2.2.6 Mortalities

Bivalves in the larval, juvenile and adult stages can die from a variety of causes, which can be environmental or biological in origin. The subject is much too large to consider in detail here but a brief synopsis is given to highlight a number of pertinent points, which could be important in hatchery operations.

The physical environment can cause severe mortalities to bivalves in all three stages. Too high temperatures or prolonged periods of cold temperatures can be lethal to bivalves as can be sudden swings in temperature. Severe extremes in salinities, particularly low salinities after periods of heavy rain or run off from melting snow, can also cause extensive mortalities. Heavy siltation can smother and kill juveniles and adults.
Pollution, particularly industrial pollution, can cause extensive mortalities in juvenile and adult bivalves. Both industrial and domestic pollution can be problems for hatchery operations and must be avoided. Domestic pollution can increase organic and bacterial loads in water as well as contributing a wide range of potentially toxic materials. Little is known of the combined effects of sub-lethal levels of the wide range of organic and organo-metallic compounds of man-made origin that may be present in such effluents.
Bivalves in the larval, juvenile and adult stages are preyed upon by a wide variety of animals that can cause severe mortalities. In the natural environment plankton feeders probably consume large quantities of larvae. In hatcheries, predation is largely a non issue since the water used is filtered and any predators are removed.
Bivalves are hosts to parasites that can cause mortalities, particularly in the adult stage. Shell boring worms, Polydora sp., and sponges burrow into the shells and weaken them, thus causing mortalities.

Probably the major cause of mortalities in bivalves, particularly of larvae and juveniles in hatcheries, is disease. Considerable research effort has been expended in studying bivalve diseases and trying to develop methods to control them.
Diseases can be devastating to adult bivalves as witness the demise of some populations in the world. A few examples include,
Dermocystidium:
a fungal disease of bivalves caused by Perkinsus marinus;
Delaware Bay Disease (MSX):
a disease caused by the haplosporidian protozoan, Haplosporidium (Minchinia) nelsoni;
SSO (seaside organism disease):
a disease caused by the haplosporidian protozoan, Haplosporidium costale, (which together with H. nelsoni has decimated large populations of Virginia oysters on the Atlantic coast of the USA and now extends northwards into Atlantic Canada).
Aber Disease:
A disease caused by the protozoan, Marteilia refringens;
Bonamiasis (Haemocytic Disease):
A disease caused by the microcell parasite, Bonamia ostreae;
(Aber disease and Bonamiasis have resulted in the virtual demise of the European oyster in some parts of Europe).
Although considerable work has been carried out on these diseases, no practical methods have been developed to control them and restore oyster populations to previous levels. The severity of these diseases points to the care that must be taken when transporting adult bivalve stock into a hatchery.
In hatcheries it appears that diseases which do occur are caused by bacteria and not by protozoans. Bacteria are present to some degree in both algal and larval cultures. Indeed, bacteria may form an important part of the diet of larvae. However, periodically, large groups of larvae will die suddenly and an entire culture is lost. High bacterial counts are almost always associated with such large-scale mortalities. Bacteria may cause mortalities (pathogenic) or they may be simply present as opportunistic bacteria (saphrophytic), feeding on the dying larvae. Bacteria that cause diseases largely belong to the genus Vibrio sp. and every precaution must be taken to prevent them from causing epidemics in hatcheries. The best method to prevent such epidemics is to observe strict hygienic procedures and ensure that the larvae are well fed with high quality food. Larvae should be inspected regularly. If a disease occurs or is suspected, tanks and equipment should be sterilized with a bleach solution and rinsed well with freshwater. To protect larvae from further contamination, tanks should be refilled with UV-irradiated or ozone treated seawater. Use of antibiotics to control diseases is largely avoided in hatcheries. They are expensive and add to cost of operations and also there is the fear of a strain of bacteria developing that will be resistant to the antibiotics, which could lead to even more severe disease problems in the future.