FISHERY BULLETIN: VOL. 76, NO. 3 



highest (34-35) at the deep stations, intermediate 

 (24-25) at the 100-m stations and lowest ( 19) at the 

 shallow station (Table 2B). Species diversity cal- 

 culated by the information function (Shannon and 

 Weaver 1963): 



M g ^pj iTipj' 



varied between 0.7 and 2.5 at the seven stations. 

 Diversity was lowest at the shallow, sand station 

 where sanddab composed 86% of the catch. Diver- 

 sity was highest at Station 2, which had the 

 largest number of species, and next highest at 

 Station 23, where the dominant species composed 

 only 23^7^ of the total number of fishes. Since the 

 number of species was similar among the three 

 100-102 m stations or the three deep (148-195 m) 

 stations, differences in diversity within these two 

 groups are due to variations in the evenness of the 

 proportions of the various species. 



The values of diversity are similar to those 

 found by others for demersal fish communities. 

 Margalef ( 1968) reported that diversity (//) of bot- 

 tom fishes trawled off the Spanish Mediterranean 

 ranged from 1.0 to 2.4. Haedrich and Haedrich 



(1974) calculated H = 0.7-1.7 for demersal fishes 

 of Block Island Sound [from the data of Merriman 

 and Warfel (1948) and Richards (1963)], and 

 H = 1.6-1.8 for bottom fishes on the continental 

 slope off southern New England. Haedrich et al. 



( 1975) give a value for H of 1.9 for 141-285 m on 

 the continental slope south of New England. 



Biomass Estimates 



Based on odometer estimates of distance 

 trawled, numbers varied from 1.4 to 4.0/100 m^, 

 and biomass ranged from 0.9 to 2.2 g m ^ (Table 

 2). Estimates of the biomass of fishes based on 

 ships's speed (3,700 m h^^), average trawling times 

 on the bottom ( 16 min) and the width of the beam 

 trawl are in agreement with the estimates that 

 used odometer readings. 



The biomass of the four common flatfishes at 

 each station shows that the Pacific sanddab com- 

 posed nearly all the biomass at Station 22 and also 

 had the largest biomass at Stations 7 and 15. 

 Dover sole predominated in catches at Stations 23, 

 2, and 8, and slender sole was the dominant fish at 

 Station 6. These trends are similar to those noted 

 earlier for species numbers (Table 2). 



Catches of demersal fishes with the small beam 

 trawl at these seven stations are low compared 



634 



with other estimates made with commercial-sized 

 otter trawls off Oregon and Washington. On the 

 basis of surveys using an otter trawl with a 23-m 

 footrope with 9.9-cm mesh (estimated to have a 

 9-m horizontal and a 1.5-m vertical mouth open- 

 ing), Demory and Hosie^ calculated that the aver- 

 age biomass of fishes on the continental shelf of 

 Oregon was 19 g m~2 in 1971-72 and 16 g m-2 in 

 1973-74. Barss et al.^ employed similar methods to 

 estimate a standing stock of trawlable fishes on 

 the continental shelf and upper slope ( 18-549 m) of 

 Washington of about 15 g m ^ . In both of these 

 studies, a large portion of the catch consisted of 

 rockfishes, Sebastes spp.. Pacific hake, Merluccius 

 productus, other roundfishes, skates, and other 

 elasmobranchs. The average biomass of flatfishes, 

 the type of fishes that their trawl was designed to 

 catch, was 5.7 and 5.4 g m ^ for 1971-72 and 

 1973-74, respectively, off Oregon and 9.0 and 7.5 g 

 m-2 for 1975 and 1976 off Washington. Alverson 

 et al. (1964), also using large otter trawls (28-m 

 footrope) with large (11-cm) mesh, provided data 

 from which a biomass of 8.0 g m-2 for demersal 

 fish and 3.1 g m^ for flatfishes can be estimated 

 for the continental shelf of Oregon and 

 Washington. Our estimates are also low compared 

 with the values given by Oviatt and Nixon ( 1973) 

 for New England waters. 



Although all these values are approximations 

 dependent on the accuracy of estimates of actual 

 distance trawled and the effective mouth area, the 

 otter trawl catches are several times larger than 

 my beam trawl estimates. Nektobenthic fishes, 

 such as hake and rockfishes, as well as large de- 

 mersal flatfishes and skates are poorly sampled 

 with the beam trawl because they are not avail- 

 able to the net or they avoid the trawl ( see also Day 

 and Pearcy 1968). Thus, retention of small species 

 of fishes and juveniles of large species by the beam 

 trawl, fishes that may escape through the meshes 

 of the larger nets, do not compensate for fishes that 

 avoid the beam trawl or nektobenthic species that 

 range above the bottom. 



Kuipers (1975) estimated that the catch ef- 

 ficiency of a 2-m beam trawl decreased exponen- 

 tially with increasing length of plaice, 

 Pleuronectes platessa, from approximately 100% 



'Demory, R. L., and M.J. Hosie. 1975. Resource surveys on 

 the continental shelf of Oregon. Fish Comm. Oreg., Anna. 

 Rep., 9 p. 



sBarss, W. H., R. L. Demory, and N. Ten Eyck. 1977. Ma- 

 rine resource surveys on the continental shelf and upper slope off 

 Washington, 1975-76. Oreg. Dep. Fish Wildl. Rep., 34 p. 



