Young, Jock W 1998
, University of Tasmania145 pp.
On the shelf the distribution and biomass of the major zooplankton species, krill (Nyctiphanes australis), was closely related to fluctuations in these water masses. In autumn, when its biomass is highest, schools of its main predator, jack mackerel (Trachurus declivis) form on the surface over the shelf where they are fished commercially. The central position of the krill 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 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 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 reason(s) for this scarcity is not clear but is probably the result of 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 offshore 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 the biomass of the shelf 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 shelf-break 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 6 midwater fish trophic categories and four main prey categories off eastern Tasmania across the shelf to the open sea. The fish groups were - large pelagic omnivores (eg. Thunnus maccoyii), pelagic omnivores (eg. Trachurus declivis), pelagic piscivores (eg. Brama brama), small mesopelagic omnivores (e.g. Diaphus danae, Lampanyctus australis and Chauliodus sloanii), neritic planktivores (e.g.Lampanyctodes hectoris) and bathypelagic omnivores (e.g. Hoplostethus atlanticus). The prey groups were - squid (e.g.Nototodarus gouldii), gelatinous zooplankton (e.g. Pyrosoma pyrosoma), oceanic (e.g.Pleuromamma spp. and Phronima sedentaria) and shelf (e.g.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 (e.g. Nyctiphanes australis with the shelf), their main predators moved freely between the inshore and offshore waters. I propose 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. As two of these species, jack mackerel and southern bluefin tuna, are the focus of commercial fisheries their effective management 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 depend upon krill stocks, which in turn depend on the fluctuations of the regional oceanography.