Using the otolith sulcus to aid in prey identification and improve estimates of prey size in diet studies of a piscivorous predator

Abstract

Diet studies are fundamental for understanding trophic connections in marine ecosystems. In the southeastern US, the common bottlenose dolphin Tursiops truncatus is the predominant marine mammal in coastal waters, but its role as a top predator has received little attention. Diet studies of piscivorous predators, like bottlenose dolphins, start with assessing prey otoliths recovered from stomachs or feces, but digestive erosion hampers species identification and underestimates fish weight (FW).To compensate, FW is often estimated from the least affected otoliths and scaled to other otoliths, which also introduces bias. The sulcus, an otolith surface feature, has a species-specific shape of its ostium and caudal extents, which is within the otolith edge for some species. We explored whether the sulcus could improve species identification and estimation of prey size using a case study of four sciaenid species targeted by fisheries and bottlenose dolphins in North Carolina. Methods were assessed first on otoliths from a reference collection (n = 421) and applied to prey otoliths (n = 5,308) recovered from 120 stomachs of dead stranded dolphins. We demonstrated in reference-collection otoliths that cauda to sulcus length (CL:SL) could discriminate between spotted seatrout (Cynoscion nebulosus) and weakfish (Cynoscion regalis) (classification accuracy = 0.98). This method confirmed for the first time predation of spotted seatrout by bottlenose dolphins in North Carolina. Using predictive models developed from reference-collection otoliths, we provided evidence that digestion affects otolith length more than sulcus or cauda length, making the latter better predictors. Lastly, we explored scenarios of calculating total consumed biomass across degrees of digestion.A suggested approach was for the least digested otoliths to be scaled to other otoliths iteratively from within the same stomach, month, or season as samples allow. Using the otolith sulcus helped overcome challenges of species identification and fish size estimation, indicating their potential use in other diet studies. K E Y W O R D S diet, digestive erosion, estimated prey size, otolith sulcus, prey identification, Tursiops truncatus | 3585 BYRD et al. ACK N OWLED G M ENTSWe thank Bridget Watts and Dean Ahrenholz (NMFS), and Craig Harms (NCSU) who were instrumental in building the reference collection used in this study. We thank Randy Gregory (NCDMF) for loaning otoliths, and

Sexual dimorphism in the otolith shape of shi drum, Umbrina cirrosa (L.), in the eastern Mediterranean Sea: Fish size–otolith size relationships

“…Fish otoliths are also a useful tool as a proxy to assess the ontogenetic life history of a fish (Campana et al ., 2000; Ding et al ., 2020). They can remain as prey parts in the digestive systems of piscivores, and corresponding studies have helped to identify the species of prey they originated from when such prey can be too digested to identify, and can help to estimate the sizes of otherwise unidentifiable prey (Byrd et al ., 2020; Fitch & Brownell Jr, 1968; Mychek‐Londer & Bunnell, 2013).…”