Given the sporadic and unpredictable nature of algal blooms and jellyfish infestations affecting fish farms it is difficult to plan for the prevention of harmful effects. However, there are steps which can be taken before, during and after an event which can minimise stock losses and also assist in the investigation of events.
Prior to Any Event and Routine Monitoring
- As a matter of routine, site staff should monitor the turbidity (cloudiness) of the water surrounding the cages on site. An increase in turbidity generally indicates an increase in the number of plankton, which may be either benign or harmful
- Collect a water sample daily from the site and preserve it in Lugol's Iodine. In the event of problems occurring on site this can prove invaluable in investigative work and may be useful in the event of an insurance claim being submitted. If site staff are adequately trained, it may be possible for an increase in harmful organisms to be spotted prior to a major problem occurring
- Prepare a contingency plan to help deal with any large-scale mortality including their removal and disposal
During an Event the Following May be Useful
- Fish farmers often stop feeding their stocks during an algal bloom. This has a dual effect - reducing the metabolic rate, and therefore oxygen requirement of the fish, and generally the fish will remain deeper in the cage. (Algal concentrations are normally highest closer to the surface.)
- Observe the stocks for changes in behaviour, signs of distress and any increase in mortality
- Monitor oxygen concentrations and turbidity
After an Event
- Remove any mortalities quickly - they are easier to handle when fresh and may have some value
- Continue to monitor oxygen levels and turbidity
- Recommence feeding
Some farms have installed systems to attempt to circumvent some of the effects of algal blooms. These systems include oxygenation systems to provide additional oxygen, water uplift systems - bringing water from below the bloom through the farm and causing a flow of water away from the farm and providing compressed air "curtains" (this system is under trial to prevent jellyfish infestations.)