Research Document - 1999/175

Age of sexual maturation and recruitment in Pacific herring.

By D.E. Hay and P.B. McCarter

Abstract

Percent at age data from the commercial herring roe fishery in northern BC indicate that herring are fully recruited at age 4. In general, there are more age 4 fish in the catch than age 3, 5, or older ages. Percent at age data are taken as evidence that in most assessment areas, a substantial part of the age 3 cohort is immature and not fully recruited to the spawning population. This is represented in the annual BC herring assessments in an age-structured model as the age-specific parameter lambda (l ) that represents the age-specific vulnerability to fishing gear. There is, however, other evidence that most age 3 fish are sexually maturing. We present 3 types of evidence. (1) Ovarian histology: analyses of maturing ovaries taken from samples collected from several years and areas in the winter months indicates that virtually all age 3 females had vitellogenic oocytes, a condition found only in sexually maturing females. (2) Gonad weights: since 1982, gonads have been weighed as part of the routine herring sample analyses. When ovary weights exceed 5% of total body weight (defined as a gonosomatic index or 'GSI' > 5) sexual maturation has started. We then show that virtually all age 3 fish collected in the late winter and spring have a GSI of 5, or greater. (3) The Hjort Maturity scale: this index is recorded for all fish, but the results have not been presented in previous analyses. Using this scale, any fish with a Hjort index of 4 or greater is sexually mature. All age 3 herring, collected in the late winter or later, have a Hjort Index of 4 or more. Therefore, there is an apparent contradiction between the area-specific percent at age data, which indicate incomplete recruitment of age 3 herring, and the observation most that age 3 are maturing. There are two mutually exclusive hypotheses to explain this contradiction (1) the 'age 3 immature hypothesis' and (2) the 'sample bias hypothesis'. The present stock assessment models assume the 'age 3 immature hypothesis' - that there are many immature age 3 herring that do not reside with the main herring stock and are not available for capture. The alternative 'sample bias' assumes that most age 3 fish are mature, but their frequency in the samples of catches is under-represented. There is no direct evidence to support the 'age 3 immature hypothesis'. If such evidence existed, it would consist of samples that consist mainly of sexually immature age 3 fish. Through reviews of the historical biological sampling data, the authors have not found such samples. Such fish have not been encountered during winter surveys of herring. On the other hand, there is evidence that samples from fishery catches may not be fully representative of the spawning populations. There are differences in the mean age of spawning fished when compared among different 'Statistical Areas', within each of the 'stock assessment areas'. In some areas and years, the duration of the sampling period often is shorter than the duration of the spawning period. Younger fish tend to spawn later so could be under-represented in samples. Therefore, there are data to support the 'sample bias hypothesis'. In general, however, it was concluded that the potential for sample bias may not fully explain the differences between the percent at age data and observed age at maturity. A new hypothesis is suggested: that there are episodic migrations of younger but mature herring from southern to northern areas. The consequence is that the observed percent at age data are valid, because the older age groups are augmented from southern mature migrants, not late maturing fish. This migration hypothesis concurs with a recent review and analyses of tagging data and would support the general validity both of the percent at age data and our observations about the age of sexual maturation. Of the competing hypotheses on this issue, the hypothesis of episodic migrations is the most parsimonious and precautionary.

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