Research Document - 2001/127

Metapopulation structure and dynamics of British Columbia herring

By Ware, D., Schweigert, J.

Abstract

The purpose of this exploratory paper is to integrate existing knowledge about the dispersal and population dynamics of the five major BC herring stocks into a structured metapopulation model (SMP). We used the model to quantify the amount of straying that occurred during the 1977-98 base period. Tagging studies indicate that dispersal rates for the five major BC herring populations during this period ranged from 14% to 36% per year, and appeared to be an increasing function of the biomass of the spawning stock in each population. The tagging studies also indicated that the dispersal pattern matched an isolation by distance model. That is, most herring dispersed to nearby populations, but a few strayed to the most distant ones. Consequently, all of the major BC herring populations are genetically linked by dispersal, and the dispersal rates are high enough to ensure that there is unlikely to be any genetic differentiation between the major populations. During the warm ocean climate regime, which prevailed during the base period, the SMP model suggests that about 25 kt/yr of adult herring migrated between the five major populations. The productive Georgia Strait population exported about 12 kt of adults per year to the other four, less productive populations.

The SMP model suggests that dispersal is important because it tends to stabilize the spatial distribution of spawners in the metapopulation and increases the persistence time of the less productive populations in two ways: 1) the density-dependent straying response increases the fidelity rate when the 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 and vacant spawning habitat. Conceptually, this enables the metapopulation to "adapt" to habitat changes, and to alter its distribution pattern in response to low frequency trends in climate, and other factors. From a stock forecasting perspective, straying probably has the largest effect on the metapopulation when an exceptionally large year-class recruits to one of the local populations. When this occurs a significant proportion of the resulting adult biomass may be exported to nearby populations, much like a radiating wave which decreases in height as it moves further away from the source population. For example, the SMP model suggested that after the exceptionally large 1977-yr class recruited to the QCI population, about 7 kt of herring might have migrated to the Central Coast population, and a roughly equal amount to the PRD population in both 1980 and 1981. The "unexpected" appearance of this amount of biomass would cause the forecasts for these stocks to be low.

Year-class strength synchrony in several nearby BC herring populations in 1977 and 1985 may have been caused by high juvenile herring survival rates over a large geographical area. However, it might also reflect significant straying by juvenile herring from a source population, which had a locally high juvenile survival rate, to nearby populations. More research needs to be done to measure juvenile herring dispersal rates, and their impact on recruitment in adjacent populations. We stress that the foregoing conclusions only apply to the warm climate regime, which prevailed during the base period analyzed here. The straying dynamics and productivity of BC herring populations during cool climate regimes will be examined in a future PSARC paper.

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