Studies of the life history of a parasite : a basis for effective population management

Abstract

Kalatäit ovat pintaloisia, jotka imevät kaloista verta. Ne liikkuvat kalan pinnalla ja vaurioittavat ihon pintaa. Ihovauriot ja niistä johtuvat bakteeri- ja sienitaudit heikentävät kalojen arvoa ja voivat johtaa kasvun hidastumiseen ja kalan kuolemaan. Kalatäiden torjunta kalanviljelylaitoksissa on ollut vaikeaa ja tehotonta, koska niiden ekologiaa ja elinvaiheita ei ole tunnettu, kertoo Teija Hakalahti. Hakalahti tutki Argulus coregoni -kalatäin ekologiaa ja käyttäytymistä loisen elinkierron eri vaiheissa, joita ovat munat, munista kuoriutuvat vapaana elävät nuoruusvaiheet ja kalassa aikuistuvat loiset.

In this thesis, the life history traits of the ectoparasite Argulus coregoni were examined during its life cycle. Water temperature was a key determinant of temporal patterns of A. coregoni population abundance and dynamics; neither parasites attached to the host nor egg-hatching were recorded at temperatures below ~10ºC. Parasite transmission to their hosts was characterized by peak recruitment in spring followed by extended, but slower recruitment. Egg-hatching was triggered by cooling treatments, the magnitude of which was dependent on the period of cooling. Each female parasite, however, produced eggs that hatched over many hatching opportunities. This delayed emergence pattern can be seen as an adaptation to unpredictable, risky environments, where such bet-hedging may allow some infection to occur even after periodic absences of hosts. Female A. coregoni preferred to lay their eggs on dark substrates in the deepest water in a 2 m-deep canal, with the egg population showing an aggregated distribution pattern. Non-random distribution of eggs coupled with variable hatching may result in differences in host exposure to parasites, which was shown to be a dominant causative factor behind the observed aggregated distribution of A. coregoni on their rainbow trout (Oncorhynchus mykiss) hosts. Rainbow trout did not seem to develop any resistance mechanisms to prevent parasite settlement after first encounters with A. coregoni nor any general trend in repeated susceptibility of fish to parasite attachment observed. Both these findings suggest that the fish were of equal genetic susceptibility. Nevertheless, the attachment of parasite infective stages, that carried a finite energy resource for host seeking, seemed to be opportunistic and non-selective. Although the infectivity of A. coregoni remained high up to death, delayed starts with respect to host searching time were reflected on growth in the altered growth rate of settled parasites. Over time, parasites that had spent more time searching for a host reached the length of parasites that attached to a host faster. A. coregoni thus appeared to compensate for delayed growth resulting from an extended period off-host by accelerated growth, although, this was shown to incur a cost through decreased life-expectancy. This study has shown that individual parasites are more flexible in their life history strategies than has hitherto been understood. The results from this thesis provide parameter estimations for a detailed host-macroparasite model that will capture both host and parasite population dynamics, and can be used for planning cost-effective management against ectoparasite infestations at fish farms