5.3.5 Disease

Mention was made in 5.3.3 of the implication of bacteria of the genus Vibrio in mass mortalities of larvae, which occur from time to time in the best run hatcheries.

Vibrio sp. may not always be the direct cause of abnormal rates of mortality, nor are they the only group of opportunistic or obligate pathogens that can contaminate cultures and present problems. Potentially pathogenic species are present within the hatchery environment year round but are for the most part held in check by being only a minor part of the bacterial flora. At other times of the year, as indicated in 5.3.3, they may proliferate and dominate the microbial flora, posing a serious threat to production.
Before ascribing mass mortalities of larvae to a disease outbreak other potential causes need to be investigated. For example, the cleanliness of pipelines and filters needs to be inspected. Likewise, equipment such as pumps and air blowers that may have corroded or be leaking oil needs to be thoroughly examined. Algal cultures may have become badly contaminated or a technician may have made an error of judgement or miscalculation and grossly overfed a culture, or have forgotten to switch on the air flow to a tank or tanks, or not rinsed a tank after bleach treatment. Only after every avenue has been investigated and discounted should the possibility of disease be considered.

Unlike disease in larval fish, the onset of disease in bivalve larvae is swift and catastrophic. Larvae rarely show protracted symptoms leading to a mass mortality situation. They might appear perfectly normal in terms of colouration and behaviour the night before, but by the next morning be on the bottom of the tank, either dead or moribund with their shells almost devoid of tissue and filled with ciliate protozoans as opportunistic scavengers. Often there is prior warning in terms of larvae not grazing the food to the extent that would normally be expected the day before a mass mortality occurs. This highlights the importance of maintaining thorough records.
Once larvae have dropped to the bottom of a tank there is little the hatchery operator can do other than add a powerful sterilizing agent such as bleach to the tank. Even though a small percentage of the larvae may still be active and appear normal, they will invariably die before they reach metamorphosis if a pathogen is implicated. The objective is to try and contain the disease and eliminate the source of infection. This may mean closing the hatchery for thorough fumigation, ensuring that all equipment is cleaned and sterilized. The hatchery is then allowed to lie fallow for a week or two before production is resumed. The use of antibiotics is inadvisable during such outbreaks. They rarely improve the situation and there is always the risk that the pathogens will gain resistance to them.
Many hatcheries concentrate their production during periods of the year when mass mortalities are unlikely to occur. In temperate regions the most reliable period is winter and early spring, i.e. before the onset of phytoplankton blooming. Late June through to the end of September is often suitable for uninterrupted production.
Further reading on disease in bivalve larvae is to be found at the end of Part 5.