42 



Fishery Bulletin 102(1) 



highly migratory species such as Pacific sardines, jack 

 mackerel, and chub mackerel. Pacific sardine, which was 

 almost completely absent from the system in the 1980s, has 

 undergone a substantial resurgence and is now one of the 

 most abundant species off the coast in summer (Brodeur 

 et al., 2000; Emmett and Brodeur, 2000; McFarlane and 

 Beamish, 2001). It should be noted, however, that some of 

 the differences between cruises could be accounted for by 

 the inclusion of substantially more offshore stations during 



Axisl (r 2 =0.31) 



CO 

 CO 

 X 



< 



Axis 2 (r 2 =0.23) 



Figure 8 



Nonmetric multidimensional scaling (NMS) ordination 

 plot of stations and nekton species with environmental 

 parameters from June (A) and August (B) 2000 GLOBEC 

 cruises. Station symbols denote: onshore tO>. mid-shelf 

 !▲). and slope (D) groupings; Species abbreviations denote 

 the following taxa: CHIN (chinook, age 0), CHIN 1 

 (chinook, age al.ll, STHD (steelhead trout). SUSM (surf 

 smelt), PSAU (Pacific saury), WOEL (wolf-eel juvenile), 

 OSM J (osmerid juvenile), REXS (rex sole, larval i, MEDF 

 (medusafish ), PSAR (Pacific sardine), .JAMA (jack mack- 

 erel), CHMA (chub mackerel), NANC (northern anchovy). 

 BLSH (blue shark). 



the second cruise. Our results from the community analy- 

 ses suggest that juvenile salmon tend to co-occur with each 

 other and with a variety of other pelagic nekton, including 

 adult salmon, and that this spatial overlap varies tempo- 

 rally. Brodeur et al. (2003), in analyzing community struc- 

 ture based on previous pelagic sampling data off Oregon 

 and Washington, arrived at similar results. In both studies, 

 the geographic boundaries of the pelagic assemblages often 

 overlap and are not as distinct as demersal assemblages. 

 However, the pelagic environment is much more spatially 

 and temporally heterogeneous than the demersal environ- 

 ment, and many of the species examined in our study are 

 highly mobile and are likely to respond quickly to changing 

 conditions. Research is presently underway to examine the 

 trophic interactions among salmonids and with other sym- 

 patric nekton species in order to determine what ecological 

 relationships (e.g. predation, competition), if any, are occur- 

 ring in this system. 



From the results of our sampling, we concluded that ju- 

 venile salmonids, with the possible exception of steelhead, 

 occupy the cool, high salinity, inshore upwelling regions off 

 the southern Oregon coast. However, particularly for the 

 June cruise, many of the coho and chinook salmon juveniles 

 collected may have recently entered the ocean with little 

 time to disperse offshore, so that the capture location may 

 not reflect true habitat preferences. Moreover, the vertical 

 dimensions of our trawl also precluded us from sampling 

 the nearshore, subtidal regions where some subyearling 

 chinook may reside shortly after entering the ocean. 



Salmon and steelhead differed considerably in stock com- 

 position. The pattern for coho salmon was similar to that 

 of chinook salmon in that fish from sources both north and 

 south of Cape Blanco contributed to our catches. However, 

 steelhead from rivers north of Cape Blanco were absent, 

 presumably having migrated offshore and north shortly 

 after entering the sea, as shown by Pearcy et al. (1990). 

 Although our stock composition estimates for steelhead 

 should be viewed with caution because of an incomplete ge- 

 netic baseline and a relatively small number of samples, our 

 findings support Pearcy et al.'s suggestion that steelhead 

 from rivers south of Cape Blanco have a unique marine 

 distribution and reside throughout the summer in the up- 

 welling zone off northern California and southern Oregon. 



Our study revealed seasonal shifts in the abundance and 

 stock composition of juvenile salmonids. Although salmo- 

 nids comprised small portions of the vertebrate catches of 

 both the June and August cruises, juvenile chinook salmon, 

 coho salmon, and steelhead were much more abundant 

 later in the summer, likely indicating that fish moving 

 into our study area are from shoreline or riverine habitats. 

 The greater abundance of chinook salmon in late summer 

 can be explained in part by the northern migration offish 

 that originated in rivers south of our study area. Chinook 

 salmon from the Sacramento and San Joaquin rivers in 

 California's Central Valley comprised substantial propor- 

 tions in the August catches both south (20%) and in nth 

 i 90' i ) of Cape Blanco. In contrast, the few chinook salmon 

 caught in June were mostly (549r ) from streams that en- 

 ter the sea immediately north of Cape Blanco such as the 

 Umpqua, Coquille, Sixes, and Elk rivers. Chinook salmon 



