APPENDIX
From: Costello, M.J. 2009. How sea lice from salmon farms may be the latest cause of wild salmonid declines in Europe and North America, and a threat to fish elsewhere.
FUTURE RESEARCH
Lepeophtheirus salmonis in the plankton
The studies on L. salmonis behaviour, dispersal models and field observations have made remarkable progress in understanding this species ecology, and the risks that farm infestations can pose to wild salmonids and other farms. More information on sea lice behaviour and host-interactions, and model refinement will further improve understanding of local situations. Models of lice dispersal need to be three dimensional to best predict the spread of larvae (Asplin et al. 2004; Gillibrand & Willis 2007; Gillibrand & Amundrud
2007). Dispersal models show that nauplii can in some instances be transported away from their point of origin (Amundrud & Murray 2009). These authors also note that better empirical data on larval swimming speed, depth distribution, salinity tolerance (or avoidance) and mortality rates are required. In addition, hydrographic models need to be extended for areas greater than 30 km, and field sampling of nauplii and copepodites is needed to test the models. Improvements to model accuracy to account for wind and freshwater forcing conditions are also necessary (Amundrud & Murray 2007). Similar models must also be developed for C. elongatus and C. rogercresseyi , and include knowledge on adult and larval dispersal behaviour. To date, the literature has assumed that lice dispersal is primarily by planktonic larvae without any assessment of the ability of pre-adult and adult lice to find new hosts. Distinguishing between larvae of caligid species can be difficult by morphology but is
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possible using PCR (polymerase chain reaction) methods (McBeath et al.
2006). This research will help explain how lice infestations from farm and wild sources may impact both farm and wild fish populations, and thus what management measures may decrease pathogenicity and the need for parasiticides. It will also aid the selection of new sites for sea cages, including the impact on lice abundance of moving farms from one location to another.
Relationship to farm production
There may also be a positive relationship between the scale of salmon production in a region and sea lice larval abundance, as found in a sea loch by Penston & Davies (2008a, b). When lice parasiticides are applied based on estimates of the average number of lice per fish rather than the total number of lice on the farm, then the number of ovigerous lice remaining will be larger on farms with more fish. As more farms develop in an area then lice transmission between farms can compromise individual farm efforts to control sea lice. The countries that produce most farmed salmonids have a lice problem (i.e. Canada, Chile, Faeroes, Ireland,
Norway, UK), whereas the minor producers do not (Costello 2009). Yet production is unlikely to be the only factor involved in problems with sea lice. In Japan, lice do not become sufficiently abundant on salmon to become pathogenic because the fish are only held in sea pens for eight months (Ho & Nagasawa 2001). However, over ten times more yellowtail than salmon are farmed in Japan and C. spinosus is pathogenic to them (Ho 2004). In Australia and Finland, and regions of other countries, sea pens are in estuarine conditions where low salinity prevents lice abundance reaching pathogenic levels (Costello 2009). Quantitative models of the relative effects of farm size, distance between farms, lice reservoirs on wild fish, and local environmental conditions that affect sea lice production (e.g. temperature, hydrography), are needed to prioritise measures to reduce sea lice impacts. However, where lice abundance on farms and adjacent wild fish populations are correlated, a precautionary
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approach to protect the wild fish would be to control lice on the farm to a level that would avoid significant risk to the wild population.
Caligus ecology and dispersal
Species of the genus Caligus are increasing in importance as finfish aquaculture develops around the world. Five Caligus species already impact farmed fish in Europe, British
Columbia, Chile, Japan, south Australia, and south-east Asia (see above), yet only speculation is possible about how larval and adult behaviour has evolved to encounter hosts, and if their host-relationships are specific or hosts of convenience. Even their taxonomy is poorly known, with initial misidentification of the species parasitizing farms in Chile
(Boxshall & Bravo 2000), and new species regularly being described (e.g. Ho 2000; Ho et al.
2004).
Lice adaptations and evolution
The breadth and depth of knowledge on sea lice in comparison to other parasites, provides opportunities for understanding how parasites may influence the evolution of their hosts, and vice-versa. While retaining the typical copepod life-cycle, sea lice have life-stages specialised for parasitism, notably the sessile chalimus, and mobiles designed to swim over and suck onto their host (Costello 2006). Krkošek et al. (2007a) described how migration of Pacific salmonids into freshwater rids adults of sea lice, and avoids parasitism of juveniles until they migrate to sea. The movement of migrating Pacific and Atlantic salmon smolts away from the coast may reduce the risk of infestation by L. salmonis , a species that has evolved to intercept its salmonid hosts in estuaries. Although sea trout stay in coastal waters with increased infestation risk (Thorstad et al.
2004), their regular return to freshwater may ameliorate lice impacts. The more limited information on Caligus species, indicates that they have a different
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strategy to L. salmonis , involving less host specificity and more mobile adults. In comparison to L. salmonis, Caligus clemensi has a diverse range of hosts year-round in coastal British
Columbia, and has not been reported to reach pathogenic levels on migrating juvenile salmon
(Krkošek et al. 2007a).
Distribution on hosts
Copepodites and chalimii can attach to any part of the Atlantic salmon, but most commonly to the fins, and in laboratory situations to the gills (Treasurer & Wadsworth 2004). They are either lost from the host or move to the body as they mature. On farm Atlantic salmon and wild sea trout, mobile (adult and pre-adult stages) L. salmonis are most abundant on the head and back of the host, whether accompanied by C. elongatus or not (Jaworski & Holm 1992;
Treasurer & Wadsworth 2004; Urquhart et al. 2008; pers. obs.). In contrast, C. elongatus, at least when accompanied by L. salmonis , are more abundant ventrally and on the caudal peduncle and tail (Treasurer & Wadsworth 2004; Urquhart et al. 2008). The distribution of
Caligus species on their hosts in the absence of other sea lice species, has not been reported.
Their distribution on Pacific salmonids has also not been described, either with or without L. salmonis.
Lice have an aggregated (clumped) distribution on hosts under natural conditions (Costello
1993, Murray 2002). This may arise because a few hosts encounter many lice, most hosts loose lice through behavioural or immunological responses, or both mechanisms. Should lice reduce host reproductive success there may be selection for hosts to avoid areas where the infective stages congregate and to develop mechanisms to remove lice from their skin (e.g. rubbing on sand, splashing at water surface, inviting cleaner-fish); and for the parasite to increase host encounter rates by congregating where the hosts congregate. It would seem that
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L. salmonis larvae concentrate to intercept hosts migrating through estuaries, and that their hosts may move away from the coast to reduce infestation. However, the significance of other host mechanisms to reduce lice burdens is unclear.
Population parameters
The available data on lice populations, such as fecundity (eggs per egg string, number of egg strings), generation time, and longevity, on different host species, in laboratory, farm and wild situations, and at different sea temperatures, are useful but are too limited across host and parasite populations under different environmental conditions (reviewed in Costello
2006; Bravo et al.
2009). Replication of studies is necessary to develop population models that can be used to predict the risks of different lice species to farm and wild fish. Sea temperatures vary significantly between salmon growing areas (Costello 2006), and have been used in some lice transmission models (Murray & Amundrud 2007). Seasonal temperature variation already varies between years and can significantly increase lice abundance (Hewitt 1971; Tully 1992), and climate change is likely to alter the geographic ranges of sea lice and their hosts (Dulvey et al.
2008; Marcogliese 2008). Indeed, initial studies indicate that most parasite impacts will be greater on hosts in marine ecosystems due to climate induced temperature warming (e.g. Harvell et al.
2002; Poulin & Mouritsen 2006).
Consequent disturbance of the host-parasite opportunities may lead to parasites switching hosts (Hoberg & Brooks 2008), and farm fish are already providing a new host for native lice species in Chile. Temperature adjusted population models will enable prediction of the risk of sea lice to farm and wild fish populations both now, in different countries, and under future climate change scenarios.
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