As sea lice do not survive in fresh water, sea trout have the potential to reduce or rid themselves of infestation by seeking low-salinity waters, such as those in rivers. Early returning sea trout have been noted in a number of areas and can be used to give an indication of the levels of lice encountered by the fish. Every May and June since 1999, trout have been collected in the lower reaches of the River Shieldaig by electrofishing1 and the presence/absence of sea lice noted.
From 2002 onwards, each fish was anaesthetised and placed in a white tray, where sea lice were counted and classified as either attached (copepodids/chalimus) or mobile (pre-adults/adults/ovigerous females). This process allows the lice burdens of each fish to be categorised as above or below a threshold equivalent to 13 mobile lice per fish (weight range 19–70 g)2. Above this level of infestation it has been suggested that the fish are subject to serious physiological stress and potential death from sea lice infestations3.
Previous analyses of these data from 1999 to 2009 have shown the proportion of sea trout with lice, and those with burdens above the threshold, to be significantly higher in the second year of fish farm production than the first1. These patterns can be seen to continue in subsequent production cycles.
The proportion of trout sampled in the lower Shieldaig with lice in relation to fish farm production cycle. Green bars are those in the first year of production, blue are in the second year.
The proportion of trout sampled in the lower Shieldaig exhibiting lice burdens above the threshold level with respect to fish farm production cycle. Green bars are those in the first year of production, blue are in the second year.
1Hatton-Ellis, M., Hay, D.W., Walker, A.F. & Northcott, S.J. (2006). Sea lice Lepeophtheirus salmonis infestations of post-smolt sea trout in Loch Shieldaig, Wester Ross, 1999–2003. Sea trout: biology, conservation and management, 372-376.
2Middlemas, S.J., Raffell, J.A., Hay, D.W., Hatton-Ellis, M. & Armstrong, J.D. (2010). Temporal and spatial patterns of sea lice levels on sea trout in western Scotland in relation to fish farm production cycles. Biology Letters, 6, 548-551.
3Wells, A., Grierson C. E., MacKenzie, M., Russon, I. J., Reinardy, H., Middlerniss, C., Bjorn, P. A., Finstad, B., Bonga, S. E. W., Todd, C. D. & Hazon, N. (2006). Physiological effects of simultaneous, abrupt seawater entry and sea lice (Lepeophtheirus salmonis) infestation of wild, sea-run brown trout (Salmo trutta) smolts. Canadian Journal of Fisheries and Aquatic Sciences, 63(12), 2809-2821.