No abstract

Abstract. This paper presents the first records of the occurrence of brown bullhead, Ameiurus nebulosus (Lesueur), in the £yna River drainage basin. In February 2010, 77 brown bullhead were collected in Lake Czarne, and one in July 2010 was caught by angling method in Lake D³ugie in Olsztyn (northeast Poland). The specimens collected in Lake Czarne range from 72 to 154 mm total length (TL) and weighed from 3.9 to 36.8 g. Among measurable characters, the greatest variability was noted in head width, preanal distance, and snout length. The least variability in measurable characters was noted with regard to dorsal fin length, eye diameter, and preventral distance. Because of the adverse impact brown bullhead would have on the indigenous ichthyofauna of the £yna drainage basin, its spreading occurrence in this region is an ecological and economic threat.

Abstract. The post-stocking dispersal of juvenile Atlantic sturgeon (Acipenser oxyrinchus Mitchill) in the Wis³oka River (southern Poland) was investigated using biotelemetry. Thirty-five hatchery-reared juvenile A. oxyrinchus were tagged with radio or acoustic transmitters and tracked using mobile surveys and fixed receivers.

Abstract. The European eel, Anguilla anguilla L., is an endangered species. Barriers to its downstream spawning migration are one of the greatest threats this species faces. There are hundreds of hydroelectric plants (HEP) on rivers in Poland (> 600), and thousands throughout Europe. Eel that pass through HEP turbines as they migrate downstream suffer high mortality, but this depends mainly on local and technical conditions. Silver eel mortality was estimated and the possibility of the fish bypassing the turbines was studied between November 2013 and June 2014 at a typical HEP in northern Poland. Two telemetry methods were used with 49 eel: passive integrated transponder (PIT) system and acoustic telemetry. Fifty five percent of eel migrated downstream in fall 2013, soon after their release, and 45% migrated the next spring. The eel did not use the fish passes designed for upstream migration; thus, they were forced to go through the turbines, which resulted in 55% mortality. HEPs cause interruptions and delays in eel spawning migrations and are responsible for high eel mortality. This can make implementing an eel restitution plan difficult or even impossible in river systems with many barriers.

In many regulated rivers, silver eels (Anguilla anguilla L.) experience significant mortality as they pass through small hydropower stations (HPS) during their downstream migration. There has been a huge collapse in the recruitment of this species, primarily because of the destruction of river–ocean connectivity with overfishing. The direct mortality caused by small hydro turbines varies between different places and depends on many factors. The aim of the present study was to assess the mortality rate of silvers eels passing through a small HPS equipped with a Francis turbine in a small Pomeranian river using acoustic telemetry. During the experiment, no direct mortality occurred as a result of passage through the HPS turbine. However, three individuals were believed to have suffered from injuries or passage trauma.