Data on the bycatch fishery and reproductive biology of mobulid rays (Myliobatiformes) in Indonesia

This paper provides some of the only fisheries and quantitative biological data for the Mobulidae (manta and devil rays), a group of large pelagic species that are widely distributed in tropical and warm temperate waters. The data were derived from 409 mobulids that were taken as bycatch of drift gillnet fisheries for the skipjack tuna Katsuwonus pelamis and subsequently examined at fish landing sites in Indonesia. The most abundant of the five species was Mobula japanica (∼50%), followed by Mobula tarapacana (∼24%), Manta birostris (∼14%), Mobula thurstoni (9%) and Mobula cf kuhlii (2%). The four most abundant species were represented by a wide size range of each species and, in the case of Mobula japanica, by embryos, neonates and fully mature individuals. The disc width at maturity (DW50) of males, derived from the proportion of males at each size class with fully calcified claspers, ranged from 1538mm for Mobula thurstoni to 3752mm for M. birostris. As the claspers of males become calcified over a relatively narrow size range, the process of maturation is presumably relatively rapid. There is an increasing demand for various body parts of mobulids. Branchial filter plates, which are used for traditional Chinese medicines, are the most valuable, fetching as much as 30 $US a kilo (dry weight). The skins are dried and deep fried and the flesh salted and dried and these are used for human consumption, while cartilage is dried for export as a filler for shark-fin soup. The very low fecundity of the large and probably long-lived mobulid rays make the stocks of their species particularly susceptible to further increases in fishing.