Can drifting objects drive the movements of a vulnerable pelagic shark?

Abstract

Juvenile silky sharks (Carcharhinus falciformis) regularly associate with floating objects yet the reasons driving this behaviour remain uncertain. Understanding the proportion of time that silky sharks spend associated with floating objects is essential for assessing the impacts of the extensive use of fish aggregating devices (FADs) in the tropical tuna purse‐seine fisheries, including increased probability of incidental capture and the potential of an ecological trap. Previous studies provided insight into the amount of time that silky sharks spent at an individual FAD but were unable to assess neither the time spent between two associations nor the proportion of time spent associated/unassociated. The percentage of time that juvenile silky sharks spend unassociated with floating objects was estimated through the analysis of horizontal movements of 26 silky sharks monitored with pop‐up archival tags. Under the assumption that a high association rate with drifting FADs would align the trajectories of tracked sharks with ocean surface currents, a novel methodology is proposed, based on the comparison of shark trajectories with simulated trajectories of passively drifting particles derived using a Lagrangian model. Results revealed that silky shark trajectories were divergent from surface currents, and thus unassociated with FADs, for at least 30% of their time. The potential of the methodology and the results are discussed in the context of increasing FAD densities in the Indian Ocean.

Behavior and Ecology of Silky Sharks Around the Chagos Archipelago and Evidence of Indian Ocean Wide Movement

“…Importantly, given our small sample size, differences observed here could be simply attributable to individual variation, especially as divergent longitudinal movements of silky sharks have been observed in individuals tagged at several other locations (Musyl et al, 2011b;Filmalter et al, 2015;Hutchinson et al, 2019). Alternatively, movement behaviors of individuals may have been artificially impacted by association with drifting FADs (Hutchinson et al, 2019), although a recent study demonstrated that at least 30% of juvenile silky shark movements were not associated with surface currents (Bonnin et al, 2020).…”

“…In the Gulf of Mexico, neonates and young juveniles have been shown to live a more coastal, demersal lifestyle than adults (Branstetter, 1987;Bonfil, 1997). However, elsewhere, juveniles have been regularly reported in open ocean environments (e.g., Filmalter et al, 2015;Hutchinson et al, 2019;Bonnin et al, 2020), often joining schools of large pelagic fishes, such as tuna (Camhi, 2009;Sánchez-De Ita et al, 2011). Silky sharks grow up to 330 cm in length (Camhi, 2009), and can be highly mobile, with maximum recorded displacement distances of over 3,000 km (Schaefer et al, 2019).…”

Quantifying longline bycatch mortality for pelagic sharks in western Pacific shark sanctuaries

“…Throughout these nations, all commercial shark fishing is prohibited, and trade in shark parts is made illegal (11). As we are just beginning to understand how much space is needed to effectively protect populations of large sharks (12,13), the true conservation benefits of these spatial management tools remain poorly understood or otherwise contested (14)(15)(16)(17).…”