Spatial ecology of mobile predators in a nearshore tropical environment and its implication for marine protected areas

Knip, Danielle Melissa (2011) Spatial ecology of mobile predators in a nearshore tropical environment and its implication for marine protected areas. PhD thesis, James Cook University.

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Nearshore environments are of high value and provide important habitat for a diverse assemblage of shark species. However, these regions are also highly dynamic, creating challenging conditions for inhabitants due to their shallow nature and close proximity to shore. This dissertation research defines the spatial ecology of sharks within a nearshore tropical environment to (1) provide a better understanding of the factors influencing species distribution, habitat use and movement patterns and (2) evaluate the effectiveness of marine protected areas (MPAs) for sheltering these mobile species from exploitation (e.g. fishing pressure).

Although shark species are highly diverse and occur in a wide range of habitats, limited effort has gone into understanding population level use of habitat. Theoretical models describing coastal shark populations have remained largely unchanged since the 1960s despite limitations in applicability to many species, with smaller-bodied species being poorly represented by the current models. Coastal shark populations have typically been described as using nearshore nursery areas as juveniles and then moving further offshore as adults. A new theoretical model is proposed in this dissertation to represent those species that spend most or all of their life within nearshore regions, but do not show use of discrete nursery areas. Description of this new model outlines the importance of nearshore regions to smaller-bodied species in particular.

Field work was conducted using passive acoustic monitoring to examine the presence and movements of two coastal shark species within a nearshore tropical environment. An array of fifty-seven acoustic receivers deployed throughout two MPA regions in Cleveland Bay, north Queensland, Australia, passively tracked pigeye Carcharhinus amboinensis and spottail Carcharhinus sorrah sharks from 2008 to 2010. These species were selected to define differences in the use of nearshore habitat between sharks that have different life history and ecological characteristics, putting the results of the theoretical population models into context. Carcharhinus amboinensis individuals consisted of juveniles from three age classes (young-of-the-year, one-year-olds and two-year-olds), and C. sorrah individuals were adults.

Juvenile C. amboinensis were present in Cleveland Bay for long periods, ranging from 3 to 429 days (mean = 88). Individuals associated strongly with shallow turbid habitats, with core home ranges consistently remaining in areas adjacent to creek and river mouths. Significant differences in minimum convex polygon measures of home range revealed that older juveniles used larger areas and undertook movements from core ranges more frequently than younger juveniles. Movements of C. amboinensis were related to the tidal cycle, but changes in water depth associated with the tide had the strongest influence on the youngest juveniles. An ontogenetic shift in depth used was also evident, with young-of-the-year individuals restricting their movements to shallower depths (mean = 236 cm) than one- and two-year-old individuals (means = 261 and 269 cm, respectively).

Freshwater influences associated with the wet season played a role in habitat use of juvenile C. amboinensis in this nearshore region. Home ranges and distribution of C. amboinensis showed distinct changes across two consecutive wet seasons, with individuals moving away from creek mouths during times of high freshwater inflow. Although juvenile C. amboinensis moved in response to freshwater inflow, home range sizes remained stable, and the amount of space individuals used did not change in relation to freshwater inflow. This result suggests that individuals use consistent amounts of space despite changes in location.

Adult C. sorrah were also present in Cleveland Bay for long periods, ranging from 8 to 408 days (mean = 190). Unlike C. amboinensis, however, this species used areas away from creek and river mouths and generally remained in deeper water habitats (mean = 421 cm). Analyses of home range revealed that individuals consistently used the same areas and similar amounts of space over time. A high level of spatial segregation occurred among C. sorrah within this nearshore region, with individuals using different types of habitat and showing high attachment to specific areas. In addition, the range of depths individuals used varied between sexes. Males tended to remain within a narrow depth range, whereas females displayed a seasonal shift in the range of depths used. Mean monthly depths varied by as much as 200 cm for females, with individuals using shallower habitats where water temperature was higher (up to 4° C) during the winter months.

Both species spent a large proportion of time inside the MPAs. The mean proportion of time C. amboinensis and C. sorrah spent inside MPAs was 0.22 and 0.32, respectively, and MPA use varied seasonally. Both species used large areas inside the MPAs, but individuals generally used only half of the available protected space. All individuals made excursions from the MPAs, however, both species exited and re-entered MPAs within consistent locations along the boundaries.

Long-term use of Cleveland Bay demonstrates that this region contains important habitat for both juvenile C. amboinensis and adult C. sorrah. However, differences in movement and habitat use suggest that this nearshore environment serves different functions for these species. Use of discrete, shallow areas is likely a refuging strategy for young C. amboinensis, and an ontogenetic shift in depth used may be a mechanism to decrease competition between different juvenile age classes using the same area. For C. sorrah, the high level of spatial segregation among individuals may be a strategy to improve foraging success by increasing separation and using more areas. Although movement and habitat use patterns varied between species, both used MPA regions for a large proportion of time. Thus, coastal MPAs provide some protection and benefits for mobile species like sharks. By defining the spatial ecology of coastal sharks, this dissertation contributes to improving the conservation and management of their populations, and provides a framework for future research on the effectiveness of MPAs for mobile species within Australia and around the world.

Item ID: 22939
Item Type: Thesis (PhD)
Keywords: acoustic telemetry, habitat use, home range, movement patterns, sharks, marine protected areas, tropical nearshore environments, near-shore environments, spatial ecology, species distribution, fishing pressure, MPAs, population dynamics, Cleveland Bay, Carcharhinus amboinensis, pigeye sharks, pig-eye sharks, Carcharhinus sorrah, spottail sharks, spot-tail sharks
Date Deposited: 17 Aug 2012 01:05
FoR Codes: 07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070402 Aquatic Ecosystem Studies and Stock Assessment @ 60%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 40%
SEO Codes: 83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8302 Fisheries - Wild Caught > 830204 Wild Caught Fin Fish (excl. Tuna) @ 60%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 40%
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