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.

Comparison of two machine learning frameworks for predicting aggregatory behaviour of sharks

“…Bull and blacktip shark occurrence and relative abundance were obtained from standardized gillnet surveys conducted by the Texas Parks and Wildlife Department (TPWD) long‐term fishery‐independent monitoring programme from 1982 to 2017. Within each estuary, monofilament gillnets were set using a stratified cluster design within a 3.4225 km 2 (1 nautical mile; Ayres et al, 2021) grid system in 10‐week spring (April–June) and 10‐week fall (September–November) sampling seasons. Gillnets were 183 m long and 1.2 m deep, with ascending 45.7 m sections of 7.6, 10.2, 12.7 and 15.2 cm stretched mesh.…”

“…Among leading conservation challenges, many animal populations exhibit migration or aggregation patterns that increase their vulnerability to disturbances wrought by habitat degradation, losses in community diversity and direct human interference (Tamario et al, 2019; Vierus et al, 2018). For many shark populations, these aggregations serve critical roles in feeding, spawning and predator avoidance (Ayres et al, 2021; Economakis & Lobel, 1998; Rowat & Brooks, 2012). Bull and blacktip sharks, C. limbatus both exhibit aggregative behaviour across their geographical ranges (e.g.…”

“…Bull and blacktip sharks, C. limbatus both exhibit aggregative behaviour across their geographical ranges (e.g. Ayres et al, 2021; Pirog et al, 2019), but it is still unclear what leads to such aggregations in many contexts beyond seasonal migrations (e.g. Kajiura & Tellman, 2016).…”

Orca (Orcinus orca) and shark predator-prey interactions within Cabo Pulmo National Park in the Gulf of California, Mexico

“…Seasonal aggregations of adult blacktip sharks (Carcharhinus limbatus) occur throughout the park each winter between December and April, a phenomenon that has been noticed by locals since 2008 (Ayres et al, 2021a) and by visual censuses carried out from a vantage point close to the shore since 2013 (Asuńsolo-Rivera, 2016;El-Saleh, 2016). Using unoccupied aerial vehicles (UAVs), sharks can be clearly observed occupying coastal shallow (< 5m) waters, with largest aggregations (over 1000 individuals) recorded in the northern sector of the park before the afternoon crepuscular period (Ayres et al, 2021a). This aggregating behavior has been linked to thermoregulation, as the shallow water sea surface temperature reaches its peak (~23.5°C) during this time between 1500 -1600hrs.…”

Manta and devil ray aggregations: conservation challenges and developments in the field

“…It is well documented that many elasmobranchs form large aggregations, varying from temporary groups to structured, longterm associations (e.g., hammerhead sharks, basking sharks, blacktip sharks, and common stingrays) (Klimley, 1987;Ward et al, 2004;Croft et al, 2006;Ward et al, 2007;Croft et al, 2011;Crowe et al, 2018;Chaikin et al, 2020;Ayres et al, 2021;Sims et al, 2022). The functions of such aggregation behaviors have been hypothesized to include reproduction (Klimley, 1987;Chaikin et al, 2020;Sims et al, 2022), feeding (De-la-Parra-Venegas et al, 2011), energy conservation, (Klimley and Nelson, 1984;Economakis and Lobel, 1998;Hight and Lowe, 2007), refuge from predators (Heupel et al, 2007;McAllister et al, 2017) and social learning (Sih et al, 2009;Brown and Laland, 2011).…”