LIVINGSTON: PACIFIC COD PREDATION ON THREE CRAB SPECIES 



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Figure 10. — Total number of red king crab. Paralithodes camfschatica, eaten by the Pacific 

 cod population dui'ing 1981, 1984, and 1985 compared with resource assessment estimates of 

 the total female red king crab population during the same years. (Note different y-a.\is 

 scales.) 



same pattern of linear decline. Removals, ex- 

 pressed as a percentage of the population, are 

 3,8%, 2.8%, and 1.4% for the years 1981, 1984, 

 and 1985, respectively. The declining percent- 

 ages removed actually indicate weak compensa- 

 tory density-dependent mortality over time, 

 which appears to be mainly due to the functional 

 response of individual cod to declining prey pop- 

 ulations (i.e., a decline in the average amount of 

 crab per predator with a decline in crab popula- 

 tion). Thus, at least over the range of female red 

 king crab population sizes considered here, it 

 appears that cod predation is not responsible for 

 the observed declines in female red king crab 

 populations from 1981 to 1985. The percentages 

 removed by cod form a small and declining part 

 of the total population decline. Since the period 

 of red king crab vulnerability to cod predation 

 for the present study included only 30 out of a 

 possible 60 days when red king crab females are 

 in the soft-shell condition, the estimated re- 

 movals could be doubled to approximate total 

 annual amounts removed by cod. This would af- 

 fect the percentage of removals in each year by a 

 factor of two but would not change the seemingly 

 compensatory density-dependent relationship 

 between crab removals and crab population size, 

 A similar comparison cannot be made directly 

 for cod consumption of the two snow crab species 

 because cod consume mostly age 1 crab, which 



are not well estimated in NMFS research sur- 

 veys. However, the numbers of age 1 snow crab 

 eaten in a particular year can be compared to the 

 number of age 3 crab collected 2 years later in 

 NMFS research surveys, which should be more 

 precise (Figs, 11, 12). Although the curves of age 

 1 crab consumed and the number of age 3 crab 

 found 2 years later in the trawl surveys appear 

 to be somewhat similar in shape for C. bairdi for 

 the 3 years, the estimated numbers of age 1 crab 

 consumed in a particular year are about two 

 orders of magnitude gi'eater than the numbers of 

 age 3 crab found 2 years later. This at least 

 indicates that cod are responsible for removing 

 large numbers of age 1 crab relative to the num- 

 ber that remain at age 3, The unequal ratios of 

 prey removed to prey remaining among years 

 could also indicate that density-dependent re- 

 movals are occurring. 



The numbers of age 1 crab in the population 

 can be reconstructed, as in Forney (1977), by 

 adding the number remaining at age 3 to the 

 number of age 1 eaten by cod. This assumes that 

 cod predation is the major source of mortality for 

 crab less than age 3 and that virtually all of this 

 mortality occurs at age 1, If removals by cod are 

 calculated as a percentage of the reconstructed 

 population size, then the values obtained for 

 1981, 1984, and 1985 are 84%, 95%, and 94%., 

 respectively. These percentages are substantial 



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