Recreational fisheries provide insights into kahawai populations
Science Centres: Fisheries
Much of the data we use to assess the status of fish stocks comes from commercial fisheries, but recreational catches may be more appropriate for some pelagic species. Some of these, such as kahawai, form schools of similar size and age, and a commercial landing often consists of fish from only one or two schools. Samples from these landings describe the schools fished, but they tell us little about the population as a whole, because they are not representative of the fish in the area.
Even the catches of fishers like this tell us something about kahawai stock dynamics.
Recreational fishers generally catch only a few fish, but as many landings are available from a wide geographic range, a representative picture of the local population can be developed. Such information is valuable in evaluating the status of the kahawai stocks – a task which up to now has been very difficult.
The results of sampling kahawai in a Ministry of Fisheries-funded project over the last two summers appear to give a far more realistic and consistent picture of kahawai population structure than that derived from commercial landings.
During the last two summers, recreational catches of kahawai have been sampled at 28 boat ramps on the northeast coast of the North Island. Fishers have been generally very supportive of the programme. After the kahawai are measured, their heads are removed and from these the otoliths (ear bones) are later extracted and used to determine the age of the fish.
There are clear differences between kahawai populations in East Northland, the Hauraki Gulf, and the Bay of Plenty. The Hauraki Gulf appears to be a nursery ground where most of the fish caught are only 2–5 years old (25–45 cm long). The East Northland population is mainly 3–7 years old (35–50 cm), and most fish caught in the Bay of Plenty are 3–12 years old (35–55 cm).