Trophic overlap in mobulid rays: insights from stable isotope analysis

Abstract

Mobulid rays, a group of closely related filter-feeders, are threatened globally by bycatch and targeted fisheries. Their habitat use and feeding ecology are not well studied, and most efforts have focused on temporally limited stomach content analysis or inferences from tagging data. Previous studies demonstrate a variety of different diving behaviors across species, which researchers have interpreted as evidence of disparate foraging strategies. However, few studies have examined feeding habitats and diets of multiple mobulid species from a single location, and it is unclear if the proposed differences in diving and inferred foraging behavior are examples of variability between species or regional adaptations to food availability. Here, we use stable isotope data from mobulids landed in fisheries to examine the feeding ecology of 5 species at 3 sites in the Indo-Pacific. We use Bayesian mixing models and analyses of isotopic niche areas to demonstrate dietary overlap between sympatric mobulid species at all of our study sites. We show the degree of overlap may be inversely related to productivity, which is contrary to prevailing theories of niche overlap. We use isotope data from 2 tissues to examine diet stability of Manta birostris and Mobula tarapacana in the Philippines. Finally, we observe a significant but weak relationship between body size and isotope values across species. Our findings highlight challenges to bycatch mitigation measures for mobulid species and may explain the multi-species mobulid bycatch that occurs in a variety of fisheries around the world.

Long-Term Photo-Identification Reveals the Population Dynamics and Strong Site Fidelity of Adult Whale Sharks to the Coastal Waters of Donsol, Philippines

“…The main driver of whale shark occurrence at Donsol appears to be prey related as sightings coincide with periods of higher productivity in the region (Gordon et al, 2011;Stewart et al, 2017). Diatom blooms appear to dominate the plankton composition in Donsol between January and August.…”

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

“…Planktivorous elasmobranchs tend to follow productive ephemeral pulses that may vary in time and space (Sims, 2003;Rohner et al, 2013). The presence of higher numbers of pregnant females in the region could be related to the high primary productivity and food availability in the area for these species Stewart et al, 2017). Investigations based on catch data coupled with GPS tracks of fishing vessels and environmental parameters (i.e., primary productivity) would provide further insight on areas with higher densities of pregnant females and this relationship with food availability.…”

“…A monitoring program of this fishery revealed that over 2,000 individuals were landed per season (Authors, unpub data). The fishing season for mobulids coincided with the northeast monsoon when increased upwelling leads to higher primary productivity in surface waters (Cabrera et al, 2011;Gordon et al, 2011), with prolific pulses of krill Euphausia diomedeae on which mobulids feed in the area Stewart et al, 2017). The four species studied here were caught with drift nets, a targeted but not selective fishing gear, resulting in the catch of M. birostris despite it being protected nationally since 1998 (Fisheries Administrative Order 193; Republic of the Philippines., 1998), and other elasmobranchs, such as thresher shark Alopias spp., juvenile silky sharks Carcharhinus falciformis, and large bony fishes such as billfishes.…”

DNA metabarcoding assays reveal a diverse prey assemblage for Mobula rays in the Bohol Sea, Philippines

“…Very few studies have described the diet of sympatric Mobula rays. Dietary analysis of Mobula species has been conducted using microscopy of stomach contents (Notarbartolo‐Di‐Sciara, ; Rohner et al, ), as well as stable isotope analyses (Sampson, Galvan‐Magana, Silva‐Davila, & Aguiniga‐Garcia, ; Stewart et al, ). Microscopy provides detailed information on taxa consumed for a particular individual at a specific period of time (Hyslop, ), while stable isotope analysis provides insights into relative trophic level and the sources of carbon supporting diets, that are integrated over time (Peterson & Fry, ).…”

“…Few fish species have been identified as prey items, with the exception of M. birostris stomachs containing myctophids (small, mesopelagic fishes) and M. tarapacana containing Sardinella and Cubiceps spp. in the Philippines (Rohner et al, ; Stewart et al, ). One individual M. tarapacana was reported to have 27 fish in its stomach (from the Gulf of California), which were thought to be carangids (family of fish containing jacks, jack mackerels, runners, and scads), or smaller anchovy‐like species (Notarbartolo‐Di‐Sciara, ).…”