ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on January 31, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(2):171-173; doi:10.1093/icesjms/fsm188
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Prevalence, abundance, and distribution of Lepeoptheirus salmonis (Krøyer, 1837) and Caligus elongatus (Nordmann, 1832) on wild sea trout Salmo trutta L.
Fisheries Research Services Marine Laboratory, PO Box 101, 375 Victoria Road, Torry, Aberdeen AB11 9BD, UK
Correspondence to K. Urquhart: tel: +44 1224 295609; fax: +44 1244 295667; e-mail: urquhartk{at}marlab.ac.uk
Urquhart, K., Pert, C. C., Kilburn, R., Fryer, R. J., and Bricknell, I. R. 2008. Prevalence, abundance, and distribution of Lepeoptheirus salmonis (Krøyer, 1837) and Caligus elongatus (Nordmann, 1832) on wild sea trout Salmo trutta L. – ICES Journal of Marine Science, 65: 171–173.Lepeophtheirus salmonis (Krøyer, 1837) and Caligus elongatus (Nordmann, 1832) were found on 100% and 90%, respectively, of 30 wild sea trout from the east coast of Scotland. Mean abundances of the same two sea lice were 7.8 and 7.7, respectively. The distribution of the two species of sea louse differed, however, with a greater proportion of L. salmonis in the posterior and anterior dorsal regions, and a greater proportion of C. elongatus in the caudal and posterior ventral regions.
Keywords: abundance, Caligus elongatus, distribution, Lepeophtheirus salmonis, prevalence, sea trout
Received 25 May 2007; accepted 7 December 2007; advance access publication 31 January 2008.
 |
Introduction |
|---|
 Top Introduction MethodsResultsDiscussionReferences |
|---|
Few surveys of wild sea trout (Salmo trutta) have considered the abundance and species of sea louse present on the fish. Some studies have reported the presence of sea lice, but few have quantified the different species of louse (MacKenzie et al., 1998; Schram et al., 1998). A survey in 1998 in Scottish waters, primarily on the west coast, found that Lepeophtheirus salmonis were more abundant on wild sea trout than Caligus elongatus. The developmental stages of each louse depended on the time of year, with reported lice loads higher on sea trout caught near Atlantic salmon (Salmo salar) farms (MacKenzie et al., 1998).
Here, we quantify the abundance and distribution of L. salmonis and C. elongatus on wild sea trout in the North Esk estuary in Angus, Scotland. Analysing the distribution of sea lice on individual sea trout from a wild population provides a novel approach to understanding the interaction of these pathogens with its sea trout host in an area remote from marine fish farming.
|
Methods |
|---|
|
TopIntroductionMethodsResultsDiscussionReferences |
|---|
In all, 30 wild sea trout (mean weight 1.16 kg ± 0.32 s.d.) were obtained from bag nets near the North Esk (NGR NO 742 625) on the Scottish east coast in May and June 2005 (Pert et al., 2006). Each fish was euthanized in the field, placed in an individual polythene bag, labelled, and returned to the laboratory in a cool box containing a freezer pack. At the laboratory, the body surface, skin, gills, and fins were examined for ectoparasites. The positions of external parasites were recorded using the zoning method of O'Shea et al. (2006), which divides the fish surface into 12 regions.
The prevalence (%) of L. salmonis and C. elongatus was estimated, with 95% confidence intervals (CIs), using the binomial distribution function. Inferences about the abundance and distribution of L. salmonis and C. elongatus were based on re-sampling methods (Efron and Tibshirani, 1993), which account for correlations in louse numbers within fish, and variation in louse numbers between fish. The mean abundance of L. salmonis and C. elongatus was estimated by the sample mean, with CIs on the means and on the difference in the means obtained by bootstrapping over fish. Differences in distribution (the proportion of lice in each part of the fish) were assessed by fitting a multinomial generalized linear model (McCullagh and Nelder, 1989) to the counts of lice in each part, first allowing the distribution of each species to vary, then constraining them to be the same, and calculating an F-statistic from the model deviances. This was then compared with a reference distribution obtained by permuting the counts of L. salmonis and C. elongatus within fish and recalculating the F-statistic 1000 times. CIs on the proportion of lice in each region, and on differences in these proportions, were obtained by bootstrapping over fish.
|
Results |
|---|
|
TopIntroductionMethodsResultsDiscussionReferences |
|---|
There were 235 L. salmonis and 230 C. elongatus in total, with at most 24 L. salmonis and 41 C. elongatus on any one fish. All lice were pre-adults or adults. The prevalence of L. salmonis and C. elongatus was 100% (95% CI, 88–100%) and 90% (95% CI, 73–98%), respectively. The abundances of L. salmonis and C. elongatus were correlated across fish (Spearman rank correlation 0.64, p = 0.0002), with means of 7.8 (95% CI, 6.0–10.0) and 7.7 (95% CI, 4.6–11.3), respectively. The mean abundances did not differ significantly (p = 0.84).
The total numbers of lice in each part of the fish are given in Table 1. The distributions of L. salmonis and C. elongatus were different (p < 0.0001), a greater proportion of L. salmonis being found in the posterior and anterior dorsal regions (p < 0.0001 and p = 0.026, respectively), and a greater proportion of C. elongatus in the caudal and posterior ventral regions (p = 0.002 and p = 0.013, respectively) (Figure 1).
|
|
|
Discussion |
|---|
|
TopIntroductionMethodsResultsDiscussionReferences |
|---|
Wild sea trout taken from the North Esk estuary in May and June 2005 had mean abundances of L. salmonis and C. elongatus of 7.8 and 7.7, respectively. This contrasts with a survey in May and June 1995 of trout from the South Esk, about three miles from the North Esk, which found no C. elongatus and a mean abundance of L. salmonis of 0.8 (MacKenzie et al., 1998). However, such variations in numbers are within the limits of natural levels of infection (Bjørn et al., 2001).
Experimental studies have shown that the preferred settlement sites of the chalimus stage of L. salmonis are on the gill and fins, but pre-adult and adult stages prefer the head and dorsal regions (Bjørn and Finstad, 1998). In this study, L. salmonis was not found in the head region, but a large proportion of the L. salmonis, all pre-adult or adults, was found in the dorsal regions.
The two species of louse occupied different regions on the fish, L. salmonis adopting more dorsal and C. elongatus more ventral and caudal positions. Factors that might affect louse settlement include the body size and shape of the sea trout, water flow, structural differences in the epidermis, differences in the size of the two species of louse (Jaworski and Holm, 1992; Dawson et al., 1997), and interspecific behavioural differences (J. Bron, pers. comm.).
To conclude, these preliminary findings concerning the natural distribution patterns of L. salmonis and C. elongatus on wild sea trout captured on the east coast of Scotland show almost equal numbers of the two species of louse co-existing on the same fish. The preferred settlement sites differ between louse species, although whether this results from behavioural or morphological differences of the lice or from interspecies competition cannot be ascertained from our data. The numbers of lice we observed in this area of the North and South Esk, east coast of Scotland, is greater than the numbers reported nine years ago, but are still within the range for natural levels of infection.
|
Acknowledgements |
|---|
We thank James Bron for advice on the draft manuscript, Una McCarthy for commenting on it, and Bob Ritchie and the staff at the Montrose netting station for assisting with the sampling of sea trout.
|
References |
|---|
|
TopIntroductionMethodsResultsDiscussionReferences |
|---|
-
Bjørn P. A., Finstad B. The development of salmon lice (Lepeophtheirus salmonis) on artificially infected post smolts of sea trout (Salmo trutta). Canadian Journal of Zoology (1998) 76:970–977.
Bjørn P. A., Finstad B., Kristoffesen R. Salmon lice infection of wild sea trout and Artic char in marine and freshwaters: the effects of salmon farms. Aquaculture Research (2001) 32:947–962.[CrossRef][Web of Science]
Dawson L. H. J., Pike A. W., Houlihan D. F., McVicar A. H. Comparison of the susceptibility of sea trout (Salmo trutta L.) and Atlantic salmon (Salmo salar L.) to sea lice (Lepeophtheirus salmonis (Krøyer, 1837)) infections. ICES Journal of Marine Science (1997) 54:1129–1139.
Efron B., Tibshirani R. J. An Introduction to the Bootstrap (1993) London: Chapman and Hall.
Jaworski A., Holm J. C. Distribution and structure of the population of sea lice, Lepeophtheirus salmonis Krøyer, on Atlantic salmon, Salmo salar L. under typical rearing conditions. Aquaculture and Fisheries Management (1992) 23:577–589.
MacKenzie K., Longshaw M., Begg G. S., McVicar A. H. Sea lice (Copepoda: Caligidae) on wild sea trout (Salmo trutta L.) in Scotland. ICES Journal of Marine Science (1998) 55:151–162.
McCullagh P., Nelder J. A. Generalized Linear Models (1989) 2nd edn. London: Chapman and Hall.
O'Shea B., Mordue-Lundtz A. J., Fryer R. J., Pert C. C., Bricknell I. R. Determination of the surface area of a fish. Journal of Fish Diseases (2006) 29:437–440.[CrossRef][Web of Science][Medline]
Pert C. C., Urquhart K., Bricknell I. R. The sea bass (Dicentrarchus labrax L.): a peripatetic host of Lepeophtheirus salmonis (Copepoda: Caligidae)? Bulletin of the European Association of Fish Pathologists (2006) 26:162–164.
Schram T., Knutsen J. A., Heuch P. A., Mo T. A. Seasonal occurrence of Lepeophtheirus salmonis and Caligus elongatus (Copepoda: Caligidae) on sea trout (Salmo trutta), off southern Norway. ICES Journal of Marine Science (1998) 55:163–175.
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||