Publikasjon

Tittel Sea Lice Working Group Report
Undertittel
Forfattere Revie , C., Dill , L., Finstad, B. & Todd , C.
År 2009
Kilde NINA Temahefte 39: 117 pp. Norsk institutt for naturforskning (NINA), Trondheim.
ISBN, ISSN 978-82-426-2048-4 0804-421X
Referat

The collective term “sea lice” is colloquially used to refer to numerous species of copepod crustaceans of the family Caligidae that are externally parasitic on the skin of marine and anadromous fishes. The most intensively studied species - Lepeophtheirus salmonis - is, as its specific name implies, a specialist parasite of salmonid fishes. It is commonly associated with a total of 12 host salmonid fish species of the genera Salmo, Oncorhynchus and Salvelinus in the Pacific and Atlantic Oceans. Along the Pacific coasts of Alaska and British Columbia, L. salmonis as well as Caligus clemensi and Lepeophtheirus cuneifer (both host generalist lice species) have been recorded on wild and farmed salmonids. In British Columbia, Chile and Tasmania Atlantic salmon (Salmo salar) is the principal salmonid species in culture. While the Tasmanian industry apparently suffers no especial problems from caligid infestation, the Chilean industry has been heavily impacted by Caligus species, initially C. teres but more recently and significantly C. rogercresseyi (both of which are host generalists). In Japan, Caligus orientalis is the most pathogenic sea louse on cultured Pacific salmon, although L. salmonis also remains a problem. L. salmonis is associated with wild chum and pink salmon in Japan, but also infests cultured coho salmon and rainbow trout. C. orientalis – like C. elongatus in the North Atlantic – is a host generalist; C. orientalis occasionally impacts salmonids, and it is an especial problem to cultured rainbow trout. The current scientific literature refers to Lepeophtheirus salmonis Krøyer as occurring on salmonids in both the North Pacific and North Atlantic Oceans. At first sight it might appear curious that the same species should occur in two separate and geographically distant oceans, but there is convincing geological, molecular and ecological evidence of past trans-Arctic connectivity of the marine fauna of the North Pacific and North Atlantic basins — and specifically of Pacific species having tended to colonize the North Atlantic rather than vice versa — following the recent opening of the Bering Strait (~5 million years ago). The presently available molecular (DNA) results cannot provide conclusive evidence regarding the specific status of Pacific and Atlantic L. salmonis, but recent DNA sequence analyses do indicate clear genetic distinction between the Atlantic and Pacific lineages. Similarly, there now is a body of mitochondrial DNA sequence and ecological (host association) data indicating that the host generalist parasite, Caligus elongatus, actually comprises taxonomically separable entities. It is, however, too early to affirm that there are definitely two (or more) species of either “C. elongatus” or “L. salmonis”; for clarity and consistency with the contemporary scientific literature we continue in the present report to refer to single species in both cases. All female caligids undergo internal fertilization of the eggs prior to their extrusion into a pair of external egg sacs or “eggstrings”. The nauplius I is the hatching stage and at this point the eggstring disintegrates and the larvae are released to become planktonic. The nauplius I molts into a nauplius II and then again into the infective copepodid stage. None of the three planktonic stages feed; all the reserves the larvae require to complete development to the infective copepodid are provided by the parent female. Initial attachment for the copepodid typically occurs on the fins of the host fish (especially the dorsal, ventral and anal fins) or to the scales. Several chalimus stages follow, attached to the host by a sort of tether. Depending upon the species of caligid there then may be two, one, or no pre-adult stages between chalimus IV and the mature adult. The pre-adult and adult stages are all mobile, i.e., able to move about on the host fish’s body. When large numbers of farmed salmon are introduced to the marine environment in open net cage salmon farms, three things are virtually inevitable for these fish: ? they will become hosts to sea lice (Lepeophtheirus spp. and/or Caligus spp.) since these occur naturally on wild host species in the vicinity of most farms; ? they will become part of a dynamic host-parasite system involving wild hosts, because they can produce large numbers of infective larvae in a restricted spatial area if gravid females are allowed to develop; and ? because they carry sea lice, and because some of these fish may escape from the farms, the dispersal of parasites is likely to be even more widespread on occasion. Given the above, it may be concluded that it is next to impossible to (1) avoid infection of farmed fish, all of which go into the pens as clean smolts, and (2) also subsequently avoid infection of wild fish that are found in the vicinity (“infective field”) of an open cage farm. A parasitic infection becomes a disease when host behavior and physiology (and ultimately host health, survivorship and fitness) are altered or compromised to an exceptional extent. For example, the increased metabolic demand exerted by the parasite may cause slower host growth, making the wild fish more likely to be captured by predators, or conversely causing them to take greater risks to feed, with the same end result. Reduced host condition also can affect swimming ability, with several negative ecological consequences ranging from reduced competitive ability to slower migration. Slower migration rates through coastal waters might elevate the risk of infestation by sea lice copepodids. Skin damage caused by the feeding behavior of sea lice can increase the physiological cost of osmotic regulation, or provide sites for secondary bacterial or fungal infection. Contrary to typical perceptions that it is not “in the interest” of parasites to kill their host, it is worth noting that sufficiently high sea lice loads will kill individual wild fish, but the definition of “high” will depend on sea louse stage, fish size and developmental stage. While not exhaustive, this list illustrates some of the direct and more subtle indirect ways that sea lice may cause disease, as defined above. Sea lice abundance on farmed salmon only rarely attains levels where the health or welfare of these fish is negatively affected. When this occurs there are legal and regulatory requirements in many countries that demand treatment, but it is clearly also in the economic interests of the farmer to treat the infection promptly and effectively. For wild fish, disease is likely to be an issue whenever sea lice intensity on individual hosts is sufficiently high as to cause significant stress, or to increase their vulnerability to secondary pathological infection or other mortality agents, as noted above. For example, newly-migrated smolts exposed to the challenge of osmoregulating in saline waters will be physiologically stressed by that environmental challenge and will be more vulnerable than post-smolts that are older and fully adapted to seawater. Sea lice disease of wild salmonids is potentially problematic in areas with intensive Atlantic salmon aquaculture in British Columbia, Canada, on the west coasts of Ireland and Scotland, and throughout Norway (Chile lacks endemic species of anadromous salmonids). In British Columbia, the focus of attention has been on the much studied and discussed Broughton Archipelago region, where there is particular concern regarding the impact of Lepeophtheirus salmonis on wild stocks, particularly juvenile pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon. Unlike Atlantic salmon, sea trout (the anadromous form of the brown trout, Salmo trutta) spend extended periods of time in nearshore or coastal waters, and this feature may render them particularly vulnerable to sea lice infestation. As has been the case in British Columbia, analyses concerning the potential interaction between farmed and wild salmonids in Irish bays, Scottish sea lochs and Norwegian fjords subject to intensive aquaculture have not been without controversy. The circumstantial evidence of farm-produced larval sea lice contributing to parasite loadings on wild sea trout in Ireland is considerable. Correlations have been drawn between abundances of sea lice on wild sea trout and on Irish farms up to 30 km distant. As is the case for sea trout, Arctic charr (Salvelinus alpinus) are effectively confined to coastal waters (often in narrow fjord systems) and these areas commonly are home to a high density of captive farmed salmon. Sea trout and Atlantic salmon are also the species of most concern with regard to detrimental effects of sea lice in Norway, although Arctic charr also are impacted by these parasites there. It has been demonstrated that salmon within a given farm site can be self-reinfesting (because hatched nauplii drift back into the net pens having completed their development to the infective copepodid stage); it is also intuitive that nauplii exported from one farm site will infect salmon being grown in neighbouring farms or free-ranging wild fish in the vicinity. Similarly, wild fish may well infect other wild fish or, if they are resident in coastal waters, adjacent farmed fish. The absolute abundances of farm and wild fish, the absolute abundances of sea lice on those fish and the relative strengths of farm-farm and farm-wild interactions (and any seasonal or annual variation thereof) will determine the overall infestation pressure on individual fish in a given locality. It is far easier to monitor and assess the outcome of interventory treatment for sea lice infestations for farmed fish than it is for wild fish, and there are potentially many more strategies available to control sea lice on farmed fish. For example, in addition to medicinal treatments, a variety of management (and even informed environmental) decisions can be made which can have impacts on the control of sea lice on farms. The challenges in managing sea lice on farmed and wild fish in an integrated manner should not, however, be underestimated. As we outline, the evidence is largely indirect or circumstantial that sea lice emanating from salmon farms can and do exert detrimental effects on wild salmonids. It is practically impossible to track larvae from release to host colonization and therefore to precisely quantify wild-to-farm versus farm-to-wild and wild-wild infestation interactions. Furthermore, in view of the diversity of life-history strategies and differential vulnerability of host species associated with sea lice in both the Pacific and Atlantic Oceans, as well as the geographic differences in the intensity of the industry and its regulation, it is not plausible to draw a single over-riding conclusion regarding the potential negative impacts of sea lice on all wild fish stocks worldwide. Nevertheless, we believe that the weight of evidence is that sea lice of farm origin can present, in some locations and for some host species populations, a significant threat. Hence, a concerted precautionary approach both to sea lice control throughout the aquaculture industry and to the management of farm interactions with wild salmonids is expedient. It is arguably the case that sea lice are one of the most studied diseases of aquaculture and, as such, thinking in terms of ‘system-wide’ management has been relatively well developed. The principles of Integrated Pest Management (IPM) have been taken from the terrestrial setting and attempts made to apply them to sea lice in an inclusive and comprehensive fashion. In addition, as mentioned earlier, sea lice infestation on salmon farms has been a matter not only of control on farms to maximize cultured fish health and well-being, but of significant public and scientific controversy; these issues arose initially and most notably with respect to wild sea trout populations in Ireland and Scotland, Atlantic salmon and sea trout in Norway and, more recently, for the case of wild Pacific salmon in British Columbia. Management of wild-farm and farm-farm infestation interactions is not a simple challenge, if only because of our present inability to reliably quantify them. Given the impossibility of directly observing and tracking individual sea lice larvae from release by the adult female to ultimate settlement on a host fish, alternative indirect analytical approaches have proven necessary to specifically assess farm-wild interactions. The utility and limitations of these various empirical methods (e.g. molecular genetics and stable isotope markers) has been reviewed. A conceptually different, but complementary, analytical approach has been the development of mathematical models to enable both a better understanding of infection dynamics and to aid decision makers in exploring assumptions regarding underlying management parameters and the effectiveness of potential intervention strategies. Once again, these models are much more diverse and complex than is typical for most pathogens within the aquatic setting. However, in further complicating the debate as to the importance of farm sources of infestation to wild fish, in a number of cases the models themselves have become a source of controversy. This is not necessarily a “bad thing” as it is arguably not the place of mathematical modeling to produce answers/solutions, but rather to encourage policy makers, commercial farmers, sport fishery managers, and scientists to think more carefully about their assumptions and the likely impact of various types of intervention. Another important issue relates to the optimal location of salmon farms; establishment of “safe sites” should lead to minimizing risks and maximizing benefits to all concerned parties. Indeed, research in this area has led to a number of recent projects – for example, the Hardangerfjord project in Norway or the Finite Volume Coastal Ocean Model in British Columbia – which have attempted to tackle aspects of the problem through the use of fjord/sea loch/archipelago–wide hydrographic modeling to improve our understanding of dispersal and colonization of sea lice larvae. This has also led to changes in policy, for example, in Scotland the Location/Relocation Working Group (LRWG) of the Scottish Government has the remit to, “prepare criteria to assess whether or not any finfish aquaculture site is poorly located, and make an assessment of the likely benefits and effectiveness of relocation of those farms that are sited close to rivers important for migratory fish” (http://cci.scot.nhs.uk/Topics/Fisheries/ Fish-Shellfish/whatwedo/whatwedo5).

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