550 



Fishery Bulletin 105(4) 



PS I- 





•A-Pjk\ 



xlOO, 



(1) 



where p = the proportion of biomass (wet weight) of the 

 /zth prey species consumed by predator spe- 

 cies i and/ 



Diet overlap values >60% were considered biologi- 

 cally significant (Wallace and Ramsay, 1983). As in 

 the AHCA, prey items of unidentified material, and 

 fish and crustacean tissue, were eliminated from the 

 calculation of PSI. 



Diets (percent wet weight of major prey taxa) from 

 selected species were compared with respect to interan- 

 nual differences. Comparisons between 2000 and 2002 

 were limited to species and life history stages of nekton 

 that were found in >5% of the total number of hauls 

 within a cruise, which left only yearling and adult Chi- 

 nook and coho salmon, and juvenile steelhead trout (O. 

 mykiss). Comparisons between years were undertaken 

 by visual inspection of plotted diet data. 



Results 



Trophic groups 



A total of 3161 stomachs from 26 species of marine nekton 

 were analyzed for diet composition from the June and 

 August 2000 and 2002 cruises. A species-specific sum- 

 mary of number of stomachs analyzed, nekton percent 

 frequency of occurrence, nekton size (mean and standard 

 deviation), and mean stomach fullness and condition 

 are presented in Table 1. A more detailed description of 

 specific prey species consumed by nonsalmonids can be 

 found in Miller (2006). Results from cluster analysis 

 and MRPP showed major trophic groups for both years 

 based on nekton diet (percent wet weight) of the fol- 

 lowing general prey groups: adult fish, larval-juvenile 

 fish, euphausiids {Thysanoessa spinifera and Euphausia 

 pacifica), decapod larvae, copepods, and other mixed 

 zooplankton groups (predominantly hyperiid amphipods, 

 gelatinous zooplankton, and phytoplankton). For 2000 

 and 2002, five significant trophic groups (2000 MRPP, A 

 statistic = 0.37, P«0.001; 2002 MRPP, A statistic = 0.31, 

 P«0.001) were observed at the cutoff level of 60% and 

 52% information remaining, respectively (Fig. 1). 



From AHCA of nekton diets for the year 2000, trophic 

 group A (blue shark [Prionace glauca] and adult coho 

 salmon. Figs. 1 and 2) had diets that consisted (62% 

 and 80%, respectively) of adult Osteichthyes (Clupeidae 

 and unidentifiable fish tissue), and to a lesser extent of 

 larval-juvenile Osteichthyes (for adult coho salmon) and 

 euphausiids (for blue shark). Trophic group B (juvenile 

 and adult salmon) had diets comprising >90% larval- 

 juvenile fish and adult euphausiids for each age class. 

 Of the euphausiids consumed, T. spinifera contributed 

 the highest proportion by wet weight to salmonid diets. 

 Trophic group C (jack mackerel. Pacific saury [Cololabis 

 saira], and Pacific sardine) had diets of 86%, 93%, and 



90% euphausiids, respectively; most euphausiids con- 

 sumed by this group were identified as E. pacifica. Tro- 

 phic group D (market squid and surf smelt [Hypomesus 

 pretiosiis] had the most diverse diet of all nekton; mixed 

 species of crustacean zooplankton (brachyuran larvae, 

 euphausiids, hyperiid amphipods) accounted for most of 

 the diet by wet weight. Trophic group E (Pacific herring 

 [Clupea pallasi], whitebait smelt [Allosnierus elongates], 

 and juvenile sablefish [Anoplopoma fimbria]), consumed 

 euphausiids that contributed 77%, 96%, and 97% to the 

 wet weight of their diets, respectively. Except for E. 

 pacifica, which was consumed most by juvenile steel- 

 head trout, T. spinifera was the dominant euphausiid 

 species consumed by nekton within this group. The 

 remaining nekton not included in the cluster analysis 

 generally consumed a mix of crustacean zooplankton 

 or crustacean zooplankton and larval-juvenile Osteich- 

 thyes (Fig. 2). 



There was a similar trend in 2002 nekton diets based 

 on the consumption of adult and larval-juvenile Ostei- 

 chthyes, euphausiids, and brachyuran larvae (Figs. 1 

 and 3). As in 2000, blue shark and adult coho salmon 

 (trophic group A, Figs. 1 and 3) diets consisted mainly 

 of adult Osteichthyes (81% and 30%, respectively); blue 

 shark diets consisted secondarily of osteichthyean tissue 

 (16%), and adult coho salmon diet consisted secondarily 

 of brachyuran larvae (69% of which were Dungeness 

 crab [Cancer magister] megalopae). Pacific hake and 

 adult chinook salmon, also in this group, consumed 

 adult Osteichthyes (predominantly Clupeidae), larval- 

 juvenile Osteichthyes (chinook salmon only), euphau- 

 siids (Pacific hake only), and brachyuran larvae (adult 

 chinook and coho salmon). Trophic group B (Fig. 1) 

 consisted of jack mackerel, northern anchovy. Pacific 

 herring, juvenile sablefish, and whitebait smelt, whose 

 diets consisted mostly of euphausiids (predominantly 

 T. spinifera) and various other zooplankton and larval- 

 juvenile Osteichthyes (Fig. 3). Juvenile chinook, coho, 

 and chum salmon (O. keta), and juvenile steelhead trout 

 formed a trophic group (C, Fig. 1), where larval-juvenile 

 Osteichthyes represented >50% of wet weight of diet for 

 all species (Fig. 3). Euphausiids were important in only 

 juvenile steelhead trout (35%). Trophic group D (Fig. 

 1) consisted of market squid, surf smelt, spiny dogfish 

 iSqualus acanthias), and Pacific sand lance [Ammodytes 

 hexapterus) which consumed a mix of crustacean zoo- 

 plankton (euphausiids, brachyuran larvae, hyperiids) 

 and in the case of spiny dogfish, gelatinous zooplank- 

 ton. Pacific sardine, juvenile widow rockfish (Sebastes 

 entomelas), and Pacific saury clustered into a group 

 (trophic group E, Fig. 1) that had mixed zooplankton 

 diets of copepods, euphausiid eggs, and euphausiids. 

 The remaining nekton not included in the cluster analy- 

 sis were juvenile rockfish (Sebastes spp.) species and 

 juvenile lingcod (Ophiodon elongatus). Juvenile lingcod 

 diets consisted mostly of large copepods (68%, Calanus 

 spp.), and larval-juvenile Osteichthyes (30%). Juvenile 

 rockfish consumed a combination of euphausiids, cope- 

 pods, hyperiid amphipods, and gelatinous zooplankton 

 or material (Fig. 3). 



