A complementary approach of stomach content and stable isotope analyses was used to characterize the foraging ecology and evaluate niche overlap between pygmy (Kogia breviceps) and dwarf (K. sima) sperm whales stranded on the U.S. mid‐Atlantic coast between 1998 and 2011. Food habits analysis demonstrated both species were primarily teuthophagous, with 35 species of cephalopods, and 2 species of mesopelagic fishes represented in their overall diets. Pianka's Index of niche overlap suggested high overlap between whale diets (On = 0.92), with squids from the families Histioteuthidae, Cranchidae, and Ommastrephidae serving as primary prey. Pygmy sperm whales consumed slightly larger prey sizes (mean mantle length [ML] = 10.8 cm) than dwarf sperm whales (mean ML = 7.8 cm). Mean prey sizes consumed by pygmy sperm whales increased with growth, but showed no trend in dwarf sperm whales. Significant differences were not detected in δ15N and δ13C values of muscle tissues from pygmy (10.8‰ ± 0.5‰, −17.1‰ ± 0.6‰), and dwarf sperm whales (10.7‰ ± 0.5‰, −17.0‰ ± 0.4‰), respectively. Isotopic niche widths also did not differ significantly and dietary overlap was high between the two species. Results suggest the feeding ecologies of the pygmy and dwarf sperm whales are similar and both species occupy equivalent trophic niches in the region.
Beaked whales are vulnerable to the impacts of disturbance from several sources of anthropogenic sound. Here we report the distribution and abundance of beaked whales off Cape Hatteras, North Carolina, U.S.A., an area utilized by the U.S. Navy for training exercises, and of particular interest for seismic geophysical surveys. From May 2011 through November 2015, monthly aerial surveys were conducted at the site. Beaked whales were encountered 74 times (n 5 205 individuals) during these surveys. Ziphius cavirostris, the most commonly encountered species, was observed in every month of the year. Mesoplodon spp. were encountered in ten months of the year. Photographs of adult males with erupted teeth permitted six sightings to be identified conclusively as M. europaeus; M. mirus was also photographed just outside the study area. Beaked whale surface densities stratified by depth (0.005-0.007/km 2 ) were among the highest reported in the world for small ziphiids. A quantitative comparison of sightings and stranding records suggests that strandings do not accurately reflect the relative abundance of beaked whale species in this area. We conclude that Cape Hatteras, at the convergence of the Labrador Current and Gulf Stream, is a particularly important year-round habitat for several species of beaked whales.
Several species of odontocete cetaceans depredate bait and catch and, as a result, become hooked and entangled in pelagic longline fisheries. The present study measured how selected commercial longline hooks, including “weak hooks”, behaved within odontocete mouths. Five hooks (Mustad-16/0, Mustad-18/0, Mustad J-9/0, Korean 16, and Korean 18) were tested on three species of odontocetes known to interact with longline fisheries—short-finned pilot whales (Globicephala macrorhynchus), Risso's dolphins (Grampus griseus), and false killer whales (Pseudorca crassidens). Specimens were secured to a stanchion, hooks were placed in the mouth at multiple positions along the dorsal lip, and the force required to pull each hook free was measured. The soft tissue lips of these odontocetes were capable of resisting forces up to 250 kg before failing. The polished steel M-16, M-18, and J-9 hooks straightened at forces between 50 and 225 kg, depending on hook gauge. When straightened, these hooks exposed the sharpened barb, which sliced through the lip tissue, usually releasing the hook intact. The K-16 and K-18 hooks behaved very differently, breaking at higher forces (110–250 kg) and consistently just at the barb; usually, there was measurable soft-tissue loss and often shards of the hook were retained within those soft tissues. The different behaviours of these two hook types—the M and J type polished steel vs. the K type carbon steel—were consistent across all species tested. Mechanical tests were also conducted to determine if hooks could fracture the mandible of these same odontocetes. Only the M-18 and K-18 hooks had sufficiently large gapes to hook around the mandible, and both hook types fractured bone in short-finned pilot whales and Risso's dolphins. These results support other lines of evidence indicating that longline hooks can cause serious injury to these species, and suggest possible steps to mitigate these impacts.
In this paper the occurrence, distribution and abundance of cetaceans in offshore waters of Onslow Bay, North Carolina, USA is described. Between June 2007 and June 2010 monthly aerial and shipboard line-transect surveys were conducted along ten 74km transects placed perpendicular to the shelf break. In total 42,676km of aerial trackline (218 sightings) and 5,209km of vessel trackline (100 sightings) were observed. Seven species of cetaceans were observed, but the fauna was dominated strongly by common bottlenose and Atlantic spotted dolphins. Both species were present year-round in the study area. Using photo-identification techniques, five bottlenose dolphins and one spotted dolphin were resighted during the three-year period. In general, the abundance of cetaceans in Onslow Bay was low and too few sightings were made to estimate monthly abundances for species other than bottlenose and spotted dolphins. Maximum monthly abundances of bottlenose and spotted dolphins were 4,100 (95% CI: 1,300–9,400) in May 2010 and 6,000 (95% CI: 2,500–17,400) in March 2009, respectively. Bottlenose dolphins were found throughout the study area, although they were encountered most frequently just off the shelf break. In contrast, spotted dolphins exhibited a strong preference for waters over the continental shelf and were not encountered beyond the shelf break.
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