Annual social behaviour of basking sharks associated with coastal front areas

Abstract

Comparatively little is known about reproductive behaviour in wild sharks as it has proved extremely di¤cult to study, especially in large pelagic sharks. Here we describe annual courtship-like behaviour in the second-largest ¢sh species, the basking shark (Cetorhinus maximus), from 25 separate episodes observed and tracked during a ¢ve-year study period (1995^1999) o ¡ south-west England. Social behaviours observed between paired, or three or four, sharks were consistent with courtship behaviours seen in other shark species, namely nose-to-tail following, close following, close £ank approach, parallel and echelon swimming. Mature individuals between 5 and 8 m total body length (LT) exhibited these behaviours whereas smaller sharks (3^4m LT) did not. Lead individuals were identi¢ed as female on a number of occasions and interactions were prolonged; the longest continuous observation of socializing was 1.8 h, although intermittent track data indicates bouts may last for up to 5^6 h. Locations of court-ship-like behaviour events were not distributed randomly and were signi¢cantly associated with thermal fronts. Our results indicate that putative courtship behaviour occurs between May and July along oceanographic fronts, probably as a consequence of individuals aggregating to forage in rich prey patches before initiating courtship. Thus, locating the richest prey patches along fronts may be important for basking sharks to ¢nd mates as well as food in the pelagic ecosystem. As courtship-like behaviours occur annually o ¡ south-west 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 was not seen at the surface