Cod migration patterns in relation to temperature: Analysis of storage tag data

Abstract

Bivariate time-series of depth (pressure) and temperature with two-hour intervals from 19-data storage tags (DST) attached to adult Northeast Arctic cod (Gadus morhua L.) released from mid-March are analysed. Interplay between migration behaviour, physiological limitation factors, environment, and ecology in the Barents Sea is investigated using geometrical and statistical methods. Thermo-stratification is identified using r(t), the ratio between temperature and depth change over each record interval. Vertical activity, act(a), in relation to physiological limitations to pressure change is measured with the ratio of the daily depth range to the free vertical range. Cycles are detected by spectral analysis. The analysis supports conclusions from large-scale studies. Cod migrate along stable thermal paths until they reach a front area (or feeding ground), where the vertical activity increases and the records of depth, temperature, and r(t) change pattern, level and range. The (semi-) diurnal vertical migration (DVM) occurs seasonally in some fish, mainly in areas with large temperature gradient. In 11 out of 12 tags where DVM is detected, this occurs during summer and autumn. In seven out of 11 tags where semi-diurnal tidal cycles are detected in the temperature series together with a significant reduction in vertical migration, this occurs during April. In some tags diurnal or semi-diurnal cycles appear in both depth and temperature series. © 2001 International Council for the Exploration of the Sea.