The white spotted eagle ray, Aetobatus narinari is a graceful animal, which greatly contributes to the beauty of coral reefs and other marine environments around the world. Unfortunately, increases in fishing pressure, habitat destruction, urbanization, pollution and climate change have contributed to significant changes in its natural ecosystem, and are likely to negatively impact on the abundance and distribution of A. narinari today and in the future. Environmental agencies and scientists are struggling to protect global flora and fauna and even with the most relevant species information at hand adequate conservation measures are hard to develop and even harder to enforce. However, in the complete absence of any scientific information, this struggle is almost futile. This study was conducted to provide much needed information on several aspects of the biology of A. narinari.
Aetobatus narinari, has been listed as ‘near threatened’ globally and ‘vulnerable’ in Southeast Asia by the World Conservation Union Red List of Threatened Species since 2006. Prior to this study, information on life history, population genetics and sensory biology was sparse or deficient, but urgently needed for realistic stock evaluation, fisheries management, and ultimately for successful conservation. To assess age and growth, reproduction and diet, 119 individuals were collected on field trips in Australia and Taiwan and purchased from commercial fishermen. A. narinari was found to grow slowly (k=0.033), mature late, and produce few young, potentially rendering the species susceptible to overfishing. Females grew slower, matured later and attained larger sizes than males. Neonate disc width averaged 54 cm, while size at maturity was estimated around 130.0 cm for males and >150.0 cm for females. The diet of A. narinari consisted almost exclusively of gastropods, bivalves and malacostracans. Minor regional and no gender specific differences were observed related to feeding and few dietary shifts occurred throughout development.
Aetobatus narinari is known worldwide throughout temperate and tropical seas, displaying a variety of phenotypes, which suggests that this species may represent a species complex. Therefore, 154 genetic samples were collected from 18 populations worldwide to determine relatedness by comparing sequence polymorphisms at the sites of Cytochrome b and ND4. Two monophyletic lineages were distinguished between Indo-Pacific and Atlantic Ocean populations. There was also some evidence for a separate Western Indian lineage. Populations in the Western Pacific were highly structured, with 77% genetic variation found among populations. The degree of genetic structuring was unexpected for a large, semi-pelagic ray exhibiting high dispersal capabilities. Additionally, two genetically distinct populations were found within the Western Pacific, centred around Australia and Taiwan. A taxonomic revision is highly recommended; nonetheless, for management purposes, populations within the Atlantic, the Indian Ocean, around Australia and Taiwan should be considered as independent ‘management units’, which are genetically distinct and may therefore vary in respective life histories. Protective measures need to suit each unit, e.g. with regard to marine park zoning, fishing quotas, catch restrictions of neonates or reproductively active segments and gear selectivity (especially if A. narinari is a common bycatch species in the area).
Olfaction is used in feeding, reproduction and predator avoidance. Since chemical signals can be detected by animals a considerable time after stimulus production and over large distances, olfaction may be of particular importance to a species such as A. narinari, which lives in the open water column, where medium or close range sensory cues may not be as crucial for survival. Therefore, A. narinari, one of the few bentho-pelagic batoids was compared to twenty benthic and bentho-pelagic shark and ray species. Four olfactory structures were used as morphological indicators of olfactory capability and compared between species and groups across a diversity of habitats and lifestyles. This enabled a comprehensive analysis of olfaction, based on both ecological and phylogenetic relationships. The study revealed that A.
narinari possesses a well-developed olfactory system and, based on morphology, it is likely to play a key role throughout development. It was also confirmed that bentho-pelagic sharks and rays possess significantly more olfactory lamellae and a larger sensory epithelial surface areas than benthic species. There was little evidence to support a correlation between phylogeny and morphology, indicating that differences in olfactory capability are the result of functional rather than evolutionary adaptations. Juveniles appeared to possess fully-developed olfactory organs and no structural differences were observed between differently-sized individuals within the same species. Across a number of elasmobranchs, olfactory organ size was positively correlated to body length and mass, although only few species, including A. narinari, showed proportional size scaling. Results indicate that while olfaction may be important to juveniles in general (subjected to heavy predation rates), in some species it may play an even more significant role in adults.
During a one month collection trip to Taiwan, a comprehensive market survey on elasmobranch catches in the local fish market in Makung was conducted. In total, 22 shark and 17 batoid species were landed, the most common ones being Dasyatis kuhlii, Scoliodon laticaudus, Rhynchobatus cf australiae and Mustelus griseus. Batoids comprised over 60% of the total catch. During the same month, aspects of female reproductive biology in Rhinobatos formosensis were assessed. Uterine fecundity ranged from 1-14 (mean 8.5) and vitellogenesis proceeded in parallel with gestation in this Taiwanese endemic guitarfish.