Bulletin of the Sea Fisheries Institute 1 (155) 2002 - CEEMaR

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BULLETIN OF THE SEA FISHERIES INSTITUTE 1 (155) 2002 Growth, sex structure and commercial significance of turbot (Scophthalmus maximus L.) in the Lithuanian EEZ Svajunas Stankus Institute of Ecology, Akademijos 2, 2600 Vilnius, Lithuania Abstract. The growth of turbot in length and weight is described using the modified von Bertalanffy equation on the basis of data obtained from back calculations. The study suggests differences in male and female growth rate in both length and weight. The rate of turbot length growth is the highest in age groups 1 and 2, while the difference between the male and female growth rate becomes more apparent in age group 3. The greatest weight increments of turbot females was observed in age group 8 and of males in age group 7. The range of turbot age groups caught in the 1998-2000 period was 1-16 for females and 1-11 for males. The amount of females in the coastal zone was on average from 1.4 to 1.9 times lower than that of males. Analysis of the turbot commercial catch age structure in different years showed a tendency towards a decline in the catch of fish from older age groups, females in particular. The CPUE index decreased from 20.3 kg per net in May 1995 to 2.1 kg per net in May 2001. Intensive turbot fishery in the coastal zone has influenced the abundance of juvenile fish. Key words: turbot, growth, sex structure, catches, the Lithuanian EEZ. INTRODUCTION Turbot is one of the most valuable flatfish species in Lithuania. However, it is one of the least investigated Baltic fish species, and its name – Rhombus, Scophthalmus, Psetta, has been changed several times due to taxonomic disputes. The geographic range of turbot is rather wide, and it occurs in the European coastal waters of the Atlantic Ocean and in the Baltic and the Mediterranean seas. Turbot is abundant in the Lithuanian Baltic Sea coastal waters from April to July (Repechka et al. 1998). Turbot biology has been most thoroughly studied in the North Sea. The relative proportions of turbot males and females from different ages groups in this area was studied by Rae and Devlin (1972). Studies on turbot growth in various areas of the North Sea were conducted by Mengi (1963), Rae and Devlin (1972), Jones (1974) and Leeuwen and Rijnsdorp (1986). The growth of Mediterranean Sea turbot was studied by Robert and Vianet (1988) and Vianet et al. (1989). Baltic Sea turbot has not been extensively investigated, but the growth of turbot in the Polish EEZ has been addressed in several studies (Ciêglewicz et al. 1969, Shlakowski 1990, Draganik et al. 1996).
54 SVAJUNAS STANKUS There is little knowledge of the biology of turbot from Lithuanian waters. The previously mentioned study by Repechka et al. (1998) as well as that by Stankus and Repechka (1998) provided some data on the sex structure and growth of turbot. The influence of commercial pressure on the turbot population in the Lithuanian EEZ was examined by Draganik et al. (1996). The present paper studies the growth and sex structure of turbot inhabiting the Lithuanian EEZ. The commercial significance of turbot and the influence of fishing on its population are also included. MATERIAL AND METHODS Turbot samples were taken from catches conducted in the coastal waters extending from Butinge to Juodkrante and in the open sea of the Lithuanian EEZ (Fig. 1). The research material was collected in 1995 and in the 1998-2000 period from May to August. Nets with a mesh size of 50-125 mm were used for coastal fishing, whereas bottom trawling was used in the open sea. Juvenile fish were caught with a 30 meter-long and 1 m high beach seine equipped with a bag. The total length of all the turbot caught was measured to the nearest mm and their weight to the nearest g. During the research period 4,125 turbot specimens were measured and sexed. Otoliths were used to determine the age structure of the turbot catches. The otoliths were taken from 720 individuals (259 females and 461 males) and stored in glycerine. The distance between the annual rings was determined using a computerized pictorial analysis technique. Otolith images were transferred to a computer with a YCH 15 video camera. o o o o o o o o o o x x x x x x x x x x Fig. 1. Sampling areas for turbot in the Lithuanian EEZ (x indicates the coastal zone and o the open sea).
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