The East Cape Current and two eddies: a mechanism for larval retention?

Current meters deployed near East Cape, New Zealand, for 1 year show large differences between the offshore flow and that inshore of the 1000 m isobath. Offshore, mean flows are to the south-east, and are the East Cape Current. Inshore, mean flows are to the north-east, indicating that there may be a persistent inshore counter current. Comparison between the offshore current meter measurements and geostrophic currents inferred from TOPEX/Poseidon altimetry shows good agreement. We use this agreement to argue that passive drifter trajectories can be modelled using the altimeter data. By simulating drifter releases into the region, and modelling their trajectories, we develop a statistical picture of likely retention times for passive drifters. Drifters can get retained in one or other of two permanent eddies: the East Cape and Wairarapa Eddies, and retention within the system can be as high as 2-3 years, but depends on release time and location. If weak-swimming larvae such as rock lobster larvae behave as passive drifters, retention and recirculation within the eddies may provide a mechanism allowing them to survive within the system long enough to recruit as juvenile lobsters.