The status of the turbot Psetta maxima (L.) stock supporting the Baltic fishery
Bulletin of the Sea Fisheries Institute 1 (164) 2005, pp. 23-53

Bohdan Draganik1, Yuryj Maksimov2, Sergej Ivanov3 and Iwona Psuty-Lipska1
1 Sea Fisheries Institute, Kołłątaja 1, 81-322 Gdynia, Poland
2 Fishery Research Laboratory, Klaipeda, P. O. Box 5800, Lithuania
3 Atlantic Scientific Research Institute of Marine Fisheries and Oceanography (AtlantNIRO), Dm. Donskoy Street 5, Kaliningrad, Russia

Key words: Baltic turbot, annual catch, growth rate, mortality rate, age structure, gillnet selectivity, minimum landing size, closed season.

Abstract.
Turbot catches in the Baltic Sea reached 1,200 tons in 1996, whereas before the 1990s the reported annual turbot catches seldom exceeded 200 tons. Half of the total annual turbot catch was taken from sub-divisions 25 and 26. In recent years, turbot catches have exhibited a downward trend, and the average catch rate index recorded in Lithuania fishery fell from 5 kg to 1.6 kg per standard net. The results of the analysis of 67,062 turbot length measurements and 5,187 fish age determinations from materials collected in 1995-2004 were used to describe the turbot population. Length ranged from 16 to 63 cm and fish age ranged from the age groups 2 to 14. The bulk of the catch was comprised of fish measuring from 32-35 cm. Of the turbot caught with gill nets used in flounder fishery (65-70 mm mesh size), 72% of fish were undersized (< 30 cm). This paper presents and discusses the effects of the mesh size used in gill net fishing on the length composition of the turbot caught. Gill nets with 110 mm mesh size used by fishermen specializing in turbot fishery proved to be the most selective. The growth rate of turbot males is slower in comparison with that of females and their maximum sizes recorded in the sampled materials were 37 cm and 63 cm, respectively. The values of the von Bertalanffy's growth equation parameters given in the literature were reviewed and those of L= 55 cm, K = 0.122, t0 = -2.569 (for females) estimated from the data collected in 2004 were used to evaluate the optimum minimum length size (tc) at the range of natural mortality coefficient 0.1-0.25. The regulatory measures for turbot fishing currently enforced in Lithuania, Poland, and Russia are reviewed and discussed in light of the authors' findings.