Abstract
Fish have critical periods during early life where mortality is extremely high. There are a multitude of factors which can influence mortality, however, parasitism is often overlooked as a causal factor of mortality during this period. Understanding the extent to which biotic and abiotic factors, such as parasites, can influence fish survival, is paramount for effective management of threatened species. The purpose of this study was to investigate the effects of a trematode parasite, Telogaster opisthorchis (a parasite known to cause malformations in young fish), on the critical larval stage of its second intermediate fish host Galaxias anomalus, a small, endemic, freshwater fish vulnerable to extinction. The influence of T. opisthorchis on the condition, size, and survival of G. anomalus fish was tested by experimentally infecting larval fish of different ages with different doses under standard conditions. Additionally, the likelihood that parasites cause malformations and their influence on the swimming ability of their hosts were also investigated. Swimming tests were carried out using a unidirectional propeller-driven flume where individual fish swimming ability was monitored over a fixed period. Results demonstrate that T. opisthorchis can affect G. anomalus survival directly via reduced condition, and indirectly via malformations and lowered swimming ability. However there was no effect of parasitism on condition or size, nor was there any effect of infection level on the presence of skeletal malformations, the reasons for which are discussed in the text. The effect of parasitism in early life stages of fish influences survival, and in wild populations, may be exacerbated by additional stressors, with consequences at both the population and community level.