McCABE ET AL.: INTERRELATIONSHIPS BETWEEN SALMONIDS 



Data Analysis 



Because subyearling chinook salmon were the most 

 frequently caught salmonids and were available in 

 sizable numbers from March through September, we 

 chose to compare all other species (including other 

 juvenile salmonid species) in relation to them. We 

 assigned one of three abundance categories: Com- 

 mon, occasional, or uncommoa A common species 

 occurred in 50% or more of the sampling efforts (in 

 which juvenile salmon were captured) and equaled 

 50% or more of the total number of subyearling 

 chinook salmon captured in that habitat. An oc- 

 casional species occurred in more than 20% of the 

 sampling efforts and equaled more than 10% of the 

 total number of subyearling chinook salmon. An un- 

 common species occurred in 20% or less of the sam- 

 pling efforts and equaled 10% or less of the total 

 number of subyearling chinook salmoa Fish were not 

 separated by age- classes, except yearling and sub- 

 yearling chinook salmoa 



Food habit data from April through September 

 were combined into two periods — spring (April 

 through June) and summer (July through Septem- 

 ber). Diet descriptions and comparisons are not pre- 

 sented for February, March, and October. Principal 

 prey items for each fish species were determined by 

 calculating the Index of Relative Importance (IRI) 

 modified from Pinkas et al. (1971): 



IRI ={N+ W)F 



where N = numerical percentage of a prey item 

 W = weight percentage of a prey item 

 F = frequency of occurrence percentage of 

 a prey item. 



Any prey item with an IRI value >50 was con- 

 sidered a principal prey for a given species. Digested 

 food was not included in this calculation. 



To assess possible food competition, diet overlap of 

 associated species was measured using the formula 

 developed by Morisita (1959) and modified by 

 Horn (1966): 



2 Z X, • Y, 



1=1 





where C\ 

 s 

 X, 



overlap coefficient 

 number of food categories 

 proportion of the total diet of fish spe- 

 cies X contributed by food category i 

 (by biomass) 



Y, = proportion of the total diet of fish spe- 

 cies Y contributed by food category i 

 (by biomass). 



Values of C x range from to 1, with indicating no 

 overlap and 1 indicating complete diet overlap. A 

 value of 0.6 is considered significant diet overlap 

 (Zaret and Rand 1971). 



RESULTS 



Juvenile chinook salmon (subyearling and year- 

 ling); coho salmon, 0. kisutch; and steelhead were the 

 most common salmonids in the estuary (Table 1). 

 Subyearling chinook salmon were the most abundant 

 and were available in quantity for the longest time 

 (March through September). Catches of juvenile 

 chum salmon, O. keta; sockeye salmon, O. nerka; and 

 cutthroat trout, 5. clarki, were small; consequently, 

 they will not be included in the analysis of inter- 

 competition. The low incidences of these species in- 

 dicate their small estuarine populations when com- 

 pared with steelhead and chinook and coho salmon 



TABLE 1 .—Numbers of juvenile salmonids collected in four habitats 

 of the Columbia River estuary from March to September 1980. 



Proportional abundances of juvenile chinook 

 salmon (subyearling and yearling), coho salmon, 

 steelhead, and nonsalmonids are shown by month in 

 Figure 2 . If fewer than 1 subyearling chinook salmon 

 were collected, then no comparisons were made. In 

 pelagic areas of the upper estuary, juvenile sal- 

 monids were numerically important from April 

 through August, with a substantial decline in Sep- 

 tember. Yearling chinook salmon were an important 

 part of the catch in April and May, coho salmon in 

 May and June, and steelhead in May. Subyearling 

 chinook salmon were important from May through 

 August. 



In the pelagic area of the lower estuary, non- 

 salmonids were clearly numerically dominant. Peri- 

 odically this portion of the estuary contained large 

 schools of marine fish, such as Pacific herring, Clupea 



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