Does the Arctic charr ( Salvelinus alpinus) exhibit a seasonal change in seawater tolerance?

Abstract

Parasites cause significant economic losses in fish farming, and knowledge of the mechanisms underlying their effects on hosts is an essential prerequisite of preventative procedures for the parasite problem in hatcheries. One such mechanism is local adaptation of parasites, which should lead to higher infectivity of parasites in sympatric host–parasite combinations compared to corresponding allopatric combinations. In this study, we investigated the infectivity of two ubiquitous species of trematode eye flukes, Diplostomum sp. and Tylodelphus clavata, in their farmed salmonid fish hosts with respect to origin of the parasite and the host. First, we exposed two anadromous and one landlocked Atlantic salmon (Salmo salar) populations to controlled infection by Diplostomum sp. and natural infection by T. clavata. The anadromous salmon populations were local to Diplostomum sp. and non-local to T. clavata. In the case of the landlocked salmon, the population was non-local to Diplostomum sp. and local to T. clavata. Second, we exposed a landlocked population of Arctic charr (Salvelinus alpinus) to Diplostomum sp. cercariae from one non-local and two local parasite populations. The results indicated that eye fluke infectivity in both Atlantic salmon and Arctic charr was 1.5–2.6 times higher in local parasite–host combinations compared to those separated by greater geographical distance. This suggests that diplostomid parasites are able to adapt to local host populations, which may have significant implications such as high abundance and prevalence of the parasites in endangered fish stocks that are maintained in hatcheries.