The study of marine fish diseases has been directly related in its development to the significance of the fisheries or latterly aquaculture systems involved with epizootics. In this respect, Scotland has played a central role because of the importance of specific diseases of salmon which have been especially prevalent in Scotland, and the advanced state of its basic veterinary and medical expertise. This has been particularly important in the past, when very little information was available and extrapolation from higher animal studies was the only relevant approach.

Herpesviruses and viruses having the typical morphological and replicative characteristics of the genus Herpesvirus from marine animals have been reviewed. These animals include one teleost fish, two reptiles, one pelecypod mollusc and an arthropod. With the exception of the arthropod, all descriptions of virus-related diseases were from animals maintained under artificial culture conditions and a common theme relates temperature, stress and crowding to outbreaks of disease. The significance of these observations in relation to the possible evolution of the herpesvirus group is discussed.

Viral erythrocytic necrosis (VEN) in fish is prevalent in a wide variety offish species. It has been detected in both wild and captive fish populations. The infection prevalence in wild populations of some fish species is correlated inversely with the age of the fish. VEN is characterised by cytoplasmic inclusions in circulating erythrocytes of infected fish; nuclear necrosis of erythrocytes may also be evident by light microscopy. The stage and degree of infection affect the type of cytology seen in VEN infections of different fish species. By electron microscopy, infected erythrocytes show spherical virions within the cytoplasm and the virion size is characteristic of the host erythrocytes infected. To date, knowledge of VEN viruses in fish is restricted to virus morphology and propagation in vivo, although preliminary studies have indicated the type of blood parameters which VEN can affect. No such virus has been fully isolated and characterised and all of Koch's postulates have not been fulfilled.

Furunculosis has caused serious economic losses in some Atlantic salmon farms in Scotland. It is considered that the disease has the potential to be a significant economic problem to the whole industry unless control measures are adopted. When salmon in a farmed population carry the disease agent in non-clinical form the bacterium has low to intermediate virulence, but in stocks with recurring clinical disease, virulence is apparently increased. The significance of these observations is discussed in terms of the major methods of control, namely avoidance, husbandry procedures, chemotherapy, vaccines and statutory controls.

The general biology and pathology of Lepeophtheirus salmonis and Caligus elongatus and the prevention and treatment of such ‘sea-lice’ infestations on farmed salmonids are described from the literature and original observations. The life-cycle of L. salmonis and probably also that of C. elongatus comprises the egg and 10 stages separated by moults, namely, two nauplius, one infective copepodite, four chalimus, two pre-adult and the adult (male and female) stages. Water temperature greatly affects the rate of development, especially for early larval stages. Heavy infestations of wild fish seem rare, and lice are lost fairly rapidly in freshwater. In Scotland at least L. salmonis shows a succession of generations on farmed salmonids; generation time is about six weeks at 9–12 C. Post-chalimus stages of C. elongatus may exchange between farmed salmonids and wild fish (especially gadoids). Epizootics (particularly with L. salmonis) cause great damage to salmonids in Norwegian and Scottish farms largely through feeding on host skin. The dermis is oedematous and haemorrhaged where lice feed, and blood seeps between scales; deaths probably result from osmoregulatory failure. Whilst prevention of infestation is difficult, a bath treatment for 1 h with 1 ppm of the organophosphorus compound Dichlorvos is effective against post-chalimus stages of L. salmonis on caged salmonids. Side effects are minimal and clearance rates from fish tissues satisfactory, but treatment may be required every 3–4 weeks.

The general epidermal morphology of teleost fish is described and comparison made in terms of its vulnerability to ultraviolet radiation (UVR) with that of the human. Whereas human skin has a natural pigmentary photoprotective (tanning) mechanism within its epidermis, in teleosts the protective pigment layer is located primarily within the upper dermis. The potential impact of solar UV on farmed fish is discussed both in relation to water penetration of UVR and the phenomenon of photosensitisation, a condition common in higher animals and which has now been shown to induce skin lesions in fish. The results of recent investigations into photosensitisation in fish are described.

The development of various secondary sexual characteristics in salmonids is described, together with a method of preventing sexual maturation by surgical gonadectomy. The major problem in the application of this technique is the regeneration of male testicular tissue, and it is suggested that the practical solution is the use of hormonal feminisation techniques prior to gonadectomy.