Temporal Variations in Lemon Shark Population Dynamics
Steve Kessel & Dr. Samuel H. Gruber
The work of Volterra (1928) long ago highlighted the importance of top predators in the functioning of ecosystems, with particular reference to population dynamics and energy exchanges between trophic levels. Top predators are no less important in the marine environment than the terrestrial (Gruber, 1982). Smith (1974), Steele (1974) and Hassell (1976), among others, have demonstrated that top predators can exert both oscillating and stabilising influences on ecosystem dynamics. Therefore, the population dynamics of these predators have a massive influence upon the environment they inhabit.
Due to its large size and lack of natural predators in the adult stages, the lemon shark Negaprion brevirostris (Poey, 1868) is considered an 'apex predator' (Gruber, 1982). Its size, abundance, generalised structure, ease of capture and adaptability to captivity makes it perfectly suited to systematic investigation (Gruber, 1982). These characteristics allow the collection of behavioural, bioenergetic and physiological data in both the field and laboratory, making the lemon shark the definite species of choice for this study (Gruber, 1982). The Bimini Islands support an abundant lemon shark population containing all life stages from newborn through mature adult (Feldheim et al 2002).
The shark population data set for Bimini, one of the most comprehensive long-term studies in the field of shark research (Gedamke and Hoenig 2005), spans from 1982 until the present day, but has never been thoroughly analysed. This data set most likely contains a wealth of information waiting to be unlocked by the proper analytical approaches. Preliminary analysis suggests that the long-line database has witnessed a long-term decline, 83% over the past 20 years, in the number of lemon sharks caught annually (Kessel et al, 2005). However, sub-adult/adult lemon sharks are still regularly sighted around the catch target areas (Kessel et al, 2005). This suggests that they may have developed some kind of avoidance mechanism to the long-line equipment, though this remains to be proven.
This project involves a continuation, extension and expansion of the current research methods used to sample the Bimini shark population as well as the implementation of new and altered methods to asses the effectiveness of the existing ones.
Aim: To identify the temporal variations in the resident lemon shark (Negaprion brevirostris) population structure, Bimini, Bahamas, and the causes behind these variations.
1.To analyse the existing 25 year long-line and 10 year-on-year lemon shark data-bases from the following view points:
1.1 To identify temporal variations in survival rates and the influencing factors;
1.2 To identify the relationships between other shark species and the lemon shark population;
1.3 To incorporate into GIS software to allow in-depth spatial analysis;
2. To investigate whether long-lining, as an investigative technique, is providing an accurate representation of the population, i.e. have the resident lemon sharks learnt avoidance of the lines?
3. To determine if 'better mothers' exist and if so, how they can be defined.
Recent years have seen a global increase in interest in elasmobranchs, particularly in relation to the need for management advice (IMM, 1997; ICES, 1997) and conservation (STECF, 2002). However, the most significant hurdle in providing this advice is a basic lack of biological and ecological data to be used in assessment (Rodriguez-Cabello and Sanchez 2005). This study will be vital in gaining data on the Bimini lemon shark population to allow for more accurate management advice.
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