Shellfish farm-induced changes to tidal circulation in an embayment, and implications for seston depletion

Abstract

Marine farm structures cause drag, which can affect currents over large areas. This study uses numerical modelling to consider the effect of long-line mussel farms on tidal currents within embayments. The modelling illustrates how currents can be altered over most of a bay by farms that occupy a relatively small (10%) proportion of the bay. The effect of several small farms located around a bay perimeter are compared to that of a large farm of equivalent area placed in different parts of the bay. While results show that current speeds are reduced in most places, particularly inside farms, different farm locations may increase velocities in some parts of a bay. Changes in currents caused by farm drag have an important influence on potential seston depletion, which is simulated using a tracer released from farmed areas. A simple decay rate is used to simulate plankton regrowth and seston replenishment. The depth-averaged modelling is based on 2 neighbouring bays: Port Ligar and Waihinau Bay, in the Pelorus Sound, New Zealand. Existing mussel farms in these bays occupy 10% of the area of each bay and are located near (< 400 m) to shore, forming a broken ribbon around the perimeter of each bay. Existing farms reduce mean current speeds by an average of 7% in Port Ligar and 3% in Waihinau Bay. The difference is attributed to the different aspect ratios of the 2 bays. Neglecting farm drag results in over-predicting dispersal and under-predicting the degree of depletion within farms. At both sites, seston depletion is least likely to occur in farms placed in the outer parts of bays where currents are faster. © Inter-Research 2011.