The IUCN/SSC Shark Specialist Group
Shark News 13: July 2001
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Western Australia's dusky shark fishery:
an example of a sustainable fishery for a long-lived, late maturing, slow growing, low reproductive rate species?
Colin Simpfendorfer, Western Australian Marine Research Laboratories
The dusky shark Carcharhinus obscurus is a species of shark with a strongly K-selected life history. The young are born at 80 to 105 cm total length (TL) and on average grow less than 10 cm TL per year. Individuals mature at approximately 20 years and may live to over 45 years. The average litter size for mature females is ten, with litters produced every third, or possibly second year. The life history of the dusky shark is therefore one that makes it particularly susceptible to overfishing by commercial fisheries. For example, in the western North Atlantic, the abundance of dusky sharks has been estimated to have declined by 60-80% between 1974 and 1991 because of heavy commercial and recreational fishing (Musick et al. 1993).
New-born dusky shark. Photo: Colin Simpfendorfer
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A commercial fishery exists in southwestern Australia for dusky sharks. Annual catches of this fishery peaked at 600-700 mt during the early 1980s, and are currently around 450-500 mt. This fishery is unlike any other for a large, long-lived species in that recently pupped juveniles are targeted using demersal gillnets with a mesh size of 16.5-17.8 cm.
The Fisheries Department of Western Australia undertakes assessment of the fishery using a combination of exploitation rates and demographic analysis. Exploitation rates are estimated from a tagging study of recently pupped dusky sharks. Age-specific exploitation rates are used in the assessment based on releases during the 1994 and 1995 pupping seasons. It was assumed that the size selectivity of the gillnets used in the fishery meant that no sharks over six years of age were caught in the fishery. The exploitation rate data are used in the demographic analysis along with life history information such as age at maturity, maximum age, litter size, reproductive periodicity, and natural mortality. For the purposes of the assessment, and the examination of uncertainty in the outcomes, a total of 17 scenarios were used (one base case and 16 sensitivity tests for variations in life history and exploitation rate data), each examining three levels of fishing: no fishing, exploitation rates experienced by the 1994 cohort, and exploitation rates experienced by the 1995 cohort.
Results of the assessment indicated that the population was sustainable at the current levels of exploitation. The annual rate of population increase without fishing was 4.3%, while with the current level of exploitation is 2.3-2.7%. Sensitivity tests indicated that only if natural mortality was above expected levels would there be a possibility that the current levels of exploitation could not be sustained.
The results of this assessment are interesting in that they indicate a possible strategy for commercially exploiting long-lived, late maturing, slow growing, low reproductive rate species. This strategy is to target fishing at the youngest age class. In the case of the Western Australian dusky shark population, only one in six individuals survive to maturity (due to the late age at maturity) so most of the neonates caught would have died anyway.
The application of this fishing strategy to other populations of long-lived, late maturing, species is probably limited. The strategy works well with dusky sharks because of their large size at birth, which provides fishermen with a product large enough to be commercially viable. In other species, individuals in the youngest age class may be too small to be economically viable as a target. It is also a strategy that applies to meat fisheries, since fin-based fisheries are most commonly driven by the desire for the largest fins.
It is important that when a fishing strategy such as this is applied that only the desired age classes are caught. There are two approaches that can be used to do this. One is the use of size-selective fishing gear (e.g. gillnets), and the other is to fish in nursery areas.
In situations where there is also capture of other age classes, the advantages of this fishing strategy quickly diminish. Exploitation rates of older age classes need only be in the order of 1% to 2% to result in over-exploitation of the stock. In any fishery that employs this fishing strategy it is important that there is an ongoing monitoring programme to estimate exploitation rates of both the target, as well as the non-target age classes. The results of the sensitivity tests also indicate that if possible an accurate estimate of the level of natural mortality will provide a decrease in the uncertainty of the assessment.
The results of the assessment of the Western Australian dusky shark fishery indicates the accepted paradigm that strongly K-selected shark species cannot withstand targeted commercial fishing pressure does not always hold true.
However, it is only an extreme fishing strategy, where the youngest age classes are caught, that may be sustainable. Such a fishing strategy has limited applicability to most strongly K-selected shark species and should be carefully examined before being implemented.
Colin Simpfendorfer
Current Address: Center for Shark Research, Mote Marine Lab,
1600 Ken Thompson Parkway,Sarasota, Florida, USA,
Fax:+1 941 388 4312. Email: colins@mote.org
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