Climate and the fisheries off Tasmania — interactions of physics, food chains and fish

Abstract

The impact of climate on the fishery for jack mackerel Trachurus declivis off Tasmania has been traced to an interaction of the 40-day oscillation in the Zonal Westerly Wind (ZWW), driven by latitudinal changes in the position of the subtropical high pressure cells over south-eastern Australia, with ocean circulation and water column stability. Nitrate concentrations are strongly correlated with temperature in these waters. High ZWW stress causes advection of colder, nutrient-rich subantarctic water up the eastern side of Tasmania and reduces water column stability. The result is periodic overturn of the water column and increased new production. Spring blooms tend to be later, but stronger, as a result of periodic mixing and restratification. Reduced ZWW stress (under the influence of high pressure) leads to incursions of subtropical water, increased water column stability and reduced biological production. In calm years spring blooms tend to be earlier, but weaker, because of the reduced nitrate availability. Changes in the strength of the 40-day oscillation cause changes in the dominant zooplankton populations: From salps in windy periods, through a mixture of krill and salps in normal years, to copepods in calm periods. Reduced nutrient availability in warm years leads to reduced new production and a drastic reduction in the biomass of larger zooplankton, especially krill. In summer, jack mackerel feed on krill in coastal waters. If krill are rare and do not swarm in sufficient numbers, the jack mackerel do not school in commercial quantities and the fishery fails. These events have occurred at about 10-year intervals during anti-ENSO events. This is a very clear example of climate-induced, “bottom up” control of the distribution of a commercial fish stock. © 1992 Taylor & Francis Group, LLC.