In this study, we used genetic-based approaches to estimate population size and structure of Eurasian otter along the Drava River in Hungary, and compared these results to traditional survey-based methods. The relative spraint density of otter was estimated based on the number of fresh (D f ) and total number (D t ) of spraints collected on standard routes over a 2-year period. Nine microsatellite loci were screened, generating 17 individual otter genotypes composed of 45 different alleles. The expected heterozygosity ranged from 0.53 to 0.89 and observed heterozygosity from 0.25 to 0.92. The mean density (D g ) estimated over six different sites was 0.17 individuals per km of shoreline. A close correlation was found between the number of genotypes and spraint counts along a standard route (fresh spraints: r P ¼ 0.85, Po0.01; total spraints r P ¼ 0.76, Po0.05). All genotypes found within the 50 km-long study area were closely related (D m ranged between 0.08 and 0.21).

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Summary Early gonad development of the Asian sea bass (Lates calcarifer) grown in a freshwater recirculation system was investigated to determine the onset of sexual maturation. Studied individuals aged between 9 and 15 months. Culture conditions were provided initially in circular tanks (4000 L) and linked to a warm‐water recirculation system (27°C, O2 = 7–8 mg L−1) until they reached 20 g wet weight. They were ongrown in raceways (54 m3; final density 80 kg m−3) connected to a recirculating system with a fluidized bed as biofilter. Their gonads were examined each month (5–12 fish) and prepared for histology. Following microscopic observations testis maturity was categorized and correlated to total length (cm) and weight (g). Gonad development started much earlier in the recirculation system than described previously in both, wild and cultured fish. We assume that spermatogenesis in these fish starts earlier than 9 month of age and the fact that mature spermatozoa were present in testes that contained mostly spermatogonia indicates highly asynchronous spermatogenesis.

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