Can the possum fur industry contribute to possum control programmes?
Possums are killed as pests in official control programmes and also harvested as a resource by commercial operators seeking their fur and skins.
Possums have been harvested for their fur in New Zealand since 1921. The industry peaked in 1981 when 3.4 million skins were exported. Since then, demand for fur-on skins has been weak, but has been replaced by an increasing demand for plucked fur that is woven with merino wool to produce high quality yarn (e.g. Merinomink™, Perino™). The demand for plucked fur has grown about 10% per annum over the last 7 years (Fig. 1).
The use of plucked possum fur as a component in blended yarn is now well established with the New Zealand yarn industry estimated to be worth $50–70 million per annum. To service the demand for fur, about 1.5 million possums are now harvested each year, with about 80% of the fibre being processed into yarn for manufacturing into garments in New Zealand.
Because possums are controlled extensively in New Zealand for both the management of bovine TB and for protection of conservation values, the yarn-based industries are concerned about security of supply and the ‘waste’ of fur when possums are killed in control operations. Consequently, commercial fur hunters ask how can the fur industry and pest control agencies work together for their mutual benefit?
To determine whether fur harvesting can provide a sustainable livelihood for trappers ‘competing’ for possums with the possum control industry, Bruce Warburton has developed a spreadsheet model that integrates the price paid for fur, and the catch rate most likely achieved by trapping, given the prevailing density of possums. The model assumed (1) a hunter’s income of $1,250 a week, (2) that hunters checked 200 traps per day, (3) that $5 worth of fur was recovered on average from each possum, and (4) that the catch on trap lines declined each night at a given rate (derived from catch-rate data from lines trapped for up to 7 nights). The model also assumed a starting density of possums (indexed using a trap-catch rate), and was varied over the range found in the field (e.g. 0–80%). For the example shown below, the model used a starting density (trap catch) of 50% (Fig. 2).
In this example, the catch on the first night was 100 possums (i.e. 50% of 200 traps) and accumulated each night (yellow line) at a declining rate because of the decline in capture rate (blue line). By the sixth night of trapping, the catch had declined to 10% (blue line on graph).
The trapper can use one of two strategies: (1) try to maximise profit (i.e. return on effort), or (2) try to maximise the number of possums harvested (and therefore reduce the population to as low a level as possible). If they choose option (1), the model suggests they should stop when the catch rate has fallen to 25%, i.e. at point (c) when the difference between the numbers caught and what they need to achieve their desired income is maximised. If the trapper chooses option (2) then they can continue to trap through to night 6 when their cumulative catch equals the possums required to match the income target set of $1,250 a week. If they trap beyond this point they will operate at a loss. At point (a), they would have reduced the catch level down to about 10% and be getting a very low rate of return on their effort expended.
So does such modelling help managers planning official control operations? If the example given above is applied uniformly across a block where official control is planned, there may be some conservation benefits to plant and animal species that are moderately susceptible to possum browse or predation, but no benefits to more vulnerable species. However, it is unlikely there will be any benefits for the managers of TB control operations because possum numbers need to be reduced to levels indicated by a trap-catch of about 2% to eliminate any transmission of the disease between them.
At present Bruce is working with the Hawke’s Bay Regional Council to try and find effective ways to integrate harvesting for fur into official control programmes. He believes his model goes some way to providing a better understanding of what possum densities (as indexed by trap catch) trappers for fur can economically operate down to, and how the confl ict between possums as pests and possums as a resource can be resolved.
This work was funded by an Envirolink Grant (HBRC53)
Bruce Warburton
Fig. 1. Possum fur harvested from 2000 to 2007, with the number of possums harvested based on the weight of fur taken (assumed 20 possums equate with c. 1 kg of fur).
Fig. 2. Relationship between cumulative catch per night, number of possums required for a sustainable industry, and resulting trap-catch index. At point (a), the number of possums caught equals the minimum number of possums required to generate an acceptable income, and trapping for more nights would result in the trapper making a loss. When the trapper gets to point (a) on night 6, the trap-catch index has been reduced to point (b), which is about 10%. The point where the difference between the cumulative catch and the number of possums required (i.e. the rate of return on effort expended) is maximised is also shown (c).
