Research Document - 2000/146

Offshore herring biology and 2001 recruitment forecast for the West Coast Vancouver Island stock assessment region.

By R.W. Tanasichuk

Abstract

An offshore trawl survey was done between July 25 and 30, 2000 to collect information on pelagic fish distribution and feeding, and to sample herring schools for a recruitment forecast for West Coast Vancouver Island (WCVI) herring in the 2001 fishing season. Herring (Clupea pallasi) dominated the pelagic biomass. Schools were numerous and distributed widely over the study area. Several sardine (Sardinops sagax) schools were observed. Pacific hake (Merluccius productus), traditionally the dominant pelagic fish, was absent. Stomach content examination indicated that herring continued to feed exclusively on euphausiids and that the daily ration estimate was similar to that for other years, in other words, feeding conditions were typical. Data analysis for the traditional (correlation) form of the recruitment forecast included three evaluations of potential bias. Results of the first test showed no effect of sampling time on forecast accuracy. The second test involved replicating the survey to compare estimated proportions of age 3 within a year. Estimated proportions were similar between replicate surveys. The third test showed no change in the forecast performance between years. The performance of the forecast did not change over the 1990's when WCVI recruitments were declining while Strait of Georgia recruitments were increasing. This suggested that any interaction between the two stocks did not affect the forecasting procedure. The forecasting procedure was re-expressed as a regression so that the uncertainty of the forecasts could be described. The forecasted biomass of recruits ranges between 700 and 1200 tonnes. This would be considered as poor based on the criteria used in the stock assessment process. A forecast based on risk analysis suggested that recruitment should be poor or average.

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