Growth, size and abundance of juvenile coho salmon in the Strait of Georgia; 1999-2001 and 2012-2014

Growth of juvenile coho salmon has been measured during the annual June/July DFO survey of the Strait of Georgia, 1999 – 2001 and 2012 - 2014. Growth was assessed by measuring plasma levels of the hormone insulin-like growth factor 1 (IGF1) from blood samples obtained during the survey. Two major patterns are apparent 1) IGF1 levels were higher 1999, 2012 – 2014 than found 2000 – 2001 and 2) for the period 2012 – 2014, growth was higher in northern regions of the Strait of Georgia as compared to southern regions. Sampling was limited to southern portions of the Strait of Georgia 1999 – 2001, precluding making a similar North to South comparison during that time period. Further analyses revealed several interesting relationships. There was a significant correlation between fork length and plasma IGF1 for coho salmon collected 1999, 2012 – 2014, suggesting that within these years larger fish had a greater ability to exploit food resources than smaller fish. Fish collected in 2000 – 2001 were relatively large but had low IGF1 values, suggesting that the overall prey field was reduced in those years (2000 – 2001). Finally, catch per unit effort (CPUE) was measured in each year for both the June/July survey and for the September survey. The ratio of September CPUE to June/July CPUE provides a simple assessment of change in abundance from June/July to September. Plasma IGF1 level measured in June/July was positively and significantly related to Sept/June CPUE, suggesting that higher growth rates during the early summer were associated with increased autumn abundance. Overall, these results suggest that there are significant inter-annual differences in juvenile coho salmon growth in the Strait of Georgia and that these differences in growth may be related to the resulting relative abundance of coho salmon in the autumn