Five decades of change in somatic growth of Pacific hake from Puget Sound and Strait of Georgia

Declines in fish body size have been reported in many populations and these changes likely have important ramifications for the sustainability of harvested species and ecosystem function. Pacific hake, Merluccius productus, have shown declines in size over the last several decades for populations located in Puget Sound (PS), Washington, USA, and Strait of Georgia (SoG), British Columbia, Canada. To examine this decrease in size, we used archived otoliths from both populations to assess when the decrease in somatic growth occurred and explored what factors and processes might explain the decline, including otolith microchemistry to infer the environment experienced by fish at different ages. Results indicated that substantial changes in juvenile somatic growth have occurred across decades. The divergence in body size occurred in the second summer, whereby SoG fish grew, on average, 18% more than PS fish. Within the PS population, somatic growth differed significantly among fish that hatched in the 1980s, 1990s, and 2010s, such that the more recently hatched fish grew 26% more in their first summer and 71% less in their second summer relative to those that hatched in the 1980s. In comparison, growth of SoG fish did not differ between those that hatched in 1970s and 1990s. For both populations growth in the first and third summer was positively and negatively related, respectively, to the abundance of harbor seals, while growth in the first and second summer was negatively related to salinity. Overall, this study highlights the complicated nature of Pacific hake population recovery under dynamic, and typically uncontrollable, variation in biotic and abiotic conditions.