Abstract
Investigating the feeding ecology of marine predators is critical for understanding their roles and functional importance in ecosystems. However, assessing the diet of large and wide-ranging predators can be challenging, particularly in the case of rare and elusive species. Killer whales (Orcinus orca) are large apex predators in global oceans, but very little is known about their feeding ecology in tropical ecosystems, particularly in the Caribbean Sea. Killer whales are distributed throughout the Caribbean, and are a regular target of artisanal whalers in St. Vincent and the Grenadines (West Indies). Here we investigate the feeding ecology of Caribbean killer whales using a combination of stable isotope analysis (δ15N, δ13C) and traditional ecological knowledge (TEK) data from whalers operating from St. Vincent. Potential prey data from the Caribbean region included large pelagic teleosts, sea turtles, oceanic sharks, cetaceans, and oceanic cephalopods. Mixing models suggest that the contribution of odontocete cetaceans to the diet of killer whales is the most important (overall mean contribution: 60.4%), particularly mesopelagic delphinids (Lagenodelphis hosei; 26.4%, SD = 0.14), large teuthophageous odontocetes (Physeter macrocephalus,Globicephala macrorhynchus; 20.0%, SD = 0.14), and epipelagic delphinids (Stenella attenuata; 14%, SD = 0.14). Oceanic sharks (Carcharhinus longimanus) had a contribution of 17.0% of the diet of killer whales (SD = 0.13), and other potential prey had a relatively marginal contribution. TEK data suggest that whalers believe killer whales feed almost exclusively on cetaceans, particularly odontocete species that they hunt (e.g.,G. macrorhynchus). Stable isotope and TEK data were consistent to some extent, although TEK data are qualitative and based on a limited number of observations. Despite some limitations (including sample size for both methods), this study highlights the value of combining independent data sources and methodologies to investigate the ecological roles of marine predators in data-poor regions.
Highlights
Investigating the feeding ecology of marine predators is critical to assess their functional roles in food webs, and to understand their ecological importance (Bowen, 1997; Heithaus et al, 2008; Kiszka et al, 2014; Estes et al, 2016)
Stable carbon and nitrogen isotope analysis has increasingly been used to investigate the trophic interactions within marine predator communities, as well as spatial, ontogenetic, and sex variations of the diet of predators across a diversity of marine ecosystems (Hobson et al, 1996; Walker and Macko, 1999; Lesage et al, 2001; Kiszka et al, 2014)
The results of this preliminary study suggest that killer whales sampled off the island of St Vincent mostly feed on cetaceans, small and larger teuthophageous odontocetes such as short-finned pilot whales and sperm whales
Summary
Investigating the feeding ecology of marine predators is critical to assess their functional roles in food webs, and to understand their ecological importance (Bowen, 1997; Heithaus et al, 2008; Kiszka et al, 2014; Estes et al, 2016) Due to their high metabolic rates and high abundance in some regions, marine mammals, such as cetaceans, have the potential to affect communities and ecosystems via consumptive and non-consumptive effects, and other processes such as nutrient translocation and recycling (Katona and Whitehead, 1988; Heithaus et al, 2008; Roman et al, 2014; Estes et al, 2016). It has been acknowledged that combining stable isotope mixing models with other independent sources of data, including other dietary indicators such as stomach contents (Jansen et al, 2013) or behavioral observations (Kiszka et al, 2014), would reinforce the credibility of these models
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