Mobulid rays feed on euphausiids in the Bohol Sea

Abstract

Mobulid rays have a conservative life history and are caught in direct fisheries and as by-catch. Their subsequent vulnerability to overexploitation has recently been recognized, but fisheries management can be ineffective if it ignores habitat and prey preferences and other trophic interactions of the target species. Here, we assessed the feeding ecology of four mobulids (Manta birostris, Mobula tarapacana, M. japanica, M. thurstoni) in the Bohol Sea, Philippines, using stomach contents analysis of fisheries specimens landed between November and May in 2013–2015. We show that the mobulids feed heavily on euphausiid krill while they are in the area for approximately six months of the year. We found almost no trophic separation among the mobulid species, with Euphausia diomedeae as the major prey item for all species, recorded in 81 of 89 total stomachs (91%). Mobula japanica and M. thurstoni almost exclusively had this krill in their stomach, while M. tarapacana had a squid and fish, and Ma. birostris had myctophid fishes and copepods in their stomachs in addition to E. diomedeae. This krill was larger than prey for other planktivorous elasmobranchs elsewhere and contributed a mean of 61 364 kcal per stomach (±105 032 kcal s.e., range = 0–631 167 kcal). Our results show that vertically migrating mesopelagic species can be an important food resource for large filter feeders living in tropical seas with oligotrophic surface waters. Given the conservative life history of mobulid rays, the identification of common foraging grounds that overlap with fishing activity could be used to inform future fishing effort.

Trophic overlap in mobulid rays: insights from stable isotope analysis

“…Importantly, isotopic overlap does not translate directly to trophic overlap, as consumers may feed on mixtures of taxonomically distinct but isotopically similar prey items that would lead to similar consumer isotope signatures. However, analysis of stomach contents in the Philippines verified that the mobulids' diets converge for at least 6 mo of the year (Rohner et al 2017), effectively ground truthing our inferences from isotope data. This demonstrates the benefits of combining these 2 approaches in dietary studies, and future analysis of mobulid stomach contents in Sri Lanka and Peru would help validate the inferences made here.…”

“…In some cases, prey sources were present in a number of different stomach content samples but in quantities that were too small to prepare for stable isotope analysis (e.g. copepods, chaetognaths, and pteropods; see Rohner et al 2017 for a detailed analysis of stomach contents). While these prey sources did not make up a substantial portion of the diet during the period when stomach contents were collected, their relative importance may change throughout the year.…”

“…We visually sorted prey collected in mobulid stomach contents and plankton tows to the species level except for copepods, which we pooled across species to obtain enough material for isotope analysis. We further sorted prey species using microscopy (details reported in Rohner et al 2017). In 2 cases, fish samples collected from M. tarapacana stomachs could not be visually identified.…”

Southernmost record of the Giant Manta Ray Mobula birostris (Walbaum, 1792) in the Eastern Pacific

“…Even though the observed individual was seen with its cephalic fins closed at 16 m deep, suggesting a non-feeding behavior (Ari & Correia, 2008) and recent work has found that M. birostris mainly feeds on deep-water zooplankton (Burgess et al, 2016;, the possibility that this muninid crustacean could be a prey of Mobula spp. shall not be completely discarded as few studies have determined that zooplankton, shrimps, crabs and small fishes are key items on giant manta rays diet (Bigelow & Schroeder, 1953;Couturier et al, 2012;Rohner et al, 2017).…”

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

“…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, ).…”