Relationship Between Sea Lice Infestation, Sea Lice Production and Sea Trout Survival in Ireland, 1992‐2001

Abstract

Abstract: This chapter contains sections titled: The relationship between sea lice infestation on sea trout with distance to salmon aquaculture sites for a broad geographic range of Irish rivers was examined over a 10-year period. Highest mean levels of total lice and juvenile (chalimus stages) lice were recorded at sites less than 20 km from farms. The mean total lice infestation was lower at sites less than 30 km from farms, and beyond 30 km, very low mean total lice levels were recorded. Chalimus lice stages dominated the sea lice population structure at distances of < 20 and < 30 km. At distances < 60 and < 100 km, chalimus and post-chalimus stages are equally represented and at sites > 100 km post-chalimus stages predominate. A model was fitted to pooled 10-year data time series for sea lice infestation and distance from marine salmon farms to indicate an overall relationship that could be used to support management actions. The average abundance of lice per fish expected very close to farms (1 km) was 50.6. Regression of log-transformed data for individual years showed significant relationships in all years except 1994 and 1999, although substantial variation existed in the data particularly close to farms. Infestations at distances greater than 25 km never reached over 32 lice per fish and were usually much lower. At distances less than 25 km the full range in infestation occurred. Sea trout have been shown to experience physiological problems and osmoregulatory imbalance at lice levels of approximately 0.7 lice larva/g fish weight. The overall mean size of trout in the present study carrying lice was 79 g, giving an indicative stress level of sea lice infestation of 55 lice/fish. Twenty nine percent of the infested trout had lice levels above this indicative stress level. For fish sampled in bays without farms, 3.4% of the infested trout were above this indicative stress level, while for fish captured in bays with farms this level rose to 30.8%. There was a relationship between the proportion offish in each sample above 55 lice per fish and distance from salmon farms. There was a significant negative relationship between sea trout marine survival and the level of lice infestation on sea trout in four bays in mid-west Ireland. A linear model of the relationship between the total number of ovigerous lice produced in two bays between March and mid-May and the average number of sea lice infesting sea trout in nearby rivers showed a significant positive relationship between lice reproductive potential and infestation of trout. The relationships shown in the present study indicate that sea lice from marine salmon farms were a major contributory factor in the sea trout stock collapses observed in aquaculture areas in western Ireland. If recovery of depleted sea trout stocks is to be achieved in this area it is critical to ensure that ovigerous sea lice levels are maintained at near-zero levels on marine salmon farms over the spring period prior to and during sea trout smolt migration. This must be achieved on a consistent annual basis for a successful sea trout recovery.