Research Document - 2002/107

Metapopulation dynamics of British Columbia herring during cool and warm climate regimes

By D.M. Ware, J. Schweigert

Abstract

In a previous paper (Ware and Schweigert 2001) we documented the dispersal and production dynamics of the five major BC herring stocks during the recent 1977-98 "warm"
climate regime, and incorporated the results into a structured metapopulation model (SMP). In this paper we analyze the dispersal and production dynamics of the five major BC herring stocks during the 1951-76 "cool" climate regime, and include the results in the SMP model to estimate dispersal during this period. We also compare and contrast the dispersal rates and production dynamics of the major BC herring stocks during the complete time series (1951-98). The dispersal rate appears to be density-dependent during both climate regimes. Consequently, dispersal tended to be higher during the warm regime, because the average biomass of spawners was larger. Recruitment to the Queen Charlotte Islands (QCI) and West Coast of Vancouver Island populations was significantly reduced during the warm regime, and decreased the productivity of these populations. The dispersal rate time series output by the metapopulation model imply that the Georgia Strait and Prince Rupert populations tend to be significant exporters of adult herring to other populations, while the Central Coast and QCI populations tend to be major importers. The appearance of a large year-class in one of the local populations will set up a dispersal wave, which radiates throughout the metapopulation until it is depleted by natural and fishing mortality. The dispersal rate time series indicate that seven, large dispersal waves probably occurred since 1951. Dispersal is an important process because it tends to stabilize the spatial distribution of spawners in the metapopulation, and increases the persistence time of the less productive, local populations in two ways: 1) the density-dependent dispersal response increases the fidelity rate when a population is declining, and (2) declining populations will tend to receive more migrants from other populations than they export. Dispersal is also important because it recolonizes new (or vacant) spawning habitat. This enables the metapopulation to adapt to habitat changes, and to alter its spatial distribution in response to low frequency trends in climate, and other factors.

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