New Zealand lakes and ponds often have large populations of koura or freshwater crayfish.
These are an important traditional food source for Maori, particularly for Te Arawa and Ngati Tuwharetoa of the Central North Island.
There is anecdotal evidence that changes in lake conditions (e.g., spread of exotic weeds, reduced bottom water dissolved oxygen due to eutrophication, exotic fish introductions) over the last 50 years have reduced koura abundance in many lakes. However, the lack of quantitative information on trends in koura abundance makes it difficult for iwi and government agencies to formulate plans for managing this taonga species. A key obstacle is the lack of sampling methods for koura that are suitable for research and monitoring the state of these resources.
We have set out to address this issue of how to monitor lake koura populations as part of research on factors influencing populations in the Rotorua/Te Arawa lakes. Several methods have been investigated and our assessments of these as practical tools for use by iwi groups are summarised in Table 1.
Table 1: Summary assessment of lake koura monitoring/sampling methods trialled in our study.
|Rama koura - shoreline spotlighting||
|Baited cage traps||
|SCUBA surveys or underwater video surveys||
|Tau koura - bracken fern bundles on a long-line||
Each of these methods has their place as koura monitoring tools, and refinements are expected as experience in their use is developed over time.
Spot lighting (rama koura) is probably limited to qualitative or presence/absence assessments.
Baited cage traps (beef liver has proved the best bait) can be deployed overnight in a wide range of habitats (including macrophyte beds) to obtain catch per trap (or unit effort) information for a limited size range of koura (determined by the trap opening and mesh dimensions). We use modified minnow traps with the openings enlarged to fit a 10 cm length of 4 cm diameter PVC tubing to reduce escapement of koura attracted into the trap. The traps are attached by rope to a surface bouy. The bait is enclosed in a muslin bag hung inside the trap below the bouy line. The total cost was about $40 per trap.
Visual surveys using SCUBA or an underwater video camera can provide quantitative information on koura densities (i.e., numbers/unit area) along transect lines or over standard search areas, but have higher costs and skill requirements. Preliminary tests indicate that underwater, black and white, cameras with 50 m of cable are suitable for use from a boat for qualitative survey of koura populations. These can be purchased for about $1000. Linking the camera to a tape or digital recorder can provide quantitative koura density counts.
Tau koura appears to be a particularly useful method for monitoring long-term patterns in catch per unit effort and koura population structure particularly where the lake bed is relatively free of tall-growing aquatic plants. A preliminary protocol for use of the Te Arawa tau koura method as a monitoring tool has been developed in collaboration with Willie Emery (Ngati Pikiao).
Appropriate use of this suite of tools should allow iwi and government agencies to develop the information base on koura populations needed for improved management of koura in our lakes.
This article is based on research by Ian Kusabs (Fisheries Consultant, Ngati Tuwharetoa, Te Arawa), Willie Emery (Kaumatua, Ngati Pikiao and Te Arawa Maori Trust Board, Rotoiti), Stephanie Parkyn, Chris Hickey and John Quinn (latter 3 all based at NIWA, Hamilton).