Possible impacts of the greenhouse effect on the distribution and fisheries of pelagic fishes off south-eastern Australia.

Predictions of greenhouse related rises in global temperature have to date mainly concentrated on air temperature. Very few extrapolate to likely effects on ocean temperatures, other than to suggest that these also will rise, albeit after some time-lag due to heat transfer effects. Sea level rises are predicted on the basis of two effects, both related to temperature: melting of the polar ice caps (long term) and, more immediately, thermal expansion of the oceans due to an overall increase in water temperature. Given that global air temperature is predicted to rise by 2-4 oC by the year 2030, a rise in average sea surface temperature of about 3 o for midlatitudes is fairly widely predicted (Stark 1988, De Sylva 1989). In this event, the marine fishes most likely to be immediately affected would be those which inhabit the surface layers of the ocean, the many and varied pelagic species. While a number of such species are small schooling fish, the so-called bait fishes, I will confine this paper to the potential impacts of the Greenhouse effect on the larger predatory fishes of the surface layer, and in particular the tunas and billfishes. These are of special concern since they form the basis of very important existing commercial and recreational fisheries off eastern Australia