Research Document - 2004/081
Stock assessment for British Columbia herring in 2004 and forecasts of the potential catch in 2005
By Schweigert, J.F.
Herring stock abundance in British Columbia waters was assessed for 2004 and forecasts were made for 2005 using an age structured assessment model for the major stock assessment regions and an escapement model for the minor stocks in Areas 2W and 27. These models have been applied to assess herring abundance since 1984. As in recent assessments a fixed spawn conversion or catchability factor was applied for the dive survey era beginning in 1988 and a free fitted parameter was estimated for the earlier surface survey period. In addition, a year specific logistic function was applied to model the availability of fish on the spawning grounds. Penalty weights adopted for the preceding assessment were again used this year. All available biological data on total harvest, spawn deposition, and age and size composition of the spawning runs were used to determine current abundance levels. No significant problems were evident in the extent and comprehensiveness of the data collections. However, fewer samples were aged in 2004 due to constraints imposed on the ageing laboratory. All available data were included in and summarized from an Access database. Coastwide, the estimated pre-fishery stock biomass for all assessment regions in 2004 was 221,000 tonnes based on the age-structured model which represents a 2% decrease from the 2003 abundance level. This reflects stable or minor increases in all stocks except the Strait of Georgia where biomass decreased slightly. Recruitment of the 2001 year-class was very poor in the north and poor to average in the southern stocks.
The estimated harvestable surplus in 2005 (20% of the 2005 forecast herring run) based on forecast abundance to the five assessment regions is 37,540 tonnes for the B.C. coast assuming average recruitment to all areas except the Queen Charlotte Islands where the stock remains at depressed levels and no surplus is available.
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