Shellfish farming structures such as intertidal racks, trestles or longlines may alter the hydrodynamics of an area (Kaiser et al., 1998), though such changes have been rarely documented (Crawford, 2001).
Anecdotal evidence suggests that the effects of racking on hydrodynamics can be seen in some culture areas in Tasmania, with organic buildup evident underneath farm structures in areas of high current flow.
Additionally, Thorne (1998) proposed that sediment accretion around racks at Pipeclay Lagoon in south-eastern Tasmania may be due to hydrodynamic modifications. Sediment accretion and compaction may also result from heavy machinery use in shellfish growing areas (Crawford, 2001).
De Graves et al. (1998) concluded that the compaction and dispersal of sediments by heavy vehicle traffic may affect the composition and abundance of benthic species.
In addition, shading by the shellfish , farm infrastructure, and farm activities such as boat and vehicular traffic may have a detrimental impact on seagrass beds. The extent of this impact will depend on the species affected.
For example, Thorne (1998) observed a reduction in seagrass cover (presumably Heterozostera tasmanica) under stocked oyster racks in Tasmania. He also noted that the seagrass appeared to recover in areas left unstocked for any length of time.
This species is known to be capable of rapid regeneration (Crawford, 2001), however the potential for permanent loss is much higher for other species more sensitive to disturbance (eg. Posidonia).
Seedstock:
Where seedstock is sourced from the wild there is potential for collection to have an impact on native stocks. This is not generally considered a cause for concern in the South-east Marine Region. Hatchery produced spat is the preferred means of collection, and where seed is sourced from the wild, such as for mussel culture, the impacts are considered low due to the scale of the collection (Gavine & McKinnon, 2001, Christine Crawford, TAFI, pers. comm.).
Genetic integrity of wild stock and the introduction/translocation of pest species:
The introduction and translocation of commercial shellfish species potentially poses a number of risks. These risks are outlined by the National Policy for the Translocation of Live Aquatic organisms (AFFA, 1999) and include environmental and ecological issues. Environmental issues include the potential for genetic shifts in wild populations, establishment of feral populations and impacts associated with the unintentional translocation of associated species.
As hatchery production of seed increases, the potential to alter the genetic characteristics of wild stocks will increase (Crawford, 2001). Translocated species may breed with other distinct populations of the same species, possibly resulting in a genetic shift in the local population, and a loss of genetic diversity. Similarly, hybridisation may occur between endemic species and translocated species where the species are genetically compatible.
The accidental introduction of exotic pathogens and subsequent infection of existing native species also poses a threat to the marine environment. These exotic pathogens may be introduced with the commercial species itself, or alternatively, shellfish culture may facilitate the transfer of endemic pathogens to new areas. The problems associated with the establishment of feral populations are described in Chapter 1.
Chemical usage:
Chemical usage on shellfish farms is minimal (Crawford, 2001). Chromium, copper and arsenic treated pine is frequently used for intertidal racking, however the treatment process prevents the accumulation of these heavy metals in the environment (Crawford, 2001).
Information Sourced From:
