Zoogeography of Cetaceans Off Puerto Rico and the Virgin Islands
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Caribbean Journal of Science, Vol. 34, No. 3-4, 173-190, 1998 Copyright 1998 College of Arts and Sciences University of Puerto Rico, Mayagüez Zoogeography of Cetaceans off Puerto Rico and the Virgin Islands ANTONIO A. MIGNUCCI-GIANNONI Department of Marine Affairs, The University of Rhode Island, Washburn Hall, Kingston, Rhode Island 02881 Present address: Red Caribeña de Varamientos • Caribbean Stranding Network, PO Box 361715, San Juan, Puerto Rico 00936-1715 [email protected]. ABSTRACT.—A zoogeographical analysis of cetaceans in the waters of Puerto Rico, US Virgin Islands, and British Virgin Islands was conducted to document the different species found, and to relate their occurrences to patterns of ocean floor topography. A total of 2,016 sighting records was entered into a specially formatted database system, and analyzed for distributional and temporal patterns. Species included 13 odontocetes and four mysticetes. The hypothesis that the spatial distribution of cetaceans is highly correlated to the area’s bathymetric relief, whether high or low, was generally supported. Through the use of a relative slope index measure, each sighting was characterized by depth classes (shelf, shelf edge, or offshore), and by sea floor relief. RESUMEN.—Para documentar la presencia de las distintas especies de cetáceos y relacionar la misma con patrones de topografía del fondo marino, se llevó a cabo un análisis zoogeográfico de los cetáceos de Puerto Rico, Islas Vírgenes Estadounidenses e Islas Vírgenes Británicas. Un total de 2,016 avistamientos fueron ano- tados en una base de datos con formato específico para avistamientos, y los datos fueron analizados en bus- queda de patrones de distribución espacial y temporal. Las especies estudiadas incluyeron 13 odontocetos y cuatro misticetos. La hipótesis de que la distribución espacial de los cetáceos está estrechamente relacionada con el relieve batimétrico del área, ya sea alto o bajo, fue apoyada generalmente. A través de una medida de índice de pendiente relativa, cada avistamiento fue caracterizado por categoría de profundidad (de plata- forma, borde de plataforma o mar afuera) y por relieve del fondo marino. INTRODUCTION 1935), including water temperature, bottom depth, tidal flow, currents, areas of Whales and dolphins are an intricate part upwelling or areas of convergence and of the marine and coastal fauna of the divergence, prey movements or concentra- northeastern Caribbean Sea, with some of tions, and sea floor relief. Of these condi- the islands serving as primary habitat for tions, water temperature seems to be the the mating and calving of endangered spe- most important factor in determining cies. Although the presence of these crea- breeding and calving grounds, at least in tures has been documented (Erdman, 1970; large whales (Gaskin, 1982). In contrast, Erdman et al., 1973; Taruski and Winn, whale and dolphin congregations in high 1976; Mattila and Clapham, 1989), there is latitude feeding grounds appear to be more still a lack of information on the basic biol- related to phenomena such as ocean fronts, ogy, life history and distribution of the spe- eddies and areas of upwelling which con- cies in the northeastern Caribbean. Two centrate prey. These three features can be of basic questions remain: where and when either of a dynamic or topographic nature, are each of the cetacean species found, and with the latter highly induced by surface which factors affect their distribution in the land masses or underwater sea floor relief northeastern Caribbean. (i.e. sea mounts, subsurface ridges and Cetacean distribution may be deter- edges of the continental shelf) (Gaskin, mined by factors which enhance the avail- 1982). ability of food and the suitable conditions There is a direct relationship between required for reproduction (Townsend, cetacean distribution and bottom topogra- 173 174 A. A. MIGNUCCI-GIANNONI phy in some areas (Hui, 1985; Kenney and between 1987 and 1989, 18 fishing villages Winn, 1987; Selzer and Payne, 1988). In were visited throughout Puerto Rico, and many instances, sea floor contour appears locals were interviewed on the occurrence to be the limiting factor affecting species of all species of marine mammals in the aggregations (Gaskin, 1971). Probably as area. State and federal government officials bottom topography increases in complex- and previous researchers (H. E. Winn, D. K. ity, so does the complexity of the total Mattila, J. C. Jiménez and D. S. Erdman) aquatic environment (Hui, 1979). This com- were also interviewed, and their marine plexity is characterized by seasonal areas of mammal data files kindly made available upwelling, bringing high levels of nutrients for analysis. which enhance primary productivity, and By its nature, most of the data presented therefore facilitating the build up of pri- here are non-random and biased in a vari- mary and secondary consumers such as ety of ways, mainly by observational zooplankton, cephalopods and fish (Hui, chance and effort. The location of some of 1979; Gaskin, 1982). Consequently, oppor- the occurrences may relate more to the dis- tunistic apex predators such as whales and tribution of human populations or activi- dolphins appear. The assumption that com- ties, and not necessarily to the distribution plex sea floor relief enhances food avail- of the animals. Therefore, conclusions ability has been well documented in the based on the statistical analysis must be western North Atlantic, especially for the treated with caution. However, since this is area off the northeastern coast of the the only available information, it serves as United States (Kenney, 1984; Kenney and a general overview of expected patterns of Winn, 1986; Hain et al., 1985), where most aggregation, spatial distribution, and sea- large species of cetaceans migrate to feed. sonality of the species. Only Wells et al. (1980), Dorf (1982) and A database management system was Baumgartner (1997), have examined under- established to file sighting records for each water topography as a factor affecting ceta- species in the study area. Opportunistic cean distribution in tropical or subtropical sightings were included after they proved areas of the western North Atlantic. Rela- to be reliable in terms of species identifica- tionships between underwater topography tion, date, and event location. A serial num- and areas of breeding and calving have not ber (NEPSGXXXX) and a quadrant number been analyzed. were assigned to each sighting in the data- The waters of the northeastern Carib- base. bean are feeding grounds for some species, The study area included the insular shelf and reproductive grounds for others at dif- waters of Puerto Rico, the US Virgin ferent times of the year. Some species do Islands, and the British Virgin Islands (Fig. not migrate, utilizing these waters for feed- 1). The area was partitioned into 864 quad- ing and reproduction throughout the year. rants, each measuring five minutes of lati- Given the presence of whales and dolphins tude by five minutes of longitude, and with off Puerto Rico and the Virgin Islands, it is an area of approximately 68 km2. The hypothesized that their spatial distribution bathymetry of the study area was analyzed is correlated with the area’s bottom topog- by calculating a Slope Index (SI) for each raphy. Some cetaceans are expected to be quadrant, following Kenney’s (1984) tech- distributed in relation to areas of high sea nique for measuring absolute bottom slope, floor relief, while in other species the factor and adapted from Evans’ (1975) Depth affecting distribution may be a low-com- Index (DI) and Hui’s (1979; 1985) Contour plexity level in bottom topography. Index (CI) formulas. These formulas char- acterize sea floor relief as a percentage of maximum depth change for a given section MATERIALS AND METHODS of the study area. Evans’ DI and Hui’s CI Sighting data were obtained from pub- measure this change as a percentage of the lished and unpublished records collected in maximum depth in that section, while the the study area up to 1989. In 1985, and SI developed by Kenney (1984) incorpo- CETACEAN ZOOGEOGRAPHY 175 N 025 British Virgin Islands Nautical Miles Atlantic Ocean Camuy Toa Baja Loíza Jost van Dyke Aguadilla Arecibo Anegada Dorado San Juan Luquillo Isla de Tortola Culebra Fajardo Isla Desecheo Virgin Gorda Ceiba Rincón Puerto Rico St. Thomas Mayagüez St. John Humacao 18 00'N Isla de ° Cabo Rojo Isla de Vieques Mona Arroyo US Virgin Islands La Parguera Ponce Salinas Guánica Caribbean Sea St. Croix Guayanilla Bahía de Jobos 67°30'W 67°00'W 66°30'W 66°00'W65°30'W 65°00'W 64°30'W FIG. 1. Study area and localities referred to in the text. rates into a single number the slope of a chi-square (c2) one-sample test of inde- specific quadrant as a percentage of change pendence (as in Evans, 1975; Hui, 1979; of the bottom topography to the whole Selzer and Payne, 1988). The null hypothe- study area. The SI appears as a dimension- sis (Ho) was: the spatial distribution, repre- less formula defined as: sented by the location of sightings of a given species of cetacean in the study area, æöMm– is random and proportional to the number SI= 100 ---------------- of quadrants in each SI class. The expected èøMsa frequency of sightings in SI class i (Ei) was where M and m is the maximum and mini- calculated following Evans (1975), and mum depth, respectively, within each quad- defined as: rant, and M is the maximum depth in the sa L entire study area (6,373 m). Quadrants which E = O æö-----i i tèø border the shoreline were assigned a mini- Lt mum depth of 1.0 m. Strandings and mortal- where O is the total number of sighting ity records were not included in the spatial t occurrences for a species in the study area, distribution analysis because it is impossible L is the number of quadrants in SI class i, to determine the origin of the animals.