Abstract
Salmon lice Lepeophtheirus salmonis pose a major threat to the sustainable development of salmonid farming. To investigate effects of farm-origin salmon lice on wild salmonids, salmon lice dynamics are typically simulated using models that depend on experimentally determined rates of development, reproduction, mortality and infestation. Several recent studies provide new estimates of how these demographic rates depend on temperature and salinity. Here, we review and synthesize these studies and test if updating a salmon lice infestation model based on the new insights improves predictions of salmon lice infestations on salmon post-smolts in experimental cages in the sea. This model predicts spatiotemporal variation in infestation pressure based on weekly monitoring data of salmon lice and sea temperature in all salmonid fish farms in Norway, here supplemented by temperature and salinity data from a regional ocean model. Using data from 2012-2017 to select model formulation, we found the largest improvement in explanatory power by incorporating a salinity-dependent infestation rate. Updating functions for temperature-dependent egg production and infestation rates led to smaller improvements. Moreover, results suggest additional effects of temperature and a possible temperature-salinity interaction effect, not captured by the modelled processes. Out-of-sample predictions for experimental cage data from 2018-2020 confirmed that the uncertainty was realistically quantified, but also showed that associations of salmon lice infestations with salinity and temperature had changed. These results provide a field evaluation of experimental data and point to a knowledge gap regarding the combined effects of temperature and salinity on salmon lice infestations.
Highlights
The salmon louse Lepeophtheirus salmonis is a parasitic copepod on salmonid fish and the dominant sea louse species on farmed salmonids in the Northern Hemisphere (Pike & Wadsworth 1999)
We considered an updated dispersal function based on analyses by Aldrin et al (2019), here scaled to be 1 at distance 0 (Fig. 2): ri,j = 0.582·exp[−0.374(di0,.j692 −1) / 0.692] (7b)
While approximating development time by using a fixed number of degree-days may be convenient in some circumstances, our results suggest that such approximation may lead to a measurable loss in the precision of estimates of salmon lice infestation pressure
Summary
The salmon louse Lepeophtheirus salmonis is a parasitic copepod on salmonid fish and the dominant sea louse species on farmed salmonids in the Northern Hemisphere (Pike & Wadsworth 1999). The environmental indicator used to regulate whether to increase, maintain or reduce production in different areas along the coast is the estimated detrimental effect that salmon lice from fish farms has on wild salmonids, with a main focus on the mortality of wild salmon post-smolts. Fish farmers are required to count and report salmon lice on a weekly basis (Ministry of Trade, Industry and Fisheries 2012; monthly reporting before 2012). Statistical analyses of these data have provided insights into salmon lice dispersal and population dynamics (Jansen et al 2012, Aldrin et al 2013, Kristoffersen et al 2014, Aldrin et al 2019). Considerable uncertainties in the mortality effects complicate interpretation and decision-making, and require a continual effort to improve the precision of the estimates
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