The need for this project arises from a number of directions. Firstly, the Exmouth Gulf region of the fishery (Zone 1) is currently experiencing a very high exploitation rate. In a review of the Zone 1 fishery in 1997, a lack of knowledge of length-fecundity relationships and Natural Mortality rates for MOP was highlighted. As a result of a high exploitation rate, it was hypothesized that the flow through of recruits to the MOP stock may be less than the Natural Mortality Rate of the MOP stock, thus placing a long-term risk on the breeding population. Therefore research into MOP stock dynamics was identified to be of critical importance to maintaining the sustainability of the harvest regime in Zone 1.
Second, there is the long-term strategic need to find an alternative supply of large nuclei to counteract the declining source from the American Freshwater Mussel industry. In anticipation of demands for harvesting of MOP for nuclei, there is an urgent need to start gathering information, to enable a sustainable harvesting regime for MOP to be implemented in 3 years time. Most likely this will result in the setting of an appropriate quota for MOP, within the current TAC allocation mechanisms in zone 2 or 3 where MOP stocks have built up.
In order to implement sustainable harvesting, we need to address a number of uncertainties. First, what is the extent of MOP stocks with respect to overall numbers of pearl shell. Secondly, what are the rates of recruitment into, and natural mortality of, the MOP stocks. These data, combined with knowledge of stock size, particularly with reference to the culture shell, will enable rates of sustainable harvest to be estimated. Finally, what is the impact of harvesting of MOP on recruitment. Recently, for example, there is good evidence from catch rate data of increased recruitment at two spatially distinct areas, 80 Mile Beach and the Lacepede Islands. Whether this has been environmentally and/or stock driven is currently undetermined, although recent data suggests a considerable environmental factor in the central area (Zones 2/3) of the fishery.
Another identified strategy for sustainable management of MOP is relocation of MOP to better growth areas. Obviously the quality of MOP shell will determine the amount of nuclei which can be produced. There is some anecdotal information suggesting that MOP shell from different areas have different quality of shell, and there is the possibility that shell growth and quality may be improved by transplanting to these areas. Ultimately, after establishing the parameters for natural mortality, fecundity, and recruitment in MOP populations, there is the potential to undertake: a) a one-off, large-scale stock survey of MOP, and b) investigate the possibility of re-locating hatchery produced shell, after they have grown too large for culturing pearls, into areas to grow to a suitable MOP size.
Final report
The West Australian Pearling industry is one of Australia’s most valuable aquaculture industries, currently generating around $120 - $160 million annually. The majority of pearl shell used to culture pearls come from the pearling beds in the inshore waters near Broome. The fishery for pearl oysters preferentially targets smaller (120mm - 165mm DVM) shell (hereafter defined as ‘culture’ shell), that are more suitable for culturing of pearls, leaving larger MOP oysters (175mm+) on the pearling grounds. These larger pearl oysters, the majority of which are female (protandric hermaphrodites), form a major component of the broodstock for the fishery. Although MOP are currently protected by the ‘gauntlet’ strategy adopted by the fishery, historically, MOP were harvested in large numbers to service the trade in shell buttons and other nacre products. The harvest of MOP, which occurred for most of the 20th century, was discontinued in the mid 1980’s as the markets declined and the requirement for ‘culture’ shells (120 mm to 170mm DVM) became into the main focus. With almost 20 years of protection from fishing mortality, there has been a build up of MOP on some pearling grounds, leading to proposals to commercialise this component of the fishery.
