Evaluating post-release behaviour modification in large pelagic fish deployed with pop-up satellite archival tags

Abstract

Post-release behaviour modification, possibly a result of capture and handling stress, was evaluated using empirical eigenfunction analysis to detect changes in vertical movement patterns recorded by 183 pop-up satellite archival tags (PSATs) deployed on large pelagic fish. Argos-transmitted summary, timed interval, and some archival data were included. Scoring of irregular post-release behaviour was based on a separation of plotted eigenfunction coefficient values by their mean, with the transection across the mean reference line denoting the duration of irregular behaviour. In all, 67 (36.6%) individual fish exhibited irregular behaviour, lasting from 3 to 60 d (mean 15.8, s.d. 10.4). An additional 27 (14.8%) displayed patterns suggestive of irregular behaviour. Data quality and quantity were important criteria for revealing behaviour patterns. Irregular behaviour was detected in 32.6% of Argos-transmitted dataseries, increasing to 60.6% in the higher-resolution archival series. Decreased vertical movement characterized the irregular behaviour of blue sharks (Prionace glauca) and porbeagles (Lamna nasus), whereas all other species showed increased vertical activity. The approach described provides a useful method of revealing behavioural modification during the post-release recovery period of PSAT-tagged large pelagic fish, although the extent of influence on normal behaviour is not fully understood.