Atlantic salmon infected with salmon lice are more susceptible to new lice infections

Repeated exposure affects susceptibility and responses of Atlantic salmon (Salmo salar) towards the ectoparasitic salmon lice (Lepeophtheirus salmonis)

Atlantic salmon (Salmo salar) is repeatedly exposed to and infected with ectoparasitic salmon lice (Lepeophtheirus salmonis) both in farms and in nature. However, this is not reflected in laboratory experiments where fish typically are infected only once. To investigate if a previous lice infection affects host response to subsequent infections, fish received 4 different experimental treatments; including 2 groups of fish that had previously been infected either with adult or infective salmon lice larvae (copepodids). Thereafter, fish in all treatment groups were infected with either a double or a single dose of copepodids originating from the same cohort. Fish were sampled when lice had developed into the chalimus, the pre-adult and the adult stage, respectively. Both the specific growth rate and cortisol levels (i.e. a proxy for stress) of the fish differed between treatments. Lice success (i.e. ability to infect and survive on the host) was higher in naïve than in previously infected fish (pre-adult stage). The expression of immune and wound healing transcripts in the skin also differed between treatments, and most noticeable was a higher upregulation early in the infection in the group previously infected with copepodids. However, later in the infection, the least upregulation was observed in this group, suggesting that previous exposure to salmon lice affects the response of Atlantic salmon towards subsequent lice infections.

The effect of different intensities of the ectoparasitic salmon lice (Lepeophtheirus salmonis) on Atlantic salmon (Salmo salar)

The effect of different intensities of the ectoparasitic salmon lice (Lepeophtheirus salmonis) on stress, growth and the expression of immune and wound healing transcripts in the skin of Atlantic salmon (Salmo salar) was investigated. Lice infection success and survival were similar at the chalimus and preadult stage in the low and high dose group, but infection success and survival were significantly lower in the high than in the low dose group at the adult stage. The expression of investigated transcripts was not correlated to lice intensities, but several of them were significantly differently expressed locally in the skin at the site of lice attachment in infected fish compared to controls. This included an up-regulation of pro-inflammatory markers at the site of lice attachment (e.g., interleukin 1-beta, interleukin 8 and the acute phase protein serum amyloid A), a reduction of markers of adaptive immunity (cluster of differentiation 8-alpha and immunoglobulin M) and decreased expression of the anti-inflammatory cytokine interleukin 10.

Larval salmon lice Lepeophtheirus salmonis exhibit behavioral responses to conspecific pre-adult and adult cues

In the larval stage of the parasitic copepod Lepeophtheirus salmonis, the free-living copepodid must locate and settle on a salmonid host. Chemosensory mechanisms play a role in determining whether a potential host is suitable for attachment, yet the full suite of chemical cues and resulting behavioral mechanisms used are unknown. After maturing, pre-adult female and adult male salmon lice aggregate upon salmonid hosts for reproduction. Copepodid salmon lice have been observed preferentially infesting hosts that harbor conspecific adults. The aim of this study was to investigate the possibility that salmon lice copepodids perceive and respond to cues from pre-adult female, adult-male, and/or gravid female salmon lice. Behavioral bioassays were conducted in vitro with copepodids exposed to water conditioned with 3 stages of conspecific lice (pre-adult female, adult male, or gravid female), and seawater conditioned with Atlantic salmon Salmo salar Linnaeus, 1758. Experiments demonstrated that copepodids exposed to water conditioned with the salmon host, pre-adult female, or adult male salmon lice significantly altered their behavior, whereas salmon lice exposed to water conditioned with gravid females did not. These results are potentially valuable in the development of novel methods for mitigation of L. salmonis in the salmon aquaculture industry.

Experimental transmission of Caligus rogercresseyi between two different fish species

Caligus rogercresseyi is the dominant sea louse parasite affecting the salmon and trout industry in southern Chile. This parasite has a wide range of native and endemic fish hosts. The Patagonian blenny Eleginops maclovinus, which is parasitized mostly by the caligid species Lepeophtheirus spp. and C. rogercresseyi, is presumably responsible for the transmission of C. rogercresseyi to salmonids. The aim of this study was to characterize the transmission of parasites between different fish species and parasite cohort development under laboratory conditions. Parasite abundances and intensities were quantified. Transmission of parasites from Patagonian blenny to Atlantic salmon Salmo salar was lower (~9%, mainly corresponding to C. rogercresseyi) than from salmon to Patagonian blenny (14.7-26.9%, where only C. rogercresseyi were observed). This suggests that the transmission of C. rogercresseyi from salmon individuals is higher than the transmission from a native fish. Parasite cohorts developed successfully on both fish species, but apparently under different developmental rates. Water temperature, oxygen, and juvenile abundances were the variables that better explained cohort development success and variation in C. rogercresseyi adult abundances over time.

Parasitic sea louse infestations on wild sea trout: separating the roles of fish farms and temperature

Background

The causal relation between parasitic sea lice on fish farms and sea lice on wild fish is a controversial subject. A specific scientific debate has been whether the statistical association between infestation pressure (IP) from fish farms and the number of parasites observed on wild sea trout emerges purely because of a confounding and direct effect of temperature (T).

Methods

We studied the associations between louse infestation on wild sea trout, fish farm activity and temperature in an area that practices coordinated fallowing in Nordhordland, Norway. The data were sampled between 2009 and 2016. We used negative binomial models and mediation analysis to determine to what degree the effect of T is mediated through the IP from fish farms.

Results

The number of attached lice on sea trout increased with the T when the IP from fish farms was high but not when the IP was low. In addition, nearly all of the effect of rising T was indirect and mediated through the IP. Attached lice remained low when neighbouring farms were in the first year of the production cycle but rose substantially during the second year. In contrast to attached lice, mobile lice were generally seen in higher numbers at lower water temperatures. Temperature had an indirect positive effect on mobile louse counts by increasing the IP which, in turn, raised the sea trout louse counts. Mobile louse counts rose steadily during the year when neighbouring farms were in the first year of the production cycle and stayed high throughout the second year.

Conclusions

The estimates of the IP effect on louse counts along with the clear biennial pattern emerging due to the production cycle of fish farms clearly indicate that fish farms play an important role in the epidemiology of sea lice on wild sea trout. Furthermore, the mediation analysis demonstrates that a large proportion of the effect of T on louse counts is mediated through IP.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3189-6) contains supplementary material, which is available to authorized users.

Susceptibility, behaviour, and retention of the parasitic salmon louse (Lepeophtheirus salmonis) differ with Atlantic salmon population origin

Atlantic salmon populations across the world have diverse ecological and evolutionary histories, from wild anadromous or landlocked, to domestication and genetic modification. The natural host behaviours confer protection from infestation by ectoparasitic salmon lice Lepeophtheirus salmonis, yet whether genetic origin results in different behaviours and thus susceptibility to infestation is unknown. In common garden experiments, we tested antiparasite behaviours, susceptibility and retention of salmon lice in wild anadromous, wild landlocked, domesticated and genetically modified domesticated strains. Within domesticated strains, we tested two infestation histories (previously infested and naïve) and a new phenotype (albino colouring). Farmed stocks initially acquired 24%-44% higher levels of parasite density than the wild and landlocked strains. Burst swimming and displacement behaviours were higher in the domesticated groups, and jumping was more prevalent in the domesticated strains. At 34 days post-infestation, domesticated strains and the wild anadromous strain did not differ significantly from each other; however, landlocked salmon had increased infestation levels considerably. Domesticated strains lost ~20% (±9.9%-16.5%; 95% CI) of their initial parasite load, while parasite load increased by 5.5% (±30.1%) for wild salmon and 20.1% (±28.5%) in landlocked salmon. This study provides early evidence for diverged host-parasite interactions associated with domestication in this system.

Invest more and die faster: The life history of a parasite on intensive farms

Organisms are expected to respond to alterations in their survival by evolutionary changes in their life history traits. As agriculture and aquaculture have become increasingly intensive in the past decades, there has been growing interest in their evolutionary effects on the life histories of agri‐ and aquacultural pests, parasites, and pathogens. In this study, we used salmon lice (Lepeophtheirus salmonis) to explore how modern farming might have affected life history evolution in parasites. We infected salmon hosts with lice from either farmed or unfarmed locations, and monitored life history traits of those parasites in laboratory conditions. Our results show that compared to salmon lice from areas unaffected by salmon farming, those from farmed areas produced more eggs in their first clutch, and less eggs later on; they achieved higher infestation intensities in early adulthood, but suffered higher adult mortality. These results suggest that salmon lice on farms may have been selected for increased investment in early reproduction, at the expense of later fecundity and survival. This call for further empirical studies of the extent to which farming practices may alter the virulence of agricultural parasites.

Parasite fecundity decreases with increasing parasite load in the salmon louse Lepeophtheirus salmonis infecting Atlantic salmon Salmo salar

Aggregation is common amongst parasites, where a small number of hosts carry a large proportion of parasites. This could result in density-dependent effects on parasite fitness. In a laboratory study, we explored whether parasite load affected parasite fecundity and survival, using ectoparasitic salmon lice (Lepeophtheirus salmonis Krøyer, 1837) infecting Atlantic salmon (Salmo salar) hosts. We found a significant reduction in fecundity with higher parasite load, but no significant effect on survival. Together with previous findings, this suggests that stronger competition amongst female lice under high parasite load is a more likely explanation than increased host immune response.