Medel Silvosa’s research while affiliated with Mindanao State University and other places

We used acoustic telemetry to investigate the roles of depth and temperature in the daytime foraging behaviours of 13 tagged pelagic thresher sharks by monitoring their fine scale vertical movements in the Philippines. Cumulatively, pelagic thresher shark dives traversed the entire water column where they encountered temperatures that ranged from 33°C at the surface to 12°C at 250 m depths throughout the day, but the movements of individuals varied in the extent of both their deep and shallow water limits. Dives were not synchronized to diurnal cycles, and periodicity reflected cycles of similar dives, the dives themselves, deviations, cruising, and individuality. Pelagic thresher shark movements between the warm surface layer and cooler waters below the thermocline (155 – 175 m) may reflect a common Alopiid strategy that balances maintaining tolerable ambient water temperatures with opportunities to search for and forage on spatially patchy distributions of prey.

Oceanic sharks are vulnerable to overexploitation due to their life-history strategies, and efforts to protect them in the wild have been stalled by transjurisdictional conflicts of interest. The pelagic thresher shark (Alopias pelagicus) is one such species that visits a seamount in the Philippines where its dependable presence has cat-alysed a burgeoning dive tourism industry and the designation of a conservation area. Nothing is known of the range and turnover of this population, but the regularity with which these sharks interact with cleaner wrasse on the seamount provides important stability for regional businesses that lack empirical knowledge of their vulnerability. We fitted 14 pelagic thresher sharks with acoustic tags and monitored their fine scale movements for 66 days (June to mid-August 2014). Individuals were present at the seamount for 32% of their days at liberty, and 42% of the tagged sharks were still being detected there at the end of the study. Thresher sharks showed preferences for visiting specific locations on the seamount where they interact with cleaner fish, and estimates of their fidelity to these sites provided scalars for visual census. Pelagic thresher sharks moved away from the seamount after early morning visits to cleaning stations using swim speeds of 3.79 km h −1 (± SD 1.43). These movements demonstrated that they have access to the jurisdictional waters of five provincial territories when dispersing from and returning to the seamount on a diurnal basis. While the seamount offers cleaner-associated services and refuge provision for pelagic thresher sharks, their scale of movement leaves them vulnerable to fisheries that operate in the region. Natural history observations provide context and reveal bias for their application in the management and conservation of this rare and vulnerable shark species.

Selected fish were measured on markets along the Davao Gulf, Philippines between 2009 and 2016, augmenting the number of Length-Weight relationships (LWR) published earlier for the same area. LWRs were calculated for 28 fishes including those of 12 firstly reported, rare species. SL-TL and SL-FL relationships were determined for 28 and 25 species (also including 8 and 12 newly reported relationships, respectively). Minimum size at which individuals start developing forked tails are provided for Cheilinus fasciatus (SL = 15.0 cm), Plectorhinchus polytaenia (SL = 27.0 cm), Pseudobalistes flavimarginatus (SL = 18.0 cm) and Thalassoma hardwicke (SL = 11.5 cm). The flatfish Psettodes erumei had a right-left eyed ratio of 0.55.

Selected fish were measured on markets along the Davao Gulf, Philippines between 2009 and 2016, augmenting the number of Length-Weight relationships (LWR) published earlier for the same area. LWRs were calculated for 28 fishes including those of 12 firstly reported, rare species. SL-TL and SL-FL relationships were determined for 28 and 25 species (also including 8 and 12 newly reported relationships, respectively). Minimum size at which individuals start developing forked tails are provided for Cheilinus fasciatus (SL = 15.0 cm), Plectorhinchus polytaenia (SL = 27.0 cm), Pseudobalistes flavimarginatus (SL = 18.0 cm) and Thalassoma hardwicke (SL = 11.5 cm). The flatfish Psettodes erumei had a right-left eyed ratio of 0.55.

Overhead tail-slap with preparation, strike, wind down recovery and prey item collection phases. Recorded in the sagittal plane on 24 May 2010, this event did not result in prey capture and was classified as unsuccessful. (M4V)

The hunting strategies of pelagic thresher sharks (Alopias pelagicus) were investigated at Pescador Island in the Philippines. It has long been suspected that thresher sharks hunt with their scythe-like tails but the kinematics associated with the behaviour in the wild are poorly understood. From 61 observations recorded by handheld underwater video camera between June and October 2010, 25 thresher shark shunting events were analysed. Thresher sharks employed tail-slaps to debilitate sardines at all times of day. Hunting events comprised preparation, strike, wind-down recovery and prey item collection phases, which occurred sequentially. Preparation phases were significantly longer than the others, presumably to enable a shark to windup a tail-slap. Tail-slaps were initiated by an adduction of the pectoral fins, a manoeuvre that changed a thresher shark's pitch promoting its posterior region to lift rapidly, and stall its approach. Tail-slaps occurred with such force that they may have caused dissolved gas to diffuse out of the water column forming bubbles. Thresher sharks were able to consume more than one sardine at a time, suggesting that tail-slapping is an effective foraging strategy for hunting schooling prey. Pelagic thresher sharks appear to pursue sardines opportunistically by day and night, which may make them vulnerable to fisheries. Alopiids possess specialist pectoral and caudal fins that are likely to have evolved, at least in part, for tail-slapping. The evidence is now clear; thresher sharks really do hunt with their tails.

Overhead tail-slap with preparation, strike, wind down recovery and prey item collection phases. Recorded in the sagittal plane on 24 August 2010, cavitation bubbles can be seen rising from the apex of the arc forming the trajectory of a pelagic thresher shark's strike. This event resulted in the shark successfully debilitating and consuming three sardines. Video material provided by Jan Acosta (© 2010). (M4V)

Overhead tail-slap with preparation, strike, wind down recovery and prey item collection phases. Recorded in the transverse plane on 9 September 2010, this successful event resulted in a pelagic thresher shark debilitating and consuming two sardines. (M4V)

Sideways tail-slap. Recorded in the sagittal plane on 14 June 2010, this event followed a previously successful overhead tail-slap. Sideways tail-slaps appeared slow and lazy compared to overhead tail-slaps. (M4V)