Biological observations from the 

 Cobb Seamount rockfish fishery 



Donald E. Pearson 



National Marine Fisheries Service, Southwest Fisheries Science Center 

 Tiburon Laboratory, 3 1 50 Paradise Drive 

 Tiburon, CA 94920 



David A. Douglas 



Oregon Department of Fish and Wildlife, Marine Field Station 

 53 Portway Street, Astoria, Oregon 97 1 03 



Bill Barss 



Oregon Department of Fish and Wildlife. Marine Region 

 Marine Science Drive, Bldg. 3 

 Newport, Oregon 97365 



The Cobb Seamount was discovered 

 in 1950 by the crew of the fishery 

 research vessel John N. Cobb 

 (Anonymous, 1950). This submerged 

 volcanic mountain (Budinger, 1967) 

 is located approximately 280 nauti- 

 cal miles (nmi) off the southern 

 Washington coast and comes to 

 within 50 meters of the sea surface. 

 At the 200-m isobath, its area is 

 approximately 32 nmi 2 (Budinger, 

 1967). Various efforts have been 

 made to study the biological, 

 oceanographic, and geologic charac- 

 teristics of the seamount since its 

 discovery (Budinger, 1967; Birkland, 

 1971; Dower et al., 1992), but these 

 efforts have been somewhat limited 

 in scope and duration. The studies 

 have shown that the seamount has 

 concentrations of commercially 

 valuable species including rock- 

 fishes (genus Sebastes). Most of the 

 information about commercial 

 fishing operations on the seamount 

 is anecdotal and speculative, pri- 

 marily because the seamount is lo- 

 cated outside the Exclusive Eco- 

 nomic Zone (EEZ) and therefore 

 fishing vessels operating there are 

 not subject to fishing regulations or 

 routine sampling. What is known, 

 however, is that bottom trawl, mid- 

 water trawl, gill net, and longline 

 fishing has occurred at various times. 



Foreign and domestic fleets have 

 fished on the Cobb Seamount from 

 at least the mid 1960's (Sasaki, 

 1986). Most of the fishing operations 

 appear to have been aimed at sev- 

 eral species of rockfish. Sasaki 

 (1986) suggests that prior to 1985, 

 there was intense fishing pressure 

 on the seamount by the Japanese 

 fishing fleet. In 1985, a few 

 fishermen from Oregon attempted 

 to trawl on the Cobb Seamount but 

 were unsuccessful. It is not known 

 why these efforts failed. Little 

 fishing activity is believed to have 

 occurred on Cobb Seamount be- 

 tween 1985 and September 1991. 



In 1991, fishermen from Oregon 

 and Washington returned to Cobb 

 Seamount; this time they were suc- 

 cessful. This success drew the at- 

 tention of fishery managers because 

 fish caught outside the EEZ are not 

 regulated by the west coast Fisher- 

 ies Management Plan governing 

 nearshore stocks. Because of the 

 regulatory problems presented by 

 this new fishery, port samplers were 

 instructed to give a high priority to 

 sampling of landings from Cobb 

 Seamount in part as an effort to 

 identify any unusual characteristics 

 which might make landings identi- 

 fiable. In this paper we present 

 some of the unique characteristics 



of the fish populations found on 

 Cobb Seamount, particularly widow 

 rockfish (S. entomelas). This sea- 

 mount is isolated enough from 

 coastal fish populations so that re- 

 cruitment dynamics, density depen- 

 dent growth, and bioenergetic stud- 

 ies could provide valuable new 

 insights into fisheries biology. 



Methods 



Landings of commercially important 

 groundfish are routinely sampled by 

 biological technicians in Oregon ac- 

 cording to standard protocols. Land- 

 ings of fish from Cobb Seamount 

 were sampled by port samplers in 

 Oregon at two or three times the 

 normal level and the majority of 

 landings were sampled. All samples 

 of widow rockfish were taken from 

 vessels by using midwater trawl 

 gear with the same mesh size as 

 used in the nearshore widow rock- 

 fish fishery. During a four-month 

 period, 11 landings were sampled 

 by taking multiple 50-pound 

 subsamples from each landing. Fork 

 lengths were obtained from 891 

 widow rockfish, otoliths were re- 

 moved from 724 of these fish for 

 age determination by the broken 

 and burnt method (Chilton and 

 Beamish, 1982). 



Mean length-at-age, age compo- 

 sition, and physical characteristics 

 of the otoliths were compared to 

 samples collected from landings of 

 widow rockfish caught in the near- 

 shore areas of the northern Oregon- 

 southern Washington coast. To 

 eliminate yearly and seasonal bias, 

 comparisons of nearshore widow 

 rockfish to Cobb Seamount widow 

 rockfish were limited to the same 

 months and years for both groups. 

 To compare mean length-at-age of 

 Cobb widow rockfish to the near- 

 shore widow rockfish, only six-year- 

 old fish could be used because of 

 the small number of fish in other 



Manuscript accepted 27 April 1993. 

 Fishery Bulletin: 91:573-576 ( 1993). 



573 



