An experimental recirculating aquaculture system was constructed under ambient seawater conditions to compare microbial community diversity of nitrifying and denitrifying biofilters that were derived from a commercial inoculum used for aquarium applications. Next generation sequencing revealed distinct and diverse microbial communities in samples analyzed from the commercial inoculant and the denitrification and nitrification biofilters. In all samples, communities were represented by a few dominant operational taxonomic units (OTUs). Bacteria having the capacity to carry out ammonia and nitrite oxidation were more abundant in the nitrification biofilter. Similarly, the proportion of the bacterial taxa known to carry out heterotrophic and autotrophic denitrification and participate in sulfur cycling were found in the denitrification bioreactor, and likely originated from the ambient environmental water source. Our results indicated that environmental seawater can be a favorable enhancement to the bacterial consortium of recirculating aquaculture systems biofilters.

The paper presents for the first time important information regarding population structure, growth rate, length-weight relationships and reproductive aspects of the Neretvan nase, Chondrostoma knerii, a fish species endemic to the Neretva River. Chondrostoma knerii is mainly distributed in the lower parts and delta of the Neretva River wetlands and its tributaries (Krupa River, Hutovo Blato wetlands) shared between Croatia and Bosnia Herzegovina. Samples were collected between 2000 and 2001 during the reproduction period of the species in the Hutovo Blato wetlands area. Population structure analysis indicates that all ages appear, from 0 to 7+. Longevity was estimated at 7.5 years. Main age classes were 3, 4 and 5 years, totalling 91.00% of all individuals. Males ranged between 2 and 5+ years of age, and females between 1 and 7+ years of age. Maximum length was 28.04 cm and maximum weight, estimated from the length-weight relationship, as 206.8 g. Found scattered in streams of high water velocity, eggs were attached to the substrate (aquatic plants or solid objects). Correlation between egg number and length and weight was extremely low. The gonad-somatic index distribution showed an inverse proportionality with total length. Main length classes for reproduction ranged from 20 to 24 cm, while fecundity ranged between 2000 and 16 000 eggs per female.

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At the beginning of the 20th century, the middle section of the Sava River in Croatia was unaffected by major human activities and rich in ichthyofauna. The Sava River was important for commercial and recreational fishing for the local population, which still remains today. However, the 1920s mining industry was established in Slovenia, which emitted carbon dust into the Sava River. At the same time, the construction of embankments to mitigate flooding started in the middle section. Furthermore, in the 1980s, the Krško nuclear power plant (NPP), and in the 2010s, the Krško hydropower plant (HPP) were built in Slovenia. These activities could have an impact on the composition of fish communities downstream from the major sources of disturbances. Therefore, the main aim of this paper were to analyze the changes in fish assemblages of the Sava River from 1978 to 2017, prior to and after the construction of Krško NPP and HPP at the Medsave site on the Sava River, 20 km downstream from the major construction operations. Collected data were divided into four sampling periods (SP): SP1, from 1978 to 1980; SP2, from 1991 to 1994; SP3, from 2001 to 2006, and SP4 from 2011 to 2017. Besides alien fish species, water quality and hydromorphological modifications were identified as significant stressors. In SP1 and SP2 limnophilic and eurytopic fish groups were predominant, and 26 different fish species were identified, but in SP3 and SP4 rheophilic fish groups become dominant, and the diversity has declined to 21 species. Threatened species blageon, Telestes souffia seems to be missing from the main course of the Sava River in last 20 years. It can be concluded that disturbances in the fish assemblage pattern have coincided with the presence of multiple stressors of human origin.

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