Don Jellyman

The Anguilla spp. migration problem: 40 million years of evolution and two millennia of speculation

Anguillid eels Anguilla spp. evolved between 20 and 40 million years ago and possess a number of remarkable migratory traits that have fascinated scientists for millennia. Despite centuries of effort, the spawning areas and migrations are known only for a few species. Even for these species, information on migratory behaviour is remarkably sketchy. The latest knowledge on the requirements for successful migration and field data on the migrations of adults and larvae are presented, how experiments on swimming efficiency have progressed the understanding of migration are highlighted and the challenges of swimming at depth considered. The decline of Anguilla spp. across the world is an ongoing concern for fisheries and environmental managers. New developments in the knowledge of eel migration will, in addition to solving a centuries old mystery, probably help to identify how this decline might be halted or even reversed.

Longfin eels need reserves: modelling the effects of commercial harvest on stocks of New Zealand eels

Freshwater eels in New Zealand are heavily exploited by the commercial fishery and, to a lesser extent, by customary fisheries. A model was developed to investigate the effect of the New Zealand commercial eel fishery on escapement of migrating longfin (Anguilla dieffenbachii) and shortfin (A. australis) eels. Exploitation rates of 5% and 10% per year reduced the spawning per recruit of A. dieffenbachii females by 83% and 96.5%, respectively, below unexploited levels. The model suggests that New Zealand longfin eels may be severely recruitment overfished and only absence of fishing in some productive areas is likely to be effective in maintaining at least 50% of spawning per recruit. Being younger at spawning, eels are less susceptible to overfishing. Using the current minimum legal weight (220 g), the spawning per recruit of A. australis females was reduced by only 48% at an exploitation rate of 10% per year.

Eel protection measures within the Manapouri hydro-electric power scheme, South Island, New Zealand

Freshwater eels are in decline worldwide. In New Zealand, although longfin eels (Anguilla dieffenbachii) and shortfin eels (A. australis) are still amongst the most common freshwater fish species, there are concerns about the scarcity of large specimens. There are also indications that recruitment, especially of the endemic longfins, is declining. To protect stocks of this species, a series of mitigation measures have been implemented at the Manapouri Power Scheme (South Island, New Zealand). These include the release of compensation flows, the reconstruction of a fish pass and a catch and transfer programme for upstream migrating juveniles and adult downstream migrants (silver eels). Acoustic telemetry studies aimed at evaluating and improving interception and/or passage for silver eels show that eels were most active when lake levels were rising. Extended searching behaviour was exhibited, especially near the natural lake outlet, and silver eels appeared reluctant to travel to the opposite side of the lake where the hydropower station is located. Overall, just over half of the 31 silver eels tagged were able to find the natural outlet, and means of improving safe passage, including targeted flow releases and more effective methods of capturing the eels in the lake for transfer downstream, are being considered.

Changes in trophic linkages to shortfin eels ( Anguilla australis ) since the collapse of submerged macrophytes in Lake Ellesmere, New Zealand

Lake Ellesmere (Te Waihora) is a nationally important coastal brackish lake in New Zealand, however degradation in water quality and loss of submerged macrophytes over past decades have raised concerns in regards to the declining status of the lake’s commercial and customary fisheries, predominantly targeted at shortfin eels (Anguilla australis). We investigated foodweb dynamics and trophic linkages to shortfin eels in Lake Ellesmere using a combination of abundance assessments, dietary studies, and stable isotope analyses. Data from our study are compared with historical data sets on benthic invertebrate community composition and shortfin eel diets to trace changes in the trophic linkages to top predators that have occurred since the late 1960s. Stable isotope analyses indicate that the foodweb is predominantly driven by epipelic and phytoplankton derived carbon sources, although it was difficult to discriminate between these two carbon pools because of wind-driven resuspension of lake sediments. Comparison of our survey results with historical data sets indicates a clear shift in benthic biota from being dominated by phytofaunal species such as Potamopyrgus antipodarum (comprising 90% of total invertebrate biomass) during the 1960s, to now being almost entirely comprised of subterranean species such as Chironomus zealandicus and oligochaetes (together comprising 82% of total invertebrate biomass). This shift in benthic communities has resulted in significant changes in the size-specific diet of juvenile shortfin eels (<400 mm) from those reported for Lake Ellesmere during the mid 1970s, with Chironomus larvae now comprising 65% of the diets of juvenile eels, whereas historically P. antipodarum was the dominant food item (>30% of total biomass). This shift towards foraging on smaller sediment-dwelling species could have implications for juvenile eel bioenergetics, and may help explain why juvenile shortfin growth rates have significantly decreased in past decades. Juvenile shortfins now appear to switch to foraging on preyfish (mainly common bullies, Gobiomorphus cotidianus) at a smaller size (≈400 mm) than historically recorded (>500 mm). Dietary and stable isotope signatures indicated that small shortfins (100–299 mm) have considerable overlap in trophic position (δ13C = −20.4‰, δ15N = 13.6‰) with common bullies (δ13C = −20.5‰, δ15N = 13.7‰), the dominant fish in Lake Ellesmere (92% of total abundance CPUE), potentially indicating that these two species may directly compete for food resources. These findings again highlighted the importance of C. zealandicus in sustaining the fish populations of the lake.

A review of radio and acoustic telemetry studies of freshwater fish in New Zealand

Radio and acoustic telemetry have been used in 24 studies of freshwater fish in New Zealand. Although most native species are too small to carry these tags, studies using telemetry have been carried out on four of the larger species, i.e. lampreys (Geotria australis), shortfin and longfin eels (Anguilla australis and A. dieffenbachii) and giant kokopu (Galaxias argenteus). Among the introduced species, telemetry has been used to study movements of brown and rainbow trout (Salmo trutta and Oncorhynchus mykiss) in both rivers and lakes, chinook salmon (Oncorhynchus tshawytscha), koi carp (Cyprinus carpio) and bullhead catfish (Ameiurus nebulosus). To date, studies have almost exclusively been of short-term movements and seasonal migrations. Study objectives range from measuring microhabitats and home ranges to determining the extent of seasonal spawning migrations. The advent of progressively smaller tags will provide increased opportunities to use this technology on a wider range of fish species. Management implications of several studies are presented. Although biotelemetry studies have not yet been used in New Zealand, there are considerable opportunities for using such technology to increase understanding of fish energetics and also stress associated with catch-and-release practices in recreational fishing.

Diel and seasonal movements of silver eels, Anguilla dieffenbachii , emigrating from a lake subject to hydro-electric control

To investigate the behaviour and fate of silver female longfin eels Anguilla dieffenbachii in relation to flow variations from New Zealands largest (700 MW) hydro-electric station, 210 A. dieffenbachii were tagged with acoustic transmitters during summer and autumn over 6 years from 2005 to 2010. A shoreline array of receivers (2005-2008) was supplemented by positioning systems near the power station and natural outlet (2009-2010) that enabled continuous tracking of A. dieffenbachii within these areas. Tagged A. dieffenbachii (total length 785-1372 mm) emigrated from the lake over an 8 month period, either in the same season they were tagged (75%) or the following spring after over-wintering for up to 7 months (25%). Most A. dieffenbachii (67%) emigrated via the natural outlet. Swimming activity was related to water temperature and was greatest during the summer months. Activity was strongly diel, although less so at the outlet where A. dieffenbachii migrating at depths greater than 20 m often did so during daylight. To analyse searching behaviour, sustained periods of continuous swimming activity (termed forays) were examined in the vicinity of the outlet. Most successful forays (where the A. dieffenbachii exited the lake via the lower Waiau River) were associated with outflows; such outflows were sometimes <10 m3 s-1 although most successful forays took place at flows >50 m3 s-1. The timing of successful forays was related to outflow, the day of year and whether the A. dieffenbachii had delayed migration or not, but not lunar phase or rainfall. Passage time from the lake to the lower Waiau River 81 km downstream decreased from an average of 58 days early in the season to <1 day in late autumn. The wide range of behaviours and responses displayed by female silver A. dieffenbachii were interpreted as an inherent form of bet-hedging to spread the risks associated with silvering and seaward migration in this slow-growing species.

Growth patterns and age validation from otolith ring deposition in New Zealand longfin eels Anguilla dieffenbachii recaptured after 10 years at large

In 1998, 9500 juvenile New Zealand longfin eels Anguilla dieffenbachii (mean total length, LT , 42 cm) captured from the lower Clutha River were transferred upstream to Lake Hawea, a high-country oligotrophic lake in the same catchment where recruitment of juvenile eels has been prevented by hydroelectric dams since 1958. A total of 2010 of the transferred A. dieffenbachii were tagged with coded wire tags. Ten years later in 2008, the A. dieffenbachii population in Lake Hawea was sampled resulting in 399 recaptures (distinguishable by the presence of tags and by LT from the remnant resident population of large old A. dieffenbachii) of the 1998 transfers; 79 (19·2%) of the recaptured fish had tags compared with 21·3% at release, indicating good tag retention and low mortality due to tagging. All recaptured tagged A. dieffenbachii were female. Mean annual growth over the 10 years since release was 3·80 cm year(-1) for all recaptures and 3·65 cm year(-1) for tag recaptures, and both were significantly greater than the estimate of 2·38 cm year(-1) at release. After release, mean condition (K) increased significantly (P < 0·001) for all recaptures and tag recaptures. Annual length growth increment was linear. Tag recaptures showed significant increases in somatic growth rate post-transfer, and otoliths from the 2008 recaptured A. dieffenbachii were examined to see whether any similar enhanced growth after transfer was incorporated into the otolith structure that would serve as a date stamp. Measurement of otolith ring radii indicated that an increase in the radius occurred on most otoliths corresponding to the year after transfer. Because there was 9 years of completed growth following the observed growth inflection on the otoliths, this was strong evidence that opaque rings were formed annually.

Efficacy of 12-mm Half-Duplex Passive Integrated Transponder Tags in Monitoring Fish Movements through Stationary Antenna Systems

AbstractGlobally, many freshwater fishes are declining in distribution and abundance, but for many species conservation measures are hindered by a limited knowledge of their ecology. Recent development of 12-mm half-duplex (HDX) passive integrated transponder (PIT) tags has broadened the potential application of this technology for studying habitat selection and migration of freshwater fishes. However, the limited read range of the smaller 12-mm PIT tags can limit their functionality in ecological studies. The present study used four migratory fish species to test the efficacy of different PIT antenna designs for detecting 12-mm HDX tags in small- and medium-sized rivers. Three types of pass-through antennae were created and coupled to either a single reader or multiplexor board. Across single-reader and multiplexor systems, the detection efficiencies for Giant Kokopu Galaxias argenteus, Shortfin Eel Anguilla australis, and New Zealand Longfin Eel Anguilla dieffenbachii ranged between 87% and 97%. Adult P...

Tracking anguillid eels: five decades of telemetry-based research

Advances in telemetry technologies have provided new opportunities to reveal the often-cryptic spatial ecology of anguillid eels. Herein we review 105 studies published between 1972 and 2016 that used a variety of telemetry technologies to study the movements of eels in a variety of habitats. Eight anguillid species have been tracked in three main geographical locations: Western Europe, the north-eastern part of North America and Australasia. Telemetry has proven to be an effective method for determining patterns of yellow eel movements in continental waters. It has also been used extensively to investigate the migratory behaviour of maturing eels as they leave fresh water to reach the sea. Among recent findings is the observation that downstream migration in continental waters is quite discontinuous, characterised by extended stopovers. Reconstructed migration routes in the open ocean obtained from satellite tags have provided indications of spawning areas, extensive vertical migrations and initial clues about the orientation mechanisms at sea. Telemetry studies have also revealed apparent evidence of predation by marine mammals and fish at sea, suggesting a significant natural source of mortality during the eel spawning migration. Finally, we discuss some limitations of telemetry technology and future directions, as well as associated challenges, to the developing field of eel spatial ecology.

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

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.