ASFB Home > Potential Changes in Prey Population Structure Following Removal of Predators by Fishing

Balancing Native Fish Diversity, Exotic Fish Impacts, and Recreational Fishing in New Zealand North Island Dune Lakes

David Rowe

National Institute of Water and Atmospheric Research, HAMILTON, NEW ZEALAND

Theme: TH1

The galaxiid fish (Galaxias gracilis) now occurs in 11 lakes on the west coast of the North Island of New Zealand. It was once abundant but is now rare in five lakes and extinct in at least two others. Its conservation is therefore a priority. Studies of its ecology and life history indicated that whereas juveniles are pelagic and planktivorous, adult fish depend on larger invertebrates for food, so feeding in the littoral zone. A six-year long experiment was carried out in two lakes where this species is rare, to determine whether predation by rainbow trout (Oncorhynchus mykiss) and/or impacts by mosquito fish (Gambusia affinis) were responsible for its population decline. Angling organisations halted trout stocking in the experimental lake, but not in the reference lake, and remaining trout were then removed. Trout removal increased the abundance of juvenile galaxiids in the experimental lake relative to the reference lake, but adult galaxiids remained rare. The reason for this was the increase in mosquito fish over summer and autumn in the experimental lake. Mosquito fish were observed attacking and killing large numbers of adult galaxiids when these fish attempted to feed in the littoral zone. As the increase in mosquito fish numbers was thought to be caused by trout removal, and was a greater threat to the galaxiids than trout predation, trout stocking was resumed to restore the balance between native fish, exotic fish impacts, and trout angling. This experiment shows the complexity involved in managing fish diversity in protected areas.