Diel vertical migration patterns in juvenile cod from the Skagerrak coast

Abstract

We demonstrated a clear year-round diel vertical migration pattern (DVM) of small (26 to 45 cm) Skagerrak coastal cod Gadus morhua on a small geographic scale. Generalized additive models (GAMs) were used to make inferences about depth movements from a large data set of repeated depth measurements of cod fitted with acoustical tags. The general pattern in vertical behaviour was closely linked to sunrise and sunset, and the timing varied with the length of the day. The absolute magnitude of vertical movement was seasonally variable and probably influenced by temperature and food availability. Cod constantly selected deeper residence depths during the day than at night, and during winter than in summer. This study uses flexible models to make generalisations of the varying nature of the diel vertical migration patterns in juvenile coastal cod.