At the Conference on Recirculating Aquaculture held last August in Roanoke, Virginia, Nick King of the Fish Vet Group in Portland, Maine presented a plea for better fish health management in marine hatcheries.
King argued that the expansion of marine (and freshwater) finfish culture will require greater hatchery production of juveniles, and an increased ability to produce them regularly and consistently in any geographic location.
Fry survival in marine fish hatcheries rarely exceeds 50%, even for commercially developed
species, and with new species where hatchery protocol development is still underway, fry survival is often less than 20%.
Through applied research on the farm—including trial and error—survival will likely increase, however, the goal of full commercialization is not only to optimize larval and fry survival and maximize production in an economical way, but to have consistent and stable results, cycle after cycle.
King notes that consistency and stability in hatchery performance can only come from a quality management program that sets measurable performance standards and accordingly minimizes risk. Protocols must arise from the methodical investigation of a predictable, calibrated system. Otherwise, the results will be too random, and the protocol will become unnecessarily complex and open to instability.
An evidence-based hatchery health plan, he says, can identify the pathogens present in the facility and the risks they present. Some pathogens — such as parasites — may affect hatchery performance and program stability even if they are only present in small numbers and do not rise to the level of a disease outbreak. Thus, an effective hatchery health-monitoring program should survey the facility for underlying bacterial, viral, parasitic and non-infectious diseases at critical points in the production cycle. This provides the foundation for biosecurity decisions that will ultimately minimize risk.
Develop a process
So how can that be done in a cost-effective manner? King suggests that sampling a fish population for pathogens or parasites requires that a sufficient number of fish be killed to provide a statistically meaningful result. A sample that gives 95% confidence of pathogen detection is recommended by the Office International des Epizooties (OIE) and American Fisheries Society-Fish Health Section, which aim to detect assumed pathogen prevalence levels (APPL) of 2-10% in the total population. Most often, it is assumed that we need to know when APPL is at 5%. For example, in a lot of 2,000 fish, sixty fish should be sampled to give 95% confidence that a pathogen would be detected, and 145 fish for detecting pathogens at 2% prevalence. The sample sizes do not change much for larger populations: numbers for a stock of 100,000 fish are 60 and 150 at 5% and 2% prevalence respectively.
But much insight can be obtained by routine observations that will show any changes in fish behavior including things like shoaling, twirling, non-feeding, aggression, cannibalism; or changes in appearance such as changes in coloration (pale or dark), deformities, swim bladder problems, eye membrane swelling caused by gas super-saturation, and other indicators of problems.
Developing a Program
In correspondence with Hatchery International King outlined a series of steps best followed in setting up a health management /risk analysis program. Ideally this would be done in consultation with a local veterinary laboratory. They include:
• Knowing the fish source, water source, new materials influx, flow of traffic.
• Creating a list of potential pathogens based on species and life-stage susceptibility.
• Establishing a surveillance and screening program.
• Developing and following strict biosecurity standards.
• Using reliable sources for all new materials (including the fish).
• Establishing disinfection protocols. Limiting site access.
• Determine a baseline mortality. (A“significant” mortality event could be an 0.05% increase in mortality over the baseline for three consecutive days).
• Develop intervention thresholds and appropriate strategies.
• Make fish husbandry a priority.
He also noted that routine fallowing and disinfection can break an infection cycle, and are important components of a management program.
Send suspicious samples to the vet
Few fish hatcheries will have the capabilities to process or read histology slides, but they certainly can collect a fish, slit the belly and drop it into a sample container to ship to back to the diagnostic lab. Some hatcheries may be able to do bacterial plating on selective agars (like TCBS for Vibrio), but wouldn’t necessarily be able to ID any bacteria on them. The follow-up is for them to send the plates to the local diagnostic lab for isolation and identification.
I am indebted to Nick King for explaining some of the subtleties of hatchery health management. For more specific information contact him at: firstname.lastname@example.org.
— Dave Scarratt