Research Document - 2001/059

Stock status of alewives and blueback herring returning to the Mactaquac Dam, Saint John River, N.B.

By Jessop, B.M.

Abstract

Linear empirical stock-recruitment relations were found between the escapement of alewives Alosa pseudoharengus and blueback herring A. aestivalis to the Mactaquac Lake on the Saint John River, New Brunswick, in year i and the year-class abundance at age 3 returning to the fishway at the Mactaquac Dam. The slopes were low but statistically significant whereas the slopes were non-significant for returns in years i+4 and i+5 in all but one case. Spawning escapements of 500,000-1,000,000 alewives produced year-class sizes at age 3 of 500,000-2,000,000 fish while blueback herring escapements of 200,000-300,000 fish produced year-class sizes at age 3 of 100,000-2,500,000 fish. The variability in returns of both species was high for a given spawning escapement, thereby reducing the value of the stock-recruitment relations as a predictive tool. Outlier years were identified by a robust regression method. Outlier removal resulted in significant stock-recruitment relations in three of six cases. No significant correlations were found between several measures of stock abundance and water temperatures at several depths in the Emerald Basin on the Scotian Shelf. Declining mean ages, lengths, and weights for both alewives and blueback herring at first spawning (sexual maturation) was probably due to the effects of fishery exploitation. The proportion of alewives first spawning at ages 3 and 4 increased while the proportion at ages 5 and 6 decreased. The proportion of blueback herring that spawned first at age 3 increased slightly, remained essentially constant (with high variability) at age 4 and declined at ages 5 and 6. The proportion of previous-spawners decreased in response to exploitation for both species and may offer a useful way of assessing stock status.

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