Aquaculture and environmental drivers of salmon lice infestation and body condition in sea trout
Samuel Shephard1, Craig MacIntyre2, Patrick Gargan1,*
1Inland Fisheries Ireland, 3044 Lake Drive, CityWest Business Campus, Dublin 24 Y265, Ireland
2Argyll Fisheries Trust, Cherry Park, Inveraray PA32 8XE, UK
ABSTRACT: Infestation of sea trout Salmo trutta L. by salmon lice Lepeophtheirus salmonis is associated with increased mortality risk and possible sub-lethal effects. Separating anthropogenic causes of infestation from background ecological variability has proved difficult. A unique 25 yr dataset was collated comprising lice counts from >20 000 sea trout sampled from 94 separate river and lake systems in Ireland and Scotland at varying distances from marine salmon farms. Statistical models were developed to explore the potential effects of distance to a salmon farm, rainfall and ambient temperature on sea trout lice infestation and body condition (weight at length). These models indicated that sea trout captured closer to salmon farms had significantly higher levels of lice infestation, and that this effect was exacerbated in warmer years. Sea trout sampled closer to salmon farms also had significantly reduced weight at length (impaired condition), with the strongest impact in dry years. The study dataset covers a broad geographic area over multiple years, and accounts for variability in temperature and rainfall. Our results imply a rather general impact of salmon farming on lice infestation and body condition of sea trout. This finding has implications for current lice control management strategies, coastal zone planning, recovery of sea trout stocks in aquaculture areas and the scale of aquaculture free zones.
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Temporal variation in sea trout Salmo trutta life history traits in the Erriff River, western Ireland
P. G. Gargan1,*, F. L. Kelly1, S. Shephard1, K. F. Whelan2
1Inland Fisheries Ireland, 3044 Lake Drive, Citywest Business Campus, Dublin, D24 Y265, Ireland
2School of Biology & Environment Science, University College Dublin, Dublin 4, Ireland
ABSTRACT: The demographic and life history characteristics of sea trout Salmo trutta L. populations can be changed by a range of pressures in both freshwater and marine environments. Few long-term monitoring programmes are in place to assess temporal change in population dynamics. We analysed a 20 yr time series (1985−2004) using 15 sea trout population response variables in the Erriff River, western Ireland. Over this period, when time was considered as a categorical variable comprising 4 sequential periods of 5 yr, important life history changes were observed. The most dramatic of these changes corresponded with the period immediately after the commencement of salmon farming in the local estuary, with significant decreases in the number and length of sea trout kelts, the estimated number of eggs deposited, the sea trout rod catch, the proportion of older (1+ and 2+ sea age) fish and the frequency of repeat spawners. We found a significant positive relationship between the number of salmon lice Lepeophtheirus salmonis in the local salmon farm and the number of lice found on sea trout collected contemporaneously in local rivers. Results of this long-term monitoring programme demonstrate that significant changes in sea trout population structure with respect to quantitative life history traits can occur over a relatively short time period and suggest that the introduction of salmon farming into the local estuary most likely contributed to the observed changes in sea trout population dynamics.
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Effects of salmon lice Lepeophtheirus salmonis on wild sea trout Salmo trutta—a literature review
Eva B. Thorstad1,*, Christopher D. Todd2, Ingebrigt Uglem1, Pål Arne Bjørn3, Patrick G. Gargan4, Knut Wiik Vollset5, Elina Halttunen3, Steinar Kålås6, Marius Berg1, Bengt Finstad1
1Norwegian Institute for Nature Research, 7485 Trondheim, Norway
2University of St Andrews, St Andrews, Fife, KY16 8LB, UK
3Institute of Marine Research, 9294 Tromsø, Norway
4Inland Fisheries Ireland, 3044 Lake Drive, Citywest Business Campus, Dublin 24, Ireland
5Uni Research Environment, 5006 Bergen, Norway
6Rådgivende Biologer AS, 5003 Bergen, Norway
ABSTRACT: Salmon farming increases the abundance of salmon lice, which are ectoparasites of salmonids in the sea. Here we review the current knowledge on the effects of salmon lice on wild sea trout. Salmon lice feed on host mucus, skin and muscle, and infestation may induce osmoregulatory dysfunction, physiological stress, anaemia, reduced feeding and growth, increased susceptibility to secondary infections, reduced disease resistance and ultimately mortality of individual sea trout. Wild sea trout in farm-free areas generally show low lice levels. In farm-intensive areas, lice levels on wild sea trout are typically higher, and more variable than in farm-free areas. Lice on wild sea trout are found at elevated levels particularly within 30 km of the nearest farms but can also extend to further ranges. Salmon lice in intensively farmed areas have negatively impacted wild sea trout populations by reducing growth and increasing marine mortality. Quantification of these impacts remains a challenge, although population-level effects have been quantified in Atlantic salmon by comparing the survival of chemically protected fish with control groups, which are relevant also for sea trout. Mortality attributable to salmon lice can lead to an average of 12−29% fewer salmon spawners. Reduced growth and increased mortality will reduce the benefits of marine migration for sea trout, and may also result in selection against anadromy in areas with high lice levels. Salmon lice-induced effects on sea trout populations may also extend to altered genetic composition and reduced diversity, and possibly to the local loss of sea trout, and establishment of exclusively freshwater resident populations.
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Evidence of salmon lice-induced mortality of anadromous brown trout (Salmo trutta) in the Hardangerfjord, Norway
ØYSTEIN SKAALA1*, STEINAR KA° LA° S2 & REIDAR BORGSTRØM3
1Institute of Marine Research, Bergen, Norway, 2Ra°dgivende biologer, Bredsga°rden, Bergen, Norway, and 3Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, A ° s, Norway
Abstract
The Hardangerfjord, western Norway, is an area with a high concentration of salmon farms, high levels of infection of salmon lice in anadromous brown trout, and declining trout populations. This study assessed the marine survival rate of anadromous trout from the River Guddalselva, in the central part of the fjord, and tested the hypothesis that trout populations in this area are depressed by salmon lice infection. From 2001 to 2011, all descending smolts and trout returning from the fjord were captured in the traps at the field station of the Institute of Marine Research. In 2004 and 2005, parts of the smolt cohorts were treated with the Substance EX to prevent sea lice infection. From 2007 to 2010, all smolts (n_3557) were also tagged with individual tags. The results show a survival rate in the sea of only 0.58_3.41% for tagged smolts, which is extremely low. The highest survival rates appeared in the years with the lowest recordings of salmon lice in spring. The survival rate of Substance EX-treated smolts and controls was 3.41% and 1.76%, respectively. These findings suggest that salmon lice infection is an important contributor to the high mortality of anadromous trout populations in the Hardangerfjord.
