The Reproduction, Growth, Feeding and Impacts of Exploitation of the Venus Tuskfish (Choerodon venustus) With some implications for its management.

Platten, John Robert (2004). The Reproduction, Growth, Feeding and Impacts of Exploitation of the Venus Tuskfish (Choerodon venustus) With some implications for its management. PhD Thesis, Centre of Marine Science, University of Queensland.

       
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Author Platten, John Robert
Thesis Title The Reproduction, Growth, Feeding and Impacts of Exploitation of the Venus Tuskfish (Choerodon venustus) With some implications for its management.
School, Centre or Institute Centre of Marine Science
Institution University of Queensland
Publication date 2004
Thesis type PhD Thesis
Supervisor Dr. Ian Tibbetts
Abstract/Summary This study examines the biology of the venus tuskfish Choerodon venustus from the southern Great Barrier Reef to enable better-informed management decisions. Venus tuskfish are taken almost exclusively by line fishing. The size of the catch is uncertain, but the annual Queensland commercial catch is probably about 30 t and the recreational catch much greater than this (possibly ca. 300 t). The species is not the primary target of fishers and can be regarded as by-catch taken while targeting other species. However C. venustus is the second or third most commonly retained species in the study area. Venus tuskfish are taken in habitats associated with coral and other reefs across a depth range from 5 m to 90 m. The species is a benthic predator, taking molluscs, echinoderms, annelids, arthropods, and fish (in decreasing order of importance). Feeding intensity was greatest in summer. There was evidence of complex feeding behaviours and a dominance hierarchy amongst feeding fish that may result in large males being more susceptible to fishing. The species is slow growing and long lived. Up to 38 bands were observed on the opercular bones of C. venustus. Back-calculation methods yielded von Bertalanffy growth parameters of L¥ =669 mm, K = 0.09 and t0 = -3.89. Male fish appear to grow faster than females. This may be related either to faster growing fish becoming males, growth acceleration following sex reversal, or both. Selective removal of faster growing males in heavily fished locations may result in a greater proportion of slower growing females in the remaining population. iv Choerodon venustus is a protogynous hermaphrodite; the proportion of males increases as the fish increase in size, transitional fish exist and remnant female tissue was evident in testes. The species is a serial spawner with an extended breeding season perhaps peaking in autumn and around the new and full moon. Females mature between 200 and 250 mm forklength (LCF). Venus tuskfish display sexually dimorphic colouration and appear to have a socio-sexual group structure. Fish occur in extended groups with several large males associated with a larger number of smaller females. Multiple sex-reversals may occur in the groups perhaps in response to the loss of the larger males. Large females produce over 100,000 eggs in the ovaries. The number of eggs released at each spawning is unknown. There is a direct cubic relationship between length and the weight of female gonads. Large females over 500 mm LCF are capable of producing over 20 times the number of eggs of small mature females (around 250 mm LCF). The locations sampled showed marked differences in fishing effort. The Capricorn Bunker Group was subject to much higher fishing effort than the Swains Reefs. Estimates of fishing mortality reflected this trend. Line fishing selects large male fish. The heavily fished Masthead Island had smaller males, females and transitional fish. Fish below 300 mm LCF were not captured efficiently. Venus tuskfish may be capable of modifying their life cycle in response to increased fishing mortality. Sex reversal may be related to the absence of large males in social groups. Hence sex ratio remains constant between fished and unfished locations. In heavily fished populations, females are smaller and consequently produce fewer eggs. Modeling suggests unfished areas may have a potential fecundity over six times that of those heavily exploited. Choerodon venustus are severely impacted by barotrauma during capture, and few captured and released fish are likely to survive. There are special challenges in managing venus tuskfish including: · Their status as a largely bycatch species; · The poor survival of released fish due to barotrauma; · A lack of good measures of catch and effort caused by inconsistent naming of the species and inherent errors in current data collection methods; · Their extended spawning period and likely widespread spawning sites and · The need to support group fecundity by protecting larger fish. A combination of a series of fish reserves, raising the minimum legal size to 36 cm TL, effective catch limits and an effective education program are likely to support the sustainability of the fishery.
Keyword Choerodon
Tuskfish
Effects of Fishing
Southern Great Barrier Reef

 
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