DISTRIBUTION AND ABUNDANCE OF SMALL FLATFISHES AND 

 OTHER DEMERSAL FISHES IN A REGION OF DIVERSE SEDIMENTS 



AND BATHYMETRY OFF OREGON 



William G. Pearcy' 



ABSTRACT 



Demersal fishes were sampled at seven stations located inshore of Heceta Bank, on Oregon's continen- 

 tal shelf, over a 2-yr period with a 3-m beam trawl designed to catch small flatfishes. Two general 

 assemblages of fishes were recognized: a shallow water (74-102 m), sandy-bottom association where 

 Pacific sanddab. Citharichthys sordidus. was numerically the dominant species, and a deeper ( 148-195 

 m) assemblage, generally on mud, where the slender sole, Lyopsetta exilis, predominated. Rex sole, 

 Glyptocephalus zachirus, was usually the second most common species at all stations. Dover sole, 

 Microstomus pacificus, ranked fourth to sixth by numbers and composed the largest biomass (wet 

 preserved weight) at three stations. Species diversity was lowest at the shallowest station where 

 sediments were well-sorted, fine sands that contained only 0.1% organic carbon. 



The biomass of all fishes captured ranged from 0.9 to 2.4 g m -. These values are low compared with 

 estimates made by others off Oregon and Washington using commercial-sized otter trawls, presumably 

 because of avoidance of the small beam trawl by large fishes. 



An analysis of variance of the catches of all fishes combined, of Dover, rex, and slender soles, and of 

 Pacific sanddab revealed few significant effects of sediment, depth or season. Sediment t3rpe had a 

 significant effect on the catches of slender sole — largest catches were on a clayey-silt bottom. Catches of 

 sanddab were inversely related to depth of water. Depth-season interactions were significant for all 

 species combined and for rex and Dover soles, numbers were higher at the deepwater stations during 

 winter than summer, indicating seasonal bathymetnc movements. Annual variations were marked — 

 total catches and catches of most species were larger for unknown reasons during 1968 and 1969 than 

 1970. 



Based on length-frequency data, age-group ( <50 mm standard length) rex sole were found in high 

 proportions at the deepest stations on the outer edge of the continental shelf Small sanddab ( <70 mm) 

 composed a larger proportion of the catch by numbers on sandy silt than on sand where larger fish 

 predominated. 



Both sediment type anci depth of water have been 

 correlated with the abundance and species com- 

 position of benthic animals. According to Thorson 

 (1957), the physical and chemical composition of 

 sediments may be the main factor in determining 

 the general pattern of distributions of infaunal 

 and epifaunal invertebrates on the level sea floor. 

 Direct influence of bottom type on demersal fishes 

 may be less than on infauna and epifauna, but 

 sediments may affect fishes indirectly by infiuenc- 

 ing the composition and abundance of available 

 benthic food. Depth of water has frequently been 

 related to faunal changes of both benthic inverte- 

 brates and vertebrates across the continental shelf 

 and slope (Sanders and Hessler 1969; Alton 1972; 

 Haedrich et al. 1975). In some studies, these 

 faunal changes were related to concomittant 



^School of Oceanography, Oregon State University, Corvallis, 

 OR 97331. 



Manuscript accepted -Januarv 1978. 

 FISHERY BULLETIN: VOL. 76. NO. 3, 1978. 



changes in both sediment type and depth ( Day and 

 Pearcy 1968). 



The influence of depth and sediments on the 

 distribution of the benthos, however, is difficult to 

 separate because these factors are usually closely 

 correlated. Sediment texture generally decreases 

 with increasing depth of water. Small particles are 

 transported from regions of high energy waves 

 and currents into deep, low energy sedimentary 

 environments, while coarse sediments, such as 

 sands, generally are deposited close to their conti- 

 nental source in shallow water. 



This study is an attempt to analyze the relation- 

 ships between sediment and depth on the species 

 composition of benthic fishes and the abundance of 

 small fiatfishes in a localized region, mainly in- 

 shore of Heceta Bank along the continental shelf 

 of Oregon. The bathymetry and sediments are var- 

 iable in this region (Figure 1). This is an important 

 factor in this study because the resulting sediment 



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