Growth pattern of flounder, Platichthys flesus (L.), from the Gulf of Gdańsk (southern Baltic Sea)

Background. It is a common perception that growth rates of many commercially-exploited fish species have been well known. It is evident, however, that in certain cases, especially in brackish water environments those growth rates may be diversified. Therefore, determination of growth patterns of various stocks of the same fish, living in different salinity regimes may be important for efficient fisheries management, especially fish-quotas negotiations. The aim of the presently reported study was to estimate the growth rates of length and weight of flounder Platichthys flesus (L.) from the Gulf of Gdańsk and to compare them with those from the Pomeranian Bay and Słupsk Furrow areas. Materials and methods. A total of 200 fish, collected in the coastal waters of Hel in 1997, was analysed. The fish were caught using flatfish gill nets. The age of flounder was determined from otoliths, whereas measurements of the annual ring increments were made using a dissecting microscope with reflected light over a dark background. The microscope was outfitted with a camera and linked to computer. The images were processed using an image-processing software “Multiscan”. The method of Vovk was employed to study the growth rate of flounder, both for the whole sample and in relation to both sexes. The calculation of the growth rates of length and weight were aided by the equations of von Bertalanffy. Results. The total length of the flounder studied ranged from 4.3 to 46.0 cm for ages from one through ten. The mean length reached 23.7 cm, while the mean age was 4.6 years. The prevalent age classes were III, IV, and V. The growth rates of length were different in males and females. Until the third year of life males grew slightly faster, while from the fourth year, the females grew more rapidly. The von Bertalanffy equations that describe theoretical growth (length and weight) of flounder, from the Gulf of Gdańsk were calculated. Conclusion. The differences found in the morphometric and biological characters between the samples collected in three different locations of the southern Baltic (Pomeranian Bay, Słupsk Furrow, and Gulf of Gdańsk) may indicate that they represent separate stocks (populations).