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Assessing and limiting predation and other sources of mortality in reseeded Trochus niloticus

Dobson, Graeme (1999). Assessing and limiting predation and other sources of mortality in reseeded Trochus niloticus. Master Thesis, Northern Territory University.

Document type: Thesis
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Author Dobson, Graeme
Title Assessing and limiting predation and other sources of mortality in reseeded Trochus niloticus
Institution Northern Territory University
Publication Date 1999
Thesis Type Master
Subjects 0699 - Other Biological Sciences
0704 - Fisheries Sciences
Abstract Trochus niloticus (Linnaeus) is a tropical marine gastropod that is of significant economic importance among the poorer maritime states of the IndoPacific region. This study was done to: • determine patterns of T. niloticus settlement and movement within a reef system • make observations of predators and their impact on juvenile T. niloticus • develop a novel packaging method to maintain juvenile T. niloticus in good condition during transit. A small reef system in the north of West Australia was divided into six distinct habitats in which a total of 123 T. niloticus, at an overall density ofO.58/m 2, were found over a 12 month period. A substrate in the inner reef, composed of Porites nigrescens and Thalassia hemprichii, was identified as a nursery habitat where 95% of the T. niloticus found (density 0.46/m2) had a shell width < 30 mm (range 4-30 mm; mean 13.9 rom). T. niloticus were found in significant numbers in only three other habitats, where 91 % had a shell width> 30 mm. T. niloticus were shown to gain a high degree of size refuge from Thalamita crenata and Gonodactylus sp. when they grew to a shell width of 25-30 mm. Observations of predators suggested that the principal benthic predators of juvenile T. niloticus are Morula margariticola, Thalamita crenata and Gonodactylus sp. The principal pelagic predator appeared to be Choerodon cyanotus. Each of these predators has a distinctive method of hunting and breaking open T. niloticus that left a characteristic damage pattern on the shell remnants. The common defence used by T. niloticus was a 'drop response'-when a T. niloticus was touched, it released its grip and dropped into the substrate. When attacked by M. margariticola, however, T. niloticus employed a range of defence measures including flight, passive defence (gripping the substrate firmly and relying on the strength of its shell) and active defence (rapidly spinning its shell in a slashing motion). The survival rate of juvenile T. niloticus after long periods in transit (> 24 hours) was improved by packing them upright on a solid substrate in rigid containers. This study is significant because it has identified a specific habitat for juvenile T. niloticus and linked the choice of that habitat to predator foraging behaviour. Applying this knowledge to future T. niloticus reseeding projects will reduce mortality from predation and ensure that the juvenile T. niloticus are placed in the most advantageous environment for their survival. The use of the packaging method described in this study should also reduce mortality rates suffered as a result of juvenile T. niloticus being in transit for long periods of time.


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