Zealand Freshwater Eels Research Articles

Evidence of different habitat use by New Zealand freshwater eels Anguilla australis and A. dieffenbachii, as revealed by otolith microchemistry

The apparent use of marine and freshwater habitats by Anguilla australis and A. dieffenbachii was examined by analyzing the strontium (Sr) and calcium (Ca) concentrations in otoliths of silver eels collected from Lake Ellesmere, which is a shallow brackish-water coastal lagoon in New Zealand. The age and growth of these eels was also examined using their otolith annuli. Size and ages of females were greater than those of males for both species. Growth rates were similar among sex and species, but the highest growth rates were observed in eels that experienced saline environments. Line analyses of Sr:Ca ratios along a life-history transect in each otolith showed peaks (ca. 15 to 21 × 10 -3 in A. australis, 14 to 20 × 10 -3 in A. dieffenbachii) between the core and elver mark, which corresponded to the period of their leptocephalus and early glass eel stage in the ocean. Out- side the elver mark, the Sr:Ca ratios indicated that eels had remained in different habitats that included freshwater (average Sr:Ca ratios, 1.8 to 2.4 × 10 -3 ), areas with relatively high salinities (aver- age Sr:Ca ratios, 3.0 to 7.4 × 10 -3 ), and in some cases individuals showed clear evidence of shifts in the salinity of their environments. These shifts either indicated movements between different loca- tions, or changes in the salinity of the lake. There were more individuals of A. australis that used areas with intermediate or high salinities, at least for a short time (85% of individuals), than A. dief- fenbachii (30%). These findings suggest that these 2 southern temperate species may have the same behavioral plasticity regarding whether or not to enter freshwater or remain in marine environments, as has been recently documented in several northern temperate anguillid species.

Migratory history and habitat use by New Zealand freshwater eels Anguilla dieffenbachii and A. australis , as revealed by otolith microchemistry

The migratory history of Anguilla dieffenbachii and A. australis, collected from a coastal lake of New Zealand, was examined using analysis of strontium (Sr) and calcium (Ca) concentrations. Line analysis of Sr : Ca ratios along the life history transect of each otolith showed a peak (Ca. 16–20 × 10−3) between the core and elver mark, which corresponded to the period of their leptocephalus and early glass eel stages in the ocean. The mean Sr : Ca ratios from the elver mark to the otolith edge indicated that eels had different migratory histories, which included freshwater residency in some eels (average Sr : Ca ratios, 1.7 × 10−3–2.4 × 10−3) but not in others (average Sr : Ca ratios, 3.1 × 10−3–6.5 × 10−3). These findings suggest that New Zealand freshwater eels have a flexible migration strategy and an ability to adapt to various habitats and salinities.

Induced spawning and early ontogeny of New Zealand freshwater eels {Anguilla dieffenbachii and A. australis)

Knowledge on the reproductive biology of New Zealand freshwater eels (Anguilla australis Richardson and A. dieffenbachii Gray) is limited to changes associated with gametogenesis in the adults; details pertaining to embryonic and larval development are not available, but are of obvious benefit for a better understanding of the eel life history and for artificial propagation programmes, whether for aquaculture or conservation purposes. Therefore, eels were artificially matured and eggs fertilised with sperm from either species to investigate early ontogeny. Only eggs from A. australis hatched successfully after c. 45 h of incubation at room temperature, yielding larvae of c. 2.5 mm in length. Larvae survived for up to 5 days, by which time they had attained a length of 5.3 mm. Scanning electron microscopy of 1‐ and 2‐day‐old larvae revealed the presence of prominent free neuromasts on the head and along the flanks. Although hatching of New Zealand eels was achieved for the first time, it is strongly recommended that hormone treatment be initiated immediately following capture, well before ovarian atresia starts.

Improved otolith preparation, ageing and back-calculation techniques for New Zealand freshwater eels

New Zealand freshwater eels are long-lived and their otoliths are difficult to age using conventional “burn-and-break” techniques. This study developed a number of new and improved techniques which increased the accuracy of age estimates and enabled growth rates to be back-calculated. The “burn-and-break” technique was significantly improved by sawing otoliths through the nucleus before burning. Criteria were developed to distinguish annuli from summer growth bands and supernumerary checks on the basis of colour, width, structure, continuity, and spacing. Annuli and supernumerary checks were most easily identified by examining thin sections stained in an acidic toluidine blue solution. Back-calculation techniques were also improved by using a curvilinear model, originating from a common biological origin, to describe the relationship between fish length and otolith radius. Annulus radii were measured accurately from the flat surface of sawn, burnt, half otoliths and back-calculated lengths were validated by comparison with the lengths of aged wild fish. Using the improved techniques developed in this study it may now be possible to determine factors influencing annual variations in growth and to study inter-relationships between sex and growth rates.

Optimum design for catch sampling of eels

Data from the first two seasons of a catch-sampling programme for New Zealand freshwater eels (Anguilla australis and A. diefenbachii) are described. These are used in two simulation experiments to provide information to optimize future sampling. Results are presented in the form of equations that predict the precision of estimates (of species composition, mean size, and mean age at two different sizes) as a function of various survey design variables. Precision typically depends more on the number of landings (catches) that are sampled than the number of eels sampled per landing. Also, the precision obtainable from a given design varies substantially from fishery to fishery. For example, if 50 eels were measured and 10 otoliths collected from each of five landings, estimated standard errors varied by a factor of 9 for species composition, 19 for mean weight, and 5 for mean age at the minimum legal size, depending on which fishery was sampled. Results for mean age estimates are more restricted and less certain (than those for species composition and mean size) because age data were fewer. Three further optimization issues are discussed: sampling costs, the importance of ‘minor’ species, and the pool of fishers from which samples are collected.

Gonad histology and plasma steroid profiles in wild New Zealand freshwater eels ( Anguilla dieffenbachii and A. australis ) before and at the onset of the natural spawning migration. I. Females

Immature and maturing male New Zealand freshwater eels, the shortfinned Anguilla australis and the longfinned A. dieffenbachii, were caught from the wild to obtain data on the natural reproductive physiology of these fish. Plasma samples were analysed for steroid hormones by radioimmunoassay and values related to the developmental stage of the testes. Our histological observations on testes largely confirmed those reported previously. Thus, the gonad of non-migrating eels often appeared undifferentiated or poorly developed, containing only type A or early type B spermatogonia. In contrast, the testes of migrating shortfins were in early spermatogenesis as evidenced by the presence of late type B spermatogonia. Similarly, early spermatogenic stages were common in migratory longfins, but eels in midspermatogenesis (all germ cell stages present) were also encountered. Unlike a previous study, patches of testicular regression were commonly seen in migrants of both species. Levels of several androgens, androstenedione (AD), testosterone and 11-ketotestosterone (KT), were elevated in migrants compared to non-migrants. AD was higher in early to midspermatogenic A. dieffenbachii (0.63 ng ml−1) than in A. australis (0.25 ng ml−1) in the spermatogonial proliferation stage, while the inverse was observed for KT (27.78 ng ml−1 and 50.52 ng ml−1, respectively). Levels of 17,20β-dihydroxy-4-pregnen-3-one were nearly undetectable (less than 0.12 ng ml−1) in all animals. Plasma 17-hydroxyprogesterone concentrations in fyke-caught eels were elevated to a greater extent in non-migrants (up to 1.92 ng ml−1) than in migrants (around 0.5 ng ml−1), and correlated well with levels of cortisol in all groups. Histological results are compared to previous studies and the presence of regression in the testes is discussed. In addition, the role of steroid hormones, in particular AD and KT, in reproduction and stress is considered.

Variability in growth rates of freshwater eels (Anguilla spp.) in New Zealand

Abstract— Both species of New Zealand freshwater eels (the shortfinned eel Anguilla australis and the longfinned eel A. dieffenbachii) are widespread and abundant, supporting important commercial and recreational fisheries. This article reviews growth studies from 35 widely distributed sites throughout New Zealand and discusses factors that influence growth rates. Length at given age is characterized by high intra‐ and inter‐population variability; growth rates for eels >30 cm are typically slow (2–3 cm per year) and linear, with females generally growing faster than males. Water temperature affects the length of the growing season, although growth rates are not correlated with latitude. Other factors suggested as affecting growth rates are eel density, quantity and quality of food, and interactions between both eel species. Al though growth of some New Zealand eel populations is the slowest recorded for any species of Anguilla, growth in culture can be rapid, similar to that of other temperate eel species.

Observations on granulocyte peroxidase in New Zealand freshwater eels, Anguilla species

Peroxidase activity in the granulocytes of eels was investigated using o‐tolidine, paraphenylene‐diamine‐pyrocatechol, and 4‐chloro‐l‐naphthol as substrates, and cyanide, azide and aminotriazole as inhibitors. Most circulating neutrophils of Anguilla australis Richardson, 1848 and A. dieffenbachii Gray. 1842 showed no peroxidase activity at pH 7.6 and pH 9.0, but a few neutrophils, thought to be mature, were positive. Another granulocyte in the anterior kidney, spleen, parasitized gill tissue and, rarely, the blood contained a cyanide‐resistant, azide‐inhibited, peroxidase and was tentatively identified as the eosinophil. Neutrophils of A. anguilla (L.) showed granular peroxidase activity which was inhibited by cyanide. The eosinophil was not observed.Absence of peroxidase from most circulating neutrophils in A. anguilla and A. dieffenbachii, and its pattern in the neutrophil precursors and mature neutrophils of A. anguilla, may be due to two morphologically indistinguishable granule types. Primary peroxidase‐negative granules occur in precursors and immature neutrophils and secondary peroxidase‐positive granules in mature neutrophils of all three eels. Circulating neutrophils in New Zealand eels seldom mature and are theretore peroxidase‐negative, whereas A. anguilla neutrophils are mature and are usually peroxidase‐positive.Impairment of microbicidal activity in neutrophils lacking peroxidase is discussed.

Hormone‐induced maturation in male New Zealand freshwater eels (Anguillaspp.)

Abstract Male adult Anguilla australis and A. dieffenbachii caught migrating to sea were held in sea water and induced to mature with injections of human chorionic gonadotrophin. Eye size increased during maturation by a factor of 1.5 in A. australis and 1.2 in A. dieffenbachii from the pre‐injection migrant stage. Spawning was observed in 4 of the 42 eels that matured. A. australis matured at between 12 and 28°C and A. dieffenbachii between 12 and 26°C. Minimum maturation times occurred at 26°C for A. australis and 24°C for A. dieffenbachii. Maturation rate and temperature were linearly related in both species. A. dieffenbachii matured more quickly than A. australis at all temperatures; this is probably related to the different state of sexual development of the experimental eels. At the time of seaward migration A. dieffenbachii are more sexually advanced than A. australis.

Timing and periodicity of migrating New Zealand freshwater eels (Anguillaspp.)

Abstract The migration of adult freshwater eels Anguilla australis and A. dieffenbachii was studied in 3 areas: Lake Ellesmere (Canterbury) (43°48′S, 172°31′E); Lake Onoke (Wairarapa) (41°23′S, 175°08′E); and the Makara Stream (Wellington) (41°13′S, 174°43′E). In Lake Ellesmere, migrating eels began congregating at the outlet area nearest the sea in January and continued until June. Eels arrived in a definite sequence: A. australis males, females; A. dieffenbachii males, females. Commercial catches exhibited a lunar periodicity with largest catches occurring just after the last quarter and smallest catches during the full moon. No obvious environmental factors were associated with the migration except that the largest commercial catches were made at times of high lake level. If the lake is opened to the sea during migration, fishing for migrating eels ceases and they escape out to sea. In Lake Onoke, aggregation of migrating eels in shallow areas nearest the sea began in February and lasted until May or J...

Morphometric changes, gonad histology, and fecundity estimates in migrating New Zealand freshwater eels (Anguillaspp.)

Abstract During transition from the non‐migrant feeding stage to the migrant stage in Anguilla australis and A. dieffenbachii the eyes increase in area by a factor of 2 in males and of 1.5–1.6 in females. A. dieffenbachii is more sexually advanced at the beginning of the spawning migration. In males the gonadosomatic ratio (GSR) was 0.5–2.7 and histological sections showed spermatocytes as the most common stage of spermatogenesis. For A. australis the GSR was 0.03–0.70, and histologically spermatogonia were predominant. In A. dieffenbachii females the mean GSR was 8.12, mean egg diameter was 0.33 mm, and histological sections showed that most eggs were at the stage of yolk deposition. In contrast, the mean GSR of A. australis females was 3.5, mean egg diameter was 0.22 mm, and few eggs were at the yolked stage. Egg counts showed that fecundity increased with total length. Fecundity estimates for 23 A. dieffenbachii (total length 711–1452 mm) varied from 1.05 million to 20.83 million and for 26 A. australis (total length 516–933 mm) from 0.46 million to 3.06 million.

Distribution of helminths in the digestive tracts of New Zealand freshwater eels 1. Distribution of digeneans

The distributions of Stegodexamene anguillae and Telogaster opisthorchis in Anguilla dieffenbachii and A. australis studied over two years are described. Peak occurrence of S. anguillae was in the anterior intestine, declining posteriorly, except for mature flukes under crowded conditions in A. dieffenbachii which predominated in the mid intestine, declining anteriorly and posteriorly. Peak occurrence of T. opisthorchis was in the rectum of A. australis and posterior intestine of A. dieffenbachii; occurrence declined anteriorly. Distributions appeared to be maintained by selective site segregation. Flukes moved down the tract, with or without subsequent loss, in both host species held live or examined after death. T. opisthorchis were lost before S. anguillae, and the larger mature flukes of both parasite species were lost before the less mobile immature flukes. Fluke loss from electric‐fished A. dieffenbachii was more rapid than from netted A. australis. Chilling and freezing retarded movement in, and ev...

Distribution of helminths in the digestive tracts of New Zealand freshwater eels

The distributions of the nematode parasites Paraquimperia sp., Spirocamallanus sp., and Cucullanus sp. in the intestines and recta of Anguilla dieffenbachii and A. australis from Lake Ellesmere and the South Branch of the Waimakariri River were studied over two years. Peak occurrence of Paraquimperia was in the anterior intestine of A. australis and mid intestine of A. dieffenbachii. Male and female Spirocamallanus sp. established in the mid intestine, but as female worms grew they attached more anteriorly and thus maintained the mid body in the mid intestine, prevented the tail end entering the rectum, and maintained the vulvae close to male worms. Cucullanus sp. was most often found in the mid intestine, but occurred throughout the intestine, possibly because of migration with food. Paraquimperia and Spirocamallanus extended their range with crowding; Cucullanus increased in density in the mid intestine. Posteriad movement, with or without subsequent evacuation, was a general feature of all species in both hosts, held live or examined after death. Chilling and freezing slowed or arrested loss from dead hosts. Larger worms (Spirocamallanus sp.) moved and were lost more rapidly than small worms (Paraquimperia sp.); worm loss was more rapid from A. dieffenbachii than from A. australis.

Distribution of helminths in the digestive tracts of New Zealand freshwater eels

Distribution of helminths in the digestive tracts of New Zealand freshwater eels

Size and age of migrating New Zealand freshwater eels(Anguillaspp.)

Migrating adult freshwater eels, Anguilla australis Richardson and A. dieffenbachii Gray, were examined during the autumnal migration from the Makara Stream (1971–73), Lake Onoke (1971, 1972) and Lake Ellesmere (1972, 1974–80). Males of A. australis migrated from the Makara Stream at a mean length of 465 mm and from Lake Onoke at a mean length of 447 mm. The difference between the mean lengths at migration of A. australis males from Lake Ellesmere between 1972 (mean 443 mm) and 1980 (mean 425 mm) were significantly different. Females of A. australis migrated from the Makara Stream, Lake Onoke, and Lake Ellesmere at mean lengths of 737, 764, and 615 mm respectively. Aging was by examination of burnt otoliths. Males of A. australis from each area migrated after spending an average of 14 years in fresh water. A. australis females migrated from the Makara Stream at an average age of 19 years, from Lake Onoke at 23 years, and from Lake Ellesmere at 24 years. A. dieffenbachii males and females were larger and o...

Scale development and age determination in New Zealand freshwater eels(Anguillaspp.)

The appearance and development of scales in Anguilla australis Richardson and A. dieffenbachii Gray were studied to assess the value of scales in age determination. Scales first appear in an area two‐thirds along the anus‐tail tip distance, when eels are 16–20 cm long. The rate of lateral and longitudinal scale spreading, scale size, and the number of scale rings were all positively related to fish length. The formation of new scale rings showed no seasonal pattern, so these rings cannot be used for age determination. Age could not be determined from length‐frequency histograms owing to overlapping of data from adjacent year‐classes. However, examination of otoliths prepared by burning showed that the otolith rings were annual in formation, and could be satisfactorily used to calculate age.

Variations in the spores ofMyxidium zealandicumHine, 1975 (Protozoa: Myxosporidea)

Abstract Myxidium zealandicum Hine, 1975 was recorded from several tissues in New Zealand freshwater eels. The gills were the most frequent site of infection, but spores also matured on the gill arch, in the skin, and occasionally in the urinary and swim bladders. Thus M. zealandicum is a histozoic species that may have evolved from a coelozoic form. Variation in spore size (8.0–11.5 jam) was found in relation to site of infection; variation in the number and arrangement of valve striations was also observed. Deposition of fibro‐blasts occurred at all sites of sporogony, except among the goblet cells of the epidermis. The small size (6.0–10.5μm) of unstriated spores occurring with normal spores in the gills of eels from Lake Otomangakau appeared to be related to the size of the cysts in which they occurred, and to the host Anguilla dieffenbachii.

First endozoic record of a trichodinid ciliate in fish (note)

Unidentified trichodinid ciliates (Protozoa: Ciliophora: Urceolariidae) were recorded within the tissues of the New Zealand freshwater eels, Anguilla australis schmidtii (Phillips) and Anguilla dieffenbachii (Gray). Organisms were small, measuring approximately 10 μm in diameter, and it is not known how they enter the eels. This is thought to be the first recording of usually ectoparasitic trichodinids occurring within their hosts. OBSERVATIONS AND DISCUSSION During a haematological study of the New Zealand freshwater eels, Anguilla australis schmidtii (Phillips) and Anguilla dieffenbachii (Gray), trichodinid ciliates were found. These occurred in routine blood films, and liver and splenic imprints. Organisms were smaller than Trichodina microdenticula from North America. T. microdenticul a was the smallest of 17 species described by Wellborn (1967), and measured 22-37 ij.m. Trichodinids from New Zealand eels had a diameter of approximately 10 jum after fixation in methanol. They possessed the characteristic adhesive disc supporting a ring of denticles, which are uniform structures in this group of ciliates. Identification relies partly upon the diameter of the adhesive disc and denticular ring. The number, length, and shape of denticles and radial pins are also important features of trichodinid taxonomy. Trichodinids are common and sometimes pathogenic ectoparasitic ciliates of marine and freshwater fishes (Jones 1974). They are also found on amphibians, coelenterates, and polychaetes, where they skim about on the mucus surfaces of skin, gills, fins, and palps. Occasionally, they are entozoic as in the urinary bladder of amphibians (Fulton 1923), but to the knowledge of the author there are no records of trichodinids being located within their hosts. How they gain entry to the host remains a mystery; they possibly entered through a lesion, for the parasites are of small size, and some fish did have scars remaining from previous small wounds. It may be significant that these organisms were located in the tissues of the reticulo-endothelial system, and that they were in the process of being filtered from the circulation. The fact that trichodinids lack marked host specificity (Lom & Homan 1964) suggests that the group are opportunistic, although it is not known if the organisms were alive when found.

Ultrastructure of the spermatozoa and spermiogenesis in New Zealand freshwater eels (Anguillidae).

The spermiogenesis and sperm ultrastructure of Anguilla australis schmidtii and A. dieffenbachii is described. During spermiogenesis the chromatin material condenses into the crescent-shaped head of the mature sperm. By elongation of the sperm head the mitochondrion is displaced to the anterior of the head remote from the posterior centriolar apparatus. A ciliary rootlet becomes segregated during spermiogenesis to form a membrane covered, striated, rod-like appendage. From the proximal centriole arise two sets of filaments, each containing three subfibrils. One group of five filaments extends anteriorly along the inner concave side of the head. A group of four modified filaments pass along the outer, convex, side. The middle subfibril B of each triplet in the group of four filaments is flattened and joins the two subfibrils A and B, forming a binocular shaped structure. The motile flagellum has a 9+0 axonemal pattern.

Three new species ofMyxidium(Protozoa:Myxosporidia) parasitic inAnguilla australisRichardson, 1848 andA. dieffenbachiiGray, 1842 in New Zealand

Abstract Three new species of Myxidium are described from New Zealand freshwater eels. M. zealandicum n.sp. is enzootic on the gills of Anguilla australis Richardson, 1848 and A. dieffenbachii Gray, 1842. It resembles M. giardi Cepede, 1906, but differs in the number of spore striations; in this respect it is similar to M. matsuii, Fujita, 1929, but the spores are smaller. The gall, swim, and urinary bladders occasionally contained spores morphologically indistinguishable from those of M. zealandicum. M. serum n.sp is described from mature cysts in the alimentary musculature, and immature cysts in the mesenteric connective tissue, of A. dieffenbachii. M. serum resembles M. matsuii and M. giardi in spore length, and M. giardi in the number of spore striations, but it is more slender in width than either species, and also differs from M. matsuii in polar capsule dimensions and number of striations. Spores of M. acinum n.sp. occur in small ovoid or elongated cysts in the gills of A. australis and A. dieffenb...