Habitat Usage and Spatial Ecology
Bryan Franks & Dr. Samuel H. Gruber
A hydrophone used for acoustic tracking
Knowledge of movement patterns (i.e. use of space and activity patterns) is essential in understanding the behavior of a species as well as defining essential habitat for that animal. An animal's movement patterns can have profound effects on its energetics, reproductive fitness, and survival (Matthews, 1990; Itzkowitz, 1991). All of the factors that affect an animal's movement patterns are not static but rather dynamic and change throughout the course of an animal's lifetime. Therefore, an individual's movement patterns will be dynamic and change both spatially and temporally throughout the animal's life.
The Magnuson-Stevens Fishery Conservation and Management Act (1996) requires the collection of data related to essential fish habitat. It stresses the importance of delineating essential fish habitat in order to successfully conserve fish species. It defines essential fish habitat as "waters and substrate necessary to fish for spawning, breeding, feeding, or growth to maturity." A major component of determining essential fish habitat is characterizing the extent and utilization of nursery areas (Merson and Pratt, 2001).
Lemon sharks as with many other shallow water or coastal sharks use nursery areas (Branstetter, 1990) during their early years of life. Studies suggest that nursery areas are chosen because of low predation rates, high abundances of prey, and suitable habitat (Branstetter, 1990; Rountree and Able, 1996). Mortality rates in sharks during their early years can be high, as high as 64% mortality in the first year for lemon sharks (Manire and Gruber, 1993). Gruber et al. (2001) found that annual survival rates for juvenile lemon sharks varied between 39% and 65%. Both of these studies suggest that lemon sharks experience high mortality rates as juveniles especially during their first year of life.
One of the major objectives of this project is to construct a GIS model of the nursery areas using biotic and abiotic factors as layers including: water temperature, salinity, water depth, dissolved oxygen, biotope, prey availability, water currents, and predator presence. This GIS model will be used to examine habitat selection of juvenile lemon sharks within the nursery areas. The GIS model will also be used to determine influences of tide, water temperature, time of day, salinity, prey availability, predator presence, and current on the movements of juvenile lemon sharks. Tracking data will be used to estimate mortality rates for juvenile lemon sharks in their nursery areas. Tracking data will also be used to estimate activity spaces and home ranges of lemon sharks around Bimini and these movement parameters will be compared for sharks of different age classes and sharks living in different nursery areas. The major hypothesis to be tested is that juvenile lemon sharks select specific habitats within the nursery areas around Bimini in order to maximize benefits such as prey availability and minimize threats such as predator encounters. These results will all help to define the essential fish habitat for lemon sharks during their early years. It has been suggested that juvenile sharks use nursery areas because of prey availability and predator avoidance. However, the reasons for sharks using nursery areas have not been well studied. This study will hopefully elucidate some the factors affecting the movements and activity spaces of juvenile lemon sharks.
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