Female Yellow Eels Research Articles

European eel (Anguilla anguilla) survival modeling based on a 22-year capture-mark-recapture survey of a Mediterranean subpopulation.

Since the 1980s, the European eel (Anguilla anguilla) has declined by over 90% in recruitment across its European and North African distribution area. This diadromous fish spawns at sea and migrates into continental waters, where it grows for three to more than 30 years, depending on habitat conditions and location. During their growth, different habitat use tactics can locally influence the life-history traits of eels, including their survival rates. Thus, the spatio-temporal dimension of this species is crucial for management. Based on a rare Mediterranean long-term survey of more than 20 years (2001-2022) in an artificial drainage canal connected to a vast brackish lagoon (the Vaccarès lagoon), we aimed to study the dynamics of one subpopulation's life-history traits. We used Bayesian multistate capture-mark-recapture (CMR) models to assess the temporal variability in survival and abundance at both seasonal and inter-annual scales, considering life-stage structure. High survival rates and low detection probabilities were found for the undifferentiated and female yellow stages. In contrast, female silver eels exhibited lower survival rates and higher capture probabilities. Estimating detection probabilities and survival rates enabled accurate assessment of relative abundance across different life stages and time periods. Our findings indicated a substantial decrease in the abundance of undifferentiated and female yellow eels in the early 2000s, whereas the abundance of female silver eels remained consistently low yet stable throughout the study period. Considering the life stage seemed essential to study the dynamics of the eel during its continental growing period. The present results will provide key elements to propose and implement suitable sustainable environmental management strategies for eel conservation.

Movement behaviour of large female yellow European eel (Anguilla anguilla L.) in a freshwater polder area

AbstractDue to a recruitment decline of more than 90% in 30 years, the European eel (Anguilla anguilla L.) has been classified by IUCN as critically endangered. Although the species has been studied intensively to obtain knowledge to improve management, studies about the resident yellow stage are relatively scarce. In this study, 52 large female yellow eels were tagged with acoustic transmitters in a Belgian polder system and tracked by a network of 23 automatic listening stations. We studied both circadian and seasonal movement patterns and the effect of environmental variables on these patterns. Large female yellow eels were most active at night in late summer and early autumn. A generalised linear mixed model showed that their movement is only slightly influenced by environmental variables. Moreover, as yellow eels show high site fidelity (i.e., the majority was detected only in the habitat type of their catch‐release location), they do not encounter many human‐induced connectivity problems in polder systems, which makes these systems highly suitable as eel growth habitat. These results can contribute to an effective eel management regarding habitat protection and restoration.

Growth of European eel Anguilla anguilla along the southern Baltic coast of Germany and implication for the eel management

To detect growth differences of European eel Anguilla anguilla along the southern German Baltic coast 728 yellow eels, with total lengths ranging from 256 to 944 mm and ages ranging from 3 to 15 years were collected from six coastal areas from 2005 to 2009. The estimation of the growth performance was based on the otolith increments. The mean growth rate of the female yellow eels varied from 56 to 62 mm year–1. No significant differences in the mean growth rate were detected between eels from inner and open coastal areas. The overall mean annual increment of eels was estimated at 59 mm year−1. Specific growth rates (SGR) of female yellow eels decreased with increasing age from 0.68 % day−1 in the first year to 0.05 % day−1 in the tenth year. Results indicate that no separation is needed in the development of population models or management initiatives based on the growth performance of eel in inner and open coastal waters of the southern German Baltic coast.

PRELIMINARY INVESTIGATION ON MORPHOMETRIC AND BIOMETRIC CHARACTERISTICS OF FEMALE SILVER AND YELLOW, Anguilla anguilla, FROM EASTERN MEDITERRANEAN (GÖKSU DELTA/TURKEY)

This preliminary study was conducted to shed light on characterization of migrating female sil-ver and sedentary yellow eels from Goksu Delta (Eastern Mediterranean) by providing some basic morphometric and biometric parameters and indices. Mean body weight and total length of migrating female silver eels were found to be 72.90 cm and 890.27 g respectively. In female yellow eels mean total body length was measured as 57.45 cm and mean body weight was found to be 384.86 g. The mean values of total length, body weigh, gonad weigh, liver weight, eye index, gonado-somatic index, and condition factor of migrating silver were significantly higher than those of yellow eels (P0.05). Gonado-somatic index was not found to be correlated to total length and eye index in female silver eels.

Growth differences of Japanese eels Anguilla japonica between fresh and brackish water habitats in relation to annual food consumption in the Kojima Bay‐Asahi River system, Japan

AbstractTo learn about the relationships between feeding and growth of temperate eels in freshwater and brackish water habitats, we analysed 533 yellow‐phase Japanese eels Anguilla japonica collected in both types of habitats in southeastern Japan. Because male eels were very rare in each habitat (FW, N = 1; BW, N = 20), characteristics of female eels were compared between the different habitats. Annual food consumption was evaluated with the consideration of instantaneous food consumption and annual activity period. Stomach fullness index (stomach content weight/body weight) was used as an indicator of instantaneous food consumption. The ratios of number of months with eel catch to those when eel sampling was conducted were used as an indicator of activity period. Female yellow eels tended to be older and slower growing in fresh water (N = 78; age, mean ± SD = 7.9 ± 2.4 years; growth rate, 59.8 ± 14.0 mm year−1) than in brackish water (N = 229; age, 5.5 ± 1.8 years; growth rate, 90.1 ± 24.4 mm year−1). Irrespective of sex, yellow eels in brackish water had a higher stomach fullness index and a greater ratio of months with eel catches, indicating greater annual food consumption by brackish water eels. These results indicate that greater annual food consumption contributes to the greater growth rates of Japanese eels in brackish water habitats.

Endocrine Evidence that Silvering, a Secondary Metamorphosis in the Eel, Is a Pubertal Rather than a Metamorphic Event

Silvering (transition from yellow to silver eel) has been traditionally considered as a metamorphosis in view of the numerous morphological, physiological and behavioral changes preparing the eel for the oceanic migration. However, some changes, such as increases in gonad weight and steroidogenesis, suggest that silvering could also be considered as a pubertal event. In order to assess which endocrine axis may be involved in the induction of silvering, we compared the profiles of pituitary and peripheral hormones during the transition from yellow to silver female eels. A strong activation of the gonadotropic axis was shown during silvering. Follicle-stimulating hormone (FSH) mRNA levels increased during the early stages of silvering, followed by a later increase in luteinizing hormone (protein and mRNA) levels. In addition, plasma levels of sexual steroids (estradiol, E2; testosterone, T, and 11-ketotestosterone) and of vitellogenin significantly increased. In contrast, thyrotropin mRNA levels did not change and no or weak variations in plasma thyroid hormones were observed, indicating no or moderate change of the thyrotropic axis during silvering. Similarly, the somatotropic axis was not activated, as shown by pituitary growth hormone expression (protein and mRNA) and plasma levels. In addition, we studied the effects of chronic treatments of female yellow eels with thyroid hormone (thyroxine, T4) and sex steroids (T and E2) on biometrical parameters characteristics of silvering. T induced an increase in eye size and a reduction of digestive tract, whereas T4 and E2 had no effect. These hormonal profiles and experimental data lead to the conclusion that eel silvering should be considered as an onset of puberty rather than a ‘genuine’ metamorphosis.

Effects of diet supplementation with carp ovary on gonad differentiation and growth of the European eel

Treating elvers of European eel Anguilla anguilla with mature carp ovary for 3 6 months during early growth induced female differentiation in 51.6-66.7% of treated animals compared with c. 5% in controls. The treatment also induced differentiation of ovaries in eels < 13 cm L T and a higher number of Syrski organs with ambisexual characters, and was most effective when administered at an early growth stage. The results could be attributed to the natural steroid content of the carp ovary. The total weight of treated animals at the end of the farm experiment was 84.7% higher than controls. The specific growth rate for weight was significantly higher in female yellow eels than in males, for both control and treated groups. The enhanced growth was related to induced feminization. A diet supplementation with mature carp ovary could be a good approach to control of sex differentiation and growth in eels.

Effects of diet supplementation with carp ovary on gonad differentiation and growth of the European eel

Treating elvers of European eel Anguilla anguilla with mature carp ovary for 3–6 months during early growth induced female differentiation in 51·6–66·7% of treated animals compared with c. 5% in controls. The treatment also induced differentiation of ovaries in eels <13 cm LT and a higher number of Syrski organs with ambisexual characters, and was most effective when administered at an early growth stage. The results could be attributed to the natural steroid content of the carp ovary. The total weight of treated animals at the end of the farm experiment was 84·7% higher than controls. The specific growth rate for weight was significantly higher in female yellow eels than in males, for both control and treated groups. The enhanced growth was related to induced feminization. A diet supplementation with mature carp ovary could be a good approach to control of sex differentiation and growth in eels.

Tagging experiments with yellow eel,Anguilla anguilla(L.), in brackish water in Norway

Abstract A total of 566 female yellow eels were tagged in Oslofjorden near Drøbak in 1983, 33 of these were recaptured during 1983–1987. An estimated instantaneous rate of fishing mortality from 0.024 to 0.079 is given. Tag loss and tagging mortality combined was estimated to be c. 17 % the first two months after tagging, and 22 % the first six months after tagging. Recaptures were made from 0 to 53 km from the site of first capture and release, and the distance between site of first capture and site of recapture increased with time.

Free amino acid content in the skin mucus of yellow and silver eel, Angvilla angvilla l

1. 1. Free amino acid contents in skin mucus extracts were analysed in sexually undinerentiated yellow eels, female yellow eels and male and female silver eels trapped at the same time from the same freshwater location. 2. 2. Total free amino acid contents in skin mucus were similar in the three groups of sexually differentiated eels (16.2–18.2μM/g dry weight). In sexually undinerentiated yellow eels, total content was higher (39.6μM/g dry weight). 3. 3. Free amino acid composition of skin mucus from male and female silver eels were similar. Compared with the two groups of yellow eels, main differences were found in taurine and non-essential amino acid relative contents. 4. 4. Taurine, glycine and alanine concentrations in skin mucus decreased as the level of sexual maturity of the eel increased. 5. 5. Possible influences of nutritional and hormonal factors on the free amino acid content of eel skin mucus are discussed. Potential consequences in osmoregulatory processes and intraspecific chemocommunication of the eel are also considered.

Temperature-dependent activity of brackish water yellow eels, Anguilla anguilla L.

. Female yellow eels, Anguilla anguilla L., were caught with fyke nets in the brackish water bight Hallangspollen, Norway during 1983 and 1984. Fishing was performed continuously for a variable number of days (defined as the fishing period), and the nets were monitored at uneven intervals during the fishing period. The eel activity, expressed as catch per unit effort (CPUE), was positively correlated (R2= 0·74; P < 0·001) with water temperature (defined as a regression of temperature variation during the fishing period). The multiple regression model of CPUE on water temperature was improved by including the number of fishing days in each fishing period in the regression model (R2= 0·91; P < 0·001). With high fishing efforts the predicted CPUE decreased below that which was predicted by the temperature model alone.