Simon R. M. Jones

Salmon lice – impact on wild salmonids and salmon aquaculture

Salmon lice, Lepeophtheirus salmonis, are naturally occurring parasites of salmon in sea water. Intensive salmon farming provides better conditions for parasite growth and transmission compared with natural conditions, creating problems for both the salmon farming industry and, under certain conditions, wild salmonids. Salmon lice originating from farms negatively impact wild stocks of salmonids, although the extent of the impact is a matter of debate. Estimates from Ireland and Norway indicate an odds ratio of 1.1:1-1.2:1 for sea lice treated Atlantic salmon smolt to survive sea migration compared to untreated smolts. This is considered to have a moderate population regulatory effect. The development of resistance against drugs most commonly used to treat salmon lice is a serious concern for both wild and farmed fish. Several large initiatives have been taken to encourage the development of new strategies, such as vaccines and novel drugs, for the treatment or removal of salmon lice from farmed fish. The newly sequenced salmon louse genome will be an important tool in this work. The use of cleaner fish has emerged as a robust method for controlling salmon lice, and aquaculture production of wrasse is important towards this aim. Salmon lice have large economic consequences for the salmon industry, both as direct costs for the prevention and treatment, but also indirectly through negative public opinion.

THE OCCURRENCE OF LEPEOPHTHEIRUS SALMONIS AND CALIGUS CLEMENSI (COPEPODA: CALIGIDAE) ON THREE-SPINE STICKLEBACK GASTEROSTEUS ACULEATUS IN COASTAL BRITISH COLUMBIA

Infections with sea lice species belonging to Lepeophtheirus and Caligus are reported from examinations of 1,309 three-spine sticklebacks collected in coastal British Columbia. Over 97% of the 19,960 Lepeophtheirus specimens and nearly 96% of the 2,340 Caligus specimens were in the copepodid and chalimus developmental stages. The parasites were identified as Lepeophtheirus salmonis and Caligus clemensi based on morphology of adult stages. Between 1,763 and 1,766 base pairs (bp) of 18S rDNA from adult specimens collected from sticklebacks and salmon differed from the GenBank L. salmonis reference sequence by a single bp and were distinct from those of 2 other Lepeophtheirus species. A 530-bp region of 18S rDNA from chalimus stages of Lepeophtheirus obtained from sticklebacks and salmon was identical to that of the L. salmonis reference sequence. The three-spine stickleback is a new host record for L. salmonis. The prevalence of L. salmonis was 83.6% and that of C. clemensi was 42.8%. The intensities of these infections were 18.3 and 4.2, respectively. There was no significant relationship between sea lice abundance and stickleback condition factor. Significant spatial and temporal variations both in abundance of sea lice and surface seawater salinities were measured. The abundance of both sea lice species was lowest in zones in which surface seawater salinity was also lowest. Sticklebacks appear to serve as temporary hosts, suggesting a role of this host in the epizootiology of L. salmonis. The stickleback may be a useful sentinel species with which to monitor spatial and temporal changes in the abundance of L. salmonis and C. clemensi.

Early development of resistance to the salmon louse, Lepeophtheirus salmonis (Krøyer), in juvenile pink salmon, Oncorhynchus gorbuscha (Walbaum)

This study examined the effect of fish weight on the susceptibility of post-emergent pink salmon to Lepeophtheirus salmonis (Krøyer). Three trials were conducted, each with two stocks of pink salmon, Oncorhynchus gorbuscha (Walbaum), at starting weights of c. 0.3, 0.7 and 2.4 g, respectively. In each trial, duplicate tanks of fish were exposed to 0, 25 (only in Trial 1), 50 or 100 copepodids per fish. Mortality in Trial 1 was c. 37%, regardless of stock following exposures to 50 or 100 copepodids. Mortalities occurred up to 26 days after exposure, and more than 80% of the lice on the dead fish were chalimus stages. Infections with adult or preadult lice were observed on c. 35% of fish surviving to 37 days after exposure. Mortality was 5% in Trial 2 and there was no mortality in Trial 3. The abundance of L. salmonis was lower in Trial 3 compared with Trials 1 or 2. Histological changes in the skin coincident with fish growth included a thickening of the epidermis, infiltration of the dermis with fibroblasts by the end of Trial 1 and the first evidence of scales by the end of Trial 2; scales were evident throughout Trial 3. These results showed that the previously reported innate resistance to L. salmonis displayed by pink salmon develops in fish heavier than 0.3 g and appears to be functional by 0.7 g. This resistance coincided with changes to the epidermis and dermis, including the formation of scales. The present results indicate that elevated risk associated with L. salmonis infection among migrating post-emergent pink salmon may occur during a relatively brief period before the fish reaches 0.7 g.

EST and Mitochondrial DNA Sequences Support a Distinct Pacific Form of Salmon Louse, Lepeophtheirus salmonis

Nuclear deoxyribonucleic acid sequences from approximately 15,000 salmon louse expressed sequence tags (ESTs), the complete mitochondrial genome (16,148bp) of salmon louse, and 16S ribosomal ribonucleic acid (rRNA) and cytochrome oxidase subunit I (COI) genes from 68 salmon lice collected from Japan, Alaska, and western Canada support a Pacific lineage of Lepeophtheirus salmonis that is distinct from that occurring in the Atlantic Ocean. On average, nuclear genes are 3.2% different, the complete mitochondrial genome is 7.1% different, and 16S rRNA and COI genes are 4.2% and 6.1% different, respectively. Reduced genetic diversity within the Pacific form of L. salmonis is consistent with an introduction into the Pacific from the Atlantic Ocean. The level of divergence is consistent with the hypothesis that the Pacific form of L. salmonis coevolved with Pacific salmon (Onchorhynchus spp.) and the Atlantic form coevolved with Atlantic salmonids (Salmo spp.) independently for the last 2.5–11 million years. The level of genetic divergence coincides with the opportunity for migration of fish between the Atlantic and Pacific Ocean basins via the Arctic Ocean with the opening of the Bering Strait, approximately 5 million years ago. The genetic differences may help explain apparent differences in pathogenicity and environmental sensitivity documented for the Atlantic and Pacific forms of L. salmonis.

THE ABUNDANCE AND DISTRIBUTION OF LEPEOPHTHEIRUS SALMONIS(COPEPODA: CALIGIDAE) ON PINK (ONCORHYNCHUS GORBUSCHA) ANDCHUM (O. KETA) SALMON IN COASTAL BRITISH COLUMBIA

In total, 23,750 specimens of the salmon louse, Lepeophtheirus salmonis, were collected from 3,907 juvenile pink and 3,941 chum salmon caught within the Broughton Archipelago during a 2-yr survey. The prevalence on pink salmon was significantly higher than on chum salmon in 2004 (62.3% and 58.6%, respectively) and in 2005 (26.4% and 23.1%, respectively). The mean abundance on chum salmon was significantly higher than on pink salmon in 2004 (7.0 ± 0.3 and 2.8 ± 0.2, respectively), whereas in 2005 the mean abundance did not differ between species (0.6 ± 0.1 and 0.5 ± 0.0, respectively). The mean intensity on chum salmon was significantly higher than on pink salmon in 2004 (12.0 ± 0.4 and 4.5 ± 0.2, respectively) and in 2005 (2.5 ± 0.2 and 1.7 ± 0.1, respectively). The prevalence, intensity, and abundance of L. salmonis were significantly higher on salmon belonging to both host species in 2004 compared with 2005. In both years, a majority of pink and chum salmon had 2 or fewer lice. In general, a decline in abundance of L. salmonis over the 3 collection periods in each year coincided with an increased percentage of motile developmental stages. The abundance was lowest on fish collected from zones in which the seawater surface salinity was also lowest. Seawater surface temperature was higher and salinity was lower in 2004 compared with 2005. The spatial and temporal trends in the abundance of L. salmonis in relation to host size, infestation rates, and seawater salinity and temperature, evident in both years, must be considered in future studies assessing the role of farmed salmon in the epizootiology of this parasite on juvenile salmon in this area.

Perspectives on Pink Salmon and Sea Lice: Scientific Evidence Fails to Support the Extinction Hypothesis

Using a Ricker (1975) model and escapement data for a subset of Broughton Archipelago, British Columbia watersheds, Krkošek et al. (2007a) predicted that sea lice infections originating on Atlantic salmon (Salmo salar) farms will cause the extinction of pink salmon(Oncorhynchus gorbuscha) in the archipelago by 2010. The purpose of this article is to examine this issue in the context of all of the escapement data available for the archipelago and to review additional scientific reports and information not discussed by Krkošek et al. (2007a). Additional research during the last five years is not consistent with the Krkošek et al. (2007a) conclusion that sea lice routinely cause in excess of 80% mortality of fry. Rather, the literature reviewed herein indicates that pink salmon fry mount an effective immune response at sizes as small as 0.7 g, resulting in the rapid shedding of lice within two weeks. Pink salmon returns are shown to be highly variable throughout the Northeast Pacific in areas without salmon farms. Following periods of high abundance, pink salmon populations typically fall to low levels, and they may remain depressed for several generations. However, in most cases, the populations then gradually increase to begin the cycle anew. An examination of returns to all of the documented Broughton Archipelago watersheds indicates that following exceptionally high returns in 2000 and 2001, the populations declined to very low numbers in 2002 and 2003. Contrary to the conclusions reached by Krkošek et al. (2007a), Broughton pink salmon returns have steadily increased since then, with no indication that they are threatened with extinction. Other unsubstantiated assumptions used in Krkošek et al. (2007a) are also discussed in light of additional scientific reports and theoretical considerations.

Differentiating size-dependent responses of juvenile pink salmon (Oncorhynchus gorbuscha) to sea lice (Lepeophtheirus salmonis) infections.

Salmon infected with an ectoparasitic marine copepod, the salmon louse Lepeophtheirus salmonis, incur a wide variety of consequences depending upon host sensitivity. Juvenile pink salmon (Oncorhynchus gorbuscha) migrate from natal freshwater systems to the ocean at a young age relative to other Pacific salmon, and require rapid development of appropriate defenses against marine pathogens. We analyzed the early transcriptomic responses of naïve juvenile pink salmon of sizes 0.3 g (no scales), 0.7 g (mid-scale development) and 2.4 g (scales fully developed) six days after a low-level laboratory exposure to L. salmonis copepodids. All infected size groups exhibited unique transcriptional profiles. Inflammation and inhibition of cell proliferation was identified in the smallest size class (0.3 g), while increased glucose absorption and retention was identified in the middle size class (0.7 g). Tissue-remodeling genes were also up-regulated in both the 0.3 g and 0.7 g size groups. Profiles of the 2.4 g size class indicated cell-mediated immunity and possibly parasite-induced growth augmentation. Understanding a size-based threshold of resistance to L. salmonis is important for fisheries management. This work characterizes molecular responses reflecting the gradual development of innate immunity to L. salmonis between the susceptible (0.3 g) and refractory (2.4 g) pink salmon size classes.

Comparative defense-associated responses in salmon skin elicited by the ectoparasite Lepeophtheirus salmonis

Susceptibility among salmonids to the ectoparasite Lepeophtheirus salmonis is related to inflammatory reactions at the site of parasite attachment. Salmon from two susceptible (Salmo salar, Oncorhynchus keta) and one resistant (Oncorhynchus gorbuscha) species were exposed to adult L. salmonis. After 24 and 48h, skin samples directly below the attachment site and at non-attachment sites were assessed for transcriptomic profiles of select innate defense genes. Abrasion of the skin permitted comparisons between abrasion-associated injury and louse-associated injury. Infection responses were consistently higher than those caused by abrasion. Temporal patterns of expression were evident in all species for the transcription factor CCAAT/enhancer-binding protein β (C/EBP-β), the cytokine interleukin-6 (IL-6) and the enzyme prostaglandin D synthase (PGDS) at attachment sites. O. gorbuscha was the highest responder in a number of genes while there was an absence of C-reactive protein (CRP) gene expression in S. salar and O. keta, indicating an altered acute-phase response. Moreover, O. keta displayed distinct interleukin-8 (IL-8) and serum amyloid P (SAP) responses. Impaired genetic expression or over-expression in these pathways may be evidence for species-specific pathways of susceptibility to the parasite. At L. salmonis attachment sites, reduced expression compared to non-attachment sites was observed for C/EBP-β (S. salar), CRP (S. salar), SAP (S. salar, O. gorbuscha, O. keta), PGDS (S. salar, O. gorbuscha, O. keta), and major histocompatibility class II (MH class II, S. salar), suggesting local immunodepression.

Infection threshold to estimate Lepeophtheirus salmonis-associated mortality among juvenile pink salmon

A threshold of lethal infection was estimated from previous controlled laboratory exposures to be 7.5 Lepeophtheirus salmonis g(-1) for pink salmon Oncorhynchus gorbuscha averaging < 0.7 g. This threshold was used to assess the risk of mortality caused by L. salmonis among pink salmon of the same size class in the Broughton Archipelago, Canada from 2005 to 2008. Virtually all (> or = 98.9%) pink salmon collected in late March belonged to this size class, and this proportion declined to < or = 1% by early July. The proportion of these small pink salmon with infections equal to or exceeding the threshold declined from 4.5 in 2005 to 0% in 2008, coincident with an overall decline in parasite prevalence and intensity during this period. In 2005 and 2006, this proportion was greatest in March (7.8 and 1.1%, respectively) whereas in 2007, the proportion exceeding the threshold was greatest in May (2.9%). In 2008, no infections exceeded the threshold. Parasite development coincided with fish migration through the study area. The declining risk between 2005 and 2008 was possibly related to changes in ocean conditions such as temperature, to changing treatment practices for this parasite on salmon farms, or to changes in the abundance or distribution of non-farmed hosts. The concept of a threshold of L. salmonis infection density may be used to assist in the management and conservation of juvenile pink salmon in the Broughton Archipelago region.

Preferential PCR Amplification of Parasitic Protistan Small Subunit rDNA from Metazoan Tissues

Abstract A “universal non-metazoan” polymerase chain reaction (UNonMet-PCR) that selectively amplifies a segment of non-metazoan Small Subunit (SSU) rDNA gene was validated. The primers used were: 18S-EUK581-F (5′-GTGCCAGCAGCCGCG-3′) and 18S-EUK1134-R (5′-TTTAAGTTTCAGCCTTGCG-3′) with specificity provided by the 19-base reverse primer. Its target site is highly conserved across the Archaea, Bacteria, and eukaryotes (including fungi), but not most Metazoa (except Porifera, Ctenophora, and Myxozoa) which have mismatches at bases 14 and 19 resulting in poor or failed amplification. During validation, UNonMet-PCR amplified SSU rDNA gene fragments from all assayed protists (n = 16 from 7 higher taxa, including two species of marine phytoplankton) and Fungi (n = 3) but amplified very poorly or not at all most assayed Metazoa (n = 13 from 8 higher taxa). When a non-metazoan parasite was present in a metazoan host, the parasite DNA was preferentially amplified. For example, DNA from the parasite Trypanosoma danilewskyi was preferentially amplified in mixtures containing up to 1,000× more goldfish Carassius auratus (host) DNA. Also, the weak amplification of uninfected host (Chionoecetes tanneri) SSU rDNA did not occur in the presence of a natural infection with a parasite (Hematodinium sp.). Only Hematodinium sp. SSU rDNA was amplified in samples from infected C. tanneri. This UNonMet-PCR is a powerful tool for amplifying SSU rDNA from non-metazoan pathogens or symbionts that have not been isolated from metazoan hosts.