FISHERY BULLETIN: VOL. 75, NO. 1 



Species Composition and Dominance 



Seventy-three taxa assigned to 19 families and 1 

 order were taken in the coastal samples (Table 2). 

 Of these, 62 were identified to species including 

 unnamed numbered larval types considered to be 

 distinct species, 7 to genus, 3 to family, and 1 to 

 order. Margalef's (1958) formula for diversity (D 

 = S — 1/ln N, where S = number of species, N = 

 total number of individuals), which provides a 

 measure of species richness, yielded a value of 7.43 

 for the coastal assemblage, which was higher than 

 that for the offshore assemblage. 



Dominant taxa within the coastal assemblage 

 were determined by a ranking method (Biological 

 Index = BI) modified from Fager (1957), which 

 takes into account both abundance and frequency 

 of occurrence. By this method, the most abundant 

 species in each sample is given five points, the next 

 four, etc. Scores for each taxon are summed for all 

 positive samples and divided by the total number 

 of samples taken. The top 13 coastal dominants 15 

 (Table 5) accounted for 91.8% of the total larvae 

 captured within 28 km of the coast over the entire 

 sampling period. These same 13 taxa were also the 

 13 most abundant, although not always in the 

 same order as dominance. 



Osmerids were overwhelmingly the most 

 dominant taxonomic group making up 50% of the 

 total larval catch. They were the most abundant 

 and most frequently taken larvae in the coastal 

 assemblage. Parophrys vetulus and Isopsetta 

 isolepis were also important in terms of abun- 

 dance. These three taxa, together with fourth 



ranked Microgadus proximus, composed 78% of all 

 larvae taken. 



Seasonality 



Obvious trends in seasonality were apparent 

 from the 1971 data, which included samples from 

 every month (Figure 6). Ninety-three percent of 

 all larvae were taken during the 6-mo period from 

 February through July. Two abundance peaks 

 occurred within that period, one in February- 

 March (24% of all larvae) before upwelling, and 

 one in May-July (68% of all larvae) during the 

 upwelling season. Larval abundance decreased 

 greatly in August and remained low through 

 December. Mean number of larvae under 10 m 2 

 was 142 in February-March, 202 in May-July, and 



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37-111 km 



Offshore 



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J I A i S '0 'N' D 



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1971 



15 Data on distribution and abundance of all 90 taxa will be 

 available in an Oregon State University Sea Grant College 

 Program Technical Report by the senior author in 1976-77. 



FIGURE 6. — Mean standardized abundance of fish larvae by 

 cruise in 1 97 1 i n the coastal assemblage ( stations 2 to 28) and the 

 offshore assemblage (stations 37 to 111). 



TABLE 5. — Coastal dominants based on all larvae collected 2 to 28 km offshore in 1971 and 1972. 

 [BI = Biological Index modified from Fager (1957)]. 



Taxa 



BI 



Rank 



order 



of 



abundance 



Total 

 standardized 

 abundance 1 



% of 



total 



abundance 



Positive 



tows out 



of 139 



Total 

 standardized 

 a bunda nce 1 

 Positive tows 



'The sum of the standardized numbers (number under 10 m 2 sea surface) of larvae from each sample. 



Months of 

 occurrence 



134 



