oa Caught in the net: A small, resident group of whale sharks feeding among fishing boats

Background Off Mafia Island, Tanzania, whale sharks are seen in a small bay that also boasts one of the country's largest coastal fisheries. We set out to determine the population structure of whale sharks here, examine their feeding behaviour and residency, and investigate the interaction between the sharks and fishers. Approach We conducted 261 boat-based surveys at Mafia Island between Oct 2012 and Dec 2015, recording all whale shark sightings as well as counting the numbers of tourist and fishing boats in the survey area during each trip. We estimated shark size visually and with laser photogrammetry and recorded sex, scars and behaviour of each shark. To examine residency, we deployed 67 acoustic tags on whale sharks and 19 receiver stations within the bay, and analysed recapture probability (both visual- and acoustic-based) with generalised additive mixing models. To investigate feeding plankton biomass and community structure, we collected 20 plankton samples when whale sharks were feeding and 202 background samples for comparison. We interviewed fishers and they recorded their fishing locations using a GPS. Results We identified 129 different whale sharks at Mafia Island, 54% of which were resighted in two or more years. Tagged sharks were acoustically detected throughout the year even though sightings were largely limited to Oct-Mar. Sharks ranged in total length from 2.5–9.7 m, were mostly male (87%), and were usually seen feeding (75%) at or near the surface. Dense patches (25 mg m^–3) of large sergestids Lucifer hanseni prompted feeding, while background plankton tows were dominated by small copepods (2.5 mg m^–3). Small and medium-sized planktivorous fishes also fed on the same patches of sergestids, often together with whale sharks. These smaller species were the main target of the local fishers. Fishing boats operated in the survey area on 90% of the time, but also made catches further afield. Fishers reported a conflict between whale sharks and their fishing operations on 36% of the time, although sharks were usually released from the nets without injury. A mean of 2.6 tourist boats per day searched the same survey area for whale sharks. Conclusions This small, resident group of whale sharks is routinely exposed to tourists, boat traffic and high fishing pressure. Whale sharks are commonly encircled in fishing nets and released, but scars indicate that they do occasionally get injured during this process. Mortalities are rare, based on interviews with fishers and long-term observers. Whale sharks at Mafia Island are unusually resident and face clear local threats. No defined management measures are currently in place, but we present some potential strategies for mitigating threats to whale sharks here.