Environmental predictors and temporal patterns of basking shark (Cetorhinus maximus) occurrence in the lower Bay of Fundy, Canada

Little is currently known about the population dynamics of basking sharks (Cetorhinus maximus) at regional or local scales. Using a long-term sighting database (1994-2012) and photo-identification of individuals, we studied the seasonal and inter-annual patterns in basking shark occurrence and site fidelity in the Bay of Fundy, Canada. Zero-inflated negative binomial models quantified spatial, temporal and environmental predictors of shark sighting rates. The probability of sighting a basking shark increased in August, and in deep water offshore; this may reflect the distribution and availability of calanoid copepod prey. Sea-surface temperature (SST) had no effects on shark sightings, but there was a negative correlation between the North Atlantic Oscillation (NAO) index and shark sightings lagged at two and four years; the former possibly due to the position of the Gulf Stream and the latter likely a result of the lagged influence of the NAO on copepod abundance. The model also showed a significant decline in the occurrence of basking sharks within the Bay of Fundy over the study period. From unique markings on dorsal fins, 98 individual sharks were identified from photographs taken between 1997 and 2012. Four of these individuals were re-sighted in subsequent years, and the longest interval between re-sightings was 9.1 years. These re-sightings suggest some site fidelity by individuals and demonstrate the longevity of some mark-types on the first dorsal fin. This study highlights the role of long-term sightings and photographic records as population assessment tools for regional scale-monitoring of a globally vulnerable species.