Research Document - 2004/008

Pacific Herring Spawn Disappearance and Recolonization Events

By Ware, D.M., Tovey, C.

Abstract

This is the fourth in a series of papers outlining the evidence that British Columbia herring are spatially structured and interact as a metapopulation. The purpose of this paper is to analyze the spawn time series for indications of “disappearance” and “recolonization” events. Some local communities in British Columbia (BC) believe that each bay where herring spawn contains a genetically discrete stock. Accordingly, if one of these “stocks” fails to return to spawn the implication is that some unique genetic diversity has probably been lost. The metapopulation concept provides an ecological basis for explaining that small, herring spawning aggregations can disappear for a time due to dispersal and other natural causes. And that vacant habitat will eventually be recolonized, when suitable conditions return. We analyzed the spawn time series (from 1943 to 2002) in 76 spatial “sections” where herring spawn in BC. We identified 82 spawn disappearance events, and found that 55% of the sections experienced one or more disappearance events in the last 60 years. We found that small sections experience more disappearance and recolonization events than sections with larger amounts of spawn habitat. Stray spawners from other areas recolonize vacant sections in 11 years, on average. Some sections were recolonized in less than five years; while at the other extreme, one section has not been recolonized for 35 years. Only 53 % of the recolonization attempts were successful. The high degree of straying between nearby sections explains why herring spawning aggregations at the "section" spatial-scale are so dynamic from year-to-year, and have a very low coherence. We found that 18 sections currently contain no spawn. These vacant sections have smaller amounts of spawn habitat, and a higher probability of disappearance events than occupied sections. The balance between the spawn disappearance and recolonization rates changed in Southern BC toward the end of the last “cool” climate regime. During the first half of the warm regime that followed the disappearance rate tended to be much higher than the recolonization rate, so the number of sections occupied by spawners declined. In the Northern sections, the disappearance and recolonization rates have tended to be more balanced. However, over the last 60-years when the disappearance (and recolonization) rates were higher in the South, they also tend to be higher in the North. In 34 of the 76 sections examined, there were no spawn disappearance events. These important sections contain about 85% of the total herring spawn habitat in BC, and therefore should be protected from shoreline development, pollution and other sources of habitat degradation. Loss of these habitats will almost certainly have a negative impact on the dynamics and resilience of the metapopulation.

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