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Most larval herring movements are consistent with a counter clock-wise, current gyre that prevails within the surface waters of the Strait of Georgia. The Fraser River plume and local, prevailing winds and tidal forces, also exert significant influence. The Fraser River, spring/summer freshet builds, peaks and diminishes from early May through to the end of July. Earlier research by Stevenson (1962) in Barkley Sound suggested that large concentrations of herring larvae, transported seaward after hatching, would die in the high salinity waters offshore of Vancouver Island. It was hypothesized that the numbers of larvae and post-larvae that were retained within inshore waters would determine the strength of the resulting year-class. Other researchers, Alderdice and Hourston (1985) describe several Strait of Georgia ichthyoplankton surveys conducted in 1981 by Mason et al. (1982) and concluded that predation was the the primary cause for larval and post-larval seasonal "disappearance" as opposed to limited food supply or seawater temperature and salinity tolerances measured in laboratory studies. These single-year, multi-species larval surveys, however, were not well-focused on the distribution and dispersal aspects of herring larvae, spatially and/or temporally and consequently, only small and peripheral concentrations were mapped. More recent, herring larval surveys conducted in the Strait of Georgia and in other BC regions (Hay and McCarter 1991, 1997) build upon these earlier studies and more accurately delineate larval retention areas and potentially important, larval-juvenile rearing sites. Herring year-class strength is likely determined in most years and areas, during early, juvenile stages (3-5 months of age) just as these small fish begin to actively school in nearshore rearing areas. Anomalous oceanographic conditions (i.e. reduced upwelling off the west coast of Vancouver Island) that can occur in some years and periods, however, may exert significant influence on the survival of younger, post-larvae in the more vulnerable (or nutrient poor) rearing areas of BC.
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