Movements and behaviour patterns of Pacific bigeye tuna (Thunnus obesus) in relation to the environment

Changes in individual fish behaviour in response to environmental fluctuations are likely to play a key role in determining spatio-temporal population dynamics, but are relatively poorly understood. Behaviour patterns of bigeye tuna (Thunnus obesus) derived from archival tags, were investigated in relation to sea surface temperature (SST) and chlorophyll „a‟ concentration. 50 individual tracks were analysed, spanning periods at liberty between 31-1508 days and totalling 15,757,614 movement “steps”. Switching behaviour between patterns of vertical space use best represented by truncated Lévy (truncated power-law) and Brownian-like (exponential) model movement patterns was demonstrated in 39% of fish. Lévy-like patterns occurred at higher frequency in warmer SSTs and low chlorophyll „a‟ concentrations, which are hypothesized to represent a sparsely distributed prey field. Larger sample sizes may add statistical significance to these trends. Lévy-like behaviour was the dominant pattern, found in 76.6% of behavioural sections, while Brownian-like patterns were found in 12.7%. The predominance of Lévy-like motion is proposed to result from fine scale environmental complexity; broadly productive areas may contain hydrographic features such as meso-scale eddies which concentrate and retain prey, resulting in a patchy prey landscape in which Lévy behaviour is optimal. 79% of individuals showed behavioural discontinuities within the first two weeks at liberty. Immediately after release, 44.7% of tuna displayed Brownian-like behaviour, in comparison to 8.3% in the subsequent section. These significant differences in behaviour are attributed to post tagging effects. This represents the first attempt using large numbers of a single species to investigate behavioural switching in response to encountered environmental changes, and has important management and conservation applications.Faculty of Scienc