Research Document - 2008/056

A review of darkblotched rockfish Sebastes crameri along the Pacific coast of Canada: biology, distribution, and abundance trends

By R. Haigh and P. Starr

Abstract

We summarize the available information on darkblotched rockfish Sebastes crameri. Specifically, this paper reviews the current data on the biology, distribution, and abundance trends, primarily for citation by COSEWIC stock status reports. This species has a mean weight of 1.318 kg/fish, representing the samples from the observed commercial fishery. Growth relationships shows no strong difference between the sexes. Allometric analyses yield curvature parameter estimates β all > 3 for males. Length-age analyses suggest females ultimately reach larger sizes than males; however, the data are very sparse and only comprise surface-read otoliths (known to overestimate young ages and underestimate old ages). Natural mortality M is not known but assumed to be 0.07, based on US observations. Maturity ogives yield lengths at 50% maturity of 32.1 cm for males and 35.3 cm for females. Using von Bertalanffy growth parameters from US models, ages at 50% maturity k are calculated to be 7.6 y for males and 8.7 y for females. Assuming k = 8 y and M = 0.07, the generation time is 22.3 y. Depth-of-capture frequency in commercial trawl tows suggests that most of the population occurs between 150 m and 435 m. Using this bathymetry interval, the estimated potential habitat is 42,848 km². The estimated area of occupancy based on trawl tow observations covers 30,760 km² using a DFO grid (0.1° longitude × 0.075° latitude) or 9,232 km² using a COSEWIC grid (2 km² × 2 km²). Within its preferred depth range, darkblotched rockfish is caught with numerous other species including Pacific ocean perch Sebastes alutus, arrowtooth flounder Atheresthes stomias, yellowmouth rockfish S. reedi, and Dove sole Microstomus pacificus. Total removal of darkblotched rockfish from BC coastal waters by Canadian and US commercial fleets since 1930 equals at least 4,200 t. Survey indices of abundance are currently not useful for assessing darkblotched rockfish population trends. The GB Reed, WCVI shrimp trawl, QCS shrimp trawl, and US NMFS surveys all generate indices with low precision. The synoptic groundfish surveys offer the best tool for monitoring this species in future. To date, the index trend in Queen Charlotte Sound from 2003 to 2007 appears flat. The commercial trawl CPUE indices coastwide show a decline of 3.9% per year from 1996 to 2006, with a flat trend from 1998 on. It is not known if the trend in CPUE indices represents a change in abundance of this species or in fishing practices associated with the introduction of IVQ management in 1997.

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