Research Document - 2011/096
Evaluation of Pacific sardine (Sardinops sagax) stock assessment and harvest guidelines in British Columbia
By L. Flostrand, J. Schweigert, J. Detering, J. Boldt, and S. MacConnachie
Abstract
Proportions of the Pacific sardine (Sardinops sagax) population off North America annually migrate from southern waters (mainly off California) where they spawn during late winter and spring to northern waters, such as off British Columbia, where they forage during summer and autumn, returning south late in the autumn. The sardine population collapsed to very low levels in the mid-20th century, disappearing altogether from Canadian waters. A coast-wide recovery of the stock began in the 1980s. The Canadian sardine fishery has harvested sardines experimentally since 1995 and commercially since they were declared not at risk by COSEWIC in 2002.
The recent Canadian Pacific sardine harvest framework has been based on an annually updated biomass estimate of the adult coastwide population from the U.S. assessment and an annually updated migration rate estimate of sardines into Canadian waters, upon which a harvest rate equivalent to the U.S. rate has been applied (a 15% Canadian harvest rate has been in place since 2002). Representatives of Canadian sardine industry have shown interest in developing a harvest policy independent of the U.S stock assessment that favours using a British Columbia seasonal estimate of sardine biomass resulting from a summer research trawl survey.
This report provides: 1) updated estimates of regional biomass and seasonal migration for the west coast of Vancouver Island from trawl survey data; 2) additional sardine biomass and migration rate estimates from extrapolating WCVI trawl survey sardine densities to other areas of the B.C. coast where sardines are frequently observed; 3) provisional maximum harvest options for the 2011 fishing season resulting from applying the previously approved harvest control rule with updated estimates of the mean seasonal migration rate (2008-2010), the recent estimate of the coastwide population from the U.S. stock assessment and a 15% harvest rate ; 4) provisional maximum harvest options for the 2011 fishing season resulting from applying an alternative harvest control rule, which is based solely on seasonal estimates of sardine biomass in B.C. waters and a 15% harvest rate; and 5) background information related to biological concerns and information gaps that should be considered when setting catch allowances in B.C. waters.
Mean sardine survey catch density and biomass estimates for the WCVI survey region declined from 2006 to 2010, as did biomass estimates for the coastwide adult population. Estimates of migration for 2008 to 2010 into the WCVI survey region were 22.9%, 27.8% and 15.3% and estimates of migration into the inshore areas outside of the WCVI survey region were 4.7%, 5.7% and 3.2 %. The 2008-2010 mean migration rate estimate was 22.0% for the WCVI survey region, 4.6% for the inshore areas outside of the WCVI survey region, and 27.2% for both areas combined. Applying these updated mean migration rate estimates to the previously accepted Canadian harvest control rule along with the recent (2010) coastwide adult sardine biomass estimate of 537,173 tonnes and a 15% harvest rate, maximum harvest options equate to 17,725 tonnes (based on WCVI survey region migration estimates), or 21,917 tonnes (based on WCVI survey region and inshore areas combined migration estimates). An alternative harvest control rule that is based on B.C. biomass estimates and a 15% harvest rate generates maximum harvest options of 12,295 tonnes (based on WCVI survey region biomass estimates), or 14,838 tonnes (based on WCVI survey region and inshore areas combined biomass estimates). This second and alternative harvest control rule has the drawback of being completely dependent on completion of a summer survey and has equal or greater risk of overshooting the target regional harvest rate when regional biomass decreases between years. Variability in regional and coastwide estimates of 2006-2010 realized harvest rates reflects changes in assessment methods and fishing effort in Canada and the U.S. Simulated harvest control rule scenarios for both nations show how harvest rates for the coastwide population can exceed a common target rate. Length and age trends from biological samples collected in coastal regions of B.C. and the U.S. show seasonal and regional variability relevant to stock structure and effects of strong and weak year classes, in particular the lack of a relatively strong year class since 2005.
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