Mariculture in Australia

Genetic Conservation & Aquatic Biodiversity

In aquaculture, alien species and genotypes, also known as introduced species and genetically improved species, are a valid means to increase production. However, there is concern that these species will adversely affect local ecosystems.

Four broad categories exist for ecological impacts:

• basic species interactions such as predation and competition
• genetic impacts
• disease impacts
• habitat alteration

Competition
Competition can occur between alien and resident species for food, habitat, mates, or other essential resources. Resident communities have evolved together and can have learned to partition resources; an invader disturbs this partitioning. Theexpansion of salmon farming in British Columbia has been stoped because evidence that exotic Atlantic salmon have escaped from cages and have reproduced in the Tsitika River. The fear is that Atlantic salmon will outcompete wild stocks or will contaminate the native gene pool. A similar fear is expressed in Norway where farmed salmon constitute about 30% of all salmon spawning and actually outnumber wild salmon in many Norwegian river systems.

The Pacific oyster, Crassostrea gigas, was introduced to Australia in the 1940s and since has spread to areas where the native C. commercialis and the Sydney rock oyster, Saccostrea commercialis, are farmed. Because of high freproductive rate and rapid growth rate, the Pacific oyster is crowding out these local species and has been declared a pest in Port Stephens (New South Wales).

Not all impacts on natural stocks are harmful. Fish farming can actually lessen the decline of fish numbers already reduced by overfishing and environmental changes. As well as decreasing the need to use natural stocks, fish farming may help to re-stock populations by the release of cultured larvae or young fish into the wild to increase natural populations.

It has been suggested, however, that the genetic diversity of natural stocks can be hurt by aquaculture. Farmed species are often bred or genetically engineered to grow very rapidly, usually at the expense of other characteristics considered unimportant in an aquaculture operation.

Genetic interactions

Possible genetic impacts from alien species include:

• loss of species integrity from mixing with alien genotypes,
• reduced reproductive efficiency from hybridizing with alien species resulting in nonviable offspring,
• decrease in fitness from incorporation of alien genes or the loss of co-adapted gene complexes, and
• indirect genetic impacts resulting from other ecological interactions e.g. competition or predation reducing a native population to the point where genetic diversity is lost or inbreeding becomes problematic. Salmon, with their homing ability, genetic sub-population structure and complicated life-history provide an ideal model to study the effects of genetic changes.

It was concluded that the mixing of farmed and wild salmonid is generally detrimental to the wild stock, but empirical evidence is not abundant.

If genetically engineered fish escape and breed with native fish, the genetic features best for fish farming may change the genetic makeup of the wild fish, reducing their chances of survival. As hatchery production of seed increases, the potential to alter the genetic characteristics of wild stocks will increase. 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.

Next: Introduction of Alien Species