The effect of salinity on the chemotaxis of glass eels, Anguilla anguilla, to organic earthy and green odorants

Synopsis The behaviour of upstream migrating glass eels to salt and brackish water solutions of 8 pure organic earthy and green odorants was investigated. In 35 salt water, IPMCET and D-MF are ineffective, while MMP, IBMP, MT, TMCE, ETMCE and L-MF are strongly repellent. Dissolved in brackish water, MMP and ETMCE become attractive. The strong attraction which glass eels show to earthy and green odorants emerges as the level of salinity is reduced, suggesting that these chemicals could be orienting cues in the last phase of glass eel migration.     

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.     

Population biology of Proteocephalus macrocephalus (Creplin) in the European eel, Anguilla anguilla (Linnaeus), in two small rivers

Seasonal changes in populations of Proteocephalus macrocephalus (Creplin) were investigated for 17 months in the European eel, Anguilla anguilla (Linnaeus), in two rivers in Devon, SW England, but no clear seasonal patterns in prevalence and abundance were apparent. Population levels of the cestode are low in both localities, and it is suggested that natural population levels of P. macrocephalus may generally be low. However, growth and maturation of the cestode show marked seasonality with both occurring mainly in early summer.     

Genetic variability is unrelated to growth and parasite infestation in natural populations of the European eel (Anguilla anguilla)

Positive correlations between individual genetic heterozygosity and fitness-related traits (HFCs) have been observed in organisms as diverse as plants, marine bivalves, fish or mammals. HFCs are not universal and the strength and stability of HFCs seem to be variable across species, populations and ages. We analysed the relationship between individual genetic variability and two different estimators of fitness in natural samples of European eel, growth rate (using back-calculated length-at-age 1, 2 and 3) and parasite infestation by the swimbladder nematode Anguillicola crassus . Despite using a large data set of 22 expressed sequence tags-derived microsatellite loci and a large sample size of 346 individuals, no heterozygote advantage was observed in terms of growth rate or parasite load. The lack of association was evidenced by (i) nonsignificant global HFCs, (ii) a Multivariate General Linear Model showing no effect of heterozygosity on fitness components, (iii) single-locus analysis showing a lower number of significant tests than the expected false discovery rate, (iv) sign tests showing only a significant departure from expectations at one component, and, (v) a random distribution of significant single-locus HFCs that was not consistent across fitness components or sampling sites. This contrasts with the positive association observed in farmed eels in a previous study using allozymes, which can be explained by the nature of the markers used, with the allozyme study including many loci involved in metabolic energy pathways, while the expressed sequence tags-linked microsatellites might be located in genes or in the proximity of genes uncoupled with metabolism/growth.     

欧洲鳗鲡血液某些生物化学指标的测定

以114尾欧洲鳗鲡为材料,测定了血液中肌酸酐,血糖、尿素氮、甘油三酯,总胆固醇,总蛋白、白蛋白、谷丙转氨酶,谷草转氨酶,乳酸脱氢酶,乙酰胆碱脂酶、γ-谷氨酰转肽酶、钠、钾、钙、镁、氯化物、二氧化碳,无机磷等血液指标值,按统计学原理进行分析。并对总体正常值范围进行估计。获得了欧洲鳗鲡血液生理生化指标参考值。     

Predicting conditions for migration: effects of density dependence and habitat quality

Migration is widespread among animals, but the factors that influence the decision to migrate are poorly understood. Within a single species, populations may be completely migratory, completely sedentary or partially migratory. We use a population model to derive conditions for migration and demonstrate how migratory survival, habitat quality and density dependence on both the breeding and non-breeding grounds influence conditions for migration and the proportion of migrants within a population. Density dependence during the season in which migratory and sedentary individuals use separate sites is necessary for partial migration. High levels of density dependence at the non-shared sites widen the range of survival values within which we predict partial migration, whereas increasing the strength of density dependence at the shared sites narrows the range of survival values within which we predict partial migration. Our results have important implications for predicting how contemporary populations with variable migration strategies may respond to changes in the quality or quantity of habitat.     

Does a 5500-km swim trial stimulate early sexual maturation in the European eel (Anguilla anguilla L.)?

The catadromous European eel (Anguilla anguilla L.) undertakes a 6000-km spawning migration from its freshwater habitats to the Sargasso Sea. In large Blazka swim tunnels of 127 l, the physiological effect of such a prolonged swimming performance on sexual maturation in adult female eels was investigated. Two groups of eels were placed in swim tunnels for 173 days, one group was able to swim at 0.5 body lengths/second (Swim group) covering a distance of c. 5500-km over the experimental period, and one group kept in static (End Control group). A control group was sampled at the start of the experiment in order to determine the initial stage of reproductive development (Initial Control group). At the end of the swim trial, the maturation parameters 11-ketotestosterone, pituitary levels of LH and plasma levels of estradiol were higher (although not significantly) in the Swim compared to the End Control group. In addition, no significant differences were observed in most measured morphometric and reproductive parameters, including eye-index, gonadosomatic index, hepatosomatic index, and plasma levels of vitellogenin, cortisol and melanophore-stimulating hormone (MSH). Also, pituitary levels of both MSH, and adrenocorticotropic hormone (ACTH) were unaffected. In contrast, the oocyte diameter was found to be significantly higher in the Swim compared to the End Control group. Based on these observations we conclude that a period of prolonged swimming might be a physiological stimulus necessary for the onset of maturation in the European eel.     

Seasonal population dynamics of Paraquimpevia tenevvima (Linstow) (Nematoda) in the European eel Anguilla anguilla (Linnaeus) in three localities of southwest England

Seasonal population dynamics of Puraquzmperia tenerrima (Linstow) in the European eel Anguillu anguillu (Linnaeus) have been investigated in three localities in Devon, southwest England. The nematode is common and exhibits very similar seasonal patterns in changes of prevalence and abundance in the three localities. The population increases over winter through to late spring or early summer as the recruitment of the new generation occurs throughout this period. Reproduction of the nematode starts in spring. A sharp decrease of prevalence and abundance accompanies or follows reproduction, and infection levels are very low in autumn and early winter. The dispersion pattern of the nematode varies with the infection levels from over-dispersion in spring and early summer to almost random dispersion in autumn and winter. It is suggested that P. tenerrimu is a common specific parasite of eels, especially in small streams or rivers and that insect larvae or species of crustaceans might serve as intermediate hosts for the nematode.     

Populations of Bothriocephalus claviceps (Goeze) (Cestoda) in the European eel, Anguilla anguilla (L.), in three localities in southwest England

The population biology of Bothriocephalus claviceps (Goeze) has been investigated in the European eel, Anguilla anguilla (L.), in three localities in southwest England. Monthly changes in prevalence and abundance show no statistically significant seasonality, in contrast to growth and reproduction of the cestode. The cestode reaches maximum size and becomes gravid mainly in summer. Gravid cestodes die and are lost soon after reproduction. This overlaps with recruitment of the new generation into eels, and may result in a rapid increase in infection levels and in a more over-dispersed distribution of the parasite in some localities, or in a more gradual change in infection levels and over-dispersion in others. Development and growth of the new generation may cease until the following spring, or continue at a slow rate over winter. Parasite numbers in each host decrease well before they actually reach maturity, since the majority of gravid individuals are present in those eels which harbour only one parasite. This suggests that few cestodes develop to maturity and most are lost from hosts as development proceeds.