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Size matters: changes in size-at-age and concurrent changes in spawning biology in Pacific herring

Time: 2017.02.09 13:00 - 14:00
Location: IMR / Pelagisk Seksjon

Spring-spawning Pacific herring spawn in nearshore inter-tidal and shallow sub-tidal habitats.  Annual estimates of spawn abundance are used as fisheries-independent indices of abundance and used in assessments of every population between California and Alaska.

Doug Hay, Scientist emeritus, Pacific Biological Station, Nanaimo, BC, Canada

Spring-spawning Pacific herring spawn in nearshore inter-tidal and shallow sub-tidal habitats.  Annual estimates of spawn abundance are used as fisheries-independent indices of abundance and used in assessments of every population between California and Alaska. In Canadian waters egg density is estimated annually by SCUBA surveys that quantify egg layers. Since the early 1990’s egg layers or egg density (n/m2) has decreased substantially in most Canadian populations.  Concurrent with this change, the mean size-at-age Pacific herring has decreased markedly in all almost populations from California to southern central Alaska. The explanation for this change is uncertain but it occurs in some areas where commercial fishing does not occur.  These two independently-measured temporal trends (decrease in egg layers and size-at-age) are significantly correlated in each of five major herring stocks in Canadian waters.  These changes are not simply related to trends in total abundance (SSB).  Also, fisheries are not directly implicated as a cause because fishing intensity has varied greatly among the stocks.  Instead, there appears to be a size-related change that, somehow, results in changes in egg density.  This presentation will examine the potential causes of these changes and implications for spawning biology of herring and other species.