Age, growth, and condition of freshwater eels (Anguillasp.) in backwaters of the lower Waikato River, New Zealand

Age and growth of longfinned eels (Anguilla dieffenbachii) in pastoral and forested streams in the Waikato River basin, and in two hydroelectric lakes in the North Island, New Zealand

“…This difference in growth is not necessarily caused by exploitation and may be entirely an effect of habitat differences. However, intensive commercial fishing, which removes large eels, is thought to cause the high densities of small eels observed in the backwaters of the Waikato River and result in slow growth (Chisnall 1989).…”

“…Growth Percentage of channel length as pool Water temperature is probably the most important factor causing habitat-specific growth differences in longfinned eels. Low water temperatures are well known to reduce eel mobility and lower growth rates (e.g., Cairns 1942;Sinha & Jones 1967;Nyman 1972;Ryan 1984;Chisnall 1989). In addition, increased light available to pastoral streams, and the generally higher trophic status of lakes, may allow greater food production in these habitats than in forested streams.…”

“…Otoliths of both shortfinned and longfinned eels show alternating concentric transparent and opaque bands; each transparent zone delimits 1 year's growth. The width of opaque bands (growth bands), appears to be correlated with habitat type, as do growth rates (e.g., Chisnall 1989;Chisnall & Hayes 1991). The main aims of the present paper were to determine to what extent growth rates of longfinned eels vary among habitat types, and to test objectively whether growth bands in the otoliths of longfinned eels have characteristic widths in different habitats.…”

“…One of the most productive eel fisheries in New Zealand is in the Waikato River basin, where longfinned eels comprise up to 30% of the 150-2001 annual catch. Longfinned eels are found throughout the Waikato River catchment but are more abundant in the upper reaches of streams and rivers, and mainstem hydro-electric impoundments, than in lowland rivers, lakes, or wetlands (e.g., Chisnall 1989;Chisnall & Hayes 1991;Swales & West 1991).…”

“…Methods for studying growth rates of New Zealand eels have been established (Jellyman 1977(Jellyman , 1979Todd 1980;Hu & Todd 1981;Chisnall 1989;Chisnall & Hayes 1991). Otoliths of both shortfinned and longfinned eels show alternating concentric transparent and opaque bands; each transparent zone delimits 1 year's growth.…”