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Title: Investigation into the behaviour and population dynamics of the lemon shark (Negaprion brevirostris)
Author: Kessel, Steven T.
ISNI:       0000 0004 2750 4989
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2010
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Coastal shark populations have been subject to increasing anthropogenic pressure over the past two decades. This study focused on two lemon shark ( Negaprion brevirostris) populations, the site-attached maturing sub-adults at the Island of Bimini, Bahamas, threatened by a large-scale resort development, and adults forming winter aggregations off the coast of Jupiter, Florida, subject to direct fishing pressure. For the sub-adult population, analysis was carried out on the long-term temporal patterns in abundance and population structure, relative to potential driving factors, and the influence of variables affecting longline catch- rates used as the basis for stock assessment. For the aggregating adult population, life-history aspects of population structure and distribution were investigated for relative implications on the species' vulnerability status. The following analysis and methodologies were utilised to investigate the two populations: longline catch records from 1982–2008 monitoring of variables potentially affecting longline catch-rates documentation of shark behavioural interactions with longline equipment using underwater video surveillance aerial surveying for abundance estimates comparison of spatial utilisation patterns with longline catch locations external tagging the utilisation of archival satellite tags passive tracking with Vemco acoustic monitoring system and research collaborations with other scientific groups utilising the same acoustic monitoring system. The key findings of this study were that in the northwest Atlantic, N. brevirostris populations are experiencing considerable anthropogenic pressure at all life-stages. In Bimini, the effects of a large-scale resort development have resulted in a significant decline in abundance, to a level (-52 individuals) well below the temporal average (-158). On the U.S. east coast, seasonal aggregating behaviour has further increased vulnerability through increased catchability, beyond the highly vulnerable status already attributed to this species, and targeted N. brevirostris fisheries appear to be currently operating at unsustainable levels. Shark longlinc catchability was noted to be significantly influenced by multiple shark presence, resulting in greater susceptibility for N. brevirostris (and other similar species) that naturally exhibit group behaviour. Incidental encounterability and predation risk significantly influenced longlinc catch-rates. Adult N. brevirostris exhibited large-scale seasonal migrations on the U.S. east coast, which, in addition to documented international transitions, supports existing evidence for genetic mixing across the distribution. Water temperature was found to be a significant driver of N. brevirostris behaviour at all life-stages, with an apparent adult temperature preference of 24°C. This study represents the first long-term abundance assessment for sub-adult N. brevirostris, and the first in-depth study to focus on an adult N. brevirostris population. The results provide essential life-history information, revealing that at all life-stages N. brevirostris appear to be highly sensitive to anthropogenic activities, relative to other species, and therefore require enhanced management for species protection. It is therefore highly recommended that N. brevirostris be added to the U.S. prohibited species list.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available