Stanley and Kronlund: Life history characteristics for Sebastes brevispims 



673 



taining the copepod parasite Sarcotaces arcticus in 

 the coelomic cavity. All the oocytes of all the female 

 gonads appeared to be in a prefertilized condition. 

 The ovaries that were used for fecundity esti- 

 mation were fixed and stored in modified Gilson's 

 solution (Leaman. 1988) and shaken weekly for one 

 year. Fecundity estimates were derived gravimetri- 

 cally (Leaman, 1988). Each ovary was drained and 

 filtered through stacked sieves (100-750 urn); each 

 clump was broken manually if possible. The ovar- 

 ian membranes and connective tissue were teased 

 away from eggs and discarded. The oocytes were 

 transferred to millipore filters, vacuumed-dried for 

 15 minutes, and the oocytes and filter were weighed 

 to 0.01 g. Four subsamples of approximately 0.1 g 

 and 1000 oocytes were weighed to 0.0001 g. Total 

 fecundity was estimated for each fish by multiply- 

 ing total vacuum-dried ovary weight by the mean 

 density of the four samples. Fecundity relationships 

 against age. weight, and length were examined with 

 a generalized additive model (GAM) (Hastie and 

 Tibshirani, 1990). An identity link with a Gauss- 

 ian error structure was used in each case. Ovaries 

 to be used for histological examination were fixed 

 in Smith's formal dichromate solution and then 

 stored in 39c formaldehyde. Histology samples were 

 imbedded, sectioned, mounted, stained with Harris' 

 haematoxylin, and counterstained with alcoholic 

 eosin (Gray, 1954). 



Spawning stock biomass per recruit (SSB/R) 



We combined estimates of instantaneous natural mor- 

 tality rate (M) of 0.06 and partial recruitment from 

 Stanley and Kronlund (2000) with our estimates of the 

 proportion mature at age and predicted fecundity at age 

 in order to derive estimates of the expected population 

 fecundity of unfished populations (Appendix 2). The 

 impact of fishing on spawning stock biomass per recruit 

 (SSB/R) can then be explored by comparing the ratio of 

 predicted cumulative fecundity of a cohort under exploi- 

 tation to predicted cumulative fecundity under no fishing 

 pressure (Gabriel et al., 1989; Clark, 1991). 



Results 



Habitat 



The commercial data indicated that the highest catch 

 rates and most of the landings of silvergray rockfish 

 come from the edge of the continental shelf or along the 

 edges of deep troughs (Fig. 1). These tows were typically 

 conducted in bottom depths of 100 to 300 m, although 

 silvergray rockfish have been reported from tows with 

 mid-point bottom depths greater than 580 m. Monthly 

 catch rates by depth indicate a seasonal trend wherein 

 peak catch rates are highest in depths of 180-280 m in 

 March and April, but highest in depths of 100-200 m 

 in September and October (Fig. 2). 



Feb Mar Apr May Jun Jul 

 Month 



Aug Sep Oct Nov Dec 



Figure 2 



Silvergray rockfish (Sebastes brevispinis) seasonal depth 

 distribution. The solid lines show the median (heavy line) 

 and 25th and 75th percentiles (thin lines) for the number 

 of silvergray rockfish catch observations (observed commer- 

 cial trawl sets) at depth, between 1996 and 2003. The dots 

 indicate the estimated depth at 7.2°C ±1 standard deviation 

 (dotted line l. 



If the shift in catch rates correctly indicates sea- 

 sonal movement, and the interpolated temperatures at 

 site Al characterize bottom temperatures on the coast, 

 together they indicate that silvergray rockfish tend to 

 maintain peak densities at bottom water temperatures 

 centered around 7.2°C (Fig. 2). The move to shallower 

 water in the late spring, however, seems to lag behind 

 the cooling of shallower water that results from sum- 

 mer upwelling (Thomson 6 ). The return to deeper water 

 in the fall is coincident with the warming of water at 

 greater depths. 



The cohabitants of silvergray rockfish were also in- 

 ferred from commercial trawl observations. For these 

 data, we selected tows with at least 50 kg of silvergray 

 rockfish. Silvergray rockfish represented 12.8% of the 

 total catch of over 36,000 t (Table 2). The dominant 

 species by weight in these tows were Pacific ocean 

 perch (Sebastes alutus), arrowtooth flounder (Atheres- 

 thes stomias), yellowmouth rockfish (S. reedi), yellowtail 

 rockfish (S. flavidus), redstripe rockfish (S. proriger), 

 and canary rockfish (S. pinniger). The species most 

 frequently co-occurring in the tows were arrowtooth 

 flounder, lingcod (Ophiodon elongatus), spiny dogfish 



6 Thomson, R. 2003. Personal commun. Institute of Oceans 

 Sciences, Fisheries and Oceans Canada. 9860 West Saanich 

 Road, P.O. Box 6000. Sidney, British Columbia V8L 4B2, 

 Canada. 



