Maturity Changes in Norwegian Spring-Spawning Herring Clupea harengus: Compensatory or Evolutionary Responses?

Engelhard, G.H. & Heino, M. ORCID: https://orcid.org/0000-0003-2928-3940 (2004). Maturity Changes in Norwegian Spring-Spawning Herring Clupea harengus: Compensatory or Evolutionary Responses? IIASA Interim Report. IIASA, Laxenburg, Austria: IR-04-001

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Abstract

The Norwegian spring-spawning herring "Clupea harengus" stock collapsed to a state of commercial extinction in the 1960s, probably due to both overexploitation and unfavorable climatic conditions. The stock has fully recovered since the 1980s. Following the collapse, the fish matured at a much earlier ages and somewhat larger sizes than previously. Currently, age and size at 50% maturity have only to some extent returned to pre-collapse levels. Two non-exclusive hypotheses may account for the maturity changes: (1) the 'compensatory response' hypothesis, predicting that reduced stock size resulting from exploitation leads to faster growth and hence earlier maturity as a phenotypically plastic response to environmental change; and(2) the 'evolutionary response' hypothesis, predicting that intensive exploitation causes selection for early maturation, since few late-maturing phenotypes survive until first spawning. Trends in length-at-age suggest a strong compensatory response, but leave the possibility of evolutionary change unaddressed. In order to disentangle phenotypic and possible genetic changes in maturation, we examined if any changes in the reaction norm for age and size at maturation have occurred. This reaction norm describes the probabilities of maturing at each relevant age and size. We found small but statistically significant changes in maturation reaction norms; these changes are restricted to two out of six maturation ages. This confirms that growth-related phenotypic plasticity has largely been responsible for the documented changes in maturity, and suggest at most a weak evolutionary response. This is in line with theoretical expectations from the dominating fishing pattern, where pure schools of only mature or only immature fish were targeted, rather than a fixed fishery on both immature and mature fish, as has been the case in cod "Gadus morhua" and plaice "Pleuronectes platessa" stocks were significant evolutionary responses have been found.

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