The distribution and abundance of euphausiids in Cook Strait

Abstract

Three currents of mixed subtropical origin reach Cook Strait. Blue subtropical D'Urville Current water curves eastward through the South Taranaki Bight and is well marked as a surface tongue in the Cook Strait Narrows. Mixing occurs with a clockwise gyral of cool green Southland Current water in southern Cook Strait, and the mixed subtropical water washes the southern coast of the North Island as it flows east. The East Cape Current has little influence apart from generating the water mass in and above the Cook Strait Canyon. Turbulent tidal flows superimposed on the oceanic circulation maintain an homogeneous layer of mixed water over the continental shelves throughout the Strait, particularly in the Narrows. Within this “tidal water” vertical and horizontal gradients in water properties are small, and the hydrology of Cook Strait is largely governed by the effect of the tides on the movement of water masses. The euphausiid fauna reflects the predominantly subtropical origin of the coastal currents and, of the species present, only Euphausia vallentini has pronounced subantarctic affinities. Nyctiphanes australis is the dominant species, with Euphausia lucens and Thysanoessa gregaria present in lesser numbers. New records included E.recurva, E.spinifera, and Stylocheiron elongatum, and the identity of Southland Current Nematoscelis euphausiids as N.megalops was confirmed. Hydrological factors are not important in limiting the distribution and abundance of the more common euphausiids in the horizontal or vertical planes. Some Cook Strait populations are distributed over different temperatures and salinities from populations of the same species in other areas, and there is evidence of acclimatization to local conditions. These physiological adaptations are paralleled by morphological and behavioural differences in the case of N.australis. This species breeds at a low intensity throughout the year, but with two peaks in larval production (unlike Australian populations) coinciding with spring and late summer diatom blooms. The Subtropical Convergence is not a barrier to euphausiid dispersal in coastal waters, and the study suggested that a broad Subtropical Convergence Region with distinctive hydrological and biological features can be recognized.