D. J. Jellyman

Sex determination in freshwater eels and management options for manipulation of sex

Catadromous eels enter fresh water as sexually undifferentiated glass eels and develop into males and females before migrating back to sea as silver eels. Females develop ovaries directly from the ambiguous primordial gonad whereas males pass through a transitional intersexual stage before developing testes. Eels have sex-specific life-history strategies. Males may grow faster than females initially, but this difference is soon reversed and females attain a greater age- and size-at-metamorphosis than males. Male fitness is maximized by maturing at the smallest size that allows a successful spawning migration (a time-minimizing strategy) whereas females adopt a more flexible size-maximizing strategy that trades off pre-reproductive mortality against fecundity. Although heteromorphic sex chromosomes have been identified in some species, the sex of developing gonads is labile and gender is determined principally by environmental factors. Individuals experiencing rapid growth prior to gonad differentiation tend to develop as males, whereas eels that grow slowly initially are more likely to develop as females. Paradoxically, males tend to predominate under conditions of high density, which may be because a male “grow quickly, mature early” strategy increases an individual’s chances of survival during periods of intraspecific competition. High temperatures and saline conditions have also been proposed to favor development as males but experimental studies have failed to demonstrate a clear effect of either on sex determination. High proportions of female silver eels migrating from some upstream areas, lakes and large rivers may be due to low population density or poor conditions for growth in these habitats. Manipulating sex ratios in favor of females has the potential to increase eel production in aquaculture and to buffer natural populations against fishing pressure. Sex steroids (oestrogens and phytoestrogens) have a strong feminizing effect on undifferentiated individuals and are most effective when targeted at younger eels and administered at high doses for prolonged periods. Modifying local environmental conditions, in particular reducing eel density and limiting interference and social stress, may also promote the development of females. Further research into the timing and mechanisms of sex determination in eels is required to effectively and efficiently manipulate sex for conservation and/or economic benefit.

Diet of two species of freshwater eel (Anguilla spp.) in Lake Pounui, New Zealand

Abstract Feeding habits of co‐existing populations of the freshwater eels, Anguilla australis Richardson (shortfinned eel), and A. dieffenbachii Gray (long‐finned eel) were studied in Lake Pounui, Wairarapa. A combination of sampling equipment gave a sample of 682 shortfinned eels and 310 longfinned eels ranging in length from 13 to 135 cm. Both species probably feed intermittently and are opportunistic feeders, consuming a wide range of food items, although feeding of individual eels was normally selective for a single prey species. Diet of each species changed with size. Smallest eels of both species ate mainly amphiphods and insect larvae; the snail Potamopyrgus antipodarum was the dominant food organism for shortfinned eels of 30–69 cm but was unimportant to longfinned eels. Longfinned eels > 40 cm were principally piscivorous but fish did not become an important part of shortfinned eel diet until eels were > 70 cm. At similar sizes, the two species had different diets. Eels of the same species but fr...

Conservation status of New Zealand freshwater fish, 2009

Abstract The threat status of 74 freshwater and estuarine fish present in New Zealand was determined. Fifty-one native taxa were ranked of which 67% were considered Threatened or At Risk. A single species was classified as Extinct, the New Zealand grayling, which has not been observed since the 1920s. Four taxa were classified in the highest threat category, Nationally Critical, and a further 10 taxa as Threatened (Nationally Endangered or Nationally Vulnerable). Twenty taxa were ranked in the At Risk group with the majority ranked as Declining. Endemic galaxiids (Galaxiidae) dominated the Threatened and At Risk taxa. The majority (68%) belonged to the Galaxias genus, comprising 81% of recognised taxa in this genus and all five species in the genus Neochanna were also ranked as Threatened or At Risk. In addition to 51 native taxa, a further three fish species were considered colonists and 20 introduced species were classified as naturalised, although two of these are considered rare. The majority of the Threatened species occur in the Canterbury and Otago regions where a suite of rare non-migratory galaxiids exist. Threat mechanisms that were identified as causal in the decline of freshwater fish species were the impact of introduced fish species, declining water quality, effects of water abstraction, loss of habitat via land-use change and land-use activities, and river modifications.

Diel and seasonal movements of radio-tagged freshwater eels, Anguilla spp., in two New Zealand streams

We studied diel and seasonal movements of 21 radio-tagged shortfinned, Anguilla australis Gray, and longfinned, A. dieffenbachii, eels in two small New Zealand streams. Movements of eels commenced at dusk, with a higher proportion of shortfinned eels moving per night than longfinned eels, and also moving greater distances. Both species often showed extensive movements immediately after tagging, but thereafter movements were limited. In the smaller stream, home ranges averaged 30 and 10 m for shortfinned and longfinned eels, respectively, but not all eels were active on every night. There were no seasonal differences in mean distances moved. In both streams, eel movement was almost exclusively bankside, and seldom cross-channel; eels also showed considerable fidelity to a particular bank. Shortfinned eels were most commonly found in runs, and longfinned eels in riffles.

Factors associated with the distribution and habitat of eels (Anguilla spp.) in three New Zealand lowland streams

Abstract The distributions of shortfinned eels (Anguilla australis) and longfinned eels (A. dieffenbachii) of all sizes in three New Zealand coastal streams were investigated during summer by sampling stratified, randomly chosen pool, run, and riffle sites from tidal to upper reaches. In all streams, both species were widespread, although a greater proportion of the total population of shortfinned eels was present in the lower reaches and small eels (<300 mm total length (TL)) had advanced further up stream than longfins in two of the streams. The biomass (g m‐2) of longfinned eels <300 mm TL was greatest in riffles, whereas that of comparable‐sized shortfinned eels was evenly distributed across habitat types in all streams; in contrast, eels ≥300 mm TL of both species showed no consistent pattern of habitat use between streams. Multiple regression analysis of species biomass (g m‐2) for four size groups (<100; 100–199; 200–299; ≥300 mm TL) against 16 habitat variables, indicated that the distribution of ...

Arrival of an Australian anguillid eel in New Zealand: an example of transoceanic dispersal

Anguilla reinhardtii, hitherto known from eastern Australia, New Caledonia, Norfolk Island, and Lord Howe Island, has recently been discovered in rivers of northern New Zealand. Identification, based on morphological and genetic characteristics, is unequivocal; eight consecutive year classes have been found. The only reasonable explanation of this occurrence is transoceanic dispersal to New Zealand, probably from subtropical oceanic spawning grounds north of New Zealand. This corroborates past hypotheses that the strongly diadromous freshwater fish fauna of New Zealand is derived by transoceanic dispersal of known marine life intervals.

Seasonal arrival patterns of juvenile freshwater eels (Anguilla spp.) in New Zealand

Abstract The arrival season of glass eels of both New Zealand species of freshwater eel, the short‐finned 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—densities were generally low in August, highest in September‐October, and declined during November‐Dece...

Distribution and early life-history characteristics of anguillid leptocephali in the western South Pacific

Freshwater eels are important fisheries species in parts of the western South Pacific, but little is known about their oceanic early life history or spawning areas. The age, growth, morphology and geographic distribution of five species of genetically identified anguillid leptocephali collected in 1995, 2000 and 2005 were compared. The sizes and ages of the leptocephali collected, Anguilla australis (n = 18), Anguilla marmorata (n = 15), Anguilla reinhardtii (n = 12), Anguilla megastoma (n = 2) and Anguilla obscura (n = 1), ranged from 19.0 to 50.9 mm and from 25 to 155 days, respectively. Leptocephali were mostly collected in the South Equatorial Current region. The total myomere ranges overlapped among species, but anodorsal myomere numbers clearly divided shortfinned and longfinned eels. The myomere ranges of the leptocephali were similar to the reported ranges of the numbers of vertebrae in adults. Larval growth rates suggested that the temperate species A. australis had slightly slower growth than the tropical species A. reinhardtii. The present study suggests that both temperate and tropical anguillid eels use the South Equatorial Current region for spawning and larval development, although some species might have different early life parameters and migration routes to their recruitment areas.

Movements of shortfinned eels, Anguilla australis, in Lake Ellesmere, New Zealand: Results from mark‐recapture studies and sonic tracking

Abstract Movements of non‐migratory and migratory shortfinned eels in Lake Ellesmere, New Zealand, were studied from recaptures of streamer‐tagged eels and tracking of eels tagged with sonic transmitters. Of the 9956 streamer‐tagged eels, 1982 were recaptured by commercial fishers over 5 years, some as many as 7 times. Most non‐migratory eels were recaptured at, or adjacent to, their original capture site, although some eels had moved to 11 of 12 regions in the lake within 3 weeks of tagging. Transplanted non‐migratory eels showed a tendency to home to their original capture site. Although most eels do not appear to move more than a few kilometres they are capable of periodic extensive movements; we conclude that there is a single population in the lake. The mobile portion of the population will be important in repopulating areas where eel numbers have been reduced by commercial fishing. Migrating eels always congregated at Taumutu, the area where the lake is artificially opened to the sea. There was some...

HABITAT PREFERENCES OF SHORTFINNED EELS (ANGUILLA AUSTRALIS), IN TWO NEW ZEALAND LOWLAND LAKES

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) 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...