Brodeur and Pearcy Trophic relations of juvenile salmon off Oregon and Washington 625 



Table 6 



Diet overlap by weight among all salmon species off Oregon 

 and Washington, for all cruises combined. 



Species 



O 40 



Coho 



Chinook 



Chum 



Sockeye 



33 



Jun May Jun Jul Aug May Jun Sep May JunSep Jun Jul Sep Jun 



1980 1981 1982 1983 1984 1985 



CRUISE 



Figure 6 



Diet overlap between juvenile coho and chinook salmon for each 

 cruise period. 1980-85. 



Prey diversity varied greatly among cruises for juve- 

 nile coho (Fig. 5). Generally lower values occurred dur- 

 ing 1981, especially in July, when pteropods were very 

 important in the diet. Coho salmon had higher diver- 

 sity values in 1983 and 1984 than in 1981. Chinook 

 salmon showed many of the same interannual and 

 seasonal trends in prey diversity as coho salmon, ex- 

 cept that prey diversity was not appreciably lower in 

 1981 (Fig. 5). 



Dietary overlap Diet overlap by weight among the 

 salmon species for all cruises combined was generally 

 low, with none of the six species pairs showing signifi- 

 cant (>60%) overlap (Table 6). The diets of juvenile 

 coho and chinook salmon were most similar (PSI = 

 54%), reflecting their common foraging on many of the 

 same euphausiid, larval decapod, and fish species. 

 Chum salmon diets showed the least similarity to the 

 other species, which may be a function of the poor 

 digestive state of the stomach contents of this species. 

 To examine the finer-scale variability in diet similar- 

 ity between coho and chinook salmon juveniles, diet 

 overlap was calculated for each cruise month (Fig. 6). 

 With the exception of 1983, which showed the highest 



overall similarity, monthly overlap values were highly 

 variable within years containing more than one cruise. 

 No consistent patterns were observed between years 

 for the same months. 



A wide range of diet overlap values was observed for 

 the 13 collections from which 10 stomachs of coho and 

 chinook were analyzed (Table 7). Overlap was highest 

 in the collections from September 1983, due mainly to 

 the common utilization of E. mordax and several 

 hyperiid amphipod species. Intermediate overlaps were 

 generally observed during June 1982, resulting main- 

 ly from consumption of the same euphausiid species by 

 both predators. There appeared to be no relationship 

 between diet overlap and the inshore-offshore location 

 of the collection. Diet overlap between juvenile coho 

 and chinook at a particular station was generally higher 

 than intraspecific overlap for either species at adjacent 

 stations (Table 7). 



Analysis of dietary variations The contingency 

 table analysis for [iresence or absence of the four most- 

 commonly-occurring major prey categories showed that 

 there were generally highly significant variations 

 (P< 0.001) in diets of coho and chinook juveniles by 



