Effect of environmental salinity on sea lice Lepeophtheirus salmonis settlement success

Abstract

The sea louse Lepeophtheirus salmonis (Krøyer, 1837) (Copepoda: Caligidae) is an ectoparasite of salmonid fish. It has earlier been proposed that the free-swimming infectious copepodid stage of L. salmonis gather at river mouths to infect wild Atlantic salmon Salmo salar L. and sea trout S. trutta L. smolts during their seaward migration. This study used aquarium-based methods to investigate the survival, infective ability and behaviour of L. salmonis copepodids exposed to short periods of low salinity levels, such as those encountered at river mouths. Survival of free-swimming copepodids was found to be severely compromised at salinity levels below 29 parts per thousand (ppt). Attachment to an S. salar host did not aid copepodid survival during post-infection exposure to low salinity environment, and a reduction in salinity appears to reduce the ability of copepodids to remain attached to S. salar smolts. Pre-infection exposure of copepodids to reduced salinity levels reduced infection of S. salar. Infection levels at reduced salinity were lower than predicted from the free-swimming survival experiment, suggesting that low salinity compromises the copepodids' ability to sense or respond to the presence of a host. In salinity gradients, copepodids demonstrated avoidance of salinities below 27 ppt, by both altering their swimming behaviour and changing the orientation of passive sinking. Avoidance of low salinity levels may be due to their adverse effects on copepodid physiology, as suggested by the reduction in survival. Sinking rates were also faster in reduced salinity, suggesting that remaining in the water column would be more energetically demanding for the copepodids at reduced salinity. These results show that both survival and host infectivity of L. salmonis are severely compromised by short-term exposure to reduced salinity levels.

High host densities dilute sea lice Lepeophtheirus salmonis loads on individual Atlantic salmon, but do not reduce lice infection success

“…The relationship between host density and infection intensity appears to depend on the transmission mode of the parasite (Côté & Poulin 1995). Sea lice copepodids are mobile crustaceans (Pike & Wadsworth 1999, Bricknell et al 2006, which actively seek their host and display a range of hostfinding behaviours when they detect host-related cues (Heuch et al 2007, Mordue Luntz & Birkett 2009). Mobile parasites searching for a host are analogous to predators seeking their prey, and similarly the probability of an individual host being 'attacked' 85 Fig.…”

“…Copepodids, the mobile infective stage of L. salmonis, can alter their position in the water column in response to environmental variables (Heuch et al 1995, Tucker et al 2000, Bricknell et al 2006) and actively seek their host (Heuch et al 2007, Mordue Luntz & Birkett 2009). As mobile parasites, an increase in host density could result in a lower infection intensity.…”

“…Most artificial infection protocols using sea lice only partially reflect how infection typically occurs in the ocean for both wild and farmed fish. Artificial infections add sea lice copepodids to tanks containing fish where water flow is turned off for several hours (Grimnes & Jakobsen 1996, Dawson et al 1997, Bjørn & Finstad 1998, Hull et al 1998, Bowers et al 2000, Tucker et al 2000, or place anaesthetized fish in a concentrated suspension of sea lice copepodids (Bricknell et al 2006(Bricknell et al , Øverli et al 2014. The static water conditions during artificial infections or immobile fish contrast with natural infection settings for both farmed and wild salmon, where water currents may change infection dynamics (Revie et al 2003).…”

How sea lice from salmon farms may cause wild salmonid declines in Europe and North America and be a threat to fishes elsewhere

“…However, in contrast to lice on fishes in sea cages, lice on wild and feral salmon may not be exposed to low salinities for long enough to affect their abundance until the fishes migrate into freshwater. Experiments in the laboratory and sea pens have shown that L. salmonis copepodites swim upwards towards light and do not cross into low salinity waters (Heuch 1995;Heuch et al 1995;Bricknell et al 2006). Plankton sampling discovered higher densities of L. salmonis copepodites in very shallow water along the seashore and in estuarine areas of Ireland and Scotland (Costelloe et al 1995(Costelloe et al , 1998McKibben & Hay 2004).…”

Seasonality of Parasitic Copepods on Bullseye Puffer, Sphoeroides annulatus (Pisces: Tetraodontidae), From the Northwestern Coast of Mexico

“…CCA indicated that the intensity of C. serratus was positively influenced by water salinity and, to some extent, by host size. Experimental studies by Bricknell et al (2006) on the caligid L. salmonis indicate that both survival and host infectivity are severely compromised by short-term exposure to reduced salinity levels. Also, water temperature has been shown to be positively associated with the development of other Caligus species such as C. elongatus and C. rogercresseyi Gonzá lez and Carvajal, 2003;Costello, 2006).…”