An estimation of the Baltic cod stock production

In this paper, the author describes several “simple production models” as formulated by Schaefer (1954), Fox (1970, 1974), and Pella and Tomlinson (1969). On the basis of fishery statistics on Baltic cod the volume of the annual maximum sustained yield was estimated. This volume varies from 145 to 160 thousand tons depending on the model applied

On optimum size limit for the Gulf of Gdańsk flounder stock

The protective size limit of flounder Platichthys flesus L. at the present exploitation level (F = 0.40) was estimated by means of two methods: the Beverton and Holt on the one hand and the Goh on the other. Regardless of the procedure used, it was found advisable to increase the present protective size limit (21.0 cm). In the author′s opinion, the Goh method assuming a limited fishing season yields more reliable results. The author proposes to increase the flounder protective size limit from 21.0 to 24.0 cm

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

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