Abstract

The negative effects of parasitism on host population dynamics may be mediated by plastic compensatory life-history changes in hosts. Theory predicts that hosts should shift their life-history towards early reproduction in response to virulent pathogens to maximize reproduction before death. However, for sublethal infections that affect growth, hosts whose fecundity is correlated with body size are predicted to shift towards delayed reproduction associated with larger body size and higher fecundity. This has been observed in Atlantic salmon and parasitic sea lice, via mark-recapture studies that recover mature fish from paired groups of control and parasiticide-treated smolts. We investigated whether such louse-induced changes to age at maturity can offset some of the negative effect of mortality on population growth rate in salmon using a structured population matrix model. Model results show that delayed maturity can partially compensate for reduced survival. However, this only occurs when marine survival is moderate to poor and growth conditions at sea are good. Also, the impact of delayed maturity on population growth when parameterizing the model with empirical data is negligible compared with effects of direct mortality. Our model thus suggests that management that works on minimizing the effect of sea lice from fish farms on wild salmon should focus mainly on correctly quantifying the effect of parasite-induced mortality during the smolt stage if the goal is to maximize population growth rate.

Highlights

  • Plasticity in life-history traits allows animals, such as salmon, to respond to a fluctuating environment (Caswell 1983; Hutchings and Jones 1998), including parasitism

  • Delayed age at maturity has been observed in Atlantic salmon in relation to parasitism by sea lice (Lepophtheirus salmonis and Caligus elongatus) via experimental studies that release tagged smolts in paired groups, one a control and the other receiving a treatment against marine ectoparasitic copepods (Vollset et al 2014)

  • The population growth rate had high elasticity to changes in the transition values from parr to adult, which is the stage transition that sea lice affect in the model

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Summary

Introduction

Plasticity in life-history traits allows animals, such as salmon, to respond to a fluctuating environment (Caswell 1983; Hutchings and Jones 1998), including parasitism. Delayed age at maturity has been observed in Atlantic salmon in relation to parasitism by sea lice (Lepophtheirus salmonis and Caligus elongatus) via experimental studies that release tagged smolts in paired groups, one a control and the other receiving a treatment against marine ectoparasitic copepods (randomized control trials, RCTs) (Vollset et al 2014) Fish impacted by these parasites (i.e. untreated individuals) are fewer (Gargan et al 2012; Jackson et al 2013; Krkosek et al 2013; Skilbrei et al 2013; Vollset et al 2015) and older, and as a consequence larger, and more fecund, when they return to the river to spawn.

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