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Study by Tom Gallagher and Michael Hutchison determining the best stocking strategies for barramundi, bass, golden perch, & silver perch in impoundments. Factors evaluated include release size, strategy, timing, and habitat. Results show that size matters in improving survival rates. Recommendations on release sizes, strategies, and habitats for different species.
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Impoundmentstocking strategies For Australian Native Species Does Size Matter? Tom Gallagher and Michael Hutchison
Aims Determine optimum strategy for stocking barramundi, bass,golden and silver perch in impoundments including... • release size • release strategy • timing of release
Methods • Micro-tag fingerlings with uncoded wire and VIE tags. Body location and/or tag colour denote batch. • 3 release sizes (20-30 mm, 35-45 mm and 50-65 mm) by three release strategies (shallow, deep and artificial cover). • Pseudo-replication by three dams. Replicate stockings over three years. • Approximately 6000 fish/species released into each dam each year. All 3 sizes stocked at same time. Total fish stocked over project 210,000 • Analysis by GLM of binomial proportions with a logit link function.
VIE tag Micro wire tagging Methods continued... Artificial cover Shallow release Artificial cover release
Methods continued... Recapture Gill netting Tagged fish were recaptured in each dam by a combination of gill netting, electrofishing and angling. Electrofishing
Methods continued... • Predation experiment • Two hours after releasing barramundi fingerlings predators were collected by gill netting and electrofishing within 50 metres of each release point. • Predators were stored on ice in bags labelled by capture zone. • Predators were frozen until examination of gut contents in lab. • Fingerlings found in guts were classified by size class & release zone. Similar experiments were conducted post-release of silver and golden perch.
Results Total recaptures of 3938 tagged fish from 210 000 released (1.9%). Recapture rates varied from 0% to 30%, depending on the species, dam, size and year of release. Significance of factors in the GLM
Mean recapture rates by size at stocking Barramundi Golden perch Bass Silver perch
Adjusted mean recapture rates by release strategy Barramundi Golden perch Bass Silver perch
Variation between dams/years may be in part accounted for by predator composition Survival of stocked silver perch
Predation of stocked fingerlings leads to variation in release strategy outcomes.
Size class comparison Size class comparison Relative survival ratio Relative survival ratio > or < > or < Cost ratio Cost ratio 50-65 mm: 35-45 mm 50-65 mm: 35-45 mm 1.16 (0.862) 1.94 (0.514) < > 1.44 1.44 50-65 mm: 20-30 mm 50-65 mm: 20-30 mm 2.38 (0.42) 2.43 (0.411) > > 2.3 2.3 35-45 mm: 20-30 mm 35-45 mm: 20-30 mm 2.10 (0.476) 1.22 (0.82) > > 1.6 1.6 Cost effectiveness of stocking Bass Barra
Size class comparison Size class comparison Relative survival ratio Relative survival ratio > or < > or < Cost ratio Cost ratio 50-65 mm: 35-45 mm 50-65 mm: 35-45 mm 2.325 (0.430) 1.098 (0.911) > > 1 1 50-65 mm: 20-30 mm 50-65 mm: 20-30 mm 1.961 (0.510) 8.107 (0.123) > > 1 1 35-45 mm: 20-30 mm 35-45 mm: 20-30 mm 1.786 (0.560) 3.487 (0.287) > > 1 1 Golden perch Silver perch
Summary and conclusions • Release size does matter • 50-65 mm fish generally survive better than smaller size classes • Stock 50-65 mm fish in dams with significant predator populations • 35-45 mm bass and silver perch may be considered in dams with few predatory species. • Shallow water releases appear suitable for all species. Artificial cover gave only marginal, but not statistically significant advantage to 3 species. • Avoid stocking bass in deep open water • Releasing in several large batches around the dam may reduce risk of mass predation.
