Lobster recirculation systems
Science Centres: Fisheries
Recirculating seawater facilities are commonly used by rock lobster fishers and processors to hold and condition lobsters for live export. Every lobster season there are unexplained mortalities either at the holding facility or during shipping. With recirculation units holding many tonnes of lobsters, and live lobsters averaging $50 per kg in export value, a system failure can result in the loss of hundreds of thousands of dollars.
NIWA, with the support of SITO, recently ran its first one-day training/refresher course in recirculation system monitoring and management in Wellington. The course was aimed specifically at training staff who manage rock lobster holding units. Eleven industry personnel attended, and there has been very favourable feedback.
Advances in understanding
Over the last 20 years significant advances have been made in understanding the design and management of recirculating seawater systems. These advances derive from a better knowledge of lobster physiology and the effect that lobsters have on the seawater chemistry in a recirculation unit. It is now possible to predict, reasonably well, the carrying capacity of individual recirculation units, and to prime these units for the shock loads that they experience when lobsters are introduced.
Whatever design of recirculating system is used, the stock husbandry and system maintenance protocols will be similar. There is nothing mystical or secret about the functioning of recirculating systems, and although there are a myriad of system designs, they all have the same basic features and work on the same basic principles. However, poor understanding and management of these systems create most of the common problems associated with recirculation technology and can put the highly valuable stock at risk of mortality, either on site or during shipping.
Monitoring water quality – the key to happy lobsters
The seawater in which lobsters live before capture is clean and well oxygenated, with low ammonia levels and a high pH. The seawater in a well designed live-holding system should be of similar quality.
The high density of lobsters held in a recirculation system will inevitably change the water quality from that in the ocean. Byproducts from lobster metabolism are released into the water and react with the seawater to change its chemical make up. The role of the recirculation plant manager is to monitor these changes and make the necessary adjustments to keep the water within acceptable limits.
Recirculation system design
Although the water in a recirculation system is reused, the metabolites must be removed and the dissolved oxygen restored. The system design needs to add oxygen and remove carbon dioxide as well as provide a means of removing suspended solids and ammonia. In most lobster holding systems oxygen is added either through venturi action or by splashing or cascading water into the stock tanks.
Carbon dioxide on the other hand is rarely considered, but is a major contributor to poor water quality in most lobster holding systems. It reacts with water to lower the pH, thus placing the lobsters under physiological stress. This needn’t be so, because carbon dioxide can be easily removed by a normal gas stripping process.
In lobster holding units, ammonia is normally removed by naturally occurring bacteria in a biofilter. These bacteria are surface colonisers and will be found on any surface where they have access to ammonia. Thus the biofilter is just an attachment area for the bacteria. The design of the biofilter will depend on the amount and the concentration of ammonia, the water flow, and whether the biofilter is going to be multifunctional and include processes such as solids removal or gas stripping and reoxygenation. The biofilter material and design will reflect its functions, but a large surface area is always needed.
Further courses
Further courses are planned to meet demand from the rock lobster industry, and a more detailed course is proposed for people wishing to know more about the use of recirculated seawater systems for the culture of fish or abalone.