Color profile: Disabled Composite Default screen 5 Non-infectious Disorders of Coldwater Fish David J. Speare Department of Pathology and Microbiology, Atlantic Veterinary College, Charlottetown, Prince Edward Island C1A 4P3, Canada Introduction Failure of Juvenile Salmon to Adapt to Marine Culture The use of cage culture technology for the commercial on-growing of finfish has The phase in which salmon smolts are proven to be economically efficient and moved from their freshwater rearing sites to it continues to expand. Part of what the marine cage culture on-growing site is a separates this production approach from period of high risk. This is also the case for land-based technologies is the range of transfer of steelhead rainbow trout juveniles non-infectious diseases that confront for on-growing in marine cages (Oorschot producers and health professionals. The and Boon, 1993). The Atlantic salmon principal Achilles’ heel of cage culture hatchery industry has largely geared its is the minimal degree of control, beyond activity towards production of a seawater- that afforded by cage site selection, over ready smolt in 1 year from egg hatching. environmental phenomena. This S1 smolt comes from the upper-modal Coldwater cage culture is dominated by growth population of juveniles in a hatch- the production of salmonid species such ery, and is judged for seawater-readiness as Atlantic salmon (Salmo salar), chinook based on anatomic, behavioural and physio- salmon (Oncorhynchus tshawytscha) and logical characteristics. Variations on the steelhead trout (Oncorhynchus mykiss) theme exist. For example, entry of smolts to in marine environments. The objectives of seawater can take place during their first this chapter are to focus on non-infectious autumn (S0.5), second autumn (S1.5) or diseases that interact with this segment of second spring (S2). As yet there is no the finfish aquaculture industry. Commer- consensus on mortality rates to be expected cial cage culture of flatfish species, cod and with each regime; however, autumn entry other marine coldwater species is develop- success is usually less than for spring entry. ing in importance, as is cage culture of Chinook salmon, depending on the strain rainbow trout and Arctic charr (Salvelinus being used, can be put to seacages as alpinus) in freshwater lakes or brackish S0 smolt (i.e. entry in the spring of the water bays. Some of the conditions year the eggs hatched). However, this discussed are also appropriate to the practice has been anecdotally implicated as developing culture of these coldwater the cause of higher mortality rates, particu- species. larly from infections with Renibacterium ©CAB International 2002. Diseases and Disorders of Finfish in Cage Culture (eds P.T.K. Woo, D.W. Bruno and L.H.S. Lim) 171 181 Z:\Customer\CABI\A4337 - Woo\A4419 - Woo Vouchers.vp Tuesday, November 05, 2002 11:34:43 AM Color profile: Disabled Composite Default screen 172 D.J. Speare salmoninarum (the causative organism to cage sites after handling may compound of bacterial kidney disease) during the the oxygen debt that fish experience, espe- remainder of the marine production cycle. cially since high loading rates (to reduce A number of factors can lead to smolt the weight of transported water) of fish mortalities soon after fish are transferred to are frequently used during shipment. Acute seawater. With spring transfer of S1 Atlantic post-transfer mortality is also linked to salmon smolts, a mortality rate during the the physiological and osmotic demands that first month post-transfer of up to 3% may not develop if smolts are handled so roughly that be regarded as unusual. This derives from an scale loss occurs. Smolts are particularly inability of some juveniles to adapt properly prone to losing scales, and this in turn to sea water. Mortality patterns after seawa- creates significant osmoregulatory problems ter entry can vary dramatically, and their on introduction to seawater. Smolt loss can analysis can be used to point to potential also occur if newly introduced juveniles aetiologies. For instance, when mortalities encounter strong currents. In reviewing occur shortly after seawater introduction, diagnostic case material from Atlantic this usually points to problems stemming Canada (Aquatic Diagnostic Services case from fish handling and transport methods. archives 1990–1997, University of Prince Acute patterns of mortality shortly after Edward Island, Canada), there have been shipment frequently stem from anoxic con- several submissions of smolts with severe ditions developing during transportation. skin lesions from net abrasions and The window of opportunity just prior to exertional muscle necrosis (Fig. 5.1) follow- smolt movement has been viewed as a ing their introduction to marine cages where period in which health checks, treatment, water current was excessive. The type and vaccination, grading and inventory assess- pattern of muscle damage are interesting ment can take place. Handling of fish is a in that ‘fingerprint lesions’, which can be stress that results in elevated oxygen con- misdiagnosed as nutritional deficiency, can sumption in the periods following handling persist in survivors. Early muscle lesions (Davis and Schreck, 1997). Transporting fish include various forms of degeneration, such Fig. 5.1. Section of epaxial musculature from an Atlantic salmon post-transfer smolt several weeks after introduction to a cage site with excessive water current speed. Extensive satellite cell, myoblast and fibroblast proliferation have replaced areas vacated by muscle fibre necrosis and dissolution. Regenerative fibres, with characteristic nuclear rowing (arrows indicate several examples), are abundant. H&E stained. 182 Z:\Customer\CABI\A4337 - Woo\A4419 - Woo Vouchers.vp Tuesday, November 05, 2002 11:34:44 AM Color profile: Disabled Composite Default screen Non-infectious Disorders of Coldwater Fish 173 as segmental and discoid myofibrillar condition deterioration and often succumb changes. Fibre necrosis is followed by to infectious diseases including those not mineralization. Survivors show a range of typically problematic in marine environ- muscle changes including presence of ments, such as costiasis (Figs 5.2 and 5.3). macrophages removing cellular debris, The role of these stressed fish as proliferation of satellite cells, myoblast bioamplifiers of disease such as sea lice elongation and fibre regeneration (Fig. 5.1). infestations, furunculosis and vibriosis is The scale and scope of post-transfer felt to be an important factor in the mortalities are of concern to the cage culture epizootiology of disease outbreaks. The term industry. In vertically integrated operations ‘lice-magnets’, to describe the effect of these there is considerable opportunity to coordi- fish prior to sea lice epizootics, has been nate protocols aimed at reducing transfer coined in reference to this effect. stress. This can involve more precise timing The rapid changes in environmental of pre-transfer activities (vaccinating, indices experienced by smolt at seawater grading), the timing of transfer itself entry have been linked with the develop- and avoiding delays during transfer. ment of epithelial hyperplastic plaques Protocols for safe smolt transfer, taking (Nowak and Munday, 1994) and increased into account pre-shipment, shipment and numbers of mucous cells (Franklin, 1990) on post-introduction activities, have been the gill filaments of Atlantic salmon. Envi- developed (see Pennell, 1991, for specific ronmentally induced precursor lesions are information). often cited as predisposing causes for In contrast to the acute mortality peaks, infectious gill conditions. With relevance which can reflect transportation problems, a to marine culture of salmonids, these post- more gradual onset of post-transfer mortality entry lesions are proposed to create a favour- is widely (but anecdotally) attributed to the able environment for the establishment of problem of partial adaptation of non-smolts Paramoeba sp. infections responsible for to the marine environment. These fish do not amoebic gill disease (AGD) in Tasmania die at the time of transfer, but fail to regulate (Nowak and Munday, 1994). An idiopathic their blood electrolyte levels effectively. gill condition known as clubbing and necro- These fish are highly stressed, they darken sis gill syndrome (CNG), which may also and hang listlessly at the water surface and act as a predisposing factor for AGD, has towards the edges of cages. Over an extended recently been described for Atlantic salmon period, they undergo advanced body smolt in brackish water cage sites in Fig. 5.2. Section of gill taken from a ‘pinhead’ Atlantic salmon smolt several weeks after seawater transfer. Virtually all lamellae are fused to adjacent lamellae in a pattern typical for costiasis. H&E stained. 183 Z:\Customer\CABI\A4337 - Woo\A4419 - Woo Vouchers.vp Tuesday, November 05, 2002 11:34:45 AM Color profile: Disabled Composite Default screen 174 D.J. Speare Fig. 5.3. Scanning electron micrograph of a gill lamellar surface, which is colonized by flagellated protozoans with a morphology typical of Ichthyobodo necator. Tasmania (Clark et al., 1997). During CNG, will accentuate differences in specific fish have diminished feed response and growth rate during the remainder of the pro- affected fish have pale gills due to excessive duction cycle. Harvesting a cage that has a branchial mucus production. As yet, the spread in size and condition affects market- specific