Stable isotope analysis for the whale sharks in the waters off Taiwan.

Abstract

Background The whale shark, Rhincodon typus, was utilized extensively in Taiwan before 1996. A quota management measure was put in place in 2002, and the whale shark fishery was completely banned in 2008. Biological studies including age and growth, and migration/ movement have been conducted. However, the feeding ecology of whale sharks in different life history stages and sexes in Taiwan waters (Northwest Pacific Ocean) is still unknown. Approach The stable isotope technique was used to analyze the tissue of whale sharks to understanding their feeding ecology. The ´13C value can be used to indicate the foraging habitat of fish, and the ´15N value can be used to estimate the relative position of the consumer in the ecosystem. The specimens (tissue) were taken from individuals entangled in set nets during the period 2008 – 2013 in Taiwan. Results In total, 66 tissue samples from 42 males and 24 females, ranging from 2.84 to 11.90 m TL (total length) were used in stable isotope analysis. Among these specimens 50 and 16 were from the eastern and western waters off Taiwan, respectively. The value of ´13C was from –13.68 to –18.42%, and the value of ´15N was from 5.17 to 13.01%. There was a positive relationship between ´13C and ´15N, and both ´13C and ´15N increased with body size. No gender or geographic difference was found in this study, but the range of stable isotope values of whale shark tissue was wider in eastern Taiwan waters. Conclusions In this study, ontogenetic changes in the diet of whale sharks were found. More specimens are needed to examine the differences in stable isotope values among different genders, seasons, and regions. The results derived from this study can provide useful information on the husbandry of whale sharks, which can help ecotourism operators become more knowledgeable about the ecology of whale sharks. In addition, the results can also be used as an important reference for ecosystem-based management in the future. Future work should focus on discussion on the habitat partition, utilization, and adaption in various marine environments for whale sharks.