Satellite tagging highlights the importance of productive Mozambican coastal waters to the ecology and conservation of whale sharks

Christoph A Rohner; Anthony J Richardson; Fabrice R A Jaine; Michael B Bennett; Scarla J Weeks; Geremy Cliff; David P Robinson; Katie E Reeve-Arnold; Simon J Pierce

doi:10.7717/peerj.4161

Satellite tagging highlights the importance of productive Mozambican coastal waters to the ecology and conservation of whale sharks

PeerJ. 2018 Jan 2:6:e4161. doi: 10.7717/peerj.4161. eCollection 2018.

Authors

Christoph A Rohner¹, Anthony J Richardson^{2

3}, Fabrice R A Jaine^{1

4

5}, Michael B Bennett⁶, Scarla J Weeks⁷, Geremy Cliff^{8

9}, David P Robinson¹⁰, Katie E Reeve-Arnold¹¹, Simon J Pierce¹

Affiliations

¹ Manta Ray & Whale Shark Research Centre, Marine Megafauna Foundation, Praia do Tofo, Mozambique.
² CSIRO Oceans and Atmosphere, Dutton Park, QLD, Australia.
³ Centre for Applications in Natural Resource Mathematics (CARM), School of Mathematics and Physics, The University of Queensland, St Lucia, QLD, Australia.
⁴ Sydney Institute of Marine Science, Mosman, NSW, Australia.
⁵ Department of Biological Sciences, Macquarie University, North Ryde, NSW, Australia.
⁶ School of Biomedical Sciences, The University of Queensland, St Lucia, QLD, Australia.
⁷ Biophysical Oceanography Group, School of Geography, Planning and Environmental Management, The University of Queensland, St Lucia, QLD, Australia.
⁸ Kwa-Zulu Natal Sharks Board, Umhlanga, KZN, South Africa.
⁹ Biomedical Resource Unit, University of KwaZulu-Natal, Durban, KZN, South Africa.
¹⁰ Shark Watch Arabia, Dubai, United Arab Emirates.
¹¹ All Out Africa Marine Research Centre, Praia do Tofo, Inhambane, Mozambique.

Abstract

The whale shark Rhincodon typus is an endangered, highly migratory species with a wide, albeit patchy, distribution through tropical oceans. Ten aerial survey flights along the southern Mozambican coast, conducted between 2004-2008, documented a relatively high density of whale sharks along a 200 km stretch of the Inhambane Province, with a pronounced hotspot adjacent to Praia do Tofo. To examine the residency and movement of whale sharks in coastal areas around Praia do Tofo, where they may be more susceptible to gill net entanglement, we tagged 15 juveniles with SPOT5 satellite tags and tracked them for 2-88 days (mean = 27 days) as they dispersed from this area. Sharks travelled between 10 and 2,737 km (mean = 738 km) at a mean horizontal speed of 28 ± 17.1 SD km day^-1. While several individuals left shelf waters and travelled across international boundaries, most sharks stayed in Mozambican coastal waters over the tracking period. We tested for whale shark habitat preferences, using sea surface temperature, chlorophyll-a concentration and water depth as variables, by computing 100 random model tracks for each real shark based on their empirical movement characteristics. Whale sharks spent significantly more time in cooler, shallower water with higher chlorophyll-a concentrations than model sharks, suggesting that feeding in productive coastal waters is an important driver of their movements. To investigate what this coastal habitat choice means for their conservation in Mozambique, we mapped gill nets during two dedicated aerial surveys along the Inhambane coast and counted gill nets in 1,323 boat-based surveys near Praia do Tofo. Our results show that, while whale sharks are capable of long-distance oceanic movements, they can spend a disproportionate amount of time in specific areas, such as along the southern Mozambique coast. The increasing use of drifting gill nets in this coastal hotspot for whale sharks is likely to be a threat to regional populations of this iconic species.

Keywords: Biotelemetry; Fishing pressure; Movement ecology; Oceanography; Rhincodon typus.

Grants and funding

Field work was supported by the Shark Foundation, Aqua-Firma, Waterlust, a Rufford Small Grant and the PADI Foundation. Christoph Rohner and Simon Pierce were supported by two private trusts. Anthony Richardson was supported by the Australian Research Council Future Fellowship FT0991722. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.