FISHERY BULLETIN: VOL. 81, NO. 4 



SUMMER 1980 



American shad 

 Peamouth 

 3-spine stickleback 

 Starry flounder 



Pacific herring 

 Surf smelt 

 Shiner perch 

 Staghorn sculpin 

 Starry flounder 



American shad 

 Longfin smelt 



American shad 

 Pacific herring 

 Northern anchovy 

 Longfin smelt 

 Shiner perch 



Subyearling chinook 







0.2 



0.4 



0.6 



0.8 



Upper 

 intertidal 



Lower 

 intertidal 



Upper 

 pelagic 



Lower 

 pelagic 



1.0 



FIGURE 6. — Diet overlaps between subyearling chinook salmon and commonly 

 associated species in the Columbia River estuary during summer 1980. Overlap 

 values were calculated using Morisita's (1959) equation modified by Horn 

 (1966). 



spinicorne and C. salmonis were found to be impor- 

 tant prey for juvenile chinook salmon (Reimers et al. 

 1978). Although the principal prey items differ in 

 each estuary, the estuaries do provide important 

 feeding habitat. Estuarine feeding and growth play 

 an important role in salmonid and nonsalmonid life 

 histories (McHugh 1967; Mason 1974; Levy and 

 Levings 1978; Healey 1980). 



Research indicates that at low prey abundances 

 various prey sizes are eaten as encountered, but at 

 higher densities larger prey are selected by predators 

 (Ivlev 1961; Werner and Hall 1974). We believe the 

 high diet overlap between fish in the spring is related 

 to the occurrence of an abundant food resource (prin- 

 cipally C. salmonis). In the summer when Corophium 

 abundance apparently was lower, predators shifted 

 to feeding primarily on zooplankton and diversified 

 their diets, thus keeping diet overlap at a minimum. 

 McConnell et al. (1978) also noted reduced abun- 

 dance of C. salmonis in the diets of subyearling 

 chinook salmon in the upper Columbia River estuary 

 during the summer. 



Information explaining why C. salmonis was an im- 

 portant prey in spring and not in summer is lacking. It 

 was apparent that migrating salmonids and many 

 nonsalmonids were intensely harvesting this food 

 resource. This predation may have affected Cor- 

 ophium abundance. Levings and Levy (1977) and 

 Nelson (1979) showed that fish predators could be a 

 controlling factor in estuarine amphipod pop- 

 ulations. Also, the huge deposition of sediment that 

 resulted from the eruption of Mount St. Helens prob- 



ably reduced C. salmonis populations in the Colum- 

 bia River estuary (Emmett 1982). 



Juvenile salmonids and nonsalmonids share the 

 same habitats. Both nonsalmonids and subyearling 

 chinook salmon utilize intertidal areas of the estuary 

 as feeding and resting areas. Undoubtedly the es- 

 tuary serves as a sanctuary for many juvenile non- 

 salmonids as well as for juvenile salmon. Intertidal 

 shallow areas of the estuary typically support rich 

 populations of benthic invertebrates, which are im- 

 portant prey items. 



Like the intertidal areas, the pelagic sections of the 

 estuary are utilized by juvenile salmonids and non- 

 salmonids as feeding places. Many of the juvenile 

 salmonids in the pelagic areas are probably migrating 

 actively to the ocean. Myers (1980) found that the 

 mean length of wild juvenile chinook salmon cap- 

 tured in the channel areas was greater than that of 

 those collected along the shoreline. Gear selectivity 

 may have caused this anomaly; however, if it did not, 

 then Myers ( 1 980) felt that the small juvenile chinook 

 salmon may be rearing along the beaches, then mi- 

 grating into channel areas at a larger size. 



The interrelationships of various species in es- 

 tuarine habitats will probably change from year to 

 year. Yet we feel the general picture of the fish com- 

 munities in the estuary in regards to juvenile 

 salmonids will remain virtually unchanged, unless 

 detrimental artificial alterations are made. This es- 

 tuary serves as a sanctuary (from other fish pre- 

 dators) for juvenile salmonids, along with being an 

 important feeding area for some subyearling chinook 



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