online March 10

INTRODUCTION

Shark populations around the world have beendecimated by decades of over-fishing (Baum et al.2003, Dulvy et al. 2008). This loss of abundanceand diversity has resulted in cascading conse-quences to ecosystems (Myers et al. 2007, Ferrettiet al. 2010). Recovery efforts for many animal pop-

ulations rely on effective management, whichbegins with robust estimations of current abun-dance and distribution (Dulvy et al. 2008). Popula-tion estimates for various shark species and popu-lations have typically been generated usingfisheries data (Musick et al. 1993, Wirsing et al.2006), although photo-identification (Holmberg etal. 2009, Rowat et al. 2009a, Chapple et al. 2011),

Inter-Research 2014 www.int-res.com*Corresponding author: westgatea@uncw.edu

Population density and abundance of baskingsharks Cetorhinus maximus in the lower Bay of

Fundy, Canada

Andrew J. Westgate1,2,*, Heather N. Koopman1,2, Zach A. Siders1,2, Sarah N. P. Wong2,3, Robert A. Ronconi2,4

1Department of Biology and Marine Biology, University of North Carolina Wilmington, 601 South College Road,Wilmington, North Carolina 28403, USA

2Grand Manan Whale and Seabird Research Station, #24 Route 776, Grand Manan, New Brunswick E5G 1A1, Canada3Department of Biology, Dalhousie University, 1355 Oxford St., Halifax, Nova Scotia B3H 4J1, Canada

4Acadia University, Department of Biology, 33 Westwood Avenue, Wolfville, Nova Scotia B4P 2R6, Canada

ABSTRACT: The conservation status of basking sharks Cetorhinus maximus in eastern Canadianwaters is not clearly understood, in part because population densities and abundances have notbeen recently estimated. On September 11, 2009 and 2011, aerial surveys of basking sharks wereconducted in the lower Bay of Fundy, Canada. Flyover tests of a wooden shark silhouette revealedthat basking sharks were visible to a depth of 5 m. The proportion of time basking sharks wereestimated between 0 and 5 m depth (availability bias) was 19% based on 1252 h of time-depthrecorder data from 13 free-swimming sharks. During the 2 surveys, 26 sharks were sighted. Usingthe program Distance, availability bias corrected densities of 0.0513 sharks km2 (2009, 95% CI =0.0188 to 0.1402) and 0.0598 sharks km2 (2011, 95% CI = 0.0358 to 0.1001) were reported. Thiscorresponds to abundance estimates of 542 sharks (2009, 95% CI = 198 to 1482) and 632 sharks(2011, 95% CI = 377 to 1058) occupying a 10 570 km2 area in the lower Bay of Fundy. Abundancewas far lower than previously estimated using indirect methods and untested assumptions (Bay ofFundy = 4200 sharks). Previously published habitat suitability models for basking sharks in theBay of Fundy predict heterogeneous habitat use, which would lower the overall abundance esti-mate. This lower population estimate for the Bay of Fundy, coupled with a very limited capacity torespond to significant levels of anthropogenic mortality, raises concerns about the conservationstatus of eastern Canadian basking sharks.

KEY WORDS: Basking shark Bay of Fundy Abundance Population Diving behaviour Time-depth recorder

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Endang Species Res 23: 177185, 2014

boat (Williams et al. 2010) and aerial surveys havealso been employed (Rowat et al. 2009b).

In marine systems, population densities can be esti-mated using line-transect surveys (Buckland et al.2001). This technique has been used to estimate den-sities of a wide variety of marine species, includingsirenians (Pollock et al. 2006), cetaceans (Barlow &Forney 2007) and seabirds (Ronconi & Burger 2009).One of the obstacles involved in using this techniqueis determining the availability bias, or the proportionof time diving animals are available to be counted, asthey spend unknown proportions of time underwa-ter. Availability bias can be estimated in a number ofways (Laake et al. 1997, Slooten et al. 2006), includ-ing collection of detailed information on the divingbehaviour of the species being surveyed.

Basking sharks Cetorhinus maximus are the sec-ond largest fish in the world and are found circum-globally in temperate and tropical oceans (Com-pagno 2002, Skomal et al. 2009). Many basking sharkpopulations have been diminished by historicallyhigh directed catches and an unknown level of by-catch in fisheries (Francis & Duffy 2002, Southall etal. 2005, DFO 2008). Like most sharks, basking sharksare extremely sensitive to exploitation be cause ofslow growth, late maturity, limited productivity andlow population abundance (Sims 2008). Additionally,robust estimates of population densities are lackingfor most regions. These factors combined haveprompted the IUCN to list basking sharks as Vulner-able as they face a high risk of extinction in the wild(IUCN 2004). The Committee on the Status of Endan-gered Wildlife in Canada (COSEWIC) recently as -sessed eastern Canadian basking sharks as a speciesof special concern (COSEWIC 2009).

Basking sharks are found off both coasts of Can-ada, but, to date, limited work has been conducted onthese populations (DFO 2008). In the Bay of Fundy,basking sharks congregate during summer months tofeed on rich zooplankton blooms, making it an impor-tant regional study site (COSEWIC 2009). Sightingrecords have shown that sharks are typically presentin the Bay of Fundy between May and October(Siders et al. 2013).

The current population estimate for Atlantic Can-ada is 10 125 basking sharks, with no error estimate.This region was defined as the waters around New-foundland, the Gulf of St. Lawrence and ScotianSlope, and the Bay of Fundy (DFO 2008). While esti-mates for the non-Fundy habitats were made usingdata collected during aerial surveys, the Bay ofFundy estimate (4200) was based on sightingsrecorded during aerial-and boat-based surveys for

western North Atlantic right whales Eubalaenaglacialis, which were then adjusted using a numberof untested correction factors relating to the relativevisibility of basking sharks compared with rightwhales (DFO 2008). It is uncertain to what degreethis 2008 Bay of Fundy estimate reflects the actualnumber of basking sharks that utilize this area dur-ing summer.

The present study aimed to produce the first empir-ical estimates of basking shark densities for the Bayof Fundy during summer time using line-transect sur-veys. In order to assess availability bias we also col-lected 1252 h of diving data from 13 free-swimmingbasking sharks to quantify the percentage of timethey spend at various depths. Lastly, we used awooden model shark to measure how deep a bask-ing shark could be seen from the aerial survey heightof 305 m.

MATERIALS AND METHODS

Surveys

The geographic area of interest, defined here asthe lower Bay of Fundy, is shown in Fig. 1. Thedelineations of this area (10 570 km2) were arbitrar-

178

Fig. 1. Lower Bay of Fundy, Canada, showing the distribu-tion of transects flown during surveys for basking sharks Ce-torhinus maximus conducted on September 11, 2009 and2011. The spatial distribution of basking shark sightings in2009 (triangles) and 2011 (circles) is also shown. The areabounded by the thin black line delineates the study area and

represents 10570 km2. GM: Grand Manan

Westgate et al.: Abundance of basking sharks

ily defined by 2 parallel lines located at the ends ofthe track-lines and a perpendicular line runningsoutheast from Brier Island, Nova Scotia, with a1 km buffer from shore. Two aerial surveys wereconducted, the first on September 11, 2009 and thesecond on September 11, 2011. Recent habitatmodeling of basking sharks in the Bay of Fundy(Siders et al. 2013) revealed that the highest densi-ties of animals were predicted in August and Sep-tember. Weather conditions on both survey dayswere similar (winds

Endang Species Res 23: 177185, 2014

directly on the trackline (please refer to Buckland etal. 2001 for a complete explanation of distance sam-pling equations). In distance sampling analysis abasic assumption is that all sharks on the transect lineare detected: g(0) = 1, where g(0) is the probability ofdetecting sharks at distance zero. There are 2 rea-sons why sharks may not be detected: (1) availabilitybias (animals are missed because they are sub-merged) and (2) perception bias (visible animals aremissed for other reasons, e.g. sea state, observererror). While we were able to provide estimates ofthe availability bias from the TDR and flyoverrecords, we were not able to examine perceptionbias. Hence, in our abundance equation the term g(0)reflects only the interaction of availability bias.Although variable, Dawson et al. (2008) concludedthat perception bias estimates for cetaceans are usu-ally >0.9. We predict that perception values for bask-ing sharks would also be high because they are verylarge, slow-moving animals that contrast sharplywith the surrounding water.

Because of the low number of sightings, data werenot truncated prior to analysis nor were any sightingsremoved from the records. Because of low samplesizes, data were pooled and global estimates of ESWwere made in the program Distance 6.0 (Thomas etal. 2010) using the conventional distance sampling(CDS) routine. Encounter rates (n/L) for each yearwere also calculated using Distance. Distance usesseveral key function and series expansion terms formodeling the detection function. The following mod-els and series expansion terms were run: uniformmodel (with cosine or simple polynomial or hermitepolynomial expansion), half-normal(with cosine or simple polynomial orhermite polynomial) and hazard-ratemodels (with cosine or simple polyno-mial expansion). Model