Statistical and ecological challenges of monitoring parasitic salmon lice infestations in wild salmonid fish stocks

Abstract

Ecological monitoring programmes should provide precise data to inform management, but the data quality is often limited by methodological challenges and the need for cost-effective sampling. Parasite infestations are particularly challenging to monitor due to complex interactions among hosts, parasites and the environment. In Norway, salmon lice infestations on wild salmonid fish have been monitored since 1992 to survey the potential transmission between farmed and wild salmonids. Here, we compared spatiotemporal variation in salmon lice levels with variations in local fjord conditions, including salinity, temperature and infestation pressure from salmon farms (measured as mean abundance of mature female lice × number of farmed fish). We tested 3 different measures of infestation with different statistical properties. Our results confirm that, even after correcting for temperature and salinity effects, infestation pressure from salmon farms significantly increases the probability of lice infestation in wild salmonids. The probability of infestation increases with fish body length, salmon farm infestation pressure and tem perature, and decreases with increasing freshwater influence. Furthermore, we found a significant interaction between temperature and infestation pressure from salmon farms. When the infestation pressure from farms is low, temperature has a strong increasing effect on the probability of infestation, but as the infestation pressure from farms increases, temperature gradually becomes less important. The exact results vary somewhat depending on the measure of lice infestations used, but the same trend can be seen in all models. We discuss the statistical and biological complexities that make monitoring of salmon lice in wild populations challenging.Variation in salmon lice levels on wild salmonids have been monitored along the entire Norwegian coast.Photo: Bengt Finstad. Map: Ingeborg P. Helland

Aquaculture and environmental drivers of salmon lice infestation and body condition in sea trout

“…Levels of lice infestation associated with salmon farms are likely to change across gradients of external stressors (e.g. Lafferty & Kuris 1999) such as temperature and rainfall (Helland et al 2015). The present study uses a large international dataset to develop statistical models describing lice infestation and body condition of sea trout.…”

In situ measurement of salinity during seaward migration of Atlantic salmon post-smolts using acoustic transmitters with data-storage capabilities and conventional acoustic transmitters

“…The salinity history of the tagged fish indicated that the post-smolts occasionally spent time in sea water with lower salinity than 20.0 psu, especially in the inner part of the fjord. The utilization of salinities below 20.0 psu may reduce infestation risk by salmon lice [22,23], and the probability of infestation is generally shown to decrease with increasing freshwater influence [35]. Salmon louse is a marine parasite, which can severely reduce Atlantic salmon populations in farm-intensive areas [36,37].…”

The major threats to Atlantic salmon in Norway

“…In farm-intensive areas, lice levels on wild salmonids are typically higher, and more variable than in farm-free areas (e.g. Serra-Llinares et al, 2014Helland et al, 2015;Thorstad et al, 2015). The individual physiological and pathological effects of salmon lice on salmonids have been thoroughly described , and population level effects have been well documented for Atlantic salmon.…”