836 



Abstract-The rockfishes of the se- 

 bastid genus Sebastes are a very impor- 

 tant fishery resource off the coasts of 

 CaHfomia and southern Oregon. How- 

 ever, many of the 54 managed stocks 

 of west coast rockfish have recently 

 reached historically low population 

 levels, leading fishery managers to re- 

 examine current management prac- 

 tices. Management of rockfish stocks as 

 multispecies aggregates, as opposed to 

 independent stocks within the ground- 

 fish fishery, can be more desirable when 

 nontargeted bycatch. discard, and man- 

 agement complexity are considered. 

 Rockfish assemblage structure and 

 species co-occurrences were determined 

 by using data from the Alaska Fisher- 

 ies Science Center triennial continental 

 shelf bottom trawl survey. The weight 

 of rockfish species in trawl catches was 

 expres.sed as a catch-per-unit-of-effort 

 (CPUE) statistic, from which species 

 spatial distributions, overlaps, diver- 

 sity, and richness were analyzed. Mul- 

 tidimensional scaling of transformed 

 CPUE data was employed in indirect 

 gradient and multivariate partition- 

 ing analyses to quantify assemblage 

 relationships. Results indicated that 

 rockfish distributions closely match the 

 bathymetry of coastal waters. Indirect 

 gradient analysis suggested that depth 

 and latitude are the principal factors in 

 structuring the spatial distributions of 

 rockfish on trawlable habitat. In addi- 

 tion, four assemblages were identified 

 through the joint evaluation of species' 

 distributions and multivariate parti- 

 tioning analyses: 1) deep-water slope; 

 2) northern shelf 3) southern shelf; and 

 4) nearshore. The slope, shelf and near- 

 shore groups are found in depth ranges 

 of 200-500 m, 100-250 m, and 50-150 

 m, respectively. The division of north- 

 em and southern shelf assemblages 

 occurs over a broad area between Cape 

 Mendocino and Monterey Canyon. The 

 results of this analysis are likely to 

 have direct application in the manage- 

 ment of rockfish stocks off the coasts of 

 southern Oregon and California. 



Distribution and co-occurrence of rockfishes 

 (family: Sebastidae) over trawlable shelf and 

 slope habitats of California and southern Oregon 



Erik H. Williams 

 Stephen Ralston 



Southwest Fisheries Science Center 



National Manne Fisheries Service 



1 10 Shaffer Road 



Santa Cruz, California 95060 



Present address (for E H Williams) Southeast Fisheries Science Center 



National Manne Fisheries Service 

 101 Pivers Island Road 

 Beaufort, North Carolina 28516-9722 



E-mail address (for E H Williams) Erik, Wiiiiams^noaa gov 



Manuscript accepted 6 June 2002. 

 Fish. Bull. 100:836-855 (2002). 



Rockfishes (family Sebastidae accord- 

 ing to recent work by Eschnieyer 

 (1998) and Kendall (2000). genera 

 Sebastes and Sebastolobus) are a very 

 important part of the groundfish fish- 

 ery off the United States west coast, 

 representing a relatively high value 

 in the market. In 1997, rockfish spe- 

 cies accounted for IT^i of total west 

 coast groundfish landings, but 33'X of 

 total exvessel value (Herrick et al.'). 

 Rockfishes on the U. S. west coast are 

 managed under the groundfish Fishery 

 Management Plan (FMP) by the Pacific 

 Fishery Management Council. Some 

 of the 52 Sebastes and 2 Sebastolobus 

 species that are included in the FMP 

 are managed as single-species stocks, 

 and other, generally less well-known 

 species, are managed as part of larger 

 multispecies aggregations. Moreover, 

 the 54 "rockfishes" listed in the FMP 

 do not include all of the Sebastes spp. 

 found in the region. More than 70 

 species are known from the northeast 

 Pacific Ocean (Eschmeyer et al., 1983; 

 Chen, 1986). 



The current status of the principal 

 west coast rockfish stocks is that many 

 have reached historically low levels and 

 the population sizes of many of the mi- 

 nor species remain virtually unknown. 

 Moreover, seven rockfishes (bocaccio (S. 

 pauaspinis), cowcod (S. levis), canary 

 (S. pinniger) darkblotched (S. crameri), 

 widow (S. entomelas), and yelloweye 

 (S. ruberrimus) rockfishes, and Pacific 

 ocean perch (S. alutus) have recently 

 been declared overfished by the Na- 



tional Manne Fisheries Service. The 

 continuing declines in rockfish popula- 

 tions and other groundfish stocks off 

 the U. S. west coast have prompted 

 changes in harvest policy and other 

 management practices (Ralston, 1998, 

 2002). 



Management of exploited fish stocks 

 on an individual basis often results in 

 discarded bycatch of nontargeted spe- 

 cies, which is wasteful. In contrast, 

 management of species as aggregates 

 or complexes can be more practical and 

 desirable (Ralston and Polovina, 1982; 

 Leaman and Nagtegaal, 1986; Fujita et 

 al., 1998). However, the multispecies 

 management approach is only as good 

 as the assemblage or group definitions 

 used, which depend on the availability 

 and accuracy of species-specific distri- 

 butional information. Most fisheries 

 managers rely on fishery-independent 

 surveys or at-sea observations of fish- 

 ery catches recorded in vessel logbooks 

 or noted by obsei'vers to provide spatial 

 information on fish abundance pat- 

 terns. Data collection for groundfish 

 population assessments off the coast 



Herrick, S. F., J. Hastie, and W. Jacobson. 

 1998. Economic status of the Washing- 

 ton, Oregon, and California groundfish 

 fisheries. In Pacific Fishery Management 

 Council. Status of the Pacific Coast ground- 

 fish fishery through 1998 and recom- 

 mended biological catches for 1999: stock 

 assessment and fishery evaluation. Pa- 

 cific Fishery Management Council, 2130 

 SW Fifth Ave., Suite 224, Portland, Oregon 

 97201. 



