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The development of GIS based models to estimate eel stocks and recruitment into the Aparima River, Southland, New Zealand.

Eric Graynoth

National Institute Water & Atmospheric Research, CHRISTCHURCH, NEW ZEALAND

THEME: ASFB

All flowing waters in New Zealand have been mapped and classified in the last three years by the National Institute of Water & Atmospheric Research (NIWA) using a Geographic Information System (GIS). Digital 1:50,000 maps with contours at 20 m were converted to a 30*30 m digital elevation model (DEM) grid to define the river network. Each river system typically consists of thousands of numbered and linked reaches. For each reach detailed information is available on climatic, topographical and geological features such as the altitude, source of flow, and size of the stream. This presentation describes the first use of GIS statistics and models to estimate the distribution and abundance of fish in a large New Zealand river. The overall objective was to determine the age composition of long finned eels Anguilla dieffenbachii and assess whether recruitment has declined in recent years as a consequence of increased commercial fishing pressure and environmental changes. Electric fishing was used to determine the abundance and size composition of eels at 89 locations distributed throughout the Aparima River catchment in Southland. Also a line of 8 to 10 baited fyke nets was set at 5 locations which were too deep to electricfish and eel densities estimated by the decline in catches over 2 to 3 nights. Eel abundance was calculated as numbers per km for 5 size classes and log linear and logistic models were developed to predict the distribution and abundance of eels throughout the catchment.