Unlike marine species it is extremely difficult to complete a large scale stock assessment for eels. This is because every fresh waterway has its own completely separate set of dynamics.
Pastoral streams and rivers are quite different and more eel productive than those running through native bush or exotic pine forest. The mainstem Waikato River has quite different dynamics from ponds or tributaries adjacent to it.
Niwa scientists have been developing an approach to quantify eel biomass by characterising shortfin and longfin eel densities in different types of habitat from alpine areas to the sea. They can then model eel stocks by applying these characteristics to fine scale ‘GIS’ based maps of New Zealand’s waterways.
Full scale stock assessment is still some way off.
In the meantime, we rely on updating various indices of abundance, recruitment and the size of eels being caught in the commercial fishery to indicate whether fisheries are improving or getting worse.
To assess the health of eel stocks in each catchment around New Zealand, the Eel Working Group employs on-going indices of CPUE (catch per unit effort), recruitment (the level of new eels entering the fishery) and size structure (vitally important to assure there are plenty of large females available to make the oceanic spawning migration which underpins sustainability). To read more about CPUE Click Here. To read more about recruitment Click Here, To read more about size structure Click Here.
North Island commercial eel fishers arguably supply the most comprehensive catch data in the world. Every commercial landing is recorded for effort and catch to establish ‘cpue’. Nearly every commercial landing is size graded to characterise the structure of the population in that catchment.
Elvers are trapped at a selection of hydroelectric dams each year. Samples are analysed for species, size and age. Catching methods are standardised and an index of recruitment builds up over a long time series.
There is very little data on eels for areas where commercial eel fishers do not operate.
Commercial eel fishing only occurs in about 22% of North Island waterways!
It is reasonable to assume that, given similar habitat, those areas not fished will be as healthy or better than those which are.
Fine Scale Reporting
EECo have been investigating fine scale reporting which is basically attaching catch and effort information to habitat type at a precise position using GPS technology.
We see enormous benefit for managing fishing and the fishery along with greater transparency of what we do.
It has, however, proved difficult to find reliable software and the hardware initially used could not handle the harsh and wet conditions. Privacy issues have to be worked through.