Three-dimensional ontogenetic shifts of groundfish in the Northeast Pacific

Abstract

It has been more than 100 years since fish were first described to move to deep waters as size increased, termed ‘Heincke's Law’. However, large-scale studies on ontogenetic shifts are rare compared with increased reports of distributional changes in response to temperature, often confounded with the ontogenetic shifts. We fill this gap by examining the distribution of ten abundant groundfish species in three dimensions, depth, latitude and longitude, at 10-cm size intervals within nine subregions of NE Pacific. Here, we utilized large, quality-controlled datasets from random depth-stratified, bottom trawl surveys consistently conducted during summer along the NE Pacific shelf from 1996 to 2015. Groundfish demonstrated complex ontogenetic movements in three dimensions across species, size class and subregion. In addition to the expected ontogenetic deepening, shoaling also occurred and some species demonstrated major ontogenetic shifts in longitude and/or latitude with limited changes in depth. Based on standardized ontogenetic shifts in three dimensions, our analyses show that there were significant differences in aggregate fish ontogenetic shifts between small (≤30 cm) and large (>30 cm) size groups. Small fish exhibited substantially larger ontogenetic shifts in depth than the large size group while both groups showed relatively small shifts in latitude and longitude. Our analyses strongly suggest that size structure and ontogenetic shifts should be included in the population distribution.