Abstract

Biosecurity, particularly as it relates to disease control, has always been a consideration when engineers have undertaken the design of hatchery and growout systems. However, it is becoming increasingly evident to engineers that interface with the commercial sector that the long term threat of biosecurity has been underestimated. Most affected are the systems that utilize natural surface waters as a means of water quality control or as makeup waters. This observation is rapidly driving the aquaculture industry towards the use of recirculating systems for broodstock maturation and fingerling production. Competition for a limited supply of freshwater, regardless of source, for increasing populations is pushing the industry toward more efficient recirculating systems. Likewise, newer marine hatchery systems are trending towards a higher degree of closure, as a result of increased biosecurity concerns and salt handling issues. These systems are demanding increased attention to nitrate management. At the same time, display aquaria and species groupings within these systems have become larger, more demanding, and more complex over the last 5-10 years, requiring newer approaches to increase efficiency but reduce proliferation and spread of infectious disease. Each challenge and solution has its unique set of problems.

The engineering community is also re-examining its filtration strategies, in light of these increased biosecurity threats. The tradition of centralizing water treatment to gain economies of scale is being challenged by biosecurity needs that would seem to dictate internal partitioning of systems as a means of inhibiting spread of disease once external safe guards have been breached. Use of airlift pumps driven by a centralized air source to circulate isolated filtration systems is suggested as a strategy to improve disease resistance while minimizing mechanical complexity. This approach demands design criteria for low head filtration components, such as, moving bed reactors, floating bead filters, rotating biological contactors, and three phase fluidized beds.

Dealing with the issues of disease control in intensive hatchery systems and recirculating systems in general will require increased participation by the veterinary and microbiological research communities. Although cognizant of the issues, the engineering community is poorly prepared to resolve the problem. Specifically, more information is required with regard to mycobacterial ecology and mycobacterial pathogenesis and epidemiology within recirculating systems. Clinical and research veterinarians working with aquatic systems can provide engineers with insight and information on outbreaks at the organism and population level within systems, and the microbiological research communities can provide pathogen level expertise.

How do different recirculating system designs and different parameters favor adhesion, growth, and spread of mycobacteria? Assistance is needed to determine the efficacy of centralized disinfection strategies, such as ozone or ultraviolet lights, or to determine the threat that may be posed by aerosols generated by airlift systems. Information is also required on the effects of certain water characteristics on the growth of mycobacteria, including reduced water turnover, differences in flow rates, and specific parameters, such as dissolved oxygen levels, ammonia, nitrite, nitrate, pH, alkalinity, hardness, temperature, dissolved and suspended organics, and other dissolved compounds. For all of these, their importance within the system as a whole and within microenvironments in the system needs to be evaluated. Are these effects significant and can they be managed easily?

Other factors to consider include the impact of different components of filtration, as well as different filtration types; physical characteristics of media (shape, composition); tank material composition, decorations (for displays) and substrate; and biofilm characteristics which favor and precipitate adhesion and proliferation of mycobacteria, as well as differences in biofilms among different system designs. Additional considerations include potential use of competing bacteria/probiotics and methods to favor nitrifying bacterial growth vs. that of pathogens such as mycobacteria.

Finally, system parameters that affect species and age class susceptibility and inter- and intraspecies interactions as well as differences in mycobacterial species with regard to ecology and infectivity need to be understood.

Closer collaboration between aquaculture system engineers, veterinarians, and microbiologists will speed the rational development of more disease resistant designs, avoiding the much slower and costly evolution that is ongoing in the commercial sector.