Long-term husbandry and physical observation of the whale sharks in Okinawa Churaumi Aquarium

Abstract

Background Okinawa Churaumi Aquarium has established a long-term husbandry program for whale sharks since 1980. Currently, three individuals (1 male and 2 female) are housed in the 7,500 m3 capacity main exhibit tank. The male shark was caught by a set-net on March 11, 1995, and it measured a total length (TL) of 460 cm; a subsequent measurement in December 2015 revealed the TL had increased to 865 cm. The corresponding TLs of the female sharks that have been housed in this facility for nine and eight years were 762 cm and 787 cm, respectively. The growth rate, hormonal trends, quantity of food ingested, behaviors, and other physical changes along with maturation of these sharks are presently monitored. Approach The total length of each whale shark was estimated by measuring the length between the snout tip and the first dorsal fin using an equation formulated by us. Periodic measurements of the girth at the posterior margin of the pectoral fin have been recorded since May 2014. For measuring hormonal levels, blood was collected underwater from free-swimming individuals and plasma samples were extracted. The plasma concentration of steroid hormone was measured using time-resolved fluorescent antibody analysis. This data has been collected since May 2008. The quantity of food consumed and the feeding behavior of the whale sharks are recorded each day. Results The TL of the male whale shark increased from 460 cm to 744 cm by March 2005, at an average annual growth rate of 28 cm/year. Since 2005, there has been a decrease in the growth rate, to 12.5 cm/year. The two females have been growing at an annual rate of 33 cm/year (TL = 762 cm) and 37 cm/year (TL = 787 cm). Our data shows that the rate of change of girth appears to be associated with calorie intake. The plasma concentration of steroid hormone of the male shark increased from June 2011 to June 2012 (21.49 ng/ml; TL: 850 cm). This appears to be in synchrony with the rapid elongation of the claspers. Additionally, since April 2012, we have been observing the male shark rotating its entire body laterally crossing both claspers, which is a behavioral indication that the male has reached maturity. Conclusions From our long-term study on captive whale sharks, details on direct observations on the maturity process, growth rate, required food consumption, behavioral patterns, and proportional measurement changes are available. Based on these details, we expect that future attempts for reproduction in captivity would be successful and would allow us to unravel many mysteries of whale shark reproduction.