ALBERS and ANDERSON: DIET OF PACIFIC COD 



for a significant difference between pink shrimp 

 length distributions measured from Pacific cod 

 stomachs and those measured from the trawl. Pink 

 shrimp lengths tested were ^16.5 mm CL. Ander- 

 son (1981) reported that shrimp <16.5 mm CL are 

 not fully vulnerable to trawl capture 



Population biomass estimates for pink shrimp and 

 Pacific cod were calculated using the area swept 

 technique (Alverson and Pereyra 1969). 



An estimate of pink shrimp biomass consumed by 

 Pacific cod in Pavlof Bay during the 112-d period be- 

 tween the late May and mid-September 1981 surveys 

 was determined through methods described by Minet 

 and Perodou (1978). Undigested weights (W) of pink 

 shrimp were determined from carapace lengths 

 using the weight-length relationship W = 0.000802 

 (CL)-'^^^ (calculated from Pavlof Bay pink shrimp 

 length-weight data). The mean weight of pink shrimp 

 per stomach for each 5 cm length group of Pacific 

 cod was calculated from stomachs where at least 

 80% of the pink shrimp were measurable (Ikble 1). 

 A stomach which contained <80% measurable pink 

 shrimp was deemed not suitable for determining the 

 weight of undigested shrimp consumed. These data 

 were then weighted using 768 cod lengths measured 

 during the three surveys. The mean weight of pink 

 shrimp in the stomach of an average-sized cod was 

 then estimated for each survey (Tkble 1). 



The average rates of elimination (r) of food from 

 Pacific cod stomachs collected during the May and 

 September 1981 surveys were calculated from Jones' 

 (1974) equation for food elimination rates from 

 Atlantic gadoids including Atlantic cod, Gadus 



morhua. Jones found that the rates of elimination 

 for the three species of gadoids studied were effec- 

 tively the same, adjusting for fish and meal size, and 

 temperature Since Pacific cod are very similar to 

 Atlantic cod, we used Jones' equation in the absence 

 of more relevant information: 



r (g/h) = 



j^QO.035 {To - Tc) J^O.46 QJ^li 



175 



where Tc = 6°C; temperature of the experiment 

 (Jones 1974). 



To = 5°C (May), 7°C (September); observed 

 temperature in Pavlof Bay from expen- 

 dable bathythermograph data. 



X = 98.9 g (May), 109.0 g (September); the 

 average weight of food found in one 

 stomach. The average weight of food 

 was estimated for each 5 cm length 

 group of Pacific cod and then weighted 

 using 502 cod lengths measured dur- 

 ing the May and September surveys. 

 Stomachs containing at least 60% of 

 the greatest weight of food encounter- 

 ed for each cod length group were used 

 to determine X. 



Q = 0.12; the average rate of elimination of 

 1 g of food from the stomach of a 40 

 cm gadoid (Jones 1974). 



L = 52.7 cm (May), 53.0 cm (September); 

 the average length of Pacific cod calcu- 

 lated from 236 fish measured in May 

 and 266 fish measured in September. 



E 

 C 



a 



o 

 o 



c 

 o 



T) 



c 



D 

 M 



< 











Qj 



800 



600 — 



400 — 



r— 600 



200 — 



400 



— 200 



o 





 Q. 



> 



cr 



c 



Q. 

 Q 



D 

 O 

 



:r 



<Q 

 N. 





 3 



1872 1874 



' I ' 1 ' T ' 

 1876 1878 1880 1882 



YEAR 



Figure 2.— Abundance of pink shrimp and Pacific cod from NMFS summer trawl survey data 



collected in Pavlof Bay, 1972-82. 



603 



