Group items tagged

SHARK Bay and the coastline off Karratha and Perth may be popular eateries for whale sharks (Rhincodon typus), satellite tracking has revealed.

SHARK Bay and the coastline off Karratha and Perth may be popular eateries for whale sharks (Rhincodon typus), satellite tracking has revealed.

Highly mobile marine megafauna species, while widely distributed and frequently threatened, often aggregate in distinct localized habitats. Implementation of local management initiatives within these hotspots is more achievable than developing effective conservation strategies that encompass their entire distributions. Such measures have the potential for disproportionate population-level benefits but rely on a detailed understanding of spatiotemporal habitat use. To that end, we examined the residency and small-scale habitat use of 51 whale sharks (Rhincodon typus) over 5 years at an aggregation site in Tanzania using passive acoustic telemetry. Whale sharks were highly resident within and across years, with a combined maximum residency index of 0.39. Although fewer sharks were detected from March to September, residency was high throughout the year. Ancillary photographic-identification data showed that individual residency persisted before and after tag attachment. Kernel utilization distributions (KUD) and movement networks both revealed the same spatiotemporal pattern of habitat use, with a small core habitat (50% KUD area for all sharks combined = 12.99 km2) that predictably changed on a seasonal basis. Activity spaces did not differ with time of day, sex, or size of the sharks, indicating a population-level pattern driven by prey availability. The small and predictable core habitat area at this site means that site-based management options to reduce shark injuries and mortality from boat strike and fishing gear entanglement can be spatially targeted for maximum effectiveness and compliance by human users.

Age and growth data are central to management or conservation strategies for any species. Circumstantial evidence suggests that male whale sharks (Rhincodon typus) grow to asymptotic sizes much smaller than those predicted by age and growth studies and consequently, there may be sex-specific size and growth patterns in the species. We tested this hypothesis by using stereo-video and photo-identification studies to estimate the growth rates of 54 whale sharks that were resighted over a period of up to a decade at Ningaloo Reef. We found that male growth patterns were consistent with an average asymptotic total length (TL) of approximately 8-9 m, a size similar to direct observations of size at maturity at aggregation sites world-wide and much smaller than the sizes predicted by earlier modeling studies. Females were predicted to grow to an average asymptotic length of around 14.5 m. Males had growth coefficients of K = 0.088 year-1, whereas limited resighting data suggested a growth coefficient of K = 0.035 year-1 for females. Other data including re-sightings of an individual male over two decades, records of sex-specific maximum sizes of individuals captured in fisheries and data from juveniles growing in aquaria were also consistent with the suggestion of sex-specific growth profiles for the species. We argue that selection for sex-specific growth patterns could explain many of the otherwise enigmatic patterns in the ecology of this species including the tendency of the species to form aggregations of juvenile males in coastal waters.

Baitfishes may aggregate around megafauna such as whale sharks (Rhincodon typus) to gain protection from larger predatory fishes. Here, we used videos from deployments of animal-borne cameras on whale sharks and provided by tourism operators at Ningaloo Reef in Western Australia to document large schools of trevally (Carangidae spp.) consuming entire schools of baitfishes (Carangidae spp.) swimming with whale sharks within 2-45 s. These videos showed that small baitfishes are still very vulnerable to predatory fishes when accompanying whale sharks, refuting the hypothesis that whale sharks provide baitfishes shelter from predators. It thus seems more likely that the association between whale sharks and baitfishes may confer other advantages such as reduced costs of locomotion and/or enhanced feeding opportunities for baitfishes rather than protection from predation.

Whale sharks off the western coast of India have suffered high levels of fishing pressure in the past, and today continue to be caught in small-scale fisheries as by-catch. Additionally, coastlines in this region host very large and growing human populations that are undergoing rapid development. This exacerbates ongoing anthropogenic threats to this species such as pollution, habitat loss, and ship traffic. For these reasons, there is an urgent need for data on movement patterns of whale sharks in this region of the Indian Ocean. Here, we address this issue by providing the first data on the horizontal movements of whale sharks tagged in the northern Arabian Sea off the western coast of the Indian state of Gujarat. From 2011 to 2017, eight individuals, ranging from 5.4 to 8 m were tagged and monitored using satellite telemetry. Tag retention varied from 1 to 137 days, with the sharks traveling distances of 34 - ∼2,230 km. Six of the eight individuals remained close to their tagging locations, although two sharks displayed wide ranging movements into the Arabian Sea, following frontal zones between water masses of different sea surface temperatures. We explore the relationship between the movement patterns of these whale sharks and the physical and biological processes of the region.

Subsea infrastructure of the oil and gas industry attracts commercial fish species as well as megafauna including sea lions, turtles, sharks and whales. Potential impacts of this attraction, whether positive or negative, are unknown. As part of a pilot study, we deployed acoustic telemetry equipment around offshore infrastructure to assess its effectiveness in detecting tagged marine animals and to gain insights into patterns of megafauna occurrence around these structures. Acoustic receivers were placed around four oil and gas platforms and on two remotely operated vehicles (ROVs) on Australia's North West Shelf. Two whale sharks (Rhincodon typus) tagged in the World Heritage Ningaloo Reef Marine Park were detected at two platforms, North Rankin A and Pluto, located up to 340 km to the northeast. The shark at North Rankin A was detected infrequently and only 15 times over ∼6 weeks. The shark at Pluto was detected each day of the 24-day deployment, in total 4,894 times. Detections at Pluto platform were highest during the day, with peaks at dusk and dawn. Our study indicates that acoustic telemetry around platforms may be an effective method for understanding how marine megafauna utilise these structures. We recommend collaborating with industry to undertake receiver detection range testing to understand the effectiveness of the method. Furthermore, future studies should co-occur with tagging programs at sites like Ningaloo Reef and around the structures themselves to maximise the probability of detecting animals at these sites, thereby improving our understanding of how marine megafauna interact with these structures.

Manta rays are large planktivores that inhabit tropical and subtropical waters globally, which are generally oligotrophic (MARSHALL, COMPAGNO & BENNETT, 2009). Therefore, to meet their metabolic needs, manta rays need to locate pulses of zooplankton productivity. Similar to other tropical planktivores, such as leatherback turtles Dermochelys coriacea (HAYS et al., 2006) and whale sharks Rhincodon typus (ROHNER et al., 2015), manta rays aggregate where and when conditions result in elevated local productivity (DEWAR et al., 2008; ANDERSON, ADAM & GOES, 2011; Jaine et al., 2012). However, these productivity 'hotspots' are ephemeral in nature and often difficult for researchers to locate and characterise (HARRIS et al., 2020; HARRIS et al., 2021), which makes the direct study of planktivore feeding ecology challenging (SIMS, 2008; ROHNER et al., 2015).

Holidaymakers' photos could help scientists track the movements of giant endangered sharks living in the waters of the Indian Ocean.

Swimming with Whale Sharks: Beneficial or Cruel?

Whale shark mugshots reveal teenage males hang around WA's coast.

Tanzania: Fishing Endangers Sharks Off Mafia.

Researchers are planning to tag two of Ningaloo Reef's most famous whale sharks - Stumpy and Zorro - in a bid to discover where the ocean giants go to mate.

Why the World's Biggest Sharks Love Mafia Island.

A technique that combines lasers and photography provides a decent measurement of the largest fish in the world.

A technique that combines lasers and photography provides a decent measurement of the largest fish in the world.

Shark outrage after endangered dead whale shark photographed in Malaysian fish market.

Swimming with humpback whales to be trialled at WA's Ningaloo marine park.

Swimming with humpback whales to be trialled at WA's Ningaloo marine park.

Researchers from The University of Western Australia and the Australian Institute of Marine Science (AIMS) have raised concerns about the whereabouts of the world's biggest whale sharks after finding that the largest sharks observed in recent years were smaller than those recorded more than a decade ago.

Researchers from The University of Western Australia and the Australian Institute of Marine Science (AIMS) have raised concerns about the whereabouts of the world's biggest whale sharks after finding that the largest sharks observed in recent years were smaller than those recorded more than a decade ago.

Whale sharks feeding in the western Indian Ocean - in pictures.

Researchers use 'biological passport' to monitor Earth's largest fish.

Swimming with giants in Madagascar.