Infection- and size-based predation risk of juvenile salmon during freshwater migration

Background/Question/Methods:  Predation plays a large role in structuring ecosystems, and predators can exert substantial selection pressures on prey. Few empirical examples exist of predators selecting for infected prey, particularly when prey are migrant. This study assessed the microparasite infection status and body size of juvenile sockeye salmon (Oncorhynchus nerka) smolts out-migrating from Chilko Lake in interior British Columbia. Metrics of condition (infection prevalence and load, and body size) were compared between smolts within the general population (sampled at random) and those predated by bull trout (Salvelinus confluentus), a predator that feeds exclusively on smolts during the spring migration. We used a novel molecular genomics platform (high-throughput quantitative real-time reverse transcriptase polymerase chain reaction [HT-qRT-PCR]) to screen smolts for the presence of > 40 microparasites found in salmon worldwide. Using the same platform, the expression of six smolt host genes associated with immune response were also assessed. Microparasite-based selection on smolts was assessed for two years (2014 and 2015) and size-based selection was assessed for three years (2013 – 2015). In total, 108 predated and 50 non-predated smolts were assessed for microparasite prevalence and load. Over 600 predated smolts were measured for body size (length).

Results/Conclusions:  Use of the HT-qRT-PCR resulted in the broadest known screening of microparasites in juvenile sockeye salmon to date, and revealed 10 microparasites. Infection by infectious haematopoietic necrosis virus (IHNV) resulted in a 34-times increase in odds of predation for smolts. The impact of this infection corroborates a prior study that associated IHNV with mortality in smolts tagged with telemetry transmitters. However, the infection was only observed in one of the two study years (2014). No links between infection and predation were found for the other nine microparasites identified, but in 2014, smolts with increased microparasite burdens (greater pathogen diversity and loads) experienced higher predation risk. In addition, regulation of two immune genes (MMP13 and MX) were impacted by predation status. Size-based selection occurred in all three years of the study, with smaller smolts at increased risk of predation. The potential consequences of infection-based selection by predators are numerous, including potential culling of pathogens from migrant populations and other impacts on prey population dynamics. Such selection can also affect the efficacy of predator control programs. Therefore, monitoring of managed populations may need to include the health or condition of individuals in addition to population-level mortality rates due to predation.