New insights into the trophic ecology of blacktip sharks (Carcharhinus limbatus) from a subtropical estuary in the western Gulf of Mexico

Abstract

As environmental change persists, understanding resource use patterns is of value to predict the consequences of shifting trophic structures. While many sharks are opportunistic predators, some exhibit prey selectivity, putting them at higher risk compared to species with greater trophic plasticity. In the Gulf of Mexico (GOM), Clupeids and Sciaenids comprise 69% of blacktip shark (Carcharhinus limbatus) diets, which is consequential considering potential responses of these prey groups to disturbance and over harvesting. We assessed if blacktips exhibit selectivity for Clupeids and Sciaenids in the western GOM based on stomach contents from sharks in coastal Texas. Clupeids comprised <2% of diets, while striped mullet (Mugil cephalus) and red drum (Sciaenops ocellatus) comprised >70% of identifiable prey. Ontogenetic shifts from smaller (Clupeids, small Sciaenids) to larger, higher trophic level (Ariidae, Elasmobranchii) prey fits our understanding of foraging among coastal sharks, and suggests our regional understanding of blacktip trophic ecology may be limited by the sizes of sampled sharks. Observed increases in blacktip densities coupled with declines in prey (Mugilids, Sciaenids) is concerning if blacktips have limited diet plasticity. Yet GOM blacktips may be more generalized than previously thought, which is promising for conservation and management.

Grow fast, die young: Does compensatory growth reduce survival of juvenile blacktip sharks (Carcharhinus limbatus) in the western Gulf of Mexico?

“…A similar assessment using Kruskall‐Wallis tests was used to identify monthly variability in the abundance of primary prey items for YOY blacktips (Gulf menhaden [ Brevoortia patronus ] based on bag seine data, and Atlantic croaker [ Micropogonias undulatus ] based on otter trawl data) to assess if parturition and immigration matched prey availability. Gulf menhaden and Atlantic croaker were chosen due to their predominance in the diets of YOY blacktips in the GOM (reviewed in Matich et al, 2021 ). Data were pooled across estuaries and years for monthly assessments, and post hoc Dunn's tests were used to identify significant differences between months.…”

“…However, cooling water temperatures require blacktips to leave their nurseries during winter months, with at least some individuals exhibiting natal site fidelity (e.g., Hueter et al, 2005 ). Spatiotemporal and interindividual variability in reproductive biology, life history, and nursery conditions lead to variability in blacktip growth, behavior, and survival across nurseries and cohorts (Baremore & Passerotti, 2012 ; Carlson et al, 2006 ; Deacy & Moncrief‐Cox, 2019 ; Matich et al, 2021 ; McCandless, Kohler, et al, 2007 ). As such, blacktips provide an opportunity to assess the factors that lead to compensatory growth in juvenile sharks and how this affects survival.…”

Elucidating shark diets with DNA metabarcoding from cloacal swabs

“…However, accurate and efficient methods to assess diet composition are currently lacking for many large‐bodied aquatic predators such as sharks, whose feeding behaviour is difficult to observe directly. Shark diet assessment and reconstruction generally rely on morphological stomach content analysis, obtained through invasive methods such as stomach eversion and gastric lavage, or lethal sampling (Barnett et al, 2010; Bornatowski et al, 2014; Cortés & Gruber, 1990; Matich et al, 2020; Papastamatiou et al, 2006; Rosende‐Pereiro et al, 2019; Trystram et al, 2017). These methods are necessary but undesirable for threatened and/or protected species (Hammerschlag & Sulikowski, 2011; Heupel & Simpfendorfer, 2010).…”

“…invasive methods such as stomach eversion and gastric lavage, or lethal sampling (Barnett et al, 2010;Bornatowski et al, 2014;Cortés & Gruber, 1990;Matich et al, 2020;Papastamatiou et al, 2006;Rosende-Pereiro et al, 2019;Trystram et al, 2017). These methods are necessary but undesirable for threatened and/or protected species (Hammerschlag & Sulikowski, 2011;Heupel & Simpfendorfer, 2010).…”

Spatial and Ontogenetic Patterns in the Trophic Ecology of Juvenile Bull Sharks (Carcharhinus leucas) From the Northwest Gulf of Mexico

“…In addition to bull sharks, blacktip sharks (Carcharhinus limbatus) are among the most commonly encountered shark species in nwGoM estuaries (Plumlee et al, 2018). Published values of δ 13 C (∼−16 ) and δ 15 N (∼16.5 ) from blacktip sharks captured along the northern and central Texas coast (Plumlee and Wells, 2016;Matich et al, 2021) compared with those in the present study (δ 13 C: ∼−17 ; δ 15 N ∼16 ) suggest that bull sharks occupy similar or higher trophic positions as blacktips. Given the similarity in trophic positions implied by similar δ 15 N values, slightly diminished values of δ 13 C in bull sharks may indicate this species exploits 13 C depleted freshwater/brackish food webs to a greater extent than blacktips.…”