High host densities dilute sea lice Lepeophtheirus salmonis loads on individual Atlantic salmon, but do not reduce lice infection success

F Samsing,D Johansson,T Dempster,F Oppedal,S Bui

doi:10.3354/aei00118

Abstract

Host density likely plays a key role in hostparasite interactions, but empirical evi- dence in marine ecosystems remains limited. Classical models predict a positive relationship between host density and parasite infection parameters, but this depends on the parasite transmis- sion mode. Evidence from systems where mobile parasites actively seek hosts suggests that num- bers of parasites per host decrease with increasing host density ('dilution effect'). Copepodids, the infective stage of the salmon louse Lepeophtheirus salmonis, are mobile larvae that display a range of behaviours to detect their salmonid hosts. We hypothesized that high host density would decrease infection intensity, prevalence and degree of aggregation, but not infection success, which reflects parasite performance. We infected multiple groups of Atlantic salmon Salmo salar post-smolts at low (12 fish; 7.9 kg m �3 ) and high (96 fish; 68.5 kg m �3 ) densities, with the same number of L. salmonis copepodids in swimming chambers to enable more realistic swimming behaviours during infection. Infection intensity was 8.4 times higher in the low density treatment, but there were no differences in infection success and degree of aggregation. We observed 100% prevalence in the low density treatment, which was significantly higher than the high density treatment (68%). The dilution effect most likely explained the negative relationship between host density and infection intensity, as the individual risk of being 'attacked' by a parasite decreased as host density increased. Host density is crucial in salmonsea lice infection dynamics, and opportunities may exist within production environments to use the dilution effect of density to improve fish welfare outcomes.

Highlights

Host density is a fundamental cornerstone of epidemiological theory and disease dynamics (Anderson & May 1991), but compelling experimental evidence is still scarce (Côté & Poulin 1995, Lloyd-Smith et al 2005)
Sea lice infection intensity differed between the 2 density treatments; low density had 8.4 times more lice per fish than high density (F1,6 = 26, p = 0.001; Fig. 2), which matches the 8 times more fish in the high density
Grouping or shoaling in fish is widely recognized as an anti-predator mechanism that exerts its effect through the dilution of the individual risk of being eaten infection success (F1,6 = 0.04, p = 0.8), which was approximately 12% in both treatments, and no difparasite interactions (Schmidt & Ostfeld 2001), where the increase in host density reduces the per capita ferences in the proportion of sea lice copepodids parasite load

Summary

Introduction

Host density is a fundamental cornerstone of epidemiological theory and disease dynamics (Anderson & May 1991), but compelling experimental evidence is still scarce (Côté & Poulin 1995, Lloyd-Smith et al 2005). Classical models (Anderson & May 1978, 1979) predict that the mean number of parasites per Publisher: Inter-Research · www.int-res.com. Côté & Poulin (1995) found that the direction of effect of host density upon infection intensity was governed by parasite transmission mode. The relationship between host density and mean number of parasites per host is positive for contagious or directly transmitted agents (transmitted through close proximity between hosts or through infected faeces), and generally negative for mobile parasites which actively seek their host. When a host is encountered, the lice attach and move into the fixed chalimus stage (Bron et al 1991). Lice become mobile in the pre-adult and adult stages, and freely move around their host (Morton et al 2004, Hamre et al 2013) or transfer to other hosts when searching for mates

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