REXSTAD and PIKITCH: FOOD CONSUMPTION OF PACIFIC HAKE 



collection (morning, afternoon, and evening), depth 

 of capture (0-100 m, 100-200 m, >200 m), and size. 

 Chi-square tests of homogeneity (Ostle and Mensing 

 1975) were performed on the frequency of occur- 

 rence data for each prey species in these categories. 



Consumption Estimates 



Using the size-specific prey consumption informa- 

 tion derived from this study, Pacific hake popula- 

 tion abundance estimates from the 1983 NMFS 

 survey (Weinberg et al. 1984; Francis fn. 4) and 

 bioenergetics estimates from Francis (1983), trophic 

 calculations were performed to estimate ecosystem- 

 level impacts of prey consumption by Pacific hake 

 in the Columbia INPFC (International North Pacific 

 Fisheries Commission) statistical area in 1983. 



Biomass estimates were derived from two distinct 

 surveys. The bottom trawl survey estimated the ben- 

 thic component of the population. Details of these 

 estimates can be found in Weinberg et al. (1984). 

 The pelagic component of the population was esti- 

 mated by hydroacoustic methods. Size composition 

 of the pelagic segment of the population was esti- 

 mated from companion midwater trawls conducted 

 from the hydroacoustic vessel. Biomass estimates 

 for each of the five size classes sampled were deter- 

 mined from estimated numbers in each centimeter 

 size interval and length-weight regressions (Fran- 

 cis fn. 4). 



Using a mean body weight for each size class, the 

 percent of total body weight consumed daily was 

 calculated based on the equations of Francis (1983). 

 This total biomass consumption was then broken 

 down into the constituent prey categories found in 

 the stomachs of fish sampled using the percent of 

 the diet by weight. These calculations were repeated 

 for each of the five classes, and both the pelagic and 

 benthic components of the population, to derive daily 

 consumption estimates. 



Residence times provided by Francis (1983) for 

 each age class within each statistical area were con- 

 verted to residence time by size class to account for 

 the migratory behavior of Pacific hake. This pro- 

 vided consumption rate estimates summed over the 

 length of time Pacific hake are found in the Colum- 

 bia statistical area. An example of the calculations 

 used to estimate total consumption of each prey item 

 category is shown in Table 1. 



Pacific Hake-Pink Shrimp Interaction 



The relationship between the abundance of Pacific 

 hake and pink shrimp was examined via regression 



Table 1.— Calculations used to compute total consumption of 

 Thysanoessa spinifera. Column E1 = A x B/100 x C/1 00. Column 

 E2 = E1 x D. Biomass is combined benthic and pelagic com- 

 ponents of the population, BWD is percent body weight consumed 

 per day, W is percent of the diet by weight composed of T. spini- 

 fera, and Days is number of days each size class resides in the 

 Columbia INPFC Area. Note total biomass differs from value given 

 in text due to biomass of population <35 cm in length. 



analysis. Data from Francis et al. (unpubl. data) on 

 Pacific hake catches in U.S. waters from 1967 to 

 1982 were converted to catch per unit effort (CPUE) 

 based on the number of days of effort of foreign 

 stern-trawling factory ships (BMRTs). Pounds per 

 hour of pink shrimp taken in the equivalent of single- 

 rigged shrimp trawls (SRE) in California, Washing- 

 ton, and Oregon from 1968 to 1984 (Saelens and 

 Zirges 1985) were used as the dependent variable 

 in regression analyses. 



Two regressions were performed. The first used 

 hake CPUE in year i to predict shrimp CPUE in 

 year i, while the second involved a 2-yr lag (i.e., 

 Pacific hake CPUE in year i versus shrimp CPUE 

 in year i + 2). 



RESULTS 



Stomach Content Analysis 



A breakdown of the stomach contents by size class 

 of Pacific hake is presented in Table 2. Euphausiids 

 dominate the diet of small hake while decapods and 

 fish become increasingly important as Pacific hake 

 increase in size. Considering percent of the diet by 

 weight, the importance of euphausiids monotonically 

 decreases from 100 to 7.9% with increasing predator 

 size. Likewise, the importance of fish rises from 

 to 87.1% with increasing predator size. Pink shrimp 

 comprise only a minor portion of the diet, the largest 

 percentage being 4.9% for the largest size class. 

 Commercially important herring comprise nearly 

 one-third of the diet of the larger size classes. 



A previously unreported prey item, the ghost 

 shrimp, Callianassa sp., appeared in the diet of the 

 Pacific hake sampled in this study. These burrow- 



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