Influence of the tidal cycle and a tidal intrusion front on the spatio-temporal distribution of coastal bottlenose dolphins

Abstract

Small scale hydrographic fronts may act as convergence zones in which organisms accumulate, and have been used preferentially by several marine predators. In this paper, bottlenose dolphin Tursiops truncatus spatial and temporal distribution was investigated in relation to the tidal cycle and to the spatial and temporal evolution of a tidal front in a narrow estuarine channel. Estimates of dolphin abundance during all states of the tidal cycle, and positions of the frontal surface features and of dolphin groups were obtained from land-based observations. Hydrographic data was obtained on the structure of the front. The results showed that the front was characteristic of tidal intrusion fronts and was predictable both temporally and spatially. Its development could be divided into 3 stages. In the first stage the front was determined by the tidal flows and by an unusual freshwater input at the mouth of the estuary. In later stages the bathymetry and morphology of the estuary dominated and the front became stationary until the tidal inflow stopped, causing the front to dissipate. Dolphins were most abundant during the flood tide, particularly during the stationary stage of the tidal front. Dolphins also showed a spatial association with the area near the surface features of the front, which could be related to increased foraging efficiency resulting from the accumulation of prey in the frontal region. Further work that takes account of the 3D distribution and behaviour of the dolphins and their prey with relation to the front is now required to fully understand the nature of this relationship.