Changing population structure of eels (Anguilla dieffenbachii and A. australis) from southern New Zealand

Abstract –  Commercial catches of longfin and shortfin eels ( Anguilla dieffenbachii and A. australis ) were sampled from southern New Zealand in the mid-1990s to determine the status of the eel population. We compared this with historical information on eel populations to highlight changes in the status, and then investigated the likely causes of these changes. A total of 20,722 longfins and 2366 shortfins were sampled from 216 landings. Longfins dominated catches comprising 53–100% (mean = 90%) of eel numbers. Shortfins were predominantly, but not exclusively, coastal in their distribution, whereas longfins were widely distributed through all reaches sampled. Mean size of shortfin was invariably larger than longfin and for both species generally increased with distance inland, particularly for longfin in the longest rivers. There was no clear geographical separation of longfin sexes although females were more common further inland in the longer rivers. Longfin sex ratio was about 4:1 (male:female), whereas in shortfin it was nearly 100% female. There have been marked changes in the status of the eel population in southern New Zealand over the last 60 years. We suggest that commercial fishing has resulted in a reduction in the size of both species, and for longfins a skewing of the sex ratio toward males – fishing may have favoured differentiation into males as well as the removal of the longer-lived longfin females. Skewed sex ratio and small size of longfins compared with those of shortfin suggest that this species has been more affected by fishing pressure than shortfins, possibly allowing shortfins to expand their range further inland.