Environmental impact of stocked Mulloway (Argyrosomus japonicus) in New South Wales estuaries
Studies were undertaken for estuarine stocking of mulloway Argyrosomus japonicus to develop efficient marking techniques; to identify key habitats and migratory patterns of stocked fish; to evaluate the diet and growth of stocked fish and recruitment to commercial fisheries; and to develop a numerical model to estimate optimal stocking density from the predatory impact of stocked fish. The 10-step responsible approach to marine stock enhancement (Blankenship and Leber 1995) was reviewed for four Australian species, and Argyrosomus japonicus and Lates calcarifer were found to be the best candidates for stocking in Australia. Immersion of mulloway in an oxytetracycline-undiluted seawater solution produced poor quality marks, whilst oxytetracycline-diluted seawater solutions and alizarin complexone-seawater solutions produced high quality marks in otoliths and fin spines. These techniques were applied to produce double marks, and in batch mode at fish densities of 30 kg m-3. Key habitats for mulloway were identified as discrete holes or basins up to 20 m deep. Hatchery-reared fish initially had significantly greater movements (greater than 10 km in 3 d) when released in shallow water, compared to hatchery-reared fish released directly over deep holes. Such targeted release strategies could revolutionise stocking of depleted fisheries in estuarine and aquatic habitats, and even oceans. Stomach content analysis of mulloway captured from a coastal lagoon and riverine estuary revealed ontogenetic changes in major prey of mysid shrimp, prawns and forage fish. Comparison between benthic resources and dietary composition in these two estuaries revealed that mulloway consume prey resources in proportion to those in the environment, and do not actively select prey. Simulations of a novel Predatory Impact Model were run for part of the Georges River Recreational Fishing Haven, Sydney, to evaluate appropriate stocking density and associated predatory impact. The 15 km stretch of the Georges River contained c. 361,000 m2 of key mulloway habitat, and 10% of mysid shrimp production within this habitat was assigned to support stocked fish. Given these values, this section of river could support c. 4,000 stocked 8 cm mulloway. Over the 4 y during which mulloway are resident in the estuary, these fish will have a maximum yearly predatory impact of 6.7 t y-1 forage fish, 1.8 t y-1 school prawns and 2.2 t y-1 cephalopods, which represents 48%, 17% and 127% of former commercial fishery in the Georges River respectively.
Estuaries -- Environmental aspects -- New South Wales, Estuaries -- Computer simulation
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