622 



Fishery Bulletin 88(4|. 1990 



MONTH 



^m SEPTEMBER (n - 296) 

 L^ JULY-AUGUST (n -268) 



JUNE (n • 612) 



MAY (n - 397) 



FISH EUPH DECA PIER AMPH. COPE. INSE OTHER 

 PREY CATEGORY 



/ 



AREA 



OREGON (n - 422) 



£^ COLUMBIA (n • 841) 

 Wtk VWSHINGTON (n • 310) 



n^'nl 



n 



y^ 



I 



»■«! 



m 



mi 



/ 



FISH EUPH. DECA PTER AMPH COPE INSE OTHER 

 PREY CATEGORY 



Figure 3 



Juvenile coho salmon diets by mimth and area for the major prey 

 categories as a percent of total IRI. Sample sizes (no. of fish with 

 food) for each subset are given in the legend. All figures include data 

 collected off Washington and Oregon, 1980-85. 



amphipods were the dominant prey in each area based 

 on IRI proportions (Fig. 3). The relative proportions 

 of the major prey categories were similar among the 

 regions, although fishes were more important off 

 Washington. 



Few differences were observed among the three 

 regions even at the lowest taxonomic levels. Engraulis 

 mordax and Sebastes spp. were the dominant fish prey 

 by weight in all three regions. The Washington area 

 differed somewhat from the other regions in that 

 Cancer larvae were the dominant invertebrate taxa 

 numerically, whereas L. helicina, T. spinifera, and//. 

 meduaarum were dominant in the other regions. 



Chinook saimon 



General food habits Fishes dominated the diet of 

 juvenile chinook salmon, occurring in over 85% of the 

 stomachs that contained food (n = 795), and accounting 

 for almost 87% of the total IRI for all years combined 

 (Appendix Table 1). The overall mean stomach fullness 



(2.8) and digestion (2.3) states were similar to that of 

 coho. Also resembling the diet of coho salmon, the main 

 fish prey identified were E. mordax, Sebastes spp., and 

 A. hexapterus, but cottid juveniles (Hemilepidotus 

 spinosus) were also frequently eaten. Pleuronectid and 

 agonid larvae were more common in the diets of juve- 

 nile chinook salmon. 



Many invertebrate species were also represented in 

 juvenile chinook salmon diets. The relative importance 

 of the various invertebrate taxa was similar to that 

 found for coho salmon. Decapod larvae, euphausiids, 

 and hyperiid amphipods were the dominant inverte- 

 brate groups consumed. The dominant species in these 

 prey categories were C. oregonensis megalopae, T. 

 spinifera, and H. medusarum, respectively (Appendix 

 Table 1). Copepods and mysids were generally more 

 important, and pteropods and insects less important, 

 in comparison with juvenile coho salmon. 



Temporal variations Between-cruise variability in 

 the consumption of the major prey taxa was less pro- 

 nounced for chinook salmon than for coho salmon 

 (Table 3). Fishes comprised 75% or more of the biomass 

 consumed during every cruise, with the exception of 

 July 1981 when euphausiids and pteropods were also 

 major prey. The only other invertebrate taxa to con- 

 tribute substantially in other cruises were cephalopods 

 (May and June 1981), euphausiids (May 1982 and July 

 1984) and decapod larvae (May and June 1983). In con- 

 trast to the diet of coho salmon, insects were unimpor- 

 tant during July 1984 (Table 3). 



Seasonally, the relative IRI proportions of fishes, 

 decapod larvae, and euphausiids were similar for 

 chinook salmon (Fig. 4). The diets contained more 

 major prey categories during July- August when small 

 zooplankton prey (copepods, pteropods, decapod larvae, 

 and hyperiid amphipods) were important numerically. 

 This may be due to the smaller mean size of chinook 

 salmon collected this period; in July-August, there was 

 an influx of subyearling chinook salmon into the sam- 

 pling area. Fishes were again the dominant prey in 

 September, although hyperiid amphipods remained 

 important numerically. 



Geographic variations The feeding patterns of juve- 

 nile chinook salmon were fairly consistent by major tax- 

 onomic categories among the three regions (Fig. 4). 

 Fishes were the major prey by frequency of occurrence 

 and weight in all three geographic areas, although the 

 dominant species varied somewhat. Off Washington, 

 A. hexapterus, Sebastes spp., and H. spinosus were the 

 main fish prey consumed. Engraulis mordax and //. 

 spinosus were the dominant prey in the Columbia 

 region, whereas E. mordax dominated the diet off 

 Oregon (>65% of the total IRI). 



