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

Ip Helland,B Finstad,Pa Jansen,Oh Diserud,Pa Bjørn,I Uglem

doi:10.3354/aei00155

Ip Helland, B Finstad + Show 4 more

Open Access

https://doi.org/10.3354/aei00155

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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 mon- itored since 1992 to survey the potential transmission between farmed and wild salmonids. Here, we com- pared 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 ma- ture female lice × number of farmed fish). We tested 3 different measures of infestation with different statis- tical properties. Our results confirm that, even after correcting for temperature and salinity effects, in - festation 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 pres- sure and tem perature, and decreases with increasing freshwater influence. Furthermore, we found a signif- icant 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, tem- perature 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.

Highlights

In order to monitor species and define nature management priorities, decision makers want information that is easy to evaluate and cost effective to gather
There was a significant interaction between temperature and infestation pressure from salmon farms
Our study shows that infestation pressure from salmon farms, temperature and salinity strongly influenced lice infestations on wild sea trout in Norway in the period between 2004 and 2010

Summary

INTRODUCTION

In order to monitor species and define nature management priorities, decision makers want information that is easy to evaluate and cost effective to gather. While wild salmonids usually reside in the fjords mainly during summer, large numbers of farmed salmon are present year round This causes a larger potential for heavy salmon lice infestations that may occur at different seasonal times compared to the natural infestation pressure (Bjørn et al 2011, Jansen et al 2012, Torrissen et al 2013, Vollset & Barlaup 2014). Since temperature is a strong driver of development and reproduction rate of salmon lice (Stien et al 2005), lice abundances follow strong seasonal cycles Salmon lice have their lowest abundance in early spring and peak during autumn, a pattern seen from lice counts both on farmed salmon (Jansen et al 2012) and wild salmonids (Bjørn & Finstad 2002, Rikardsen 2004, Serra-Llinares et al 2014). In our analyses as sea trout, since brown trout constitutes 95% of our data

MATERIALS AND METHODS

RESULTS

DISCUSSION