Age-specific differences in the seasonal spatial distribution of butterfish (Peprilus triacanthus)

The spatial distribution of butterfish (Peprilus triacanthus) in the Northwest Atlantic Ocean was investigated using a suite of spatial indicators based on Northeast Fisheries Science Center spring and fall bottom trawl survey data, 1982-2013. In the spring, ages 2 and 3 were found farther northeast and deeper than age 1 butterfish, while in the fall, age 3 butterfish were found farther northeast and deeper than ages 0 and 1. There was no significant northward movement of butterfish in spring or fall over the course of either time-series. However, there was a significant increase in the area occupied by ages 1-3 in the spring that was correlated with surface temperature. This illustrates that responses to climate change may be manifested as range expansions, rather than poleward movement of the centre of gravity (i.e. bivariate weighted mean location of the population). Two changes were observed over the course of the fall time series, both for ages 1 and 2: increased spatial dispersion; and a decrease in depth. The former result would have been masked, while the latter would have been erroneously generalized to all age classes, if an age-specific analysis had not been done. This study demonstrates the importance of an age-based and seasonal analysis. It is also shown how a spatial distribution analyses can inform stock assessments by providing insights into diverging survey indices and availability to surveys in general. Similarly, spatial distribution analyses can be used to verify the spatial equilibrium assumption for the calculation of biological reference points.