The effectiveness of glass eel stocking in the Vistula Lagoon, Poland

Background. Eel recruitment and catches have decreased drastically throughout Europe since the beginning of the 1960s (recruitment) and the 1980s (catches). Until the end of the 1990s, European eel, Anguilla anguilla, the highest priced fish, was the basis of the fishery in the Polish part of the Vistula Lagoon. The lagoon was stocked with glass eel from 1970 to 1994 with the aim of enhancing the stock of this species. Due to political and economical changes after 1989, national stocking funds were stopped. The aim of the presently reported study was to estimate the effectiveness of stocking glass eel in the Vistula Lagoon, with regard to the limited reliability of official landings data. Materials and methods. Eel catches in the Polish part of the Vistula Lagoon were reconstructed from landings statistics. Corrections of Polish landings were only based on grey literature request; political and socio-economical changes were explained in personal reviews with fishermen and fishery inspectors. The effectiveness of the stocking was estimated with the simplified method of calculating the accumulated biomass of fish from stocking with the assumed, constant coefficients F and M (fishery and natural mortality, respectively). The sensitivity analysis of F and M was estimated using the input of different parameters. Results. Changes in human history resulted in different biases in official landings statistics. The change in fishermen cooperative organisation and the martial law in Poland, during the period of 1981–1984, resulted in significant changes of the eel official landings. The reconstructed catches in this period (1980–1985) do not exceed 30% of the total exploitable biomass of the reserve for silver eel escapement, even under intensive exploitation. With assumed M and F, the exploitable stock biomass increased sharply to the peak value during the 10 years of stocking. Uncertainty concerning M for the first year of stocked eel results in a difference in the estimated biomass as high as 700 t. Conclusion. Under great uncertainties and lack of “hard data”, social science methodologies could help to estimate basic parameters for assessment models. Glass eel stocking at the Vistula Lagoon was effective both for the enhancement of the eel population and maintenance of an intensive eel fishery

Changes of the eel fishery patterns in the Vistula Lagoon, Poland

Background. The decline in the eel population is reflected in the decreased incomes of fishers. This has led to changes in the structure and size of the fishing effort. The fyke nets used in eel catches are multi-species gear. Changes in the fishing intensity and location of this gear impacts local fish resources, as does changing preferred gear for other types. Materials and methods. The data on the eel landings and the numbers of licences from all the fisheries bases were obtained from the Regional Sea Fisheries Inspectorate. Direct observations of the spatial and temporal deployment of the fishing effort was observed monthly in the 2000–2007 period. The trends of changes in relative eel abundance were identified from fyke net complexes deployed in the eastern fishing grounds. Results. Landings and directed fishing effort differed both seasonally and spatially. The most effective fishing grounds were in the eastern (near the border with Russia) and western parts of the lagoon. The discontinuation of stocking resulted in decreased catches and then in fishing effort. Decreases in eel catches and fishing efficiency were evident except in 2003 at the Piaski and KÄ…ty Rybackie fisheries bases (the northern part of the lagoon). Conclusion. The availability of eel stocks had the greatest impact on fishing efficiency and gear distribution patterns. The reduction of fyke net fishery because of low incomes shifted the focus of the fishery to gill nets targeting pikeperch

The effectiveness of glass eel stocking in the Vistula Lagoon, Poland

Background. Eel recruitment and catches have decreased drastically throughout Europe since the beginning of the 1960s (recruitment) and the 1980s (catches). Until the end of the 1990s, European eel, Anguilla anguilla, the highest priced fish, was the basis of the fishery in the Polish part of the Vistula Lagoon. The lagoon was stocked with glass eel from 1970 to 1994 with the aim of enhancing the stock of this species. Due to political and economical changes after 1989, national stocking funds were stopped. The aim of the presently reported study was to estimate the effectiveness of stocking glass eel in the Vistula Lagoon, with regard to the limited reliability of official landings data. Materials and methods. Eel catches in the Polish part of the Vistula Lagoon were reconstructed from landings statistics. Corrections of Polish landings were only based on grey literature request; political and socio-economical changes were explained in personal reviews with fishermen and fishery inspectors. The effectiveness of the stocking was estimated with the simplified method of calculating the accumulated biomass of fish from stocking with the assumed, constant coefficients F and M (fishery and natural mortality, respectively). The sensitivity analysis of F and M was estimated using the input of different parameters. Results. Changes in human history resulted in different biases in official landings statistics. The change in fishermen cooperative organisation and the martial law in Poland, during the period of 1981–1984, resulted in significant changes of the eel official landings. The reconstructed catches in this period (1980–1985) do not exceed 30% of the total exploitable biomass of the reserve for silver eel escapement, even under intensive exploitation. With assumed M and F, the exploitable stock biomass increased sharply to the peak value during the 10 years of stocking. Uncertainty concerning M for the first year of stocked eel results in a difference in the estimated biomass as high as 700 t. Conclusion. Under great uncertainties and lack of “hard data”, social science methodologies could help to estimate basic parameters for assessment models. Glass eel stocking at the Vistula Lagoon was effective both for the enhancement of the eel population and maintenance of an intensive eel fishery

Gillnet selectivity to roach, Rutilus rutilus, from the Szczecin Lagoon, Poland

Background. Roach, Rutilus rutilus, comprise a large proportion of the total catch in the Szczecin Lagoon, and are caught mainly as bycatch in gillnets targeting perch, Perca fluviatilis. The minimum landing length for roach and perch is 17 cm TL and the minimum mesh size is 30 mm bar length. However, little is known about the selectivity of roach, which may have implications for implementing effective regulations. Consequently, the aims of the study were to estimate roach selectivity curves and to compare the results with perch selectivity. Materials and methods. The study was based on the length frequency of catch from standardized fishing practice with commercial gillnets of mesh sizes ranging from 25 to 35 mm (bar length) and modelled using various generalized linear models. The best model was selected using deviance criteria. Results. The log-normal selectivity model provided the best fit, with spread relevant to mesh size under the assumption of equal fishing power. The estimated parameters were 18.7 cm and 1.5 (mode and spread of selectivity curve for the smallest mesh size). Conclusion. Although the most efficient mesh size for roach was 26 mm, its use in the Szczecin Lagoon is limited by impact on perch. All the studied mesh sizes prevented catches of undersized roach, while mesh sizes from 28 mm prevented catches of undersized perch (MLL = 17 cm TL). The authors concluded that the current 30 mm minimum mesh size is likely effective at preventing the harvest of undersized perch and roach

Preliminary investigations on implementation of technology of broadband signals for marine biology and sediments recognition

This paper describes potential applications of broadband acoustic signals for marine biology and fisheries and acoustic precise profiling of the upper layer of sediments. Comparisons of the results of conducted experiments employing the same experimental setup but with different signals are presented. In each of experiments short pulses of CW signals and long pulses of up/down chirp signals were used. The basic theory of FM signals as well as some of the factors that must be considered when implementing this type of signal in actual systems are described

Microbiological studies of the round goby (<i>Neogobius melanostomus</i>) from the Puck Bay, southern Baltic Sea

In summer 2017 numerous dead round gobies (Neogobius melanostomus) and individuals covered with white coating were observed in the Puck Bay. The aim of our research was to determine the microbiological composition of the round goby from the Puck Bay, focusing on the presence of pathogens. Bacteria were identified by biochemical methods and, by sequencing of 16S rRNA. The dominant bacterial species were Shewanella baltica, Pseudomonas spp. and Aeromonas spp. - opportunistic pathogens, commonly present in many fish species, which may become harmful for the organism in unfavorable conditions. It was the first trial to determine the composition of the bacterial flora of N. melanostomus from that area

Recent invasion by a non-native cyprinid (common bream Abramis brama) is followed by major changes in the ecological quality of a shallow lake in southern Europe

We present an example of how an invasion by a non-native cyprinid (common bream, Abramis brama (Pisces: Cyprinidae), hereafter bream) in a natural shallow lake in southern Europe (Lake Montorfano, northern Italy) may have adversely affected the state of the lake’s ecosystem. In less than two decades, bream became the most abundant species and characterized by a stunted population with asymptotic length 33.5 cm, an estimated mean length at first maturity of 19.6 cm, a total mortality rate of 0.64 year−1 and a diet overwhelmingly dominated by microcrustaceans. Following bream establishment, nutrients and phytoplankton biomass rose, the proportion of Cyanobacteria by numbers increased markedly and water transparency decreased. Total zooplankton abundance increased with a marked increase in small cladocerans and copepods, whereas the abundance of large herbivorous cladocerans did not change. The coverage of submerged macrophytes declined, as did the abundance of native pelagic zooplanktivorous fish. The composition of the fish community shifted towards a higher proportion of zoobenthivorous species, such as bream and pumpkinseed (Lepomis gibbosus). Our results indicate that bream affected water quality through bottom-up mechanisms, while top-down effects were comparatively weak. Selective removal of bream and perhaps stocking of native piscivores might improve the ecological status of the lake