Annual social behaviour of basking sharks associated with coastal front areas

Comparatively little is known about reproductive behaviour in wild sharks a s it has proved extremely difficult to study, especially in large pelagic s harks. Here we describe annual courtship-like behaviour in the second-large st fish species, the basking shark (Cetorhinus maximus), from 25 separate e pisodes observed and tracked during a five-year study period (1995-1999) of f south-west England. Social behaviours observed between paired, or three o r four, sharks were consistent with courtship behaviours seen in other shar k species, namely nose-to-tail following, close following, close flank appr oach, parallel and echelon swimming. Mature individuals between 5 and 8 m t otal body length (L-T) exhibited these behaviours whereas smaller sharks (3 -4 m L-T) did not. Lead individuals were identified as female on a number o f occasions and interactions were prolonged; the longest continuous observa tion of socializing was 1.8 h, although intermittent track data indicates b outs may last for up to 5-6 h. Locations of courtship-like behaviour events were not distributed randomly and were significantly associated with therm al fronts. Our results indicate that putative courtship behaviour occurs be tween May and July along oceanographic fronts, probably as a consequence of individuals aggregating to forage in rich prey patches before initiating c ourtship. Thus, locating the richest prey patches along fronts may be impor tant for basking sharks to find mates as well as food in the pelagic ecosys tem. As courtship-like behaviours occur annually off south-M;est England we speculate that this region may represent an annual breeding area for this protected species, but mating itself probably takes place at depth as it wa s not seen at the surface.