Abstract.- Gastrointestinal tract contents were evaluated from Prey selection by northern fur seals 73 female andjuvenile male north ern fur seals (Callorhinus ursinus) fCallorhinus ursinusJ in the eastern for analysis of their diet in the Bering Sea. Fur seals were col Bering Sea lected from August to October of 1981, 1982, and 1985. Juvenile Elizabeth Sinclair walleye pollock (Theragra chalco gramma) and gonatid squid were Thomas Loughlin the primary prey. Pacific herring National Marine Mammal Laboratory. Alaska Fisheries Science Center; (Clupea pallasi) and capelin National Marine Fisheries Service. NOM
144 Sinclair et al.: Prey selection by Ca/lorhinus ursinus 145
.,...... __- .....-----...... ------.....--.....-----""""1- 590 N
• seal samples (all years) o Marinovich mIdwater 1raWIs 6 Diamond mldwater trawls
Middle Shelf Damein '11-71/
.. 11-17 " ~St.P8uII. .-'7 45-62 0"-'1/ ~ . - '7• 3-'~ • 35 St. GeooQe I. --__ .~...... '%~_ ~ ~ 34 V _ !~/.:.::.:~ ~2B ~ ... '. =,30 " ~-'~" Ii:> .~---- ••_., .'-" l Ail..·,------·, ...- 560 .'e(,...._..._....._...~.._" \ 41-44 i "'flo, t...... _...... , \~ DO ,...... _...J \ .1/3 • 1/1/ 4to.DO ~. 0 • 1/1 55 Oceenlc Domein 20 1' " _.:::.:, "",J III (/ .....+' "'...... (~:.~-_ ....-_. __ ._---_.:) ~ ...~ ~...... /.----.... 0. 6-:>«!/J4' -----.....--__.---.....--__-- --....;.f'---...,..;...... ;;;...;._-----+ 540 173 0 W 1720 171 0 1700 1690 168· 1670 1660 1650 164 0
Figure 1 The study area with midwater trawl locations and northern fur seal collection positions. All midwater trawls were conducted in 1985. Seal numbers 1-17 were collected in 1981, 18-40 were collected in 1982, and 41-83 were collected in 1985.
1982; and from 6 to 16 August 1985. Collections statoliths were preserved in 70% isopropyl alcohol. were made within 185 km of the Pribilof Islands Prey identification was based on all remains, in over the continental shelf, continental slope, and cluding otoliths. Otoliths were not used for fish iden oceanic domain of the eastern Bering Sea (Fig. 1). tification in earlier fur seal diet studies because Seals were shot from a small craft and returned stomach samples were stored in formalin, which to the NOAA ship Miller Freeman (65-m stern dissolves otoliths. Techniques and references for the trawler) for examination within 1.5 hours ofcollec identification of prey based on otoliths include Fitch tion. The esophagus of each seal was checked for and Brownell (1968), Morrow (1979), Frost and food as an indication of regurgitation, and the gas Lowry (1981), and otolith reference collections (see trointestinal (GI) tract was removed and frozen. Acknowledgments). References for cephalopod beak Gastrointestinal tract contents were later thawed and statolith identification include Clarke (1962), and gently rinsed through a series ofgraded sieves Young (1972), Roper and Young (1975), Clarke (0.71, 1.00 or 1.40, and 4.75 mm in 1981 and 1982; (1986), and beak and statolith reference collections 0.50, 1.00, 1.40, and 4.75 mm in 1985). Fleshy re (see Acknowledgments). A tooth was collected from mains were preserved in 10% formalin. Fish otoliths each fur seal that was shot and ages were derived and bones were stored dry. Cephalopod rostra and from direct readings ofcanine tooth sections follow- 146 Fishery Bulletin 92( JI. J994 ing Scheffer (1950b). In the analysis ofdata, males cannot presently be separated based on beak struc and females of all ages were treated as one group ture alone (Clarke, 1986). because of small sample sizes. The highest number of either upper or lower Trawl collections of potential seal prey cephalopod beaks and left or right otoliths was re corded as the maximum number of each species Trawls were conducted throughout the study area present. If deterioration made some left and right from the Miller Freeman between 1900 and 0600 otoliths of a species indistinguishable, they were hours within the vicinity of seal collections (Fig. 1). counted and the total was divided by 2. The fre Both bottom and midwater trawls were conducted quency of occurrence and number of individuals to provide a relative measure ofthe availability and from each prey taxon was calculated for each seal. size of potential fur seal prey species. Bottom trawls The fork length (FL) ofpollock and dorsal mantle were made at 52-498 m (x=139 m) depths with an length (DML) ofsquid was measured directly when 83/112 Eastern bottom trawl n 7-m width, 2.3-m whole prey were present in the stomachs. In the height mouth opening; 3.2-cm codend liner mesh; absence of whole prey, body size was estimated by 360-mesh circumference; 200-mesh depth; 30-m measurement of otoliths and beaks. The maximum bridle). Thirty-nine bottom trawls were conducted length of pollock otoliths and lower rostral length in 1981 (14 October-4 November), 51 in 1982 (24 (LRL) ofgonatid squid beaks were measured to the September-8 October), and 26 in 1985 (5 August nearest 0.05 mm with vernier calipers. Squid DML's 22 August). Seven 1985 trawls were made beyond were estimated by comparison of LRL measure maximum recorded dive depths ofadult female seals ments to the LRIJDML relationship of 51 gonatid (257 m; Ponganis et aI., 1992), They were included squid caught in trawls conducted in the vicinity of in analyses because the species and size offish and seal collections. Walleye pollock fork lengths were squid caught were consistent with those caught by estimated by regression against otolith length (Frost bottom trawl within seal dive depths. and Lowry, 1981). For otoliths measuring: Collection and sorting methods and calculation of bottom trawl catch per unit of effort (CPUE) values followed Smith and Bakkala (1982). The total bot > 1O.0mrn,(FL) Y =3.175X -9.770 (R =0.968) tom trawl catch was randomly split into a sample ~ 1O.0mrn. (FL)Y= 2.246X -0.510 (R= 0.981). of about 2500 kg. Individual species of fishes were identified and weighed (wet) and CPUE (no./ha) was Walleye pollock ages were estimated from these estimated based on distance trawled. In 1981 and lengths based on length-age relation described by 1982, cephalopods were classified as squid or octo Smith (1981) and Walline (1983) for walleye pollock pus and discarded. In 1985, all cephalopods were from the Bering Sea. identified, sexed, weighed, and frozen whole. Beaks Otoliths may dissolve or erode to varying degrees were extracted and stored in 70% isopropyl alcohol. depending on their size and duration in fur seal Sex and age determination and body length mea stomachs. We evaluated the bias introduced in FL surements were made on a subsample of up to 200 estimates due to eroded otoliths by assigning walleye pollock from each trawl. Fork lengths were otoliths to four condition categories (excellent, good, measured to the nearest centimeter. Saccular fair, and poor) based on amount ofwear. After qual otoliths were collected for age determination (Smith ity categorization, the maximum lengths ofotoliths and Bakkala, 1982) and stored in 70% isopropyl (except those in "poor" condition) were measured for alcohol. Walleye pollock CPUE was calculated by age estimation ofbody length by regression, and length and body length. For purposes of this study, age frequencies ofeach category were determined inde length frequencies for male and female walleye pol pendently. lock were combined for each of the three years. Cephalopod beaks are more resistant to digestion Midwater trawls were made in 1985 with a Dia than otoliths and were typically identifiable. Beaks mond midwater net (n=8) no-16 fm mouth opening; with chipped, worn, or broken rostra were rare and 3.2--cm codend liner mesh; 354--mesh circumference; were not measured. Cephalopod beaks were identi and 160-mesh depth with 2-m bridles) and a fied to species when possible, but most were catego Marinovich herring trawl (n=15) (6.1-m width, 6.1 rized into two groups referred to as Gonatopsis bo m height mouth opening; 1-em codend liner mesh; realis-Berryteuthis magister or Gonatus madokai 150-mesh circumference; and 350-mesh depth with Gonatus middendorffi. The two individual species 10-m bridles). Specific trawling positions were cho within each group can be separated based on their sen within the vicinity of northern fur seal collec external morphology and statolith structure, but tion areas based on the presence offish or squid as Sinclair et aJ.: Prey selection by Callorhinus ursinus 147 indicated on 38 kHz echosounders and a chromoscope. excluded from the analysis. Of the 73 animals in Midwater towing depths measured by an attached cluded in the analysis, 13 were juvenile males, 3 transducer ranged from 22 to 340 m (x=143 m). were juvenile females and 57 were adult females. All species of fish and cephalopods collected in Most fur seals were collected over depths less than midwater trawls were identified and counted. The 200 m within the outer shelf domain (Fig. 1). CPUE and frequency of occurrence of each species, Fish represented 89% and cephalopods 11% of LRL and sex of gonatid squid, and walleye pollock prey numbers for all three sample years combined. frequency ofoccurrence by age and length were cal One-hundred percent of the GI tracts had fish re culated separately for each trawl type. mains and 82% of all samples contained walleye pollock. A total of2,658 walleye pollock otoliths were measured. In all years combined, juvenile walleye Comparison of seal diet and trawl collections pollock (3-20 cm FL) were the most numerous and The Odds Ratio (Fleiss, 1981) was used to compare frequently occurring prey species. Sixty-five percent prey availability (as determined by midwater and of prey walleye pollock were from the Q-age group bottom trawls) with selection ofprey by fur seals for (3-13 cm FL) and 31% were from age group 1 (13 each sample year: 20 cm FL). Only 4% of prey pollock were from age group 2 (20+ cm FL) and older. plq2 Gonatid squids occurred in 36% of the samples, 0=-2-1' pq but in comparison with pollock, they were not con sumed in large numbers (Fig. 2). Gonatus madokai where p1 = % of diet comprised by a given prey G. middendorffi and Gonatopsis borealis-Berry taxon, teuthis magister were the second most frequently q1 % of diet comprised by all other prey = occurring prey in all years combined. Seventy-nine taxon, percent ofthe 389 beaks measured were from squid p2 % offood complex in environment com = 5-12 cm DML. prised by a taxa, and Northern smoothtongue (Leuroglossus schmidti), q2 % offood complex in environment com = a bathylagid deepsea smelt, was the second most prised by all other taxa. numerous fish prey overall (Fig. 2) even though it Values were calculated for number of each prey was found only in 1985 (Table 1). Northern smooth species and percent frequency of occurrence among tongue composed a higher percentage of the total seals, and CPUE values (no./ha) for each trawl type. number offish than walleye pollock ~2 years old for Values for p2 and q2 were also calculated for the all sample years combined. Atka mackerel trawl types combined in order to provide a compre Percent number/frequency o 10 20 30 40 50 60 70 80 90 100 walleye pollock (Theragra chalcogramma) gonatid squid (Gonatidae) Atka mackerel (Pleurogrammus monopterygius) northern smoothtongue (Leuroglossus schmidti) • Percent of total number of prey, all years Salmoniformes • Percent frequency of (Osmeridae) occurrence of prey, all years Figure 2 Percent of total number and frequency of occurrence of primary prey in northern fur seal (Callorhinus ursinus) gastrointestinal tracts for sample years 1981, 1982, and 1985 combined. Species shown include the top three prey from each sample year. borealis-Berryteuthis magister. Seals collected over Calculation of the Odds Ratio and Z-statistic on the continental shelfcontained the remains ofwall 1985 data with midwater and bottom trawl catch eye pollock ofall ages and squids, especially Gonatus combined showed statistically significant positive madokai-G. middendorffi. Adult walleye pollock, selection by fur seals for age-O pollock (P=0.0002I, although rare in stomach contents, were found in age-1 pollock (P Table 1 Gastrointestinal contents of73 northern fur seals (Callorhinus ursinus) collected from the Bering Sea in 1981 (n=7), 1982 (n=23), and 1985 (n=43). Tentative identifications are designated as (t). % number in each year % frequency occurrence Prey species 1981 1982 1985 1981 1982 1985 Fish Clupea pallasi 0.1 4.4 Osmeridae (t) 8.7 42.9 Salmonidae 5.4 42.9 Leuroglossus schmidti 12.7 9.3 Gadus macrocephalus (t) 0.1 7.0 Theragra chalcogramma 54.4 87.3 74.1 100 95.7 72.1 3-5cm fork length (8.8) (5.7) 5-10cm fork length (4.3) (63.9) (2.3) 10-20cm fork length (55.6) >20cm fork length (38.0) (1.4) n.7) T. chalcogramma (t) 0.1 0.1 8.7 4.7 unidentified Gadidae 0.9 20.9 Lycodes sp. 1.1 0.5 14.3 Pleurogrammus monopterygius 23.9 0.1 71.4 4.4 P. monopterygius (t) 0.1 4.4 unidentified percoid 1.1 14.3 unidentified fish 5.4 0.4 0.5 14.3 13.0 25.6 Squid Gonatus berryi 0.1 2.3 G.pyros 0.1 2.3 G. tinro 0.1 2.3 G. tinro (t) 0.1 2.3 Gonatus madokai-middendorffi 0.1 4.8 4.4 34.9 Gonatus sp. 0.1 2.3 Berryteuthis magister 0.6 8.7 Gonatopsis borealis-B. magister 10.2 6.4 17.4 20.9 unidentified Gonatidae 0.1 7.0 unidentified squid 1.0 34.8 Total number prey 92 1638 2189 Total number fish 92 1445 1936 100 100 100 Total number squid 0 193 253 0 52.2 46.5 Discussion strong and the 1985 year class was considered av erage CBakkala et aI., 1987). Similarly, walleye pol The modal size distribution ofwalleye pollock in GI lock as prey in 1981 were primarily adults 3 and 4 contents of female and juvenile male fur seals re years ofage (from the 1977 and 1978 year class); in flected year-class strength projections of walleye 1982, seals ate age-O pollock exclusively; and in pollock (Fig. 7). Walleye pollock have highly variable 1985, prey pollock were primarily from the 1984 recruitment rates (Smith, 1981), and year-class year class. The concordance of pollock recruitment strength varied five-fold between 1977 and 1982 and fur seal GI content analysis indicates that the (Bakkala et aI., 1987). Population estimates based variable recruitment ofwalleye pollock affects prey on bottom trawl and midwater acoustic surveys in consumption by northern fur seals. the eastern Bering Sea indicated that the 1980 year The three basic dive patterns described for adult class (age 1 in 1981) was about half the average females in the Bering Sea are shallow, pelagic night year-class size; the 1981 year class (age 0 in 1981) time diving (most commonly to 50-60 m); deep day was the weakest observed prior to 1983; and the and-night diving over the continental shelf (most 1978 year class (age 3 in 1981) was the strongest commonly to 175 m); and some combination ofboth, observed. The 1982 and 1984 year classes were including shallow diving over the continental shelf 150 Fishery Bulletin 92111. 1994 Based on fur seal and trawl collections in this study and on distributional information ofprey (Smith. 1981; Dunn, 1983; Kubodera and Jefferts. 1984; Lynde, 1984), shallow diving fur seals over the continental shelf concentrated on juvenile walleye pollock and juvenile gonatid squid (Gonatus madokai-G. middendorffi), while shallow divers off-shelf targetedjuvenile gonatid squid tBerryteuthis magister-Gonatopsis borealis) and bathy lagid smelt. Daytime deep diving over the continental shelf would be advantageous to seals concentrating on prey (i.e., adult wall eye pollock) that tend to school at depth during daytime hours and disperse as they rise in the water column at night. Adult gonatid squid probably occur in schools at the bottom on the continental shelf and re main deep along the shelf edge during both day and night. The location and degree of concentration of prey may be closely associ ated with the hydrography of the foraging region. The hydrography of the foraging re gion may have the most direct influence on the diving patterns of fur seals. Hydrographic characteristics ofthe Bering Sea continental shelf, include a two-layered midshelf and a three-layered outer shelf • 1981 n.39 domain that may stratify and concentrate li!!I 1982 prey by species and age in a vertical plane. n_1191 Nishiyama et a1. (986) proposed that ver o 1985 ....1428 tical stratification within the eastern Bering Sea shelf serves as a "nursery layer" to con Figure 3 fine young-of-the-year pollock in the upper 40 m ofthe water column within the bound Age-length frequencies of walleye pollock Percent number 100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100 northem{~KIrmidII}smoothtDngue i'iiiiiiiiiiiiiii(-.L-----l-....L-.L-----l----L-L---L-...l.----.J gonatid aquid (~btJtrIMiW 8BnyIeu1hls ",..,) gonatid aquid (GonII"1II8dDW1nilflf."lfllrlll) OCeanic domalnlContinentai slope ContInental shelf l~) • seals • midwater trawls I • seals • midwater trawls I Figure 4 Primary species identified in fur seal ICallorhillus ursinusl gastrointestinal tracts and midwater trawls col lected on and off the eastern Bering Sea continental shelf in 1985. o 20 40 60 80 100 walleye pollock .. ------::::J (T/rfIragra cha/cognlmma) lanternlish (Myctophidae) northem smoothtongue {LeulD(IIossus schmidli} Marinovich midwater trawl gonatid squid D (Gonatidae) • Diamond midwater trawl o 50 100 150 200 waneye pollock iiiiiiiiiiliiiiiiiiiiiiiiiiiiiiiiiiii.------'------' (Theragra chalcogramma) yellowlin sole (PlIlUIOIIfICf8S asper) rock sole Bottom trawls (LepidopSlItta blllMata) • 1981 1982 Paciliccod (Gadus trI/ICI'OC8Ph8/us) •D 1985 Mean CPUE values (no./ha) Figure 5 Catch per unit of effort (CPUEl numberlhectare (no.lhal values for species caught in 1985 midwater trawls and bottom trawls in 1981. 1982, and 1985. 152 Fishery Bulletin 92( 1J. 1994 In terms ofprey species composition, the 400-,------, summer diet of female and juvenile male 1985 Trawls northern fur seals does not appear to have changed dramatically since the turn ofthe I~=I century. Pollock and gonatid squid are still Na4140 the predominant prey ofnorthern fur seals 300-+------1 in the eastern Bering Sea. More subtle changes, such as a decrease in pollock size may have occurred (Smith, 1981; Swartz t man and Haar, 1983) and could playa criti °200-+------11 cal role in foraging success of northern fur seals. Unfortunately, records of prey size I in historical fur seal diet studies are incom plete. It should be noted that Pacific herring 100 -+--1---....1I.;------t and capelin were absent from fur seal di ets in this study, despite collections in ar eas where they occurred as important prey in the past. Fluctuation in the population 0 status ofPacific herring and capelin in the 5 10 15 20 25 30 35 40 45 50 Bering Sea has been attributed to the spo Fork length (em) I I II radic and localized nature of their abun 0 2 3 4 5 dance (Turner, 1886; Meek, 1916; Favorite Age (years) et a1. 19777; Lowe 19918), overharvesting Figure 6 and displacement by walleye pollock (Wespestad and Barton, 1981; Swartzman Age-length frequencies of walleye pollock (Theragra chalcogrammal collected in bottom and midwater trawls in and Haar, 1983; Wespestad and Fried, 1985. 1983; Bakkala et aI., 1987), and/or environ- mental change such as the pronounced warming in the Gulf ofAlaska and Bering Sea over the past decade (Royer, 1989). The also form boundaries that concentrate prey. Diving absence of these previously important prey may be depths of 175 m coincide with the depth break ofthe critical to seals during successive years of weak outer continental shelf. Diving depths of 50-60 m walleye pollock year-class abundance. coincide with the depth break ofthe frontal systems Fur seals select juvenile walleye pollock as prey between the midshelf and inner shelf. despite a wide availability ofother prey types within Previous analyses of fur seal diet in the eastern their dive range. Fur seals may select their prey by Bering Sea were based primarily on a sample of size and schooling behavior, whether the prey are 3,530 stomachs collected pelagically in 1960, 1962 myctophids in oceanic waters off Japan cal records of northern fur seal diet are inadequate 100 to either support or refute an "alternate prey" ar 90 gument. However, we suggest that when juvenile walleye pollock are unavailable, such as in our 1981 80 -0 sampling season, female and juvenile fur seals se ~ ... '5>< 70 lect other specific prey ofthe same size and eat adult C::I/) lD c:: walleye pollock only ifthese other preferred prey are ii~ 60 1/)'- not available. lll~ 50 u.! During their summer breeding season, northern GiE 40 fur seals consume the most abundant and available ~e ~c:: fish and squid in the eastern Bering Sea. Walleye 8,g 30 'g.!!! pollock make up an estimated 50% of the ground a...&. 20 fish biomass in the eastern Bering Sea andAleutian 8. 10 Islands area (Walters et aI., 1988) and dense aggre gations ofO-age pollock occur off the Pribilof Islands o ~n1m1~1~1~1~1~1~ June through mid-August 10 in GI contents of seals collected between California and Alaska. Nonetheless, fur seals concentrate on 0 I- r-I an average of three primary species within each 19n 1979 1960 1981 1982 1983 1984 1985 oceanographic subregion (Perez and Bigg, 1986). In Pollock year class addition, fur seal consumption of walleye pollock, Figure 7 gonatid squid, and bathylagid smelt in the eastern Estimates of walleye pollock ITheragra Bering Sea is consistent throughout historical chalcogramma> year-class strength 1978 records, despite the wide variety of prey available 85 IBakkala et aI., 1987>, and the relative to fur seals within their diving range. Based on this abundance of specific year classes in study, we conclude that female and young male fur northern fur seal gastrointestinal tracts. seals select juvenile and small-sized fish and squid. despite the availability of larger prey types within their diving range. This study demonstrates that is size and the tendency to form dense schools. In female and young male fur seals are size-selective this sense, a "juvenation" of walleye pollock in the midwater shelf and mesopelagic feeders, at least Bering Sea (Swartzman and Haar, 1983) may have during the breeding and haul-out season in the east provided fur seals with a newly abundant but un ern Bering Sea. stable resource, due to large fluctuations in the annual year-class strength of walleye pollock and due to potential displacement of other prey species Acknowledgments (Pacific herring and capelin). During years of low pollock recruitment, fur seals may switch to other Otolith identifications for the 1981 samples were prey such as capelin and Pacific herring, and expe made by the late J. Fitch. Otolith identifications for rience food limitation only if these alternate prey 1982 and 1985 were based on the otolith reference resources have been displaced or depleted. Histori- collections at the National Marine Mammal Labo- 154 Fishery Bulletin 92( 11. 1994 ratory (NMML) and Los Angeles County Museum Clarke,M. R. (LACM). Cephalopod identifications were based on 1962. The identification ofcephalopod "beaks" and the reference collections of the NMML and Oregon the relationship between beak size and total body State University (OSU). Voucher specimens of prey weight. Bull. Br. Mus. (Nat Hist.l Zool. 8:419 material (statoliths, beaks, otoliths, teeth, and 480. bones) are archived at the NMML. Identifications of Clarke, M. R. (ed.) 1986. A handbook for the identification of cepha squid and squid beaks were confirmed by C. Fiscus lopod beaks. Clarendon Press, Oxford, 273 p. (NMML, retired), K. Jefferts (OSU), and W. Walker Dunn, J. R. (LACM). Identification of fish otoliths and bones 1983. Development and distribution of the young were confirmed by G. Antonelis Jr. (NMML) and J. of northern smoothtongue, Leuroglossus schmidti Dunn (University of Washington) respectively. (Bathylagidae), in the Northeast Pacific, with com Voucher samples of juvenile pollock otoliths were ments on the systematics of the genus confirmed by A. Brown (Alaska Fisheries Science Leuroglossus Gilbert. Fish. Bull. 81(1):23-40. Center [AFSCn, K. Frost (Alaska Department of Fiscus, C. H., and H. Kajimura. Fish and Game [ADF&G], and L. Lowry [ADF&G])' 1965. Pelagic fur seal investigations. 1964. U.S. Gary Walters (AFSC) helped interpret bottom trawl Fish. Wildl. Serv. Spec. Sci. Rep. Fish. No. 522,47 values, and W. Carlson and C. Leap of the AFSC p. Available: Alaska Fish. Sci. Cent., NOAA, Graphics Unit helped produce the figures. NMFS, 7600 Sand Point Way NE, BINC15700, The following individuals contributed to the qual Seattle, WA 98115-00700. ity and content ofthis manuscript: G. Antonelis Jr., Fiscus, C. H., G. A. Baines, and F. Wilke. 1964. Pelagic fur seal investigations, Alaska wa J. Baker, L. Fritz, R. Gentry, P. Livingston, and two ters, 1962. U.S. Fish. Wildl. Servo Spec. Sci. Rep. anonymous reviewers. These data were first pre Fish. No. 475, 59 p. Available: Alaska Fish. Sci. sented in part as a Northwest and Alaska Fisher Cent., NOAA, NMFS, 7600 Sand Point Way NE., ies Center Processed Report (Hacker and Antonelis, BINC15700, Seattle, WA 98115-0070. 19869) and in full as a Masters Thesis from Oregon Fiscus, C. H., G. A. Baines, and H. Kajimura. State University (Sinclair, 1988). 1965. Pelagic fur seal investigations, Alaska, 1963. U.S. Fish. Wildl. Servo Spec. Sci. Rep. Fish. No. 489, 33 p. Available: Alaska Fish. Sci. Cent., NOAA, NMFS, 7600 Sand Point Way NE., Literature cited BinC15700, Seattle, WA 98115-0070. Fitch, J. E., and R. L. Brownell Jr. Bailey, K., R. Francis, and J. Schumacher. 1968. Fish otoliths in cetacean stomachs and their 1986. Recent information on the causes ofvariabil importance in interpreting feeding habits. ity in recruitment ofAlaska pollock in the eastern J. Fish. Res. Board Can. 25(12):2561-2574. Bering Sea: physical conditions and biological Fleiss, J. L. interactions. Int. North Pac. Comm. Bull. 1981. Statistical methods for rates and proportions, 47:155-165. 2nd ed. John Wiley & Sons, 321 p. Bakkala, R. G., 'Yo G. Wespestad, and J. J. Traynor. Fowler, C. W. 1987. Walleye pollock. In R. G. Bakkala and J. W. 1985. An evaluation ofthe role ofentanglement in Balsiger (eds.), Condition of groundfish resources the population dynamics of northern fur seals on of the eastern Bering Sea and Aleutian Islands the PribilofIslands. In R. S. Shomura and H. W. region in 1986, p. 11-29. U.S. Dep. Commer., Yoshida (eds.), Proceedings ofthe workshop on the NOAA Tech. Memo. NMFS FINWC-117. fate and impact of marine debris; 27-29 Novem BiU, M. A., and I. Fawcett. ber 1984, Honolulu, HI., p. 291-307. U.S. Dep. 1985. Two biases in diet determination ofnorthern Commer., NOAA Tech. Memo. NOAA-TM-NMFS fur seals (Callorhinus ursinus). In J. R. H. SWFC-54. Beddington, R. J. H. Beverton, and D. M. Lavigne Fowler, C. W., and D. B. Siniff. (eds.), Marine mammals and fisheries, p. 284 1992. Determining population status and the use 299. George Allen and Unwin Ltd., London. ofbiological indices for the management ofmarine mammals. In D. R. McCullough and R. H. Reginald (eds.), Wildlife 2001: populations, p. 1051-1061. Elsevier Science Publishers. London, 9 Hacker, E. S., and G. A. Antonelis Jr. 1986. Pelagic food hab England. its of northern fur seals. In T. R. Loughlin and P. Livingston Frost, K. J., and L. F. Lowry. (eds.), Summary of joint research on the diets of northem fur seals and fish in the Bering Sea during 1985, p. 5-22. NWAFC 1981. Trophic importance ofsome marine gadids in Proc. Rep. 86-19, Alaska Fish. Sci. 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