400 



Diet changes of Pacific cod 



(Gadus macrocephalus) in Pavlof Bay 



associated with climate changes 



in the Gulf of Alaska between 1980 and 1995 



Mei-Sun Yang 



Alaska Fisheries Science Center 



National Marine Fisheries Service, NOAA 



7600 Sand Point Way NE 



Seattle. Washington 98115 



E mail address mei-sunyangffinoaa gov 



The diet of Pacific cod (Gadus mac- 

 rocephalus) in the area of Pavlof Bay, 

 Alaska, was studied in the early 1980s 

 by Albers and Anderson (1985). They 

 found that the dominant prey spe- 

 cies were forage species like pandalid 

 shrimp, capelin iMallotus villosus), 

 and walleye pollock [Theragra chal- 

 cogramma). The shrimp fishery in 

 Pavlof Bay began in 1968 and closed 

 in 1980 because of low shrimp abun- 

 dance (Ruccio and Worton 1 ). Survey 

 data indicate that, during the period 

 between 1972 and 1997, the abun- 

 dance of forage species such as pan- 

 dalid shrimp and capelin declined 

 and higher trophic-level groundfish 

 such as Pacific cod increased. There 

 is a general recognition that a long- 

 term ocean climate shift in the Gulf of 

 Alaska has been partially responsible 

 for the observed reorganization of the 

 community structure (Anderson and 

 Piatt, 1999). 



Because there has been an appar- 

 ent shift in the abundance of both 

 predators and prey in Pavlof Bay, it is 

 important to understand how trophic 

 relationships may also have changed. 



1 Ruccio, M. P.. and C. L. Worton. 1999. 

 Annual management report for the shell- 

 fish fisheries of the Alaska peninsula 

 area, 1998. In Annual management 

 report for the shellfish fisheries of the 

 westward region. 1998. Regional Infor- 

 mation Report 4K99-49, 312 p. Alaska 

 Department of Fish and Game, Division of 

 Commercial Fisheries, 211 Mission Road, 

 Kodiak, Alaska 99615. 



In order to partially address this ques- 

 tion, stomach samples of Pacific cod 

 and other groundfishes were taken in 

 1995. By performing a comparison of 

 the diet of Pacific cod right after the 

 climate shift with Pacific cod and other 

 groundfishes well after the shift, this 

 analysis may demonstrate how the 

 relative abundance of prey in the Gulf 

 of Alaska may have changed. 



Methods 



Stomachs of Pacific cod, walleye pol- 

 lock, and arrowtooth flounder (Atheres- 

 thes stomias) were collected by National 

 Marine Fisheries Service (NMFS) sci- 

 entists on board the chartered vessel 

 FV Arcturus conducting a trawl survey 

 in Pavlof Bay, Alaska. (Fig. 1) from 5 

 August to 7 August 1995. The survey 

 targeted shrimp and used a high- 

 opening net with small mesh (32-mm 

 stretched mesh). Each tow was about 

 1.2 km in length. The average depth of 

 the 13 hauls where stomachs were col- 

 lected was 108.9 (±9.5) m with a range 

 from 90 to 123 m. When a sampled 

 stomach was retained, it was put in a 

 cloth stomach bag. A field tag with the 

 species name, fork length (FL in cm) of 

 the fish, and haul data (vessel, cruise, 

 haul number, specimen number) was 

 also put in the bag. All the samples col- 

 lected were then preserved in buckets 

 containing a 10% formalin solution. 

 When the samples arrived at the labo- 

 ratory, they were transferred into 70% 

 ethanol before the stomach contents 



were analyzed. In the laboratory, the 

 stomach was cut open, the contents 

 were removed and blotted with a paper 

 towel. Wet weight was then recorded to 

 the nearest 0.1 g. After obtaining the 

 total weight for a stomach's contents, 

 the contents were placed in a Petri dish 

 and examined under a microscope. 

 Each prey item was classified to the 

 lowest practical taxonomic level. Prey 

 weights and numbers of commercially 

 important fish were recorded. Stan- 

 dard lengths of prey fish and carapace 

 width of crabs were also recorded. The 

 diet of Pacific cod was summarized to 

 show the percent frequency of occur- 

 rence, the percentage by number, and 

 the percentage of the total weight of 

 each prey item found in the stomachs. 

 Stomach contents of walleye pollock 

 and arrowtooth flounder were ana- 

 lyzed for comparisons. 



Results 



Of 130 Pacific cod stomachs analyzed. 

 129 contained food. Pacific cod sizes 

 ranged from 40 to 80 cm FL (fork 

 length); a mean size was 55.4 (SD 

 ±7.2) cm. 



Polychaetes, crangonid shrimp, pea 

 crab, and clams were the most fre- 

 quently found prey items in Pacific cod 

 stomachs (Table 1). However, in terms 

 of weight, eelpouts (zoarcids). Tanner 

 crab (Chionoecetes bairdi), crangonid 

 shrimp, hermit crab, and polychaetes 

 were the most important prey of Pa- 

 cific cod. Pandalid shrimp, spinyhead 

 sculpin (Dasycottus setiger), prickle- 

 backs (stichaeid). Pacific sandlance 

 (Ammodytes hexapterus), arrowtooth 

 flounder lAtheresthes stomias), and 

 flathead sole (Hippoglossoides elasso- 

 don ) were minor prey. 



Invertebrates (mainly crangonid 

 shrimp, polychaetes, and crabs) were 

 the principal prey of Pacific cod smaller 

 than 60 cm (Fig. 2). There were nine 

 prey categories as shown in Figure 2. 

 The miscellaneous prey included Si- 

 puncula, Echiura. fish offal (processed 



Manuscript approved for publication 

 2 I I Vcember 2003 by Scientific Editor. 



Manuscript received 20 January 2004 

 at NMFS Scientific Publications Office. 



Fish. Bull. 102:400-405(2004). 



