FISHERY BULLETIN: VOL. 87, NO. 4, 1989 



square where stomachs were sampled within a 

 crab consumption area. Since cod sometimes 

 consume only the legs of red king crab, these 

 stomachs were not included in the estimation of 

 P because this behavior may not contribute to 

 predation mortality. 



Because recent studies indicate that individual 

 prey size or weight is a more important factor 

 influencing gastric evacuation rates in fish than 

 prey type (Ursin et al. 1985; Jobling 1987), Pa- 

 cific cod daily ration (R) was calculated using 

 mean stomach content weight (S) in gi-ams for 

 each year and cod size group in the following 

 equations from Ursin et al. (1985). These equa- 

 tions describe daily ration for Atlantic cod, 

 Gadus morhua (whose diet and morphology are 

 very similar to Pacific cod), as a function of mean 

 individual prey weight (i<;) in gi'ams, and bottom 

 temperature (T)'m°C: 



R = aS 



(2) 



where a = ao w"^ and a^ = Ooo e°^^''^ and where 

 ao = 0.61 d"\ aoo = 0.33, and Oi = -0.36 for 

 North Sea temperatures were adjusted for the 

 Bering Sea using the average bottom tempera- 

 ture for Pacific cod stations sampled in each year 

 from expendable bathythermographs obtained 

 at most stations. 



The number of days (D) when snow crab were 

 vulnerable to predation was considered to be 

 the whole sampling period of May through Sep- 

 tember (153 days) because predation on hard- 

 shell juveniles of both species has been shown 

 to occur throughout this period (Livingston et 

 al. 1986). Although the Pacific cod's diet con- 

 tains large amounts of snow crab during other 

 times of year, geogi'aphic distribution of sam- 

 ples taken in other periods was not sufficient to 

 include in the current study. Because red king 

 crab in the hard-shell state are very spiny, cod 

 probably consume adults only when they are in 

 the soft-shell state. During the period sampled 

 in this study, only female red king crab are 

 molting (males molt earlier in the year), and 

 their molt extends only through May of the 

 study period (D = 30). 



Pacific cod biomass was estimated using data 

 collected simultaneously with stomach collec- 

 tions during resource assessment surveys con- 

 ducted each year by the Resource Assessment 

 and Conservation Engineering (RACE) Division 

 of the Northwest and Alaska Fisheries Center 

 (NWAFC). The catch per unit of effort (CPUE) 

 of cod in kg/nmi^ was calculated using the area 



swept method for each 20 nmi wide square 

 where resource assessment trawls were per- 

 formed in each crab consumption area. The cod 

 CPUE was then separated into the CPUE for 

 each cod size gi'oup using the length-frequency 

 information from resource assessment surveys. 

 Total biomass for each cod size gi'oup could then 

 be calculated as the sum of the CPUEs multi- 

 plied by the area of 20 nmi wide square (400 

 nmi"). 



Population estimates and size distributions of 

 crab from assessment surveys were provided by 

 Robert Otto^ Population assessment methods 

 for crab are described in Otto (1986). 



Although size-at-age determinations are un- 

 certain for snow crab, crab were separated into 

 age classes using the following carapace width- 

 at-age tables for C. opilio and C. bairdi com- 

 monly used by crab biologists at the NWAFC (J. 

 Reeves"). 



Carapace width (mm) 



Ice edge locations were taken from ice edge 

 atlases (Joint Ice Center 1981, 1984, and 1985). 

 These data come principally from satellite im- 

 agery. In each year, I chose the last observed 

 southernmost ice edge extent during spring be- 

 fore the permanent ice retreat. 



RESULTS 



Geographic Distribution of Crab 

 Consumption by Year 



In all three years, red king crab were con- 

 sumed by cod in the crab consumption area 

 bounded by long. 165°00'W in the west and lat. 

 58°30'N atdepths of 31-100 m (Fig. 2). Red king 

 crab were found in stomachs from May through 



'R. Otto. Kodiak Laboratory. RACE, Northwest Alaska 

 Fisheries Center, National Marine Fisheries Service, 

 NOAA, Kodiak, AK 99615, pers. commun. 1986. 



-J. Reeves, Northwest Alaska Fisheries Center, National 

 Marine Fisheries Service, NOAA, Seattle, WA 98115, pers. 

 commun. May 1988. 



810 



