Abstract
Age, growth, mortality, reproduction and feeding habits were analysed for Lithognathus mormyrus collected in the coastal waters of the Thracian Sea from November 1997 to September 1999. Specimens ranged from 42 to 341 mm in total length. Weight increased with size allometrically (b = 3.242) for immature individuals and isometrically (b = 2.960) for males, females and intersexuals. Growth was described by the standard form of the von Bertalanffy growth equation and the estimated parameters were k = 0.21, t 0 = –0.996 and L ? = 309.4. Total and natural instantaneous rate of mortality was found to be Z = 0.79 year -1 and M = 0.61 year -1 . Sex inversion occurred mainly between 210 and 300 mm (4-9 age classes). Males reached sexual maturity at 162.1 mm (2.5 years) and females at 190.40 mm (3.6 years). The spawning period occurred from May to September, while the gamete emission peaked in June-August. Stomach content analysis revealed that L. mormyrus is a carnivorous species feeding on benthic invertebrates, mainly polychaeta and bivalve molluscs. Ontogenetic variation in the diet composition showed that while growing, the fish become more generalist feeders. We also found that in the summer season the fish become more selective feeders.
Highlights
Lithognathus mormyrus (Linnaeus, 1758) is common in the Mediterranean Sea, the AtlanticOcean from the Bay of Biscay to the Cape of GoodHope, the Red Sea and the south-western IndianOcean
Analysis of the length-weight relationship of the immature L. mormyrus (42 to 243 mm in total length) from the coastal water of the Thracian Sea showed a positive allometric growth ind,icating that in the juvenile period the species becomes heavier with increasing length
This is in agreement with the results of Koutrakis and Tsikliras (2003), who published a length-weight relationship of L. mormyrus in the northern Aegean estuarine systems using 73 samples ranging in size between 21 and 143 mm
Summary
Lithognathus mormyrus (Linnaeus, 1758) is common in the Mediterranean Sea, the Atlantic. Ocean from the Bay of Biscay to the Cape of Good. It is a demersal species that inhabits the rocky and sand-muddy bottoms and seagrass beds down to 80 m (Mediterranean) and 150 m (Atlantic) in depth. It is a gregarious species, entering exceptionally in coastal lagoons (Bauchot and Hureau, 1986; Suau 1970; Tortonese, 1975). In the Mediterranean Sea, L. mormyrus is a highly prized species and an important catch for the coastal fishery. In the study area L. mormyrus is landed year-round with a peak in winter
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