Fatal predation of oilfish, Ruvettus pretiosus (Gempylidae), by a young short-finned pilot whale, Globicephala macrorhynchus (Delphinidae)

We found a dead whale (Globicephalus macrorhynchus) whose probable cause of death was an internal injury produced by a 1m-long oilfish (Ruvettus pretiosus) that had bitten the whale ́s stomach from the inside. This is the first record of an oilfish in Mexican Caribbean waters.

The ecological interaction between large mesopelagic fish and small odontocetes is often assumed to be based on competition.For example, the oilfish Ruvettus pretiosus Cocco (1833) and the short-finned pilot whale Globicephala macrorhynchus Gray (1846) occur together and use the same resources: they both prey on squid and fish (Overholtz & Waring, 1991;Vasilakopoulos, Pavlidis & Tserpes, 2011).The oilfish is mesopelagic; it ranges between 100 and 800m deep, but it is more common not deeper than 400m (Nakamura & Parin, 1993).Pilot whales can dive up to 500m or more (Baird, Borsani, Hanson & Tyack, 2002).
Globicephala macrorhynchus is common in the Caribbean.Ruvettus pretiosus occurs worldwide in tropical and temperate seas, although this fish is rarely recorded in the Caribbean (Claro, 1994).
Although competition between these species could be expected, predation by the odontocete on the oilfish is also possible, given the difference in size.However, attacking a large fish entails a hazard for the whale.Apparent teaching by experienced, older individuals has been documented repeatedly in cetaceans; this effort ensures safety during the acquisition of hunting skills (Boran & Heimlich, 1999).
The objective of this short contribution is to present and discuss a case of probable attempted predation by a young short-finned pilot whale on an oilfish.
On April 26th, 2009, around 17:30h, a dead short-finned pilot whale was found on the beach at Puerto Ángel, 8km south of the port of Mahahual, Caribbean coast of Mexico 18º37'N, 87º44'W.The following day, a necropsy was performed on the site.
The cetacean was a 2,94m long male (Fig. 1).This is likely a young individual, since males in this species usually attain sexual maturity at a minimum length of 3,20m (Kasuya & Marsh, 1984).
The probable cause of death was an internal injury produced by a 1m-long specimen of oilfish (Fig. 2), which had bitten the stomach of the whale from the inside.The whale also suffered two traumatisms near the head and some lacerations on the body, probably from thumping against a reef (either the extant reef barrier, or a deep-water fossil reef ) while struggling with the fish.Otherwise, the specimen displayed good body condition, without symptoms of any malady.
Short-finned pilot whales are gregarious; according to local residents, there were other individuals in the water near the area at the time of the stranding.Perhaps the fatal attempted predation of this large oilfish was due to inexperience of the young whale.
Pilot whales prefer to feed on deep-water squid off the continental shelf (Mintzer, Gannon, Barros & Read, 2008), which in the stranding area occurs at less than 1-2km from the coast.It is presumed that the encounter occurred in this zone.This is the first record of an oilfish in Mexican Caribbean waters; the only gempylids previously known from the area were Gempylus serpens Cuvier (1832) and Nesiarchus nasutus Johnson (1862) (Schmitter-Soto et al., 2000).The oilfish had been recorded before in Mexican waters of the Atlantic, but only in the Gulf of Mexico (McEachran & Fechhelm, 2005); in the Caribbean it is known at least from Belize (Claro, 1994), Guadeloupe (Froese & Pauly, 2011), Puerto Rico (Erdman, 1961), andVenezuela (Cervigón, 1994).
A second oilfish was recently captured about 150m deep by a long-line fisherman on February 2013 off Banco Chinchorro, ca.30km west of where the attempted predation discussed here occurred.This specimen, 1,20 m long, was not preserved, but it was thoroughly explored for parasite helminths (D.González-Solís, pers. comm.).
The skeleton of the pilot whale is on exhibition at the offices of the Parque Nacional Arrecifes de Xcalak (National Park Xcalak Reefs, part of the Mexican official protected areas network), whereas the oilfish, fixed in formalin, is preserved and catalogued (ECOCH 6333) in the ichthyological collection of ECOSUR at Chetumal, Mexico.

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FIG. 1.Globicephala macrorhynchus, a young male, 2,94m long, dead on the beach in Puerto Ángel, Mexican Caribbean, just before the necropsy.