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Ecosystem monitoring of subtidal reefs in different management zones of the Jurien Bay Marine Park 1999–2007

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Edgar, GJ and Barrett, NS and Crane, K and Bancroft, K (2009) Ecosystem monitoring of subtidal reefs in different management zones of the Jurien Bay Marine Park 1999–2007. Project Report. Tasmanian Aquaculture and Fisheries Institute, Hobart.

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Abstract

Floral and faunal communities associated with shallow reefs in the Jurien Bay Marine Park were investigated using underwater visual census methods on six occasions from October 1999 to November 2007. Densities of fishes, mobile macro-invertebrates, sessile invertebrates and macro-algae were quantified at a total of 42 sites. The magnitude of changes in density at zone locations between years was generally considerably less than spatial variability between zone locations, although some species and community metrics examined showed consistent increases or decreases in numbers over the survey period. The biotic community at different sites showed a major biotic subdivision between reefs located within a kilometre of the coastline and outer lagoonal reefs. Inshore reefs were characterised by sheltered conditions, water with moderate turbidity, and by macro-algae such as Dictymenia sonderi and Neurymenia fraxinifolia. Outer reefs were characterised by clear water, wave-exposed conditions, the kelp Ecklonia radiata, and the red seaweeds Pterocladia lucida and Hennedya crispa. Outer reefs were largely homogeneous with respect to associated plants and animals, whereas inshore reefs showed considerable variation between sites and between management zone locations. Scientific reference zones, in which rock lobster fishing is permitted, largely include outer reef systems. As a consequence, scientific reference zones possess a biota that differs little between zone locations. By contrast, sanctuary zones, where all forms of fishing are prohibited, include only shallow reefs, hence possess a biota that varies greatly between locations, and also differs from the biota encountered in scientific reference zones. The lack of overlap in reef communities between sanctuary zones and scientific reference zones complicated analysis of effects of restrictions on fishing that were enacted in these two zone types in December 2005. Regardless, very few observable ecological changes associated with new fishing restrictions were identified. The strongest relationships associated with fishing restrictions were found in correlation analysis when comparing the level of change from the periods before and after fishing restrictions with areal extent of protected zone. In this analysis, the mean size of breaksea cod (Epinephelides armatus) and silver trevally (Caranx georgianus) was found to increase significantly in the larger protected zones, as did the abundance of large (>250 mm) exploited fishes as a group and also dhufish (Glaucosoma hebraicum). We consider that the trends in breaksea cod size and numbers of exploited fishes are probably real, given that adequate data were available for statistical tests and trends were consistent between years. However, the trends in silver trevally size and dhufish abundance relate to highly patchy data, with the significant correlations best regarded as flags for confirmation or otherwise through the longer term. Several management recommendations arose from the study: Sanctuary zones should be extended to the outer reef area. Extra sites should then be added to the JBMP reef monitoring program to allow improved evaluation of effects of fishing restrictions, particularly with respect to assessment of the ecosystem role of rock lobsters. Other than for an extension of sanctuary zones to the offshore region, the system of protected zones in the JBMP should be maintained with as few changes to regulations and zone boundaries as possible through the long term. This is particularly important for zones that include sites surveyed as part of the long-term JBMP reef monitoring program. Surveys of fishes and mobile invertebrates should be repeated on an annual basis, and surveys of plant assemblages be conducted on a biennial basis, for at least five years from the time of enforcement of fishing restrictions in 2005. The frequency of surveys should be reviewed in 2010 to assess whether a longer period between surveys is warranted on grounds of cost-effectiveness. Monitoring should nevertheless continue through the longer term at least until biotic changes associated with MPA protection stabilise, probably longer given the unique value of the data set in tracking ecological effects of climate change.

Item Type: Report (Project Report)
Publisher: Tasmanian Aquaculture and Fisheries Institute
Date Deposited: 30 Jun 2009 04:21
Last Modified: 18 Nov 2014 04:01
URI: http://eprints.utas.edu.au/id/eprint/8826
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