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Technical annual report 2000-01



Technical annual report 2000-01


Dept. of Regional Development, Primary Industry, Fisheries and Resources technical annual report; Department of Regional Development, Primary Industry, Fisheries and Resources technical and annual report; Reports; PublicationNT; Technical bulletin (Northern Territory. Dept. of Primary Industry and Fisheries) ; no. 295




Made available via the Publications (Legal Deposit) Act 2004 (NT).






Agriculture -- Northern Territory -- Periodicals; Fisheries -- Northern Territory -- Periodicals

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Dept. of Primary Industry and Fisheries

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Technical bulletin (Northern Territory. Dept. of Primary Industry and Fisheries) ; no. 295



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Department of Primary Industry and Fisheries 280 Habitat issues Spanish mackerel is typically caught around reef fringing headlands and shoals. Spanish mackerel adults are thus found offshore. However, juveniles are found in estuarine and near shore conditions (such as mangrove creeks) and thus are dependent on the quality of these habitats. Environmental Influences Recruitment and production (the combination of growth and survival) of Spanish mackerel is likely to vary with productivity of inshore and oceanic waters. There has yet to be an investigation of such relationships. Recreational Fishery: Background Spanish mackerel is a valued light game fish, pursued for its fighting response to bait and lure, and for its excellent table qualities. Fishing tourism and guiding is a rapidly developing sector of the fishery. Commercial fishing guides are required to be licensed and to submit detailed logs of fishing activity. Catches recorded by fishing tour operators (FTOs) have increased substantially in recent years to about 3000 fish. Around 64% of these fish are released. The recreational fishery is regulated by means of a possession limit, currently set at five fish per person per day. There are no size restrictions for Spanish mackerel in the NT. Research Activity 2000: Stock Assessment Stock assessment of Spanish mackerel relies on analysis of trends in the commercial catch and effort data. Auxiliary information is provided from sampling of catches for length and sex composition. Daily logbooks (a subset of the commercial fishery information) provide mean size information by fishing session. Additionally, samples of otoliths (ear bones) are taken to provide information on age structure. Assessments during the early 1990s, 1997 and 2000 were limited by the poor information content of catch and effort data. Age structure data supports a hypothesis that the fishery has been recovering from heavy fishing by Taiwanese gillnetters during the 1970s and 1980s, explaining increases in CPUE during the late 1990s and the absence of older fish in the age structure data collected in the early 1990s. There is also some genetic evidence for this hypothesis. Modelling suggests that current harvests are near the limits of sustainability: the increasing CPUE of recent years and age structure information are consistent with the fish population recovering but the extent to which the population might continue to grow cannot be determined from existing information. It is important to note that, without alternative information on harvest rates to constrain our models, it is possible that the increasing trend in catch rates of recent years really reflects increasing efficiency of operators, and/or trends in environmental factors. Monitoring: Fishery Monitoring As well as compilation and analysis of catch and effort statistics, length composition and reproductive status data were collected on samples of nearly 2000 fish during 2000 (these represented about 7% of the landed catch). In a cooperative program, such information is collected by researchers onboard commercial vessels, or by their crews. Species, sex and size composition information is also gathered opportunistically at recreational fishing competitions. This information is summarized in Buckworth and Clarke (2001). It is planned that a tagging program will also be developed as a method of tracking harvest rates. Fishery Independent Monitoring There has been no fishery independent monitoring of the Spanish mackerel fishery - the species is not amenable to most methods of survey. Biological Research: For Spanish mackerel, most of the basic biology - growth, diet, reproduction - is well known. However, stock structure the degree to which fish mix together over spatial scales, has been largely unknown. Thus the extent to which fishing in any one area of northern Australia might affect fishing in another, by either taking part of a highly migratory stock, or by impacting on the production of larvae, is unknown. Thus the scale at which management should be set has really been unknown. In response to growing fisheries in the NT,