Authors

Discussion

Parasitic sea lice are ectoparasites infesting Atlantic salmon. In Scotland, the species of interest are the salmonid specialist L salmonis and the generalist C elongatus. Mobile stages move across host surface feeding on its skin and blood. In recent decades, wild Atlantic salmon populations have declined globally, corresponding with an increase in salmon cultivation since 1960s which has escalated host and parasite densities. This led to the interest in sea lice – wild salmonid interactions due to their potential impact on wild returns. Laboratory experiments show that sea lice elicit sub-lethal effects causing stress, a reduction in growth and condition.We evaluated the influence of sea lice on body condition of returning salmon using data from Strathy Point (N Scotland). We show that sea lice reduce condition by 4 % (1-11). Applying this condition effect to female salmon indicates an ova reduction by 3 % (0-11). Being more likely to die at sea, fish in poor condition (potentially highly infested) are underrepresented. Thus it is crucial to note, that the described effect from sea lice on condition (and thus fecundity) is likely underrated. This is the first study revealing a non-lethal impact from sea lice in wild Atlantic salmon. Furthermore, we also derive a useful proxy (i.e. condition) for fecundity incorporating both fish weight and length, which can improve current management practise. We show that a sea lice-mediated condition-effect has the potential to diminish Atlantic salmon stock components and thus can influence population dynamics.