Key words: fish stocking, threatened fish, post release survival, training, Native Fish Strategy
Fish stocking can be used to help restore threatened fish stocks. However hatchery-reared fish can have some behavioural deficits related to domestication which hinder their survival in the wild. Pond-reared fingerlings seem to retain live food foraging skills and some bird avoidance behaviours, but they are naïve in avoiding predatory fish. Fish reared to larger sizes in grow out facilities tend to be fed on artificial pellet diets and are protected from birds and other predators, so are inexperienced in foraging for live foods and poor at avoiding predatory birds in the wild. Pre-release training of hatchery-reared and grow-out facility reared fish is one strategy available to improve survival after stocking into the wild.
Broad aim and specific objectives: The value of pre-release training was evaluated in this study. Specific objectives were:
- to determine if hatchery reared threatened fish species native to the Murray-Darling Basin can be trained to reduce hatchery domestication effects and test if this leads to improved survival in the wild.
- to determine if release in predator exclusion cages (soft release strategy) to overcome transport stress, leads to improved post stocking survival.
Methods: The effectiveness of tank-based training was assessed by exposing fingerlings of Murray Cod (Maccullochella peelii), Silver Perch (Bidyanus bidyanus) and Freshwater Catfish (Tandanus tandanus) en masse to predatory fish and chemical alarm signals from fish skin extract. Sub-adult Murray Cod and sub-adult Silver Perch from grow-out facilities (where they were reared on pellet diets and protected from bird exposure) were also trained to avoid simulated cormorant (Phalacrocoracidae) attacks (Figs 1-3). Training used a combination of: bird models to harass and chase fish; cormorant odour; and, alarm signals from fish skin extract. Sub-adult Murray Cod and adult Silver Perch (~300 mm total length) from grow-out facilities were also trained to take live food. To assist this process a wild Murray Cod or Silver Perch was introduced into each training tank to help cue the behaviours of the fish from the grow-out facilities.
Stocking trials at three sites in the northern Murray-Darling Basin were used to test if pre-release training improved survival of stocked fingerlings of Silver Perch and Murray Cod. Predator free release cages were also tested as a stocking method to improve survival.
Figure 1. A juvenile Murray cod with VIE tag ready for release. Note green tag at base of anal fin. (Photo courtesy of Michael Hutchison.)
Figure 2. Training tank within which predators were separated from fingerlings. (Photo courtesy of Michael Hutchison.)
Findings: Tank-based validation experiments confirmed that training significantly improved the predator response behaviour of all three species compared to untrained fish. At least 72 hours training was required for Murray Cod and Silver Perch fingerlings and 48 hours training for Catfish fingerlings to significantly change predator avoidance behaviour.
Trained sub-adult Silver Perch showed significant behavioural changes in response to simulated Cormorant attack compared to untrained groups. However sub-adult Murray Cod showed no significant change in behaviour. Sub-adult Silver Perch readily adapted to taking live shrimp in the training tank, but pellet-reared cod failed to take live shrimp over a one month training period.
Predator release cages seemed to disadvantage the survival of stocked Murray Cod fingerlings. Predator free cages neither advantaged nor disadvantaged stocked Silver Perch.
Figure 3. Silver Perch fingerlings in a training tank, separated from predator by partition. (Photo courtesy of Michael Hutchison.)
Lessons learned and future directions: Pre-release training of fingerlings led to a significant improvement in survival of trained Murray Cod compared to untrained control fish. At locations where predators were more abundant, the survival of trained Murray Cod was up to four times higher than untrained Murray Cod. Across all locations the average survival rate of trained Murray Cod was twice that of untrained Murray Cod.
There were no significant differences detected between trained and untrained Silver Perch fingerlings stocked into the wild. This may have been because of the schooling nature of this species, and amalgamation of Silver Perch into mixed schools of trained and untrained fish, leading rapid social learning of the untrained fish from the trained fish.
Predator abundance had a significant impact on survival outcomes for both Murray Cod and Silver Perch fingerlings. Survival was lowest in locations with high predator abundance. The patchiness of predator distributions within a site means it is best to use several release points at a site.
Sub-adult and adult Silver Perch seem to be highly trainable, but sub-adult Murray Cod are not. Silver Perch are a social schooling species and this may enhance training. In contrast Murray Cod tend to be territorial and solitary. The use of long term pellet reared sub-adult Murray Cod in conservation restocking programs should be avoided. If large fish are required for conservation stocking, translocation of wild caught sub-adults or adults may be a better option.
Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy and undertaken by a research team from the (now) QLD Department of Agriculture Fisheries and Forestry.
Contact: Michael Hutchison, Queensland Department of Agriculture, Fisheries and Forestry, +617 3400 2037, Michael.hutchison@daff.qld.gov.au
EMR Project Summaries 