Effects of oestradiol—enriched diet and of feeding with porcine testicular tissue on macroscopic gonadal sex in European eels

Oestradiol–enriched food (25 mg kg^-1 oestradiol) fed for 91 days to small eels (2 or 7 g) with undifferentiated gonads (UD) significantly favoured development of gonads with the macroscopic appearance of ovaries (♀-gonads) during the next 8 months. Fifty-four and 51% of control eels and 75 and 78% ofoestradiol-treated eels developed ♀-gonads. Minced porcine testicular tissue fed to 2-g eels for up to 371 days had effects similar to oestradiol-enriched food (68% with ♀-gonads), whereas feeding for only 91 days had no effect during the next 8 months. Oestradiol-enriched food was also fed to larger eels with already macroscopically differentiated gonads (30–70 g; 95% with ♀-gonads, 5% with ♀-gonads). After 27 days significantly more eels with a ♀-gonad or with a mixture of ovary-like and testis-like regions in the gonads (♂+♀-gonads) were found. After 96 days there were 44% with ♀-gonads, 40% with ♂+♀-gonads and 16% with ♀-gonads. Oestradiol thus had a feminizing effect, not only on morphologically undifferentiated gonads but also on morphologically differentiated ♀-gonads. The presence of sex steroid hormones or their precursors in porcine testicular tissue may also exert a feminizing influence. In all experiments the hormone-fed groups showed a tendency (not significant) towards increased growth rate. In small eels early rapid growth and differentiation of ♀-gonads were clearly correlated, both in hormone treated and in control eels. Otherwise no correlation was found between growth rate and gonadal sex.     

Measurements on ground or sectioned otoliths: possibilities of bias

Back-calculation usually requires measurements of growth marks revealed on otoliths by specific preparations. The standardization of the grinding (or sectioning) plane is necessary, but difficult, especially along the antero-posterior axis. In order to show the importance of the grinding plane, tetracycline labelling of eel otoliths ( Anguilla anguilla L.) has been used. This marking has a calcio-traumatic effect on otoliths, which can be revealed with staining techniques. Unless the grinding plane is incorrect, the tetracycline labelling and the staining of the ground surface are then superposed.     

Is proximity to the North Atlantic Drift and the Continental Shelf Current sustaining freshwater European eel populations in western Scotland?

1. We report on freshwater resident eel numbers in western Scotland based on two time series of data, independent of each other, spanning 28 years and that do not rely upon fisheries information. 2. Data from eel captures on trash screens of a pumping station (1982–2003) on Loch Lomond and electrofishing data from a stream in Lochaber, the Allt Coire nan Con (1989–2010), are compared with similar time series eel population data from elsewhere in the British Isles and more widely in Europe. 3. Over the period of the study, indices of eel numbers from across Europe declined by between 72 and 95%; in stark contrast, neither time series from western Scotland showed evidence for decline between 1982 and 2010. 4. We provisionally conclude that freshwater populations in western Scotland are being maintained by regional processes directly related to the proximity of the leading edge of the North Atlantic Drift and the Continental Shelf Current and the direction of prevailing winds.     

Experimental test of environmental factors influencing the seaward migration of European silver eels

In this study we test which environmental factors are influencing the migratory behaviour of seaward migrating silver European eets Anguilla anguilla in a Norwegian river. We test this by transplanting tagged silver eels upriver after catching them in a trap at the outlet of the river. We did this at four fixed dates (fixed day lengths) during 5 years. Over 74% of the variation in time from release to subsequent recapture could be accounted for by variation in time at release, water discharge at release, and moon phase at release (in decreasing order of importance). The number of days to median day of recapture of each batch decreased as the season progressed and decreased with increasing water discharge. Water temperature did not explain any of the remaining variation in the model, even if there was a significant correlation between water temperature at release and median day at recapture of each batch (in simple linear regression model). Recapture rate during the same migration season varied strongly with water temperature, but at most 33% of the variation could be explained by this factor. It was aiso evident that the recapture rate increased when the number of downward migrating untagged silver eels was high. Day length (i.e. time at release) did not explain any of the remaining variation in recapture rate.     

Growth rate and age at migration of Anguilla anguilla

The length, age and growth rate were investigated for downstream migrating male and female eels in the unexploited Burrishoole system, western Ireland. Significant differences were found in the age and length at migration with the larger, older female eels also showing faster annual growth as early as the first year in fresh water. Female eels normally migrated at lengths from 40.5 cm, exceptionally to 92.9 cm, and male eels at lengths between 28.9 and 46'0 cm. Back-calculation showed an irregular pattern of fast and slow annual growth. Mean annual growth increments were almost always greater for females than for males.     

Growth and Production of Yellow Eels and Glass Eels in Ponds

Two experiments (I and II) were performed in drainable ponds. Yellow eels Anguilla anguilla (L.) were stocked in early June at three biomasses: 10, 20 and 60 kg · ha⁻¹ in experiment I; and 10, 20 and 40 kg · ha⁻¹ in experiment II. The mean body weights were 27.0 and 24.2 g respectively. Glass eels were stocked only in experiment II at equal densities of 1600·ha⁻¹. In both experiments each biomass of yellow eel was combined in a factorial design with three cyprinid communities differing in biomass and in species- and size-composition. The ponds were drained in autumn. The final body weights at draining ranged from 25.9 to 63.6 g for yellow eel and from 3.9 to 8.8 g for glass eel. The final body weights of yellow eel and of glass eel decreased with increasing biomass of yellow eel. No significant relation was found between the bream Abramis brama (L.) biomass and the growth of eel. The growth rates of yellow eel and glass eel were positively correlated in experiment II. At higher biomasses of yellow eel the percentage females decreased slightly. The recapture rates of yellow eel in experiments I and II amounted to 69.4 ± 9.8 % and 92.2 ± 4.9% (mean ± sd) respectively. The lower recapture rates in experiment I were caused by the inappropriate draining technique used. The glass eels were recaptured with 75.0·5.6% efficiency. The maximum net production of yellow eel occurred at a biomass of 20–40kg·ha⁻¹ and amounted to 19 kg·ha⁻¹.     

The crystalline ultrastructure of otoliths of the eel (A. anguilla L. 1758)

The ultrastructure of the organic and the mineral components of eel ( Anguilla anguilla L. 1758) otoliths was studied using sections through the nucleus and observed by scanning electron microscopy. The otoliths were composed of crystalline material (aragonite) running radially from the centre to the edge. Structural discontinuities were described: seasonal rings, checks and daily growth increments.     

Scanning electron microscopic analysis of annulus microstructure in otolith of European eel, Anguilla anguilla

To validate the yearly periodicity of annulus formation in the otolith of the eel, the structure of annuli in otoliths of the European eel, Anguilla anguilla , stocked for 7 and 12 years in Lake Ommen on the east coast of southern Sweden, was examined. The population was stocked from elvers imported both from France (Bay of Biscay) in April 1979 and England (River Severn) in March–April 1984. The microstructure of an annulus consisted of single, double and/or composite tings depending on the location in the otolith. The counts of annuli in otoliths of these eels were approximately consistent with the expected age. However, supernumerary false annuli and/or annulus underestimation frequently occurred. The methodology for annulus discrimination with light and scanning electron microscopes is described.