Trochus - Policy Framework for Enhancing Reefs

The licensing pathway that would allow groups to enhance and harvest from specific reef areas involves the Department of Fisheries in formal interaction with other departments that also share our duty of care for maintaining biodiversity and quality of these reefs. A series of principles have been established which relate to reseeding molluscs in general and specifically to enhancing trochus fisheries.

1. Impact on genetic diversity

Juvenile tropical abalone, trochus and other mollusc stock to be placed into the marine environment should originate from broodstock collected from a nearby viable population, taking into account the planktonic life cycle of the species and the hydrodynamics of the area to be stocked. If the life cycle of the species or hydrodynamics of the area are poorly known, broodstock (for relocation or use in hatcheries) should be collected from areas within about 100 km from the area to be stocked. It is important that a large number of broodstock be used to ensure that the genetic profile of the juveniles is representative of wild populations.

In the case of trochus, high current speeds and annual cyclonic conditions probably favour natural mixing of populations in the Kimberley region.

2. Risk of the introduction of disease and pests to wild populations

All mollusc spat, to be placed into the marine environment after being produced in a hatchery, must be certified as disease-free and pest-free by the Fish Health Section of the Department of Fisheries prior to release. The known and perceived disease risk with trochus is considered low as there are no known serious diseases of trochus either in the scientific literature or in stocks examined by the Fish Health Section. However, diseases caused by opportunistic organisms could occur if trochus were cultured intensively without appropriate management practices.

3. Physical disturbance to the reef during reseeding operations

For some species in certain areas, the use of retrievable and non-polluting cages for holding juveniles on reefs for a limited period of time is recommended. To maintain reef quality, impacts from such structures or equipment should be minimised. In the case of reseeding intertidal reef systems with juvenile trochus, juveniles are usually placed onto the reef by hand at low tide. The reefs in the north of the State that are proposed for reseeding are either sandstone or limestone and are subject to long periods of aerial exposure associated with large tidal ranges. Reef walking appears to have minimal impact on the biota of these northern reefs. Coral coverage is naturally low and the algae that dominate the reef surface are resistant to physical damage. However, to minimise damage to coral, algae and encrusted sponges, workers can wear dive booties and need to be briefed on which biota are fragile and how damage to them may be avoided.

4. Impact on the ecology of the reef system

A key issue is the potential ecological impact of the restocking program on the reef community. This includes the possible impact on naturally occurring populations of the species stocked (such as trochus) and other species by competition for food or space. In cases where enhancement of mollusc fisheries is proposed to being undertaken in areas where the environmental quality of the habitat is degraded or the stock is depleted (for example, trochus reseeding), the activity aims to restore the natural balance of the reef. Given this, and provided the recommendations set down in the four points below have been taken into account, the requirement to undertake complex environmental assessments on the impact on reef ecology in these circumstances is not considered by the Department of Fisheries research scientists to be critical or indeed feasible for aquaculture licence holders.

1. Enhancement should take into account the existing stocks of trochus as there is little point in reseeding areas with normal densities of trochus, that is, reefs that have not been over-fished of trochus.
2. Where feasible, a subsample of the trochus to be released should be tagged or visually marked to allow identification and assessment of the effectiveness of reseeding. This is difficult with very small juveniles but can be done with advanced juveniles grown in cages. Broodstock moved to denuded reefs should be tagged and not targeted by fishers.
3. Once stock numbers have recovered in the nominated habitat, it is important that environmental quality is maintained through ecologically sustainable management principles, such as appropriate harvest rates, so that the trochus population on that reef does not become denuded again.
4.  Potentially adverse effects on other ecologically similar organisms, such as other molluscan grazers, should be considered. However, there has been no apparent change in community structure where the Department of Fisheries scientists have relocated trochus broodstock.

A number of scientific studies into the effectiveness of stocking reef systems with molluscs have used active management strategies such as the control of naturally occurring predators around the release site. Such management strategies are not permitted in this state. In addition, to reduce nutrient inputs, artificial feed should not be used. Currently, artificial feeds are not used in producing or restocking trochus in the Kimberley region.

Informtation courtesy of the WA Department of Fisheries