What makes a house a home: Examining the relationship between growth and predation risk in a coastal elasmobranch, the lemon shark (Negaprion brevirostris)

Hamilton, Ian R. (2014) What makes a house a home: Examining the relationship between growth and predation risk in a coastal elasmobranch, the lemon shark (Negaprion brevirostris). Masters thesis, Memorial University of Newfoundland.

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Abstract

Many shark species are known to use nursery grounds, though few use coastal habitats in the same manner as juvenile lemon sharks. Lemon sharks pup in shallow, coastal areas and although the reasons for this behaviour are unknown, the two most likely explanations are access to food and avoidance of predators. This thesis aimed to examine these ideas by testing the hypotheses that a) sites with high resource availability will yield high juvenile lemon shark abundance and b) an increase in predation risk will result in decreased juvenile lemon shark abundance and increased individual growth rates. Seine nets were set to capture juvenile lemon sharks and their fish prey. Drum lines were used to measure predator abundance outside of each of the nine study inlets, while ARCGIS mapping software was used to assess site-specific refuge availability across the entire tidal cycle. Juvenile lemon shark abundances correlated strongly with mojarra (Gerres spp.), their preferred food item. Lemon sharks also showed a weak, but non-significant positive relationship with refuge availability, but not with predator abundance. Individuals in areas with high predator presence however, showed increased growth rates. Our data suggest that juvenile lemon shark populations exhibit frequency-dependent habitat selection with respect to prey availability. Moreover, the results indicate that of the variables studied, food availability primarily determined habitat quality in nursery grounds, with a less important role for predation pressure. If lemon shark populations are constrained by juvenile recruitment, appropriate management of their coastal nursery areas offer a powerful conservation tool.

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