The impact of trout on galaxiid fishes in New Zealand

Compared with the effect of invaders on the native terrestrial fauna of New Zealand, interactions between native fishes and introduced trout (sports fish in the genera Salmo, Oncorhynchus and Salvelinus) are less well known and there have been fewer efforts to remedy their effects. Trout have caused widespread reductions in the distribution and abundance of native galaxiid fishes, a family dominated by threatened species. The effects have been most severe on non-diadromous species (those lacking a marine migratory stage), which are commonly eliminated from streams by trout. Galaxiid populations in lakes, and those with migratory ‘whitebait’ stages, have also been affected, but the extent of the impacts are less understood. The mechanisms controlling negative interactions between trout and native fish, and how the environment modifies those interactions, will be important for future management. Experiments and field comparisons indicate size-specific predation by trout is the main driver of negative interactions. Large trout (>150 mm long) do the greatest damage and small galaxiids (those with adult sizes <150 mm long) are the most at risk. The fry stage of non-diadromous galaxiids is particularly vulnerable. Despite galaxiid fry production in some trout-invaded reaches, often no fry survive making them population ‘sinks’ that must be sustained by adult dispersal. Trout are also associated with changes in galaxiid behaviour and alterations to stream benthic communities. However, effects on galaxiid growth and fecundity have been little studied. Recent work also indicates that habitat conditions, especially floods, low flows and natural acidity, can mediate trout–galaxiid interactions. We argue that managers should be more proactive in their response to the plight of galaxiids, and we identify avenues of research that will benefit native fish conservation activities in the future.