Abstract
Commercial fishing often reduces densities and changes the structure of target populations. Within these target populations, the transmission dynamics of parasites are usually strongly dependent on host densities. Because parasites have direct impacts on their host and also indirect effects on interspecific interactions and ecosystem function, harvest-induced reductions in host density can have wide repercussions.
The present research investigates the effect of commercial harvesting on echinostome parasite infection within the Otago clam fishery. Clams (Austrovenus stutchburyi) have been commercially harvested from Blueskin Bay since 1983, and since 2009, experimental harvesting has extended to beds within Otago Harbour. Parasite loads (numbers of echinostome metacercariae per clam) within areas of high fishing pressure were compared to adjacent unharvested areas to assess the effects of harvesting on parasite abundance in clams. At 14 sites within Blueskin Bay and Otago Harbour, a subset of shellfish of varying age and sizes were analysed for parasite load. Unparasitised juvenile clams were also collected and caged in situ over a winter and summer period to monitor spatial and temporal patterns of parasite infection. Finally, the effect of parasite load on the growth rate of clams was monitored both in situ and in a laboratory experiment, in which juvenile clams were infected with varying levels of parasite and mortality, body condition and foot length was quantified.
Overall, commercial harvesting resulted in a 36% increase in average parasite load compared to control areas. Although mortality was unaffected by parasite infection, high parasite loads were found to deleteriously affect the growth, body condition and foot length of clams. Therefore, harvesting has the potential to alter both the local transmission of echinostome parasites and their impact on individual hosts. Increased parasite load not only affects clam physiology, but can also cause behavioural changes resulting in broader impacts on the surrounding ecological community.
Harvesters may be able to reduce their influence on parasite infection levels in clam populations by harvesting less intensively, and allowing more time between harvests for clam biomass to regenerate.