NIWA seeks help to count kahawai

Photo/file

Recreational anglers may be asked to donate heads from kahawai that they land to science over summer, as part of a NIWA survey of New Zealand's kahawai population.

NIWA staff will be spending weekends between January and April next year interviewing anglers at 21 boat ramps between Mangonui in east Northland and Ohope in the Bay of Plenty.

Fisheries scientist Bruce Hartill said staff want to collect kahawai otoliths, or ear bones, from recreational kahawai landings from East Northland, Hauraki Gulf and the Bay of Plenty, which are examined to monitor changes in the structure of the kahawai population.

"Fishing parties may be approached by an interviewer when they return to a boat ramp, and asked about their fishing trip. We will also ask them if we can measure their catch," Mr Hartill said.

"If they land any kahawai, we will ask if we can cut the heads off the fish, so that we can remove the otolith ear bones in a lab at the end of the survey."

Recreational landings are sampled because the data they provide gives a representative snapshot of the local kahawai population, which is not available from any other source.

Otoliths are collected by scientists to determine the age of a fish. The otoliths are calcified structures that develop annual growth rings similar to tree growth rings that can be counted.

Determining the age of a fish contributes to the monitoring and management of this iconic species. Ageing a range of fish across a population enables scientists to determine how productive that population is, and how much catch can be taken sustainably.

"Kahawai swim in small groups and in schools that can be in excess of half million fish," Mr Hartill said.

They eat other fish, but mainly live on krill. The average size of a kahawai is 40-50 cm and 1-2 kg in weight. Females grow larger (up to 60 cm in length), and can weigh up to 3 kg, often half a kilo heavier than males. The recreational catch limit for kahawai is 20 fish.

This is the 11th consecutive summer the kahawai survey has taken place and NIWA staff hope to interview between 4000 and 5000 fishing groups during that time.

"It's a voluntary survey and we really appreciate the public's co-operation as they play a crucial part in monitoring the fishery and ensuring that there are plenty there for future generations."

Mr Hartill said information and modelling to date suggests the kahawai stock has been rebuilding over the past 15 years and is now in a reasonably healthy state.

The survey has been commissioned by the Ministry for Primary Industries.