Understanding Natural Shellfish Colonisation Processes

Natural recolonisation of a suitable habitat may occur if larvae and juveniles originating from elsewhere are transported by currents to the area and settle.

This has the advantage of being a natural process, although it may take some time to see the populations growing.   There are many methods to determine whether natural recolonisation of a species is likely to occur, these include:

  • Field observations and field and laboratory experiments to determine how long and how far juveniles of a particular shellfish species are capable of moving (i.e. use of a flume)
  • Modeling the hydrodynamic conditions of the estuary, including currents and tides, to determine where in the estuary larvae and juveniles might originate from and/or be transported to.

Movement of Juvenile Shellfish

University of Waikato flumes (Lundquist, 2008)

Are the shellfish capable of naturally recolonising the site? As already mentioned, a successful restoration project requires a good understanding of the mobility/motility of the species. NIWA has conducted field studies to determine which New Zealand estuarine shellfish species move around by drifting in the water column (Cummings et al. 1995), and to estimate the distances these shellfish might be moving (Norkko et al. 2001). NIWA has also conducted laboratory studies to determine the mode of transport and so how far juvenile shellfish are likely to be moving.

On the right you can see a laboratory flume, which is a large aquarium with propeller-drive water flow that mimics the movement of water near the sea floor.  NIWA scientists, in collaboration with scientists at the University of Waikato, have used the laboratory flume located at the University of Waikato Aquatic Centre to look at the post-settlement transport of juvenile wedge shells (Macomona liliana) and cockles (Austrovenus stutchburyi) (see Lundquist & Pilditch, 2006)

Modelling Hydrodynamic Conditions

Figure: Dispersal of larvae (yellow) from the relase point (blue arrow)

A) Parua Bay, B) Whangarei West, C) McDonald Bank & D) Snake Bank, under weighted conditions (Lundquist et al, 2008)

The ability to predict shellfish dispersal patterns in estuaries can guide restoration efforts. Knowledge of shellfish dispersal patterns can result in better integration of whole estuarine restoration schemes, as locations may be chosen for restoration projects based on both their ability to self-seed, and their likelihood of dispersing larvae elsewhere in the estuary (Lundquist & Broekhuizen, 2008; Lundquist et al. 2009). For example, active reseeding effort can be reduced at a site where natural recolonisation may be expected or, conversely, targeting an area where it might not be.

Scientists at NIWA have developed a combined hydrodynamic and particle tracking model to predict the transport of larval cockles in Whangarei Harbour. Hydrodynamic modelling involves the use of computer technology to combine real information on movement of currents, tides, and shellfish larval biology. Results indicate that larvae released from sites within the more sheltered parts of the estuary travelled short distances compared to those released from sites closer to the mouth. In addition, some areas of Whangarei Harbour receive extensive numbers of larval recruits and are more likely to be naturally restored and remain independent, self-seeding shellfish populations.