Spatial diversity of Pacific herring (Clupea pallasi) spawning areas

Eastern Pacific herring spawn in intertidal and shallow subtidal areas. Spawning sites are conspicuous: milt turns coastal waters white, sometimes for distances of many kilometres. This attribute has enabled biologists to document spawning distributions for more than 70 years throughout the 29 500 km coastline of western Canada. Spawning distributions and spatial diversity have varied over time. When aggregated over 70 years (1938-2007), spawning occurred along 5574 km or similar to 20% of the total coastline. Cumulative annual spawn length ranges from 131 (in 1966) to 770 km (in 1992). We examined annual changes in spawn distribution using spatial units of variable size, ranging in area from a maximum of >1000 km(2) to a minimum of <0.1 km(2). Assessment of spatial diversity varied with the size of the spatial unit. Spatial diversity estimated from small spatial units (area <0.1 km(2)) was significantly correlated with spawning-stock biomass (SSB). In contrast, there was no correlation, and sometimes opposite temporal trends, between SSB and all larger spatial units (mean area >0.3 km(2)). The choice of spatial scale can affect the results from analyses of other factors, such as SSB, that could affect spatial diversity of spawning areas.