PYGMY SPERM WHALE (Kogia breviceps): Northern Gulf of Mexico Stock 2009

STOCK DEFINITION AND GEOGRAPHIC RANGE The pygmy sperm whale appears to be distributed worldwide in temperate to tropical waters (Caldwell and Caldwell 1989; Bloodworth and Odell 2008). Sightings of these animals in the northern Gulf of Mexico (i.e., U.S. Gulf of Mexico) occur primarily in oceanic waters (Figure 1; Mullin et al. 1991; Mullin and Fulling 2004; MazeFoley and Mullin 2006). Pygmy sperm whales and dwarf sperm whales (Kogia sima) are difficult to differentiate at sea, and sightings of either species are often categorized as Kogia sp. Sightings of this category were documented in all seasons during GulfCet aerial surveys of the northern Gulf of Mexico from 1992 to 1998 (Hansen et al. 1996; Mullin and Hoggard 2000). The difficulty in sighting pygmy and dwarf sperm whales may be exacerbated by their avoidance reaction towards ships, and change in behavior towards approaching survey aircraft (Würsig et al. 1998)). In a study using hematological and stable-isotope data, Barros et al. (1998) speculated that dwarf sperm whales may have a more pelagic distribution than pygmy sperm whales, and/or dive deeper during feeding bouts. Diagnostic morphological characters have also been useful in distinguishing the 2 Kogia species (Barros and Duffield 2003), thus enabling researchers to use stranding data in distributional and ecological studies. Specifically, the distance from the snout to the center of the blowhole in proportion to the animal’s total length, as well as the height of the dorsal fin, in proportion to the animal’s total length, can be used to differentiate between the 2 Kogia species when such measurements are obtainable (Barros and Duffield 2003). The Gulf of Mexico population is provisionally being considered a separate stock for management purposes, although there is currently no information to differentiate this stock from the Atlantic Ocean stock(s). Additional morphological, genetic and/or behavioral data are needed to provide further information on stock delineation.