oa Habitat conditions and potential food items during the appearance of whale sharks (Rhincodon typus) in Probolinggo waters, Madura Strait, Indonesia

Background In Probolinggo coastal waters, part of the Madura Strait, the appearance of whale sharks (Rhincodon typus) is a common sight especially during late December to March. The aggregation of these large vertebrates in this area have become highly attractive for local coastal tourism. Yet there has been limited scientific information regarding their population, from which better conservation management might result. The present study partly dealt with an exploration of whale shark habitat conditions at their arrival in this region. Approach Twelve days of whale shark sightings were made on board a wooden fishing boat (8 m length), coupled with weather data based on Beaufort's scale and plankton sampling at the time of shark surfacing. Observations were performed from early morning to late afternoon (7 am to 4 pm). The animals were counted and their length estimated by placing the boat and a whale shark in parallel positions. Zooplankton was sampled by filtering 100 L seawater using a 250 ìm mesh-size plankton net. Site marking was done at each sight using GPS. The combination of primary data and secondary supporting information was combined to understand why whale sharks are present in the area. Results There were 72 individual whale sharks recorded during 12 d of observation, of which 94% appeared between morning to late morning. The highest number of sharks was found on day 6 with 14 individuals. Shark size ranged between 2 – 8 m, with most animals between 3 – 6 m, indicating the population might by dominated by immature individuals. During observation, there was no preferred temperature for whale shark sightings, with temperatures ranging between 28.5 – 30.0°C. Instead, whale shark presence coincided with relatively calm weather (1 – 3 Beauforts' scale). Whale sharks were absent on days 3, 7, and 12, when weather conditions were between 4 – 5 Beauforts' scale. The sharks swim as close as 1 nm to shore, in 5 – 10 m water depths. Among potential food items, there were 5 dominating zooplankton groups, i.e. crustaceans, sagittoideans, urochordatans, hydrozoans, and scyphozoans. In addition, fish eggs and larvae were also found. Regarding crustaceans, the copepods were the dominant group, especially genera Lucifer, Mysis, Sergia, and nauplii larvae forms. Conclusion The aggregation of whale sharks in Madura Strait is likely feeding-based in this area where wind-driven oceanographic conditions cause food to be in high abundance. Research on ID-based population structure of this whale shark aggregation is required.