Variation in Population and Life History Traits of the American Eel, Anguilla rostrata, in Four Rivers in Maine

We examined population traits of yellow American eels from nine sites with similar habitat characteristics in each of four rivers in Maine, U.S.A. Migrating silver eels were also collected to compare sex ratio, age and size at migration among the four rivers. Population density and biomass were not significantly different among rivers with mean ranges of 8.4–21.8 eels 100m^–2and 380–1485gm^–2. Pairwise comparisons of the slopes of weight–length relationships of log transformed data (pooled data: intercept = –6.007, slope = 3.094, r²= 0.99, and n = 3116) revealed no significant differences among rivers. Length–age relationships (pooled data: intercept = 87.826, slope = 23.444, r²= 0.76, and n = 2325) also showed no statistically significant pairwise differences in slopes among rivers. In all rivers, sexual differentiation was complete by 270mm total length and age eleven. The sex ratios of migrating silver eels were not correlated with yellow eel sex ratios among the four rivers. Mean age at migration among the four rivers was significantly different for males only, with a range of 1.3years. Both sexes had some significant differences in size at migration among rivers, but the biological importance of the differences is tenuous (male range: 15mm, female range: 36mm). The yellow and silver eel population traits from these four rivers showed little variation when riverine habitat was isolated. Variations in traits appeared to be greater when eels from non-riverine habitats may have been present.     

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.     

Distribution of Adult Male and Female Baccharis concinna (Asteraceae) in the Rupestrian Fields of Serra Do Cipó, Brazil

Abstract: This study focuses on the sex ratio and spatial distribution of males and females in three populations of the endemic and restricted tropical dioecious shrub, Baccharis concinna (Asteraceae) in the mountainous region of Serra do Cipó, southeastern Brazil. The proportion of female plants in the population at lower elevation (1000 m a.s.l.) was significantly greater than of male plants. At this elevation of P/N and Ca/Al ratios in the soil were also greater indicating better nutritional status of the soils. The concentration of aluminium increased significantly with the elevation ( p < 0.001), perhaps rendering soils less conducive to female plants at higher elevations. Female plants are possibly adversely affected to a greater extent by soil quality than male plants. The spatial distribution of the populations within habitat was tested by the K(t) function, where the neighbourhood of a given individual was defined by a circle with a radius (t) up to 3 m. Despite the strong tendency for aggregation, the distribution of the sexes within habitats was random and the hypothesis was not supported. The independent distribution of the sexes within habitats may be explained by nutrient homogeneity of the soils, as well as by an absence of antagonism between the sexes. Nevertheless, we found a trend for males and females to be aggregated according to their gender.     

Interspecific and sexual differences in riverine distribution of tropical eels Anguilla spp.

A total of 261 individuals of the four tropical eel species, Anguilla celebesensis, Anguilla marmorata, Anguilla bicolor pacifica and Anguilla interioris, were collected from 12 locations around Sulawesi Island, Indonesia, to gain knowledge about the riverine distribution of tropical eels. Anguilla marmorata was predominant in the lower reaches of Poso River (94·4% of total eel catch in the sampling area), Poso Lake (93·3%), three small inlet rivers of Tomini Bay (100%) and Laa River (92·3%). Anguilla celebesensis occurred frequently in the inlet rivers of Poso Lake (63·5%). Anguilla bicolor pacifica and Anguilla interioris were rare (1.5 and 0.4%, respectively). Otolith Sr:Ca ratio electron‐probe micro analysis (EPMA) for individual migratory histories revealed that 15 A. celebesensis caught in Poso Lake and its inlet rivers were categorized into 14 river eels (Sr:Ca < 2·5) showing upstream migration seemingly at their elver stage and only one sea eel (Sr:Ca ≥ 6·0) that stayed in the marine habitat for the majority of its life after recruiting to Sulawesi Island before its late upstream migration. In A. marmorata, 19 examined eels from Poso Lake and its inlet rivers were all river eels, while 17 eels from the lower reaches of Poso River were two river eels, six sea eels and nine estuarine eels (2·5 ≤ Sr:Ca < 6·0) that mostly lived in the brackish water. The sex ratio of A. celebesensis was highly skewed towards a dominance of females (99%). In A. marmorata, females were predominant in Poso Lake (95·2%), its inlet rivers (94·7%) and Laa River (100%), while males were more frequent in the lower reaches of Poso River (76·5%) and small inlet rivers of Tomini Bay (94·1%). These results indicate that the riverine distribution pattern of tropical eels differs among species and between sexes.     

Colonisation of freshwater habitats by the European eel Anguilla anguilla

1. The spatial distribution of European eels in 18 U.K. rivers was related to distance from tidal limit using a negative exponential model. This function accounted for between 19 and 90% of the variation in eel density where quantitative data was available. For semiquantitative data the negative exponential function was a significant predictor of eel densities in only six out of 10 cases, although all rivers showed a consistent decline in abundance with distance upstream from the tidal limit. 2. The spatial distribution of different age groups of European eel in River Severn showed an initial rapid dispersion into freshwater followed by a much slower dispersion rate. Movement of the population upstream by a wave‐form migration process does not occur in this system. Instead colonisation of freshwaters can be seen as a two‐phase dispersion. Phase‐1 is a rapid dispersion upstream driven by density at the point source. Phase‐2 commences once the eels become yellow eels and is equivalent to random diffusion of particles. 3. These processes have important implications for the penetration of freshwaters with reduced numbers of eel larvae arriving on the coast of Europe and North America. Eel abundance will decrease more in freshwaters in an upstream direction whilst it may remain stable or decrease to a lesser extent in estuaries. They are also able to explain the demography of eels migrating upstream over weirs and the observations of varying sex ratios within catchments. We conclude that a dispersion model dependent on age, temperature, difficulty of migration, habitat quality and density of eels should be an important part of freshwater eel management.     

Comparative proximate body composition of Atlantic salmon with emphasis on parr from fluvial and lacustrine habitats

The nutritional status of Atlantic salmon Salmo salar parr was assessed by examining how proximate body composition and energy content (kJ g⁻¹) were associated with the habitat rearing origin of parr utilizing fluvial or lacustrine areas. Comparisons were made among sampling sites within the two habitats, as well as between the two major habitat types applying both fixed effect and random effect models. Mean protein concentration varied from 13·87 to 15·67% among fluvial sites, and from 14·96 to 16·38% among lacustrine sampling locations. Mean fat concentration ranged from 1·71 to 3·32% and from 2·22 to 4·29% among fluvial and lacustrine locations, respectively, with energy concentration, on average, c . 11% higher in lacustrine parr. Interpretation of results of comparisons between habitats was dependent upon the statistical model used. Benefits associated with lacustrine rearing, such as greater body size and nutritional status, have implications for overwintering survival in fresh water as well as subsequent survival during initial sea entrance as smolts.     

Evidence of silver eels contamination by microcystin‐LR at the onset of their seaward migration: what consequences for breeding potential?

Thirty migrating silver eels Anguilla anguilla were collected in a river system where algal blooms occurred yearly. Fifty per cent of eel livers were contaminated by microcystin-LR (mean ± s . d . toxin level: 28·1 ± 22·4 ng g⁻¹). Contaminated silver ( v. healthy) eels had lower fish condition. Consequences of this impact for the breeding potential of these migrating eels are discussed.     

Discrimination of New Zealand stream waters by glass eels of Anguilla australis and Anguilla dieffenbachii

This study tested the hypotheses, using glass eels of longfin eels Anguilla dieffenbachii and shortfin eels Anguilla australis migrating into fresh waters, (1) that both species prefer water from their river of collection to well water, (2) that shortfin eels prefer lowland, pastoral stream water to mainstem river water, (3) that longfin eels are attracted to both waters but do not prefer one to the other, and (4) both prefer water scented with geosmin, a widely occurring metabolite of bacteria and algae, to well water. Glass eels of both species from a river on the west coast of South Island, New Zealand, and shortfin glass eels from an east coast river significantly preferred water from their river of capture to well water. Two to three times as many eels chose their own river water as chose well water. Longfin eels were rare in the east coast river. Shortfin glass eels from the two rivers chose lowland stream water to mainstem river water about two to one in three experiments with different pairs of waters to which they had no prior exposure. Longfin glass eels significantly chose mainstem river water over lowland water in one pair but showed no preference when presented with a different pair. Reactions to solutions of geosmin at concentrations of 10^–5-10^–7 mg 1^–1 were inconclusive, with geosmin being preferred significantly, by shortfin eels, in only one experiment. The interspecific differences in discrimination of natural waters demonstrated in this study, with shortfin eels preferring lowland waters and longfin eels more indifferent to water types, are in broad agreement with both the distribution of adults and observations on their habitat preferences.     

Variation in the sex ratio, size and age of longfinned eels within and among coastal catchments of south‐eastern Australia

Longfinned eels Anguilla reinhardtii were captured by both fishery‐dependent and independent sampling methods from three rivers in New South Wales, south‐eastern Australia. Sex ratios, catch per unit effort and population age and total length structure were examined in three zones (fresh water and upper and lower tidal) in the Hacking, Hawkesbury and Clarence Rivers. Females were found in relatively high proportions in all zones, ranging from 97% in a freshwater (non‐tidal) site down to 59% in a tidal site. Males were found primarily in tidal zones (only two of the 677 longfinned eels caught in non‐tidal fresh water were males), with the greatest proportions being found in the brackish upper tidal areas. The mean number of fish captured per trap was higher in the fresh water and upper tidal zones than in the lower tidal zones. The mean ±  s . e . age, 17·9 ± 0·3 years, and age range, 5–52 years for females were significantly higher than those of males 12·2 ± 0·4 years; range 5–22 years, which is typical of other anguillid species. Longfinned eels captured in fresh water were found be significantly larger and older than those in tidal zones due to the almost exclusive predominance of females.     

Survival, growth and sex ratios of gynogenetic diploid honmoroko

Survival, growth and sex ratios of gynogenetic diploid honmoroko Gnathopogon caerulescens induced by blocking the release of the second polar body were examined. Mean survival of gynogenetic juveniles at 130 days after hatching was about 33% lower than that of the controls. No significant difference was seen in early growth between control and gynogenetic diploids. Standard length and body weight in six groups of gynogenetic progeny were significantly greater but in two groups were significantly smaller than in the controls. Although 69% of gynogenetic diploids had well-developed gonads, the remaining 30% had undeveloped gonads (small in size or thread-like), and those gonads were divided into four types. The mean proportion of females in the 10 gynogenetic groups was 87·2% which was significantly ( P <0·01) higher than in the controls (44·7%). Gynogenetic diploids included 3·0–35·3% males. Most of those males produced a high proportion of female progeny, but the proportion of male offspring varied widely. From these results, the sex determining mechanism in honmoroko was presumed to be female homogamety, but other factors resulted in the production of males.     

Evidence for Environmental Sex Determination in the American eel, Anguilla rostrata

Males have predominated among migrating silver eels in the Annaquatucket River, Rhode Island, for at least two decades, with no significant variation in mean total length in either sex. Because the species is panmictic (random breeding), this consistency suggests environmental sex determination (ESD). Most yellow (feeding phase) eels <300mm total length in the Annaquatucket are sexually undifferentiated, and in contrast to all other published sex ratios, males greatly outnumber females (3:1) among differentiated yellow eels. Estimates of yellow eel population densities are 4–10 times greater than published values for other habitats. We propose that this crowding results in a long period of undifferentiation and the suppression of femaleness. Published field and experimental evidence indicates that high population density results in high proportions of males in Atlantic Anguilla, and that low population density results in the predominance of females. This ESD may be adaptive, resulting in vast numbers of small males in coastal habitats, relatively close to the spawing area, and much larger and more fecund females that occupy most of the available eel habitat.     

Age, sex ratio and timing of the catch of kelts and ascending Atlantic salmon in the subarctic River Teno

By 15 June, 82% of the catch of Atlantic salmon Salmo salar kelts had been taken from the middle part of River Teno, northern Scandinavia. The median date of capture was 4 June for males and 8 June for females. Salmon of 1–4 sea–winters (SW) of both sexes survived spawning to return to sea as kelts. Among males, 1 SW kelts were caught earliest in the spring and 3 SW latest, but among females 4 SW were earliest, then 3 SW and finally 1 and 2 SW. There were 17 river and sea–age combinations among the kelts compared with 23 among the ascending salmon. The smolt age distribution and the mean smolt age differed significantly only between female 2 SW ascending salmon (3·97 years) and kelts (4·14 years). The proportion of 1 SW females was higher and that of 3 SW males lower among kelts than among ascending salmon. The proportion of males among 1 SW ascending salmon was 80% but among kelts only 57%. Similarly, the proportion of males among 3 SW fish was 21% for ascending salmon but only 7% for kelts. Hence overwinter mortality was higher among males. Male and female kelts of 1 and female kelts of 2 SWhad a greater mean length than ascending salmon in corresponding groups indicating a better survival of larger fish within an age group. Grilse ascend rivers after most kelts have left, but the main catch of ascending 2–3 SW salmon takes place concurrently with kelts leaving the river, inadvertently targeting kelts in the fishery.     

Smolt size in relation to age at first maturity of Atlantic salmon (Salmo salar): the role of lacustrine habitat

Back-calculated growth and size of Atlantic salmon smolts were compared in two groups of river systems in Newfoundland. One group consisted of rivers dominated by lacustrine habitats while the other had rivers characterized by fluvial habitats. Back-calculated length at each age and smolt size was significantly higher for the rivers dominated by lacustrine habitats. Associated with this was a lower proportion of maiden large salmon in adult returns. These findings are discussed in relation to density in fresh water, environmental conditions at sea, and life-history strategy.     

Population variation and ecological correlates of tychoparthenogenesis in the mayfly, Stenonema femoratum

Species in which both sex and parthenogenesis co‐occur are extremely valuable for investigating ecological conditions favouring sex. Tychoparthenogenesis is a breeding system characterized by hatching of a small proportion of unfertilized eggs (typically < 10%) from females of sexually reproducing species. With tychoparthenogenesis, both sexual and parthenogenetic reproduction co‐occur within the same population. To identify ecological conditions that may favour this breeding system, I quantified population variation in females’ capacity for tychoparthenogenesis and investigated biotic and abiotic correlates of tychoparthenogenesis. I estimated tychoparthenognetic capacity (proportion of unfertilized eggs hatching) for females from 12 Missouri populations of the mayfly, Stenonema femoratum (Ephemeroptera: Heptageniidae), across three different habitat types – temporary streams, permanent streams and lakes. Tychoparthenogenetic capacity, measured as the population mean hatch success of unfertilized eggs, ranged from 3.8 to 10.7%. Tychoparthenogenetic capacity varied among habitats in 1996, but not in 1997. In 1996, temporary streams showed hatch success of unfertilized eggs twice that of permanent streams and lakes. Tychoparthenogenetic capacity also varied among sampling dates within years. Temporary streams also showed extremely low nymph densities compared to the other two habitats. However, habitats did not differ in adult density. Furthermore, in all populations nymphs showed significantly female‐biased sex ratios. In contrast, adult sex ratios were equal or slightly male biased. Tychoparthenogenetic capacity was negatively correlated with nymph density in 1996, but not in 1997, suggesting possible reproductive assurance in some years. Adult densities also suggested that there may be certain times of year when tychoparthenogenesis may provide benefits of reproductive assurance. Although habitats differed significantly in their abiotic characteristics, tychoparthenogenetic capacity was correlated significantly with water temperature only. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 75 , 101–123.     

When to estimate sex ratio in natural populations of insects? A study on sex ratio variations of gall midges within a generation

Precise estimation of arthropods' sex ratio is an important issue in a wide range of ecological studies and biological control programs. Although, in many cases changes in arthropods' sex ratio may be under the control of parents or some symbiotic microorganisms, biased sex ratios in some other species are caused by some extrinsic factors, neglect of which may lead to under/overestimation of true sex ratio. In this paper, we pursued those factors that cause false estimation of sex ratio in insects' species. We studied the predatory gall midge, Aphidoletes aphidimyza Rondani (Diptera: Cecidomyiidae), an important biological control agent of aphids, that shows protandry (i.e. early male emergence), differential lifespan of sexes, and differential distribution of sexes across habitat. Ten populations of A. aphidimyza were released separately in transparent cages and their sex ratio variations were recorded every 12 hours. The primary sex ratio in this species seems to be slightly male‐biased (52.41% males), however early emergence of males biases the sex ratio up to 72% males in a few hours after emergence. Shortly after the emergence of females, the sex ratio reaches its primary situation, but as a result of male‐biased mortality after mating, the proportion of females increases gradually to 97% by the fourth and fifth days after emergence. These results explicitly suggest that direct estimation of sex ratio in natural populations may be affected by some secondary factors such as differential mortality of sexes, protandry, and differential distribution of males and females over time and/or across habitat.     

Factors determining survival of European eels in two unexploited sub‐populations

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The effects of depth and salinity on juvenile Chinook salmon Oncorhynchus tshawytscha (Walbaum) habitat choice in an artificial estuary

The energetic cost for juvenile Chinook salmon Oncorhynchus tshawytscha to forage in habitats of different salinity and depth was quantified using a behavioural titration based on ideal free distribution theory. When given a choice between freshwater habitats of different depths (>0·83 or <0·83 m), a greater proportion of fish used the deeper habitat. When the deeper habitat was saltwater, the proportion of fish using it increased. When food was added to both the shallow freshwater and deep saline habitats, however, fish distribution returned to that observed when both habitats were fresh water. This indicates that the preference for deep saline habitats during the stratified phase was driven by some benefit associated with residency in deeper water, rather than salinity. The low perceived cost of low salinity might be in part due to the fish's ability to minimize this cost by only making brief forays into the alternate freshwater habitat. When the food ration delivered to the more costly, shallow habitat was 50% greater than that delivered to the less costly, deep habitat, fish distributed themselves equally between the two habitats, presumably because of equal net benefits. This study demonstrates that juvenile Chinook salmon prefer deep saline habitat to shallow freshwater habitats but will make brief forays into the freshwater habitat if food availability is sufficiently high.     

Reproductive biology of the threatened golden galaxias Galaxias auratus Johnston and the influence of lake hydrology

Golden galaxias Galaxias auratus (31–235 mm fork length, L _F) were collected monthly from littoral habitats in Lakes Crescent and Sorell, Tasmania, Australia, between July 2000 and December 2002. Spawning habitats were identified and monitored in both lakes, and surveyed in Lake Crescent. Trends in gonado-somatic indices and reproductive stages of development indicated that gonad development in both sexes begins in midsummer and peaks in late autumn to early winter. Males mature at smaller sizes (50% at 52 mm L _F) than females (50% at 76 mm L _F), larger individuals are predominately females (95% of fish ≥138 mm L _F), and overall male to female ratios are female biased ( c . 1:2). Spawning occurs late autumn to early spring (water temperatures = 1·4–9·7° C) with peaks in spawning activity in winter (mean water temperatures <5° C). Demersal adhesive eggs ( c. 1·5 mm diameter) were found on cobble substrata ( c. 20–250 mm diameter) in littoral areas ( c. 0·2–0·6 m deep) and fecundity of fish 71–181 mm L _F ranged from 619 to 14 478 eggs. The rate of change in water level over the 20 days prior to monthly sampling was important in explaining the occurrence of spent fish and this accounted for temporal differences in spawning between the populations. Lake hydrology influences the reproductive cycle of G. auratus by possibly providing a stimulus for spawning and it controls the availability of spawning habitat in Lake Crescent. Seasonal hydrological cycles ( i.e. rises during late autumn to winter) and a minimum water level of 802·20 m Australian Height Datum in Lake Crescent during autumn (above which littoral areas of cobble substratum are inundated) are critical to G. auratus populations.     

Decline in non-native freshwater eels in Japan: ecology and future perspectives

The occurrence, distribution, and biological characteristics of non-native freshwater eels were analyzed using 5524 eels collected from 16 sites in Japan between 1997 and 2005. Three hundred seventy-four fishes (6.8%) were identified as non-native European eels, Anguilla anguilla, while the remainder (93.2%) were native Japanese eels, A. japonica. The European eel was found at 7 sites (44%), including 3 rivers, 2 freshwater lakes, one brackish lake, and one sea bay, suggesting a wide rage of habitat use. This variability of habitat use was also evidenced by the otolith microchemistry, which showed that they had lived in not only freshwater but also in seawater habitats. The sites with European eel were localized within the vicinity of southern Japan where a number of these eels were cultivated in the early 1970’s, suggesting that some had escaped from the culture ponds or were released intentionally into nearby natural waters. The large body size (mean total length: 803 mm), pigmented skin, enlarged eyes, and relatively matured gonads (mean gonad somatic index: 1.9) found in non-native European eels indicated that most had metamorphosed into the migratory silver phase, suggesting their ability to initiate spawning migration. However, the proportion of European eels in Mikawa Bay in 1997 was more than 12%, which decreased markedly to less than 2% after 2001, corresponding to the recent decline in import of European glass eels for aquaculture. This suggests that the population of European eels will decrease in Japanese waters in the future.     

Salinity-linked growth in anguillid eels and the paradox of temperate-zone catadromy

Temperate-zone anguillid eels use both saline (marine or brackish) and fresh waters during their continental phase, but use of fresh waters is paradoxical because on average these fishes grow more rapidly in saline than in fresh waters. Based on data from anguillid eels whose habitat-residency histories had been determined by Sr:Ca otolithometry, superiority of growth rates in saline water is much greater in American eels Anguilla rostrata in north-eastern North America (mean saline:fresh growth rate ratio 2·07) than in European Anguilla anguilla , Japanese Anguilla japonica and shortfinned Anguilla australis eels (range of mean ratios 1·12–1·14). Data from A. rostrata in the Hudson Estuary, U.S.A., and Prince Edward Island, Canada, were used to test adaptive explanations of catadromous migrations. The hypothesis that lower mortality in fresh water offsets faster growth in saline water was not supported because loss (mortality + emigration ) rates did not vary between saline and fresh zones of the Hudson Estuary. Hypotheses that anguillid eels move to fresh water to escape from larger anguillid eels in saline water or to evaluate habitat quality were not supported by size and age distributions. Catadromy in temperate-zone anguillid eels increases the diversity of occupied habitats and therefore lowers fitness variance caused by environmental fluctuations. Catadromy in temperate-zone anguillid eels could be due to natural selection for maximum geometric mean fitness which is sensitive to fitness variance. Temperate-zone catadromy might also be maladaptive, at least in local areas, due to shifts over time in selective pressures or to inability of panmictic genetic systems to adapt to local conditions.