Coarse fish: the demise of plants and malaise of lakes?

Winton, M. de; Dugdale, T.; Clayton, J.

`Coarse fish' were introduced to New Zealand for angling and ornamental purposes, but their presence may have far-reaching consequences within our water bodies. Through their deleterious impacts on water quality and direct disturbance of submerged plants, large populations of these alien fish have the potential to cause substantial and undesirable changes to lake and river ecosystems. Moreover, the continued influence of coarse fish may prevent the recovery or restoration of such water bodies. In this paper, we review international findings on the incompatibility of coarse fish and submerged vegetation resources that raised an alarm for the New Zealand situation. We also present recent results from NIWA's research programme. Experiments on feeding preference confirm that herbivorous rudd (Scardinius erythropthalmus) present a particular risk to native submerged plants and may facilitate invasion by less palatable alien weeds. Rudd consumption rates were found to be seasonal and we confirmed a grazing preference for native plants over weed species. Fish exclusion from areas of a shallow eutrophic lake (Hamilton Lake, or Lake Rotoroa) that supports an abundant coarse fishery, was found to substantially increase the early establishment and survival of submerged plants. The complexities of plant-fish relationships and their outcome for vegetation development and persistence are considered using a simple, illustrative model. Some management issues are considered in the light of research results and NIWA's past experiences with aquatic plant pests. Finally, some future research priorities are suggested.