aggregations among juvenile king crabs (paralithodes camtschatica, tilesius) kodiak, alaska

AGGREGATIONS AMONG JUVENILE KING CRABS (PARALITHODESCAMTSCHATICA, TILESIUS) KODIAK, ALASKA

BY GUY C. POWELL AND RICHARD B. NICKERSONAlaska Department of Fish and Game, Kodiak Research Centre, Kodiak, Alaska .

IntroductionJuvenile king crabs form spherical aggrega-

tions which have been studied in the littorialzone. These groups are called pods and areanalogous to other animal congregations illus-trated by herds, flocks, and schools (Fig . 1, PlateIX). Pod investigations may provide valuable lifehistory information for establishing indices offuture abundance of commercial size crabs .United States commercial king crab catches fromAlaskan waters increased from 2 million roundpounds in 1952 to 55 million pounds in 1961(Gray et al.).* Presented in this report arespecific measurements and observations of onepod studied by divers during 57 days in 1960, andalso a survey of unpublished data obtained bypersonal correspondence .The most serious limitation in studying

juvenile king crabs is the lack of suitable fishinggear selective for small crabs . The distribution ofpods is unknown except for the littoral zone andthe proportion which these represent remains tobe learned .

The Japanese king crab tangle net fishery inthe eastern Bering Sea from 1956 to 1959 cap-tured 5,495 juvenile crabs 2 to 33 millimetres incarapace length (Anonymous, 1959) . Only 15of these crabs were of the size found in pods ;the rest were smaller. All were among "seeminglymarine Hydrozoa" which had become attachedto the nets. They were caught 75 to 115 milesnorthwest and seaward of Port Moller, Alaska,at an average depth of 30 fathoms . Since tanglenets are 18 inch stretched mesh and highlyselective for large crabs, few pod-size crabs werecaught, apparently because they seldom clingto hydroid colonies . The population of pod-sized crabs therefore, is believed to have beengreatly in excess of their representation.

Adult king crabs are known to inhabit deepwater as evidenced by the large scale commercialharvest from depths of 100 to 150 fathoms dur-ing late summer. Adults also continue to form*Gray, George W. Jr., Robert J . Simon, Robert Roys,and Dexter F. Lall (1964) . Development of the KodiakIsland king crab fishery . Alaska Department of Fish andGame, Kodiak, Alaska. pp . 1-14 . Typewritten .

374

aggregations which have been observed asstructurally and numerically less dense thanthose of juveniles. On one occasion numerouspiles of large, adult, new shell males were ob-served by the senior author diving at a depth ofsix fathoms. The low pyramid-shaped pileswere 25 to 50 feet apart and each contained 40to 50 individuals in three distinct layers. Com-mercial fishermen often mention dense schoolsof large crabs, but the community arrangementis believed dissimilar to podded juveniles, partlybecause the protective demands of adults differ .Adults moult annually or less frequently, andmature females moult on the spawning groundsattended only by solitary males . Underwatertelevision has been tested and shows possibleapplication to future studies .t

Although this study does not delve into thepsychology of conscious versus unconsciousbehaviour, the authors point out that an inherentmechanism exists which is basic to the gregariousnature of king crabs . Villee, Walker & Smith(1958) state "when genes governing a tendencytowards aggregation arise in a species prove tohave survival value, natural selection will tendto preserve this inherited behaviour pattern" .W. C. Allee (1949) defines unconscious co-opera-tion as "an intra-specific relationship in the formof an animal aggregate providing survival valuefor the species" .

Carlisle (1957), discusses pod communitiesof the spider crab, Maia squimado, and mentionsthat these omnivores congregate during ecdysiswith soft-shelled individuals located centrally,probably to avoid predation from octopi andcarnivorous crabs . He states that the primarypurpose for podding is protection ; the secondaryfor mating. The primary purpose for king crabpods is believed to be protection, the secondarypurpose is thought to provide biological organ-ization and control .

Podding is believed to increase survival valueby minimizing the vulnerability to predators .tCruise report-Alaska Exploratory Cruise 64-1 UnitedStates Department of the Interior, Fish and WildlifeService, Bureau of Commercial Fisheries, ExploratoryFishing and Grea Research Base, Juneau, Alaska .

POWELL & NICKERSON : AGGREGATIONS AMONG JUVENILE KING CRABS

Table I . Record of King Crab Pod Observations in Alaska .

375

*Aggregates were elongate dome shaped piles orientated parallel to the beach . The 500,000 comprised two distinct sizeclasses, average lengths of 61 and 84 millimetres .

1Two Frimacrus isembechii were observed nearby, each eating a small king crab .2Crabs were scattered in intertidal area and were assumed to be a pod that dispanded during ebb tide .2The entire pod was captured and every crab studied .40bserved 10 pods but did not present any quantitative data .

On one occasion two horse crabs, Erimacrus

observed a pod in Akutan Bay in 15 feet ofisembeckii, were observed preying upon juvenile

water during April 1957 (Table I) .*king crabs when a pod disbanded after being

From April 1958, through June 1959, whiledisturbed by divers . Sculpins, Hemilepidotus

conducting growth studies of small king crabshemilepidotus, are known predators of post-

by skin diving at Unalaska Bay, Weber observedlarval king crabs 10 millimetres in length, but

two pods of crabs and was able to obtain moreof several hundred sculpin stomachs analysed

quantitative data . The pods were observed fromduring the past seven years no pod-size king

February 11th through February 15th, 1959 .crabs have been found. Another suspected

Sampling periods during which pods were notpredator of king crabs is Octopus hongkongensis

observed were : April 22nd to May 17th, 1958 ;although supporting evidence is lacking, for

June 2nd to June 5th, 1958 ; September 17th toonly the empty shells of bivalve mollusks and the

September 27th, 1958 ; and May 25th to Juneexoskeletons of E. isenbeckii have been observed

7th, 1959. Each time that pods were found theynear dens adjacent to pod locations. Halibut,

were of both sexes, maximum and minimumHippoglossus stenolepis, also prey on king crabs

lengths were 69 and 29 millimetres respectively .(Gray, 1964) .

The first effort to study pods occurred inPods of small crabs have been continually

Kachemack Bay, 700 miles north east ofreported along a major portion of the Alaskacoast for many years, and to date only limited

*Personal conversation with Douglas Weber at theeffort has been devoted to their study . The first

United States Bureau of Commercial Fisheries, Mont-lake Laboratory, Seattle, Washington, March 30th,

official record appears to be that of Weber who

1964 .

Date of RecorderLocation Estimated

numberCrabs insample

Averagecarapacelength ofsample

Carapacelengthrange

Method ofobserva-

tionArea Depthobservation and Agency of crabs (number) (mm .) (mm .)

5/27/63 Powell, Alaska Kodiak 30 ft . 1,5001 100 34 29-47 diving

4/25/62

Dept. Fish andGame

same

Island

same 60 ft . 500,000* 318 70 34-98 same

2/19/61 same same 20 ft . 6,000* 360 30

23-40 same

12/13/60 Sheridan, same same 3 ft . 450 156 - 18-432 skiff

5/18/60 Powell, same same 10 ft . 5263 526 36 24-16 diving

2/20/59 Weber, U .S . Fish Unalaska 4 ft . 1,000 372 51 42-69 I same

2/18/59

& Wildlife Service

same

Bay

same 8 ft . 2,000 746 41 33-62 1 same

March Bright University Kache- 10 ft . - - - 20-25& skiffthrough of Southern mack 60-90Aug. 19584 California Bay

9/11/57 Weber, U .S . Fish Akutan 15 ft . 500 242 34 29-39 beach& Wildlife Service Bay seine

l

3 7 6

ANIMAL BEHAVIOUR, Xlli, 2-3

Unalaska. Bright et al. (1960)* did not have thebenefits of diving and conducted the survey byskiff during 1958 and 1959 . During 1958 theyobserved 10 different pods throughout a six-months period but the following year withgreater effort they were unable to find anypods. Quantitative data are lacking from theirreport . They state that pods comprised of crabs20 to 40 millimetres were common and those of60 to 90 mm . scarce, but no tabular measure-ments are presented .

The SCUBA gear sampling methods employedduring the present study have obvious benefitscompared to working from a skiff . Diving en-abled observation in greater structural and be-havioural detail to be made, but descriptions ofpods are similar for all areas .

The study pod was first observed in 7 feet ofwater on May 5th, 1960, at the eastern shore ofHoliday Island, near the city of Kodiak, Alaska .The pod was subsequently studied at depths of1 to 6 feet on 9 different occasions during the57 day period ending June 30th, 1960, when itcould no longer be found (Table II) .

Table II . Dates and Depths of Pod Observation .

*The only occasion the pod was observed disbanded .

MethodsCapture and observations of the pod were

made by SCUBA equipped biologists usingneoprene-rubber "wet" suits (Fig . 2, Plate X) .

Carapace measurements to the nearest milli-metre were taken with vernier calipers. Weightsto the nearest gramme were obtained with alaboratory animal balance .

The behaviour and structure of the pod werestudied prior to capture . Caution was used toavoid disturbing the crabs lest they disband .After recording the observations, the pod was*Bright, Donald B., Floyd E. Durham & Jens W. Knud-sen (1960). King Crab Investigations of Cook Inlet,Alaska . Department of Biology, Allan HancockFoundation. University of Southern California, LosAngeles 7, California. pp . 1-180. (Mimeographed) .

encircled with a 25 foot beach seine, brought byskiff to the Kodiak boat harbour, and containedwithin a surface holding pen for five days whilenecessary data was obtained (Fig . 3, Plate X) .The length of each crab and the weight of eachgroup of fifty was recorded (data on file atKodiak Research Centre, Kodiak, Alaska) . Also,a length-weight relationship was establishedusing thirty-seven crabs of various sizes . Routineobservations including shell age and leg regen-eration were recorded . Subsequently, 502 of thecrabs were returned to the exact area of capture ;24 were retained for growth studies .

The pod was the only one observed and identi-fication was not considered necessary since thecrabs remained in the same location . Experi-mental tagging was conducted in live pens todetermine possible future use and revealedsuccessful retention of tags throughout ecdysis .

Length increase per moult was studied byplacing twenty-four purple-coloured crabs inseparate containers and measuring them againafter edcysis . The purple colour indicates thatmoulting is imminent. This system is believed tobe adequate in providing accurate growth data,because retarded increments resulting from pro-longed captivity are eliminated .

Formation of PodsYoung king crabs, 1 to 12 months old are

2 .5 to 12 .0 millimetres in carapace lengthrespectively and live in seclusion among rockcrevices, kelp patches, and other protectiveniches where they settle as larvae . They arecommonly found in the accessible littoral zoneThe maximum depth at which post-larval .crabs smaller than 16 millimetres have beencaptured was 58 fathoms (Reynolds & Powell,1963).t Crabs 9 to 19 millimetres in length arecommon on barnacle encrusted dock pilingswhere they are often found between the rays ofstarfish . These crabs are assumed to feed uponfood particles dislodged by their commensalhosts .

Between 12 and 24 months of age juvenilecrabs abandon their numerous hiding placesand congregate to form pod communities .Present data indicates that podding continuesthroughout the third year . Pod crabs have been

fReynolds and Powell (1963) . King Crab, Paralithodescamtschatica (Tilesius), Trawl Survey of Long IslandBank, East of Kodiak Island, Alaska . Alaska Depart-ment of Fish and Game. Kodiak Research Center.9 pages . Typewritten.

Date Depth Date Depth

5/5/60 7 feet 6/11/60 3 feet

5/12/60 1 foot 6/12/60 2 feet

5/13/60 1 foot 6/16/60* 5 feet

5/19/60 6 feet 6/17/60 6 feet

6/7/60 3 feet

observed only from December through Septem-ber and have always been less than 69 milli-metres in carapace length .

The only author to mention pods comprisedof four-year-old crabs, those approximately60 to 90 millimetres, was Bright et al. (1960) .Since measurements were not presented andobservations were from a skiff the larger crabsare believed to have been in elongate pile form-ations rather than pods .

Pod StructureThe study pod contained 526 crabs,247 males

and 279 females . Average length for each sexwas 36 millimetres . The size range for malesand females was 28 to 47 millimetres and 24 to 49millimetres respectively . Length-frequency dis-tribution is presented in Fig . 4. Few crabs weresmaller than 29 millimetres or larger than 45 .

Total weight of the pod was 17 .43 milligramsfor an average weight per crab of 33 grammes .The length-weight relationship established froma sample of 37 crabs of both sexes (Table III) isbest described as a straight line (Fig . 5) .

Weight deviation among similar sized in-


377

CARAPACE LENGTH (mm .)

Fig. 4 . Length frequency distribution of an entire pod of juvenile king crabs, 247males and 279 females .

dividuals was attributed in part, to varyingstages of development within the moult cycle,and to missing and regenerating appendages .

Each member of the pod faced outwards ; theonly crabs able to feed while podded were thoseon the bottom, and food was acquired by probingwith the chela into the mud .

After approximately a year, pods are believedto merge, forming large piles of crabs . On twooccasions the senior author observed elongateddome shaped piles of crabs lying parallel tothe beach at depths of 4 to 35 feet . These pilesappear to be numerous pods joined in line . OnFebruary 19th, 1961, the first group of an estim-ated 6,000 crabs averaging 30 millimetres wasobserved and described in field notes as a longwindrowed pile. A similar observation on April25th, 1962, revealed an estimated 500,000 crabs34 to 98 millimetres in carpace length havingmodes at 61 and 84 (Table 1) . One foot from anend of the 12 foot zig zag pile was a pod ofapproximately 1,000 crabs giving rise to theidea that spherical pods form elongate pileswhen they congregate . Thousands of other crabsremained scattered and feeding .

Disbanded PodPodded crabs usually dis-

band in layers ; the upper-most crabs moving downand over the lower individ-uals until each member ison the bottom . Central andlower crabs appear to waituntil they are free to move .

Crabs disband either tofeed, or to change location .Pod crabs also disbandedwhen observed closely bydivers. The study pod wasfound disbanded only onceduring the nine field tripsoccupying an area of ap-proximately fifty square feetand were grazing in a herd-like manner with all individ-uals moving in the samedirection . The group of crabswas easily herded to the leftor right when divers movedclose on either side . Whenapproached by divers indi-vidual crabs would defendthemselves by rearing back

37 8

on their third and fourth periopods withraised, outstretched chela and slash with theirdactyls .

GrowthThe 25 crabs kept in cages for growth deter-

minations all moulted within 10 days aftercapture and again 67 days later . Carapace lengthincreases for the first moult ranged from 6 to 8millimetres and averaged 21 per cent. for bothsexes (Table IV and Fig . 6) . Increases from thesecond moult are omitted due to possible ad-verse effects of prolonged captivity . These dataare compatible with growth investigations by -~ 3 sWeber & Miyahara (1961) and Powell (1964) .

Both studies revealed that crabs of both 533

sexes attain approximate lengths of 12, 35, 61, 3 31

84, and 100 millimetres respectively during thefirst 5 years . Frequency of moulting is approxim-ately 9, 4, 2, and 1 time each year, respectively .The difference between the smallest and largestpod crab was 25 millimetres, a variance at-tributed to 3 moults . The pod crabs were all ofthe same year class, 24 months of age. Ratherthan exhibiting a peak moult, pod crabs moultcontinuously . Thirty per cent . of the crabs wereeither preparing to moult or had moulted re-cently. Average length of pre-moult crab was

Fig. 5 . Length-weight relationship of 25 male and 12female king crabs .

EE

45

ANIMAL BEHAVIOUR, XIII, 2-3

53

51

49

47

45

43

41

39

37

29

27

25

23

21

19

IT

1s

31

32 33 34 35 36 37 38 39 40 41 42 43 44

CARAPACE LENGTH (mm.)

- 0

0

C9

00

0

f +

T OD

000+0

+ + +0

00

0

0

OMALE

+ FEMALE

0 350.

0

0 FEMALE

•

MALE

Fig . 6 . Carapace lengthincrease per moult 30(mm .) of 14 male and 20 25 30 35 40 mm10 female king crabs . PRE-MOULT LENGTH


Table III. Length-weight Relationship of 25 Male Crabs and 12 Females .

Table IV. Growth of 24 King Crabs taken from a Pod, 14 Male and 10 Female, Moulting during May 18-27th, 1960 .

3 7 9

Males

Females

Carapace length Carapace length Carapace lengthWeight (Gm.) Length (mm.) Weight (Gm .)

Length (mm)

Weight (Gm .) Length (mm .)

53 44 . 0 35

38 .0

48 43 .0

48 42 . 0 35

37.0

48 43 .0

47 40 . 0 34 37 .0

46 40.0

46 42 . 0 34 37 .0

38 41 .0

46 42 . 0 31 35 .0

37 39.0

42 41 . 0 30 36 .0

36 40.0

41 40 . 0 30 36 .0

32 37.0

41 40 . 0 29 36 .0

30 36 .0

38 38 . 0 27 36 .0

29 35.0

38 40 . 0 26 33 .0

28 33.0

35 37 . 0 25 33 . 0

28 36.0

35 38 . 0 24 34 .0

28 34.0

19 31 . 0

19 32.0

Premoult carapaceI

length Postmoult carapace length

Length increase Percent. increase

Males Females Males Females I

Males Females Males Females

mm. mm . mm. mm.

mm. mm. ° %

36 36 43 44

7 8 19 22

35 36 42 44

7 8 20 22

35 36 43 44

8 8 23 22

35 36 41 43

6 7 17 19

34 35 42 42

8 7 24 20

34 31 41

37

7 6 21 19

34 31 41

37

7 6 21 19

33 31 41

37

8 6 24 19

33 30 40

37

1

7 7 21 23

33 25 39

31

6 6 18 24

33 41

8 24

32 39

7 22

32 39

7 22

31 37

6 19

470 327 569 396

99 69 21 • 21'

380


less than that of post-moult crabs being 33 and40 millimetres, respectively .

Pod AbundanceThe number of pods in an area is probably

not a reliable index of year class abundance un-less the number of crabs is considered, for poddensity varies up to approximately 3,000 in-dividuals. On one occasion the pod under studydisbanded, and upon subsequent reforming itremained as two separate pods for only a day .Most observations suggest that pods soon unitewhenever they occupy the same immediatearea. In instances where pod size crabs wereobserved amassed in numbers exceeding 3,000individuals they formed elongate, dome-shapedpiles rather than pods . These piles comprisedas many as one half million crabs .Pod abundance varied in Kachemak Bay

during 1958 and 1959 according to Bright et al.(1960). During March through August 1958,ten pods were observed, but in 1959 after an in-tensive search of the same areas, no pods werefound. These data may indicate that survivalof the 1956 juvenile cohort may have been moresuccessful than that of 1957 or that the progenyof 1957 were distributed in an area where ob-servations were not conducted .

The ability to predict levels of year classsuccess will become a valuable tool for managingthis important fishery when concurrent investi-gations of juvenile distributions are conducted .

Summary1 . Juvenile king crabs were studied by bio-

logists diving with the aid of SCUBA equip-ment .2. The first record of an observation of a pod

of king crabs was made in 1957 . Since that timeapproximately 25 additional pods have beenrecorded .

3 . One-year-old king crabs, 3 to 12 milli-metres in carapace length, are found solitarilyin the protective niches of the littoral zone

especially beneath large rocks and among bottomvegetation .

4. Crabs 9 to 19 millimetres in carapacelength are common on barnacle encrusted dockpilings .

5. The smallest and largest crabs found inany pod are 24 and 69 millimetres in carapacelength, respectively . Pods therefore form duringthe latter part of the second year, exist through-out the third year, and continue a short timeinto the fourth. When the density of crabsapproaches 6,000, pod structures transforms intoelongate piles . Dome shaped piles do notcommonly occur until the fourth year whencrabs are 60 to 97 millimetres .

6. Intermingling among the first, second, andthird age classes has not been observed, probablybecause they occupy biological niches . Four-and five-year-olds intermingle, and are oftenobserved by divers at a depth of 60 to 100 feet,more often disbanded and moving, rather thanaggregated and quiet .

REFERENCESAlle, W. C. (1949) . Principles of animal Ecology . Phila-

delphia: W. B. Saunders .Anonymous (1959). King crab research by the United

States and Japan . International North PacificFisheries Commission, Annual Report for theyear. 72-78; 113-116 .

Carlisle, D . B . (1957) . On the hormonal inhibition ofmolting in Decapod Crustacea. The PlymouthLaboratory. Journal Marine Biological Association,U.K., 36, 292-307.

Gray, George W ., Jr . (1964). Halibut preying largecrustacea . Copeia (in press) .

Powell, Guy C. (1960) . Investigation of the Growth Ratesof King Crab in the Kodiak area. Alaska Depart-ment of Fish and Game, Kodiak Research Centre .

Villee, Claude Alvin (and others) (1958). General zoology .Philadelphia : W. B . Saunders .

Weber, Douglas D. & Takashi Miyahara (1962) . Growthof the adult male king crab, Paralithodes camts-chatica (Tilesius). U .S . Fish and Wildlife Service .Fishery Bull ., 200, Vol. 62, 75 p .

(Accepted for publication 1st December, 1964 ;Ms. number : 524) .


PLATE [X

Fig . 1 . Pod of king crabs on muddy bottom four feet beneath the surface .

Powell & Nickerson : Anim. Behav ., 13, 2-3


PLATE X

Fig . 2 . Biologist with diving equipment used in king crab research studies .(Reproduced from a photograph by Alaska Department of Fish and Game)

aggregations among juvenile king crabs (paralithodes camtschatica, tilesius) kodiak, alaska

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