Reduced Lepeophtheirus salmonis larval abundance in a sea loch on the west coast of Scotland between 2002 and 2006

Abstract

A survey for planktonic sea louse larvae was carried out in Loch Shieldaig, Scotland, between 2002 and2006, and spanned 2 successive production cycles (Cycles 1 and 2) at a local Atlantic salmon Salmo salar L. farm. The vast majority of the caligid copepodids recovered were Lepeophtheirus salmonis; however, the methodology was unable to determine the species of the caligid nauplii. Greatest densities of nauplii were found at the sampling station adjacent to the salmon farm, and larval densities were low during the fallow period of both cycles. Peaks in nauplius densities occurred around the same time in the 2 cycles, but the peaks were significantly lower during Cycle 2 than Cycle 1. Lepeophtheirus salmonis copepodid densities varied temporally, but not spatially. During most of Cycle 2, copepodid densities were significantly lower than those recovered during Cycle 1. Numbers of gravid L. salmonis at the local salmon farm correlated significantly with densities of louse nauplii and L. salmonis copepodids in the water at time lags of 0 and 1 wk, and 1 and 2 wk, respectively. This survey demonstrated a reduction in densities of L. salmonis larvae in the plankton (an indication of L. salmonis infectious pressure) between the 2 cycles and indicated that the farm was an important source of L. salmonis larvae. The application of anti-louse treatments using emamectin benzoate reduced the numbers of gravid L. salmonis at the farm, and this was the main factor influencing the apparent reduction in L. salmonis infectious pressure in the loch between cycles. KEY WORDS: Lepeophtheirus salmonis · Sea lice · Larvae · Salmon farm · Scotland Resale or republication not permitted without written consent of the publisherDis Aquat Org 81: [109][110][111][112][113][114][115][116][117] 2008 duration of each stage of development is strongly dependent on temperature (Johnson & Albright 1991a). Under average sea surface temperatures in Loch Torridon (around 10°C), L. salmonis nauplii and copepodids would last approximately 3 and 8 d, respectively (Johnson & Albright 1991a), but may become less capable of finding a host after just a few days as finite energy reserves dwindle (Tucker et al. 2000).Few published accounts have described the behaviour of Lepeophtheirus salmonis larvae in the wild. Costelloe et al. (1998a,b) frequently recovered L. salmonis larvae near salmon farms in Ireland. They suggested that most of these larvae were probably of farm origin and that intermittent pulses of L. salmonis larvae that occurred near river mouths were probably of wild origin. A study in Loch Shieldaig, Scotland (Penston et al. 2008), suggested that the majority of louse larvae recovered near a salmon farm were the progeny of L. salmonis on farmed salmon. In contrast to Costelloe et al. (1998a,b), who concluded that farms only influence L. salmonis populations in their immediate environs, Penston et al. (2008) suggested that L. salmonis larvae can have a widespread effect in the surrounding waters.As with most salmon farms in Scotl...

Using sentinel cages to estimate infestation pressure on salmonids from sea lice in Loch Shieldaig, Scotland

“…These results are consistent with Penston et al (2008b) and Penston & Davies (2009), who found significant correlations between L. salmonis copepodid densities in the water column and gravid lice counts on nearby farms. However, when the de-trended data were examined, there was no evidence that directed changes in counts at the farms, brought about through treatment events, led directly to changes in infestation pressure at the sentinel cages.…”

“…1), and given the low number of returning salmonids in the Torridon system (Penston et al 2008b), it is probable that the numbers of gravid lice on the fish farms were the dominant drivers of infestation pressure at the sentinel cages. Formally, however, the data provide no evidence of causality, only of association.…”

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

“…Costelloe et al (1996) also found a similar pattern with samples collected closer to an Atlantic salmon farm having a greater proportion of nauplii to copepodids than samples collected distant from that farm. The ratios presented in the present study are consistent with those seen previously in this area; higher close to the farms and lower at stations further away from them (Penston et al 2004(Penston et al , 2008b. This consistency in the pattern observed using this nauplius fraction over time and space supports its wider application as an index of proximity to sources of lice.…”

“…In the Loch Torridon system between the production cycles of 2002-2003 and 2004-2005 (the latter spanning the present Cycle 1), a significant decrease in the densities of Lepeophtheirus salmonis larvae was described by Penston et al (2008b). This decrease was attributed to applications of ema mectin benzoate at Farm 1.…”

Dynamics of outbreak and control of salmon lice on two salmon farms in the Broughton Archipelago, British Columbia

“…In the Broughton Archipelago, the abundance of lice on wild juvenile salmon was markedly reduced in 2006 (Jones & Hargreaves 2009) relative to 2004 (Jones & Hargreaves 2007), indicating treatment on farms may extend to reductions of lice in the surrounding environment. These findings are consistent with reductions in naupliar and copepodid stages of lice in plankton samples near salmon farms following treatment in Scotland (Penston et al 2008) and British Columbia (Morton et al 2010). Given the 1 to 3 mo timescale of delousing salmon farms we have estimated and the beginning of the juvenile salmon outmigration in this area in March (M. Krkošek pers.…”