Agonistic interactions between Gambusia affinis and Galaxias maculatus: implications for whitebait fisheries in New Zealand rivers

Laboratory tank studies were used to identify the outcome of behavioural interactions between Gambusia affinis (gambusia) and Galaxias maculatus (inanga), whereas field data on spatial distribution were used to determine whether gambusia could be affecting inanga populations in the wild. Mortality rates for juvenile inanga (mean length 42 mm) exposed to gambusia (mean length 32 mm) for 15 days at 10°C were low (<10%) and no greater than those for control fish. However, at temperatures between 15 and 25°C, gambusia nipped the fins of juvenile inanga, resulting in immobilisation and death. Cumulative mortality rates over 15 days at these temperatures were over 60%. The survival of larger (mean length 58 mm) adult inanga was also reduced by gambusia. Adult inanga occurred in 15% of shallow (<0.5 m) sites where gambusia was present compared with 33% of shallow sites where it was absent. Furthermore, the mean depth of sites containing inanga in swamps and wetlands where agonistic interactions would be greatest was 1.8 m where gambusia was present compared with 1.0 m where it was absent. Displacement of inanga into deeper waters is therefore likely to be occurring as a consequence of gambusia aggression. The spread of Gambusia in New Zealand has the potential to reduce the use of shallow-water habitats by inanga. This would further exacerbate the historic decline of these galaxiid fisheries related to land-use changes.