Variation in sea lice infestation on Atlantic salmon smolts in Killary Harbour, West Coast of Ireland

Abstract

Approximately 300 salmon smolts were stocked in each of four cages moored at various distances between an Atlantic salmon farm and a river system in Killary Harbour on the West Coast of Ireland. Samples of fish were taken from each of the cages on three occasions and examined for the presence of sea lice (Lepeophtheirus salmonis). Results from one-way ANOVA tests and a multiple comparison of the means of infection |evels in the cages indicated that there was a significant difference between the infection levels in the cages on each of the sampling dates. Cage 1, positioned nearest the river, exhibited the highest levels of infection on the first sampling date and was highly significantly different (p -0.001) from the other three cages which showed no significant difference between each other. This situation was again repeated on the second sampling date. On the final sampling date, cage 4, positioned nearest the salmon farm, was highly significantly different (p -~ 0.001) from the other three cages; cage 1 was significantly different (p -0.05) from cages 2 and 3 which showed no significant difference from each other.The population structure of the lice recovered from the smolts indicated that the lice settled in pulses rather than continuously over the trial period. The results are discussed in relation to the hydrography of Killary Harbour, the output of lice larvae from the farm, the impact that these may have on infestations in the inner area of Kil|ary and probable areas of lice larval production.

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

“…Thus copepodites would be subjected to dispersal by wind-driven surface currents, and nauplii most abundant near their sources (e.g. farms), as Costelloe et al (1995) found in Ireland and Penston et al (2008a,b) in Scotland. Comparable plankton sampling has not been reported in Canada or Norway, nor for any Caligus species.…”

“…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). While the same copepodite distribution may be expected in British Columbia, it requires confirmation because the Pacific and Atlantic populations of L. salmonis are genetically distinct (Tjensvoll et al 2006;Todd et al 2006) and may have different evolutionary adaptations.…”

Densities of planktonic Lepeophtheirus salmonis before and after an Atlantic salmon farm relocation

“…Previous surveys for planktonic louse larvae determined that the regular recovery of nauplii, which only persist for about 4 d at 10°C before developing to the copepodid stage (Johnson & Albright 1991), is indicative of an important source of louse larvae nearby to those sampling stations (Costelloe et al 1995, 1998a,b, Penston et al 2004, 2008a. To investigate whether the relocation of the farm had any effect on the distribution of larval sea lice at the vacated site (Stn H), we compared the densities and stages of the larvae at that location before and after the relocation.…”

“…Lepeophtheirus salmonis infection spreads primarily via the infective copepodid stage , Costelloe et al 1995, Finstad et al 2000, although adults, particularly males, are also known to transfer between hosts (Connors et al 2008). Previous plankton surveys in Loch Torridon (Mc Kibben & Hay 2004, Penston et al 2004, 2008a) and a coupled hydrographic-particle tracking model de veloped for that area (Murray & Gillibrand 2006, Amundrud & Murray 2009) indicate that louse larvae can be trans-ported several kilometres.…”

Identifying epidemiological factors affecting sea lice Lepeophtheirus salmonis abundance on Scottish salmon farms using general linear models

“…Number of daylight hours that a farm experiences at certain times in the production cycle Strength and direction of prevailing winds at a site in particular periods of the year Justification Given that lice may be carried to and from a site, the hydrographic pattern may have an effect on overall lice abundance levels (Løland 1993, Wallace 1998 Current speed may inhibit or enhance lice attachment and infestation (Boxshall 1976, Brenton-Davie & Kreiberg 1999, Mustafa et al 2001 Sea lice develop faster when temperatures are high, so sites with high mean seawater temperatures could experience higher levels of lice infestation (Boxaspen 1997) Broad loch-level hydrographic characteristics can have an effect on lice recruitment and retention at a site (Costelloe et al 1995) Sea lice have a preference for saline environments and fish in freshwater lose lice burdens. Different levels of salinity seen across sites might be expected to affect the numbers of lice present (Heuch 1995) Sites are fallowed before each production cycle and fish are free of parasites.…”