Ecology of midwater zooplankton and fishes off eastern Tasmania, Australia

This thesis examines the ecology of zooplankton, midwater fishes and top predators off the east coast of Tasmania in relation to the regional oceanography. This work was completed from small (<15 m) to large (60 + m) fishing and fisheries research vessels using nets ranging from fine-meshed plankton nets (100 µm) to large midwater trawls (cod end mesh size 10 mm). Further data was obtained from the Maria Island hydrographic station and the AFZ observer program. The study area was situated in and around the northern edge of the subtropical convergence zone. This zone separates the most southern edge of the East Australia Current from the broader subtropical convergence and subantarctic water to the south. The latitudinal position of this front depends not only on the time of year - in the summer it extends southward and retreats in winter - but also on interannual cycles such as the El Nino Southern Oscillation. On the shelf the distribution and biomass of the major zooplankton species, Nyctiphanes australis, was closely related to fluctuations in these water masses. This species is central to the shelf food web - it is, at times, the main prey for predators ranging in size from larval fishes (eg. those of jack mackerel Trachurus declivis) to large southern bluefin tuna (Thunnus maccoyii). In autumn, when its biomass is highest, schools of its main fish predator, jack mackerel form on the surface over the shelf where they are fished commercially. The central position of N. australis in the shelf food web was demonstrated in the summer of 1988/89 when warm waters from an anti El Nino event flushed the shelf resulting in the disappearance of the krill and the subsequent collapse of the jack mackerel fishery. Over the continental slope a suite of myctophid species dominated the water column. These lanternfish aggregate in dense schools over the slope in spring and summer, migrating to the surface at dusk and descending before dawn to depths between 300- 500 m. Their horizontal distribution is restricted mainly to a thin band (-500 m wide) over the 300 m contour. One of these, Lampanyctodes hectoris, was central to the upper slope food web. Examination of the reproductive cycles of L. hectoris and other abundant lanternfish showed multiple spawning over winter. The winter spawning is presumably timed so that juveniles can take advantage of increased prey levels, mainly euphausiids, which are generated by the spring bloom. Growth in L. hectoris, which lives up to 3 years, is fastest at this time. In winter they are scarce over the slope. The reasons for this scarcity are not clear, but it is at least partly due to the massive predation on them. However, lack of feed - copepods are the main prey over winter - may limit the size of aggregations that can be sustained. The concentrations of lanternfishes during spring and summer form the basis of a food chain supporting larger fishes such as blue grenadier (Macruronus novaezelandiae) and jack mackerel, and during summer, other larger lanternfish such as Diaphus danae. Offshore, a more diverse community of midwater fishes was identified, which, although similar among the different water masses, was significantly different in the surface waters of the East Australia Current, the result of a relative increase in species of subtropical origin. Densities were an order of magnitude lower than that found for the slope lanternfish. The diets of lanternfish ~ff shore were dominated by calanoid copepods, particularly those of the genus Plueromamma. Non-myctophids within the same size range ate a wider range of prey. Some, such as the Stomiatoid genus Chauliodus, were entirely piscivorous on lanternfish. Estimation of the relative biomass of zooplankton and micronekton from the main geographic and oceanographic regions of the area showed that shelf biomass was significantly higher than that offshore. This increased biomass appears to be derived from a mixture of subtropical convergence water washing over the shelf and shelfbreak upwelling. The higher biomass over the shelf was reflected in the daily ration of shelf-caught southern bluefin tuna (Thunnus maccoyii), which fed mainly on jack mackerel and had rations -3 times that of offshore-caught tuna. I identified 10 trophic categories off eastern Tasmania across the shelf to the open sea. These were-large pelagic omnivores (eg. Thunnus maccoyii), pelagic omnivores (eg. Trachurus declivis), pelagic piscivores (eg. Brama brama), small mesopelagic omnivores (eg. Diaphus danae, Lampanyctus australis and Chauliodus sloanii), neritic planktivores (eg. Lampanyctodes hectoris), bathypelagic omnivores (eg. Hoplostethus atlanticus), squid (eg. Nototodarus gouldii), gelatinous zooplankton (eg. Pyrosoma pyrosoma), oceanic (eg. Pleuromamma spp. and Phronima sedentaria) and shelf (eg. Nyctiphanes australis) zooplankton. A common theme, which has run through many of these studies, was the link between inshore and offshore processes. Although the dominant zooplankton and micronekton species were usually restricted in their distributions (eg. Nyctiphanes australis with the shelf), their main predators moved freely between the inshore and offshore waters. I believe that the movements of these predators are determined largely by prey availability. Thus, prey availability, itself dependent upon seasonal and interannual cycles in the regional oceanography, appears to drive much of the seasonal and interannual patterns of abundance of larger fishes in the region. Two of these species, jack mackerel and southern bluefin tuna, are the focus of commercial fisheries. Their effective management therefore will need to consider (1) the dependence of the tuna on the mackerel and (2) that both species in this area depend, either directly or indirectly on krill stocks, which in tum depend on the fluctuations of the regional oceanography. Finally, over the period of this study there have been a number of advances both in techniques and technologies in the study of midwater communities. I have therefore summarised those techniques and approaches to studying the midwater that I think have benefited this area of research. I have also identified some of the difficulties that need addressing.