Seasonal aggregations of blacktip sharks Carcharhinus limbatus at a marine protected area in the Gulf of California, assessed by unoccupied aerial vehicle surveys

Abstract

No-take marine protected areas (MPAs) can provide spatial refuge for species throughout all or part of their life cycles. Cabo Pulmo National Park (CPNP) is a no-take MPA located on the south-east coast of the Baja California Peninsula, Mexico, where there has been an increase in the abundance and diversity of elasmobranch species since its closure to fishing. An unoccupied aerial vehicle was used to complete weekly aerial surveys over sandy habitat in CPNP to determine the relative abundance of blacktip sharks Carcharhinus limbatus over a 1 yr period in 2019. C. limbatus were only observed during winter months, and abundance per survey ranged between 7 and 1086 sharks (289 ± 59, mean ± SE), yielding a maximum density of 4827 ind. km-2. A generalized additive model determined that sea surface temperature (SST), time of day, photoperiod and wind speed were significant influencers of C. limbatus abundance. According to this model, higher abundance is expected when SST is lower than 25°C, during the afternoon, at low wind speeds (<4 knots) and when photoperiod is between 12 and 13 h. The close proximity of C. limbatus to the shoreline is likely a result of a refuging behaviour, with sharks utilising the shallow sandy habitat to behaviourally thermoregulate and/or to avoid predators.