Population size, growth and movements of Anguilla australis in a small lake

Abstract

To study growth rates, movements and estimate population size of shortfin eels Anguilla australis in a small lake (2·5 ha) near Christchurch, New Zealand, 617 A. australis were tagged with PIT tags. Tag retention was high (95%) and over the seven recapture events spread over 2 years, 55% of tagged A. australis were recaptured. Growth of recaptured A. australis averaged 13·1 mm year(-1) and declined slightly with increasing total length. Distance moved from original capture site increased with increasing time at large. Population estimates of A. australis > 400 mm (susceptible to capture by fyke net) from recaptures of individuals averaged 1451 A. australis, with a biomass of 170 kg ha(-1) . An average of 6·6% of the estimated total population matured as male silver A. australis each year. Results from radio-tracking of four A. australis gave an average nightly foraging area of 2780 m(2) , and there was no apparent preference for inshore movement (within 5-6 m of the shoreline) or offshore movement. Fyke-net efficiency (total catch relative to the estimated total population available to each net) measured over four consecutive nights fishing was 88%. The lack of precision of the shoreline triangulation system used, ±10 m, meant that the positional data were considered too coarse to be used in a proposed novel population estimation technique based on determining population size within foraging areas.

Thirteen novel ideas and underutilised resources to support progress towards a range‐wide American eel stock assessment

“…Lentic (non‐flowing) waters are a major growth habitat for eels (Trancart et al, 2018), but abundance measurements are more difficult. Mark‐recapture experiments may fail to obtain the minimum of recaptures needed to produce estimates with reasonable confidence levels (Diekmann et al, 2019; Jellyman & Crow, 2016) (Table 4). In unbounded systems (large lakes, estuaries and bays) conversion of mark‐recapture population estimates to densities is uncertain due to difficulties in establishing effective home range size (Walker et al, 2014).…”

Surgical implantation of radio tags in three eel species (Anguilla spp.) in South Africa

“…In New Zealand, Jellyman and Sykes () observed a tag loss rate of surgically implanted tags of 25% for the shortfin eel A. australis Richardson 1841 and 23% for the longfin eel A. dieffenbachii Gray 1842. Low expulsion rates (5%) have also been observed for A. australis with injected passive integrated transponders (Jellyman & Crow, ). In our present study, with two cases of tag expulsion, we reached 12.5% tag loss for A. marmorata and 11% for A. bengalensis , while no tag loss was suspected for A. mossambica .…”

Can non‐native perch (Perca fluviatilis) support native eel populations in a wetland complex?

“…To determine the size of the longfin and shortfin eel population in each wetland, we conducted consecutive depletion surveys over four nights (Jellyman & Crow, 2016; Stewart et al, 2022). After each overnight set, we cleared the nets and separated the eels by species (longfin vs shortfin) – identified according to morphological features (McDowall, 1990).…”