Life History, Growth, and Reproductive Biology of Four Mobulid Species in the Bohol Sea, Philippines

Abstract

In light of the global decline of mobulid populations and the necessity for sustainable fisheries management, baseline data for population dynamics were collected from a targeted fishery in the Bohol Sea, Philippines. This study focused on life-history parameters and reproductive cycles of four mobulid rays (Mobula thurstoni, Mobula japanica, Mobula tarapacana, and Manta birostris), and re-estimated their intrinsic population growth rates. Size and reproductive data were collected from 1,509 specimens (30% of catch) landed in two fishing seasons in 2015 and 2016. Size-at-birth was reviewed, and analysis of the embryos and follicles did not show any clear seasonality in the reproductive cycle, but supported an interbreeding interval. Females of all species matured at a larger size than males, and exhibited a larger size-at-pregnancy than -at-maturity. This delay in reproduction resulted in population growth rates lower than the actual r max when based on size-at-pregnancy (r mat = r max = 0.016-0.055 year −1 and r preg = 0.008-0.044 year −1 ), and a population doubling time of 15.8-86.6 years. This study suggests that population growth rates previously reported were overestimated. In light of the Convention on International Trades of Endangered Species (CITES) and Convention on Migratory Species (CMS) assessments, while fisheries management should reflect the delayed maturation of these species and the slower population growth potential, at the current status of these population, the sustainability of any exploitation level seems unrealistic and strongly discouraged.

Pregnant giant devil ray (Mobula mobular) bycatch reveals potential Northern Gulf of California pupping ground

“…Using photogrammetry, I estimated the disc width of two of the pregnant adult female rays to be 1.97 m and 2.17 m, and one of the pups to be 0.91 m. These sizes are similar to disc‐width measurements of this species in the Philippines, with females reaching sexual maturity at an average of 2.18 m, and newborn pups at an average of 1.00 m (Rambahiniarison et al. ). I observed one captured sexually mature male, and every observed female ray being cut open was pregnant with one pup.…”

Research Priorities to Support Effective Manta and Devil Ray Conservation

“…Aging elasmobranchs using vertebrae relies on access to fished specimens and therefore is not appropriate in many wild mobulid ray populations. Direct estimates of growth rate can be obtained in-situ from morphometric measurements of resighted individuals over known time increments, based on visual size estimates (Kitchen-Wheeler et al, 2012;Kashiwagi, 2014;Stevens, 2016), paired laser photogrammetry (Deakos, 2010), or 2,000 (Rambahiniarison et al, 2018;Philippines).…”

Contrasting Habitat Use and Population Dynamics of Reef Manta Rays Within the Nusa Penida Marine Protected Area, Indonesia

“…Several known aggregation areas exist within the Coral Triangle region, particularly within Indonesia (Marshall et al, 2009;Couturier et al, 2012;Germanov and Marshall, 2014). Manta rays are long-lived, slow to mature, and exhibit low fecundity (Marshall and Bennett, 2010;Stevens, 2016;Rambahiniarison et al, 2018). Owing in part to these conservative life history traits and to overfishing across their range (Dulvy et al, 2014), both Mobula alfredi (Krefft, 1868) and the larger M. birostris (Walbaum, 1792) are currently listed as vulnerable to extinction on the IUCN Red List of Endangered Species (Marshall et al, 2011).…”