Growth rates of New Zealand endemic longfinned eels (Anguilla dieffenbachii) from streams in pasture and indigenous forest, and from two hydroelectric lakes (Lakes Karapiro and Matahina), were estimated by otolith examination. Habitat-specific growth was further investigated with measurement of widths of annual bands in otoliths. Longfinned eels 170-1095 mm in length ranged between 4 and 60 years old (N = 252). Eels in pastoral streams grew faster (mean annual length increment ±95% CL = 24 ± 3 mm to 36 ± 7 mm) than eels in streams in indigenous forest (annual length increment 12 ± 2 mm to 15 ± 3 mm). Eels from the hydro-electric lakes had growth rates (annual length increments 19 ± 4 and 19 + 7 mm) similar to eels from pastoral streams. Otoliths of most eels showed annual band widths that indicated growth in several different habitats, corresponding to growth during upstream migration, and limited movement among adult habitats. Estimated age at marketable size (220 g) ranged between 7 and 26 years. The particularly slow growth of longfinned eels in streams in indigenous forest has considerable implications for management. The fast growth rates of eels in hydro-electric lakes provides

The downstream migrations of mature longfinned eels (Anguilla dieffenbachii Gray, 1842) and shortfinned eels (Anguilla australis Richardson, 1848) were investigated at Aniwhenua Dam on the Rangitaiki River between 1992 and 1998. Migrants were mostly females over 1000 mm total length (TL) with otoliths indicating rapid growth rates. Migrations, which occurred on a few nights each autumn, generally began once water temperatures began to decline and ended when temperatures dropped below c. 11 °C. Rainfall and flow increases were found to be key factors triggering migration events. Rainfall exceeding a cumulative total of 40 mm over 3 days accounted for 60% of migrant eels arriving at Aniwhenua. It is proposed that such rainfall triggers could be used as predictors to instigate mitigation activities that would allow mature eels to proceed uninjured past barriers such as hydro-electric dams.

Forty-seven eels were captured, tagged, and released into a 420 m lowland section of the pastoral Ahirau Stream, North Island, New Zealand. Eels were marked with jet inoculations of alcian-blue dye, and plastic tube tags were surgically inserted into the gut cavity. Oxytetracycline hydrochloride was injected intraperitoneally to provide markers on otoliths. Over 3 years, 25 tagged eels were recaptured in the same stream reach, 16 of these within 20 m of their original sites of capture. All recaptured eels retained internal tags which had been incorporated into intestinal mesentary membrane, and 57% of eels recaptured over the first 2 years retained dye marks. Annual length increments, determined from measurement of tagged individuals, were 65 ± 15 mm (mean ± 95% confidence limits) for longfinned eels, and 29±18 mm for shortfinned eels. Translucent zones counted after single fluorescent rings corresponded to the number of years eels had been at liberty. The occurrence of the OTC fluorecence within the translucent zone confirms that these zones are formed in winter in both longfinned and shortfinned eels. Comparison of rings on unburnt and burnt otoliths from tagged eels validated the break-and-burn technique of age determination for both species of New Zealand eel. M93001

The arrival season of glass eels of both New Zealand species of freshwater eel, the shortfinned eel (Anguilla australis (Richardson)) and the longfmned eel (A. dieffenbachii (Gray)), was studied by electric fishing of 13 streams/rivers throughout east and west coasts of both North and South Islands at 14-day intervals. Sites were usually located at the most downstream riffle exposed at low tide, and sampled by single-pass electro fishing. The species composition was dominated by shortfins, with exceptions being west coast sites, one in the North Island and two in the South Island. From a comparison of species proportions in adjacent catchments, it was concluded that glass eels make choices about entry in particular water-types. As the stage of pigmentation advanced during the arrival season, "early stage" glass eels were defined and densities of these compared to show seasonal arrival patterns- M98073

No abstract

The habitats used by shortfinned eels (Anguilla australis (Richardson)) in Lakes Ellesmere (Canterbury) and Waahi (Waikato), New Zealand, were determined using a variety of capture techniques during the summers of 1994/95-1997/98. The most successful technique used to catch juvenile eels (<300 mm total length (TL)) in Lake Ellesmere was a 2-m wide beam trawl; larger eels were captured mainly by fine-meshed fyke nets. Trawl catches during the night exceeded catches during the day by a factor of 2.4. In Lake Ellesmere, juvenile eels were mainly caught in the depth range 0.6-1.2 m, on a gravel and/or mud substrate, and within 1 km of the shore. In contrast, larger eels (>300 mm) preferred sandy substrates, but showed no marked preference for particular depths or distances offshore. The spatial distribution of both size groups was non-random. Although water temperature did not influence catch rates (CPUE, catch-per-unit-effort) of either size group, catches of the smaller eels were greater during the new moon phase than during the other phases-catches of larger eels were unaffected by lunar phase. Length-frequency distributions of eels from Lake Ellesmere were strongly bimodal, with eels 200-300 mm poorly represented-this probably M98068 Received 22 September 1998; accepted 15 December 1998 reflects poor recruitment for several years, either because lake opening times did not coincide with the availability of glass eels, or because overall numbers of glass eels were low. Juvenile eels in Lake Waahi, caught in fine-meshed fyke nets and in special brush collectors, were also inshore residents; unlike Lake Ellesmere, juvenile eels could be caught at the lake margins, probably because Lake Waahi is not subject to the same wind-induced water level fluctuations as Lake Ellesmere. Recruitment into Lake Waahi commenced in mid December, but there was evidence of low recruitment in past years for this lake also. An important outcome of this research is that estimates of the number of juvenile eels to be transplanted during stocking programmes should be made using the area of the shallow littoral rather than the area of the whole lake.

Growth rates of shortfinned eels (Anguilla australis) from various locations in the Waikato basin, New Zealand, were determined by otolith examination. Two colour-morphs from the Whangamarino Swamp exhibited different growth rates. Darkcoloured eels (96% of the catch in low water levels) resided in the swamp; their growth was much slower than that of light-coloured eels (40% of the catch in floods) which probably entered this habitat in flood waters from Lake Waikare, the Whangamarino River, or main wetland channels. Overall, growth rates were fastest in Lake Waikare followed by Lake Waahi, Lake Whangape, and Hakarimata pastoral streams. Dark-coloured swamp eels were the slowest growing. Relative annual length increments ranged between 10 and 47 mm. Fast growth of eels in Lake Waikare may be the result of low densities owing to poor recruitment and heavy fishing pressure in recent years. Estimated age at threshold commercial size varied by 11 years throughout the Waikato.

SynopsisWe compared the fish communities of two shallow lakes in the lower Waikato River basin, North Island, New Zealand, to determine the effects of elevated suspended solids (SS) and collapse of submerged macrophytes. Lake Waahi was turbid (20-40 g me3 SS) and devoid of submerged macrophytes whereas Lake Whangape was clearer (5 g me3 SS) and dominated by submerged macrophytes. The lakes had similar fish species richness and had nine major species in common; representing eight families including Anguillidae, Retropinnidae, Galaxiidae, Eleotridae, Mugilidae, Ictaluridae, Poeciliidae, and Cyprinidae (two species). The only major fish that was absent from Lake Waahi was a lacustrine form of the common smelt, Retropinna retropinna, which disappeared after the lake became turbid in the late 1970s. CPUE, condition, and size of most species in Lake Waahi were similar to, or greater than, those in Lake Whangape. Lake Whangape clearly exceeded Lake Waahi only for CPUE of two species. Within Lake Whangape two species displayed significantly greater condition, and one species greater size, in a turbid arm of the lake than in the main basin. Apart from lacustrine Retropinna retropinna, the fish in these lakes appear well adapted to cope with, or to avoid, the direct toxic effects of suspended and settleable solids on sensitive early developmental stages. In Lake Waahi loss of cover and food provided by submerged macrophytes appears to have been compensated for by increased turbidity and an associated increase in the biomass of the mysid, Tenagomysis chiltoni (a major prey item).