Fish Production Cycle
General Production Cycle
Salmonids spawn in freshwater and are on-grown in freshwater or seawater. Seawater cage production is the main system used in Tasmania. Atlantic salmon are transferred to sea as smolts, from autumn to spring. Trout strains used are non-smolting so are transferred as large parr in autumn.
Broodstock are either maintained in freshwater or transferred from seacages and acclimated to freshwater for a few months prior to spawning. Salmon are spawned artificially in May while trout can be spawned between May and August, but principally June and July.
Production Strategies
Eggs and milt are mixed together in clean containers and are left to fertilise for 5 minutes and are subsequently washed and water hardened prior to being placed in egg incubators in a hatchery. Here they remain for periods up to 1-2 months in an undisturbed and darkened environment. On hatching the alevins do not feed for a couple of weeks, living on their yolk reserves. At first feeding, fry are offered artificial diets around 400-600 um, and can be provided with 20-24 hours continuous light, and optimal water temperatures and densities to maximise growth. It is important to provide a good start to production.
Smolts
Fish are graded and transferred to larger tank systems for between 8-15 months before being transferred as out-of-season smolts (autumn) or spring smolts. Fish range in size from 60-200 grams. Trout are transferred as parr (juveniles) in autumn and winter (sea) and throughout the year (estuaries) at 300 grams+.
Fish Harvesting
The aim of the grow-out cycle is year-round harvesting. Stock grow-out is manipulated to achieve this aim. The main methods implemented include controlled feeding, even growth, use of out-of-season smolts, pre-smolts, spring smolts and triploid smolts. Generally harvesting of a year-class is conducted prior to maturation, which reduces the attractiveness of appearance and flesh quality.
Fish are harvested as near as possible to market requirements in terms of average size and weight.
Salmonids are stocked at initial densities of <1kg/m3 and reach a maximum at harvest of 12-15 kg/m3 or less. This density is lower than those used overseas. Although this may raise the apparent cost of production, in reality, faster growing fish and healthier appearance probably offset the extra carrying cost.
Fish Health
Tasmania's salmon do not display diseases such as IPN, furunculosis, sea lice or BKD which cause concern in overseas salmon industries. The industry does experience amoebic gill disease (AGD) and occasional incidences of Yersinia ruckeri, Flexibacter and Vibriosis. Antibiotics use is strictly controlled with most growers avoiding their use. Instead, growers elect to prevent outbreaks through good husbandry practice rather than treating either after expression or prophylactically. Freshwater baths are use d to reduce the incidence of AGD, a vaccine has been developed for Vibrio anguillarum and vaccines are being developed to prevent other infections.
Overall losses in the industry due to disease are probably around 1-8%/year. Fish health issues are monitored closely by personnel from the DPIE Fish Health Unit.
