NOTE Pearson et al.: Cobb Seamount rockfish fishery 



575 



The argument for differences in fishing behavior has 

 some merit. Fishermen report that the terrain on the 

 Cobb Seamount is very rugged and it is possible that 

 the fishermen tend to keep their nets higher off the 

 bottom to avoid snags than they would in the nearshore 

 areas. This could result in a different age composition 

 if young fish tended to disperse higher into the water 

 column than older fish. 



The argument for the difference in age composition 

 being due to a difference in discard practices also has 

 some merit. Fishermen operating under trip limits 

 in the nearshore areas, and having to deal with mar- 

 ket demands for larger fish, would tend to discard 

 smaller fish. The problem with this argument is that 

 it would tend to explain a higher proportion of small 

 fish in the Cobb Seamount widow rockfish landings, 

 but it would not explain the virtual absence of older, 

 larger fish which certainly would be retained by the 

 fishermen. 



The possibility that the age composition of the two 

 areas is actually different also has merit. It is not 

 known whether the population of widow rockfish on 

 the Cobb Seamount is self sustaining or if the fish 

 recruit to the seamount from the nearshore popula- 

 tion. Juvenile rockfish have an extended pelagic phase. 

 During this stage they can be advected offshore. Re- 

 cent studies of juvenile rockfish off the central Califor- 

 nia coast have found juvenile rockfish more than 200 

 miles offshore 1 . If this also occurs off Oregon, then it is 

 possible that some fraction of the existing population 

 on the Cobb Seamount originated as juveniles or lar- 

 vae, or both, in the nearshore area. While it cannot be 

 completely discounted that adults migrate to the sea- 

 mount it seems unlikely because adult widow rockfish 

 are not considered pelagic and the seamount is more 

 than 200 miles from the nearest suitable habitat. It is 

 possible that older fish either emigrate from the sea- 

 mount or experience high mortality for some unknown 

 reason. It is also possible that there was a massive kill 

 of widow rockfish on the Cobb Seamount prior to 1985. 

 Another possibility is that there never were widow 

 rockfish on the Cobb Seamount prior to 1985. This 

 latter possibility may have some merit since yellowtail 

 rockfish appear to be totally absent from the Cobb 

 Seamount; yet, they are quite abundant in the 

 nearshore area, although the habitat at the Cobb Sea- 

 mount would seem to be ideal for them. If getting to 

 the Cobb Seamount is a fortuitous event, then it is 

 possible that yellowtail rockfish just have not been 

 lucky yet and that widow rockfish have only been lucky 

 recently. Only further studies can determine the cause 

 for the apparent differences in age composition 



between the Cobb Seamount and the nearshore 

 populations. 



Otoliths from many species can undergo a process of 

 vaterite replacement ( Gauldie, 1986 ) in which the arago- 

 nite in the otoliths is replaced by vaterite. This process 

 results in a quite distinctive otolith morphology (Fig. 2) 

 and has been observed by the authors in many species of 

 rockfish, flatfish, and other groundfish species. Vateritic 

 otoliths are sometimes called "resorbed" or "crystallized" 

 owing to their somewhat crystalline appearance. In 

 nearshore widow rockfish sampled in Oregon from 1991, 

 only 2.5% of the fish had vateritic otoliths. In contrast, 

 28 percent of otoliths from Cobb Seamount widow rock- 

 fish are vateritic. Gauldie (1986), working with salmon, 

 suggested that vaterite replacement is under single lo- 

 cus genetic control but can be overridden by temperature 

 extremes. It is possible that Cobb Seamount could be 

 subject to temperature extremes during certain oceano- 

 graphic events like El Ninos. 



Three other interesting observations have been made 

 about rockfish from Cobb Seamount during this fishery. 

 Rosy rockfish (S. rosaceous) was caught; thus a north- 

 ern range extension for this species was created.- Har- 

 lequin rockfish have been caught; thus, their southern 

 range has been extended from its previously reported 

 southern limit of Queen Charlotte Sound, British Co- 

 lumbia (Eschmeyer et al., 1983). Five shortbelly rock- 

 fish (S. jordani) were caught, three of them in excess 

 of 34-cm fork length, which exceeds the largest fish 

 previously known to the authors (33 cm). Studies on 

 other seamounts in the Pacific have shown that rare 

 species, exceptionally large specimens, and range ex- 

 tensions occur on other seamounts (Hughes, 1981). 



The Cobb Seamount undoubtedly holds many other 

 surprises for fisheries biologists. Studies of rockfish 

 communities on the Cobb Seamount and how coloniza- 

 tion occurs could lead to a better understanding of the 

 population dynamics of this large, valuable group of 

 groundfish. One possible way colonization could occur 

 would be if it were found that strong cohorts on the 

 Cobb Seamount were weak cohorts in the nearshore 

 area and vice versa. This would suggest the possibility 

 that advection of juveniles has an important role in 

 determination of cohort strength which would have 

 major ramifications in management of nearshore 

 fisheries. Continued examination of fish with vateritic 

 otoliths could yield new information, particularly if 

 other populations of fish are found to have a high per- 

 centage of vateritic otoliths. Remotely operated vehicle 

 studies would be a useful approach to examine the 

 benthic community to determine its similarity to 

 nearshore areas. 



■Stephen Ralston, Nat. Mar. Fish. Serv., Tiburon, CA 94920. Pers. 

 Commun., March 1993. 



-R. Lea. Marine biologist. Calif. Dep. of Fish and Game, Monterey, 

 CA 93940. Pers. commun. June 1993. 



