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DIET of FREE-RANGING and STRANDED SPERM WHALES (Physeter

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DIET of FREE-RANGING and STRANDED SPERM WHALES (Physeter DIET OF FREE-RANGING AND STRANDED SPERM WHALES (Physeter macrocephalus) FROM THE GULF OF MEXICO NATIONAL MARINE FISHERIES SERVICE CONTRACT REPORT Submitted to: Dr. Keith D. Mullin National Marine Fisheries Service Southeast Fisheries Science Center PO. Drawer 1207 Pascagoula, MS 39568-1207 Submitted by: Dr. Nelio B. Barros Mote Marine Laboratory Center for Marine Mammal and Sea Turtle Research 1600 Ken Thompson Parkway Sarasota, FL 34236-1096 (941) 388-4441 x 443 (941) 388-4317 FAX May 2003 Mote Marine Laboratory Technical Report Number 895 ABSTRACT Sperm whales are common inhabitants of the deep waters of the Gulf of Mexico. To date, no information is available on the diet of sperm whales in the Gulf. This study sheds light into the feeding habits ofthese whales by examining data collected from free-ranging and stranded animals. Prey species included a minimum of 13 species within 10 families of cephalopods, the only prey type observed. The most important prey was Histioteuthis, a midwater squid important in the diet of sperm whales worldwide. Most species of cephalopods consumed by Gulf sperm whales are meso to bathypelagic in distribution, being found in surface to waters 2,500 deep. Some of these prey are also vertical migrators. The diet of Gulf sperm whales does not include species targeted by the commercial fisheries. INTRODUCTION Until fairly recently, little was known about the species of whales and dolphins (cetaceans) inhabiting the deep waters of the Gulf of Mexico. Most of the information available came from opportunistic sightings and occasional strandings. In the early 1990' s large-scale dedicated surveys were initiated to study the distribution and abundance of marine mammals in the deep Gulf. Surpringly, early results of those surveys indicated a much richer marine mammal fauna than previously reported (Jefferson and Mullin 1995, Mullin and Hansen 1999, Wiirsig et al. 2000). Despite their rare occurrence in the stranding record (Odell 1991), sperm whales are the most common of the large whales to occupy the deep waters of the Gulf of Mexico (Mullin et al. 1994, Jefferson and Mullin 1995, Jefferson and Schiro 1997). The distribution of sperm 2 whales in the Gulf suggests that they may prefer areas of high productivity, and their distribution may be influenced by Gulf oceanographic features such as the Loop Current or wann-core mesoscale eddies (Mullin et al. 1994, Baumgartner et al. 2001, Davis et al. 2002). Studies on the occurrence, movements and site fidelity of individually-identified sperm whales in specific areas of the northern Gulf of Mexico have shown that the continental slope off the Mississippi River delta and the Mississippi Canyon areas constitute ecologically important habitats (Weller et al. 2000, Mullin et al. 2003). Throughout the world, sperm whales are known to make prolonged (up to 90 min), deep (up to 3,200 m) dives in search for food, which includes several species of deep-water squid and fish. To date, no dietary data are available for sperm whales inhabiting the Gulf of Mexico. This study examines the food habits of free-ranging and stranded animals through the analyses of fecal material (collected at the surface where whales were present) and stomach contents, respectively. MATERIALS AND METHODS Specimens examined in this study originated from two different sources: stomach contents were obtained from stranded animals, collected by participants of the Southeastern U.S. Marine Mammal Stranding Network (Odell 1991); floating fecal material were collected from free-ranging animals during ship surveys in the period 2000-2001 (Mullin et al. 2003). Prey identification was made using published guides (Clarke 1980, 1986) and using a Gollection of cephalopod beaks (the "roston" described in Clarke 1986: 17) including specimens from the eastern North Atlantic, south Atlantic (South Africa) and south Pacific (New Zealand). Stomach content analyses are a common method of studying trophic ecology of cetaceans (see review in 3 Barros and Clarke 2002) and fecal sample analyses of sperm whales have also been conducted elsewhere (Smith and Whitehead 2000). Back-calculations of reconstructed size and biomass were made using the equations presented in Clarke (1986). Samples available for this study are shown in Table 1. The stranded animals were young and mature individuals of both sexes. Sex and maturity/size class data for the free-ranging animals were not available at the time of writing. RESULTS Only remains of cephalopods were observed in the specimens examined. Samples obtained from free-ranging sperm whales included a single group of cephalopods, Histioteuthis sp. (Table 2). Prey of stranded sperm whales was more diverse, comprising a minimum of 13 species within 10 families (Table 3). Histioteuthis was also present in samples from all stranded whales, and was the most numerically important prey group as well. Stranded and free-ranging whales preyed on Histioteuthis squids of similar sizes (average lower rostral lengths of beaks: 5.0 mm). Using regressions provided in Clarke (1986) for Histioteuthis dojleini (= H arcturi), these squids had average mantle lengths of80 mm (range: 63-90 mm) and weighed on average 194 g (99-303 g) Other important species for stranded sperm whales included Discoteuthis iaciniosa, Octopoteuthis sp. and the cranchiid species Teuthowenia megalops and Megalocranchia sp. (Table 3). DISCUSSION 4 Sperm whales in the Gulf of Mexico have a diet comprised exclusively of cephalopods, consuming some 13 species ofthe 70-75 species oflisted for the Gulf of Mexico (Voss 1956, Salcedo-Vargas 1991). Most of these prey are midwater in distribution (Table 4). Histioteuthids occur worldwide from the surface to possibly 2000 m. Members of this family show association with slopes of continents, islands, and submarine mounts and ridges. Diel vertical migration patterns appear to be widespread in this family (Voss et al. 1992, 1998). Both Histioteuthis corona corona and H arcturi are known to occur in the Gulf of Mexico (Voss 1956, Salcedo­ Vargas 1991), with the former being the most common and abundant, particularly off the mouth, of the Mississippi River (Voss et al. 1998, N. Voss, pers. commn.). Histioteuthids are taken by predators at the surface, in midwater and near the bottom in epipelagic to bathypelagic oceanic zones of the world's oceans. They play an important role in oceanic food webs, as they constitute prey of cetaceans, pinnipeds, sea birds, fishes and other cephalopod species (Voss et aI., 1998). Cranchiids occur in oceanic waters worldwide, from the surface to unknown depths beyond 2,000 m. Life history of all species includes ontogenetic descent to deep water where maturation occurs; they exhibit various degrees of diel vertical movement (Voss 1980, 1985, Voss et al. 1992). Evidence gathered from other studies of sperm whale worldwide (see review in Whitehead 2002) suggests that these whale feed frequently in deep midwaters and near/on the bottom in depths of 1,000 to 2,000+ m. There are biases associated with inferring diet based on analyses of floating fecal materials. Smith and Whitehead (2000) pointed out that beak material from fecal samples gives a bias estimate of sperm whale diet, reducing the frequencies of very small and very large cephalopods. Interestingly enough, these authors also found Histioteuthis squids to be the most dominant prey group (62%) of sperm whales in the Galapagos Islands, followed by 5 Ancistrocheirus and Octopoteuthis, species which were also identified in this study. Analyses of stomach contents of stranded whales may also present biases (e.g. , retention of cephalopod beaks, different digestion rates of various prey, etc) but this method has several advantages over others (see discussion in Barros and Clarke 2002), some of which include the low cost of obtaining the samples and the resistance to sample degradation, commonly observed in stranded animals. Two of the stranded whales examined in this study were young animals, possibly dependent calves of2-3 years of age (Table 3). The smallest whale (a 610 cm female) stranded in Hom Island, Mississippi in 1994 (peterson and Hoggard 1996). Spenn whales are thought to eat solid food before they are 1 yr old but may continue to suckle for several years (Whitehead 2002). Considering the potential depth in which these deep-water prey were consumed, it would seem that spenn whales are capable of deep dives at a fairly young age. The potential for deep-sea exploration of the Gulf of Mexico for oil development and fisheries has received considerable attention (Carney 1998). The main species of cephalopods consumed by spenn whales in the Gulf of Mexico are not commercially explored. Existing commercial fisheries exploit shelf species such as loliginid squids and octopus, and there is potential for fisheries targeting muscular squids of the family Ommastrephidae (Voss and Brakoniecki, 1985, Salcedo-Vargas 1991). LITERATURE CITED Barros, N.B. and M.R. Clarke. 2002. Diet. Pp: 323-327 In: Perrin, W.F., B. Wfusig and H.G.M. Thewissen (eds). Encyclopedia of Marine Mammals. Academic Press: San Diego, CA. 6 Baumgartner, M.F., K.D. Mullin, L.N. May and T.D. Leming. 2001. Cetacean habitats in the northern Gulf of Mexico. Fishery Bulletin 99:219-239. Carney, R.S. 1998. Is the Gulf of Mexico ready for deep-ocean environmental regulation? Gulf of Mexico Science 16:106-108. Clarke, M.R. 1980. Cephalopoda in the diet of sperm whales of the southern hemisphere and their bearing on sperm whale biology. Discovery Reports 37:1-324. Clarke, M.R. 1983. Cephalopod biomass - estimation from predation. Memoirs of the National Museum Victoria 44:95-107. Clarke, M.R. 1986. A handbook for the identification of cephalopod beaks. Oxford: Clarendon Press. Davis, R.W., J.G. Ortega-Diaz, C.A. Ribic, W.E. Evans, D.C. Biggs, P.H. Ressler, R.B. Cady, R.R.
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