This study aims to determine the age and growth of Anguilla anguilla from Lake Bafa, Türkiye. A total of 128 samples of European eel were purchased from a small-scale fisherman who caught them in Lake Bafa in November 2016 and 2017, using 34-mm mesh size and 15-m-long fyke nets. The total length (TL) and total weight were measured, and the sex, age, and life stage were identified. The mean TL for females was 54.1 ± 10.1 (36.1–76.2) cm, and for males 43.6 ± 3.7 (35.7–47.8) cm. Yellow and silver eel mean TLs were 48.9 ± 6.6 (35.7–63.7) and 61.9 ± 10.8 (39.9–77.2) cm, respectively, and for pooled data 53.3 ± 10.3 (35.7–77.2) cm. The growth for pooled samples was positive allometric. The age of eels varied from 2 to 9 years. The Von Bertalanffy growth parameters for pooled specimens were determined as L∞ = 71.9 cm, K = 0.25, and t0 = −0.11. The sex ratio (M:F = 1:11.2) was biased in favour of females. It is inferred from this and other studies that eels in Türkiye have comparable length and age distributions in similar habitats.

A 30 years long data series on the infection dynamics of European eel (Anguilla anguilla L.) with the non-native invasive nematode Anguillicola crassus Kuwahara, Niimi & Hagaki, 1974 is presented. Parasite burden was evaluated for 30 years in inland and coastal waters in Mecklenburg-Western Pomerania from 1991 to 2020. The total prevalence, mean intensity and damage status of the swim bladders were very high during the first decade (1991–2000), and significantly decreased in both marine and freshwater eel populations in the following decades (2001–2010, 2011–2020). The parasite intensity of eels in coastal waters was significantly lower compared with the freshwater systems (61.3% vs 79.5% in the first decade), indicating the vulnerability of the parasites to brackish water conditions and the fact that the life cycle of A. crassus cannot be completed under high saline conditions. Eel caught in the western part of the Baltic Sea (west of Darss sill) had the lowest mean infection (51.8% in first decade) compared to the eastern part with 63.8%. Thus, besides different infection patterns caused by the environmental conditions, a temporal trend towards a reduced parasite intensity and a more balanced parasite–host relationship developed in the 30 years of interaction after the first invasion. Possible reasons and mechanisms for the observed trends in parasite–host interactions are discussed.

Although differences in visual pigments between developmental stages of the European eel are well known, the expected differences in spectral sensitivity have not been demonstrated at the electrophysiological level. In fact, one past electroretinographic study led to the conclusion that in eels there is no change in scotopic sensitivity, with increasing sexual maturity. In the present experiments, electroretinograms (ERGs) were recorded from in situ eyecups of immobilized eels Anguilla anguilla (L.) caught in coastal running waters. It was shown that the ERG b-wave is as good an indicator of spectral sensitivity as the unmasked late receptor potential (LRP) which directly reflects the responsiveness of photoreceptors. Complete spectral-sensitivity curves, based on b-wave thresholds and on thresholds of LRP subsequently isolated by means of sodium iodate, have been obtained in the same eel. Using fitted amplitude-log intensity functions for threshold calculation, and two models for computer-assisted fitting of spectral-sensitivity curves, significant differences in λmax were found between yellow and silver developmental stages of the eel, identified by ocular index measurements.