The horse mackerel Trachurus mediterraneus is a commercially important pelagic fish species in the Black Sea fishery. The present investigation was carried out between May and December 2010 along the Bulgarian coast of the Black Sea. Population parameters of horse mackerel from the trawl and fishing nets catches were estimated from length frequency data, by using ELEFAN-I computer program. The ELEFAN-I analysis gave the following Von Bertalanffy Growth Function (VBGF) parameters: the asymptotic length ？cm, growth coefficient value was , and the hypothetical age at which length is zero was . Based on these growth parameters, the total mortality coefficient ( ) during the study period was estimated to be 2.99. The estimated value for natural mortality ( ) was 1.08; hence, the fishing mortality coefficient ( ) was 1.91. The estimated value for the exploitation rate ( ) using the length converted catch curve was 0.64. The estimated sizes of T. mediterraneus at 25, 50, and 75 percent probabilities of capture were 16.80？cm, 18.72？cm, and 20.64？cm, respectively. 1. Introduction Family Carangidae in the Black Sea is represented by Trachurus trachurus and Trachurus mediterraneus ponticus. In the Bulgarian Black Sea territorial waters, only Trachurus mediterraneus ponticus is present. Entering of separate Trachurus trachurus specimens to the Black Sea from the Sea of Marmara is a quite rare phenomenon [1]. The systematic situation of the Black Sea mackerel was carefully examined by [2–4]. The same author stated that in the Black Sea, the species was represented by four local subpopulations: the south western (Bosporic), the northern (Crimean), the eastern (Caucasian) and the southern (Anatolian) each one with its own biological characteristics such as wintering grounds, fat content, spawning patterns, age composition, growth rate, feeding patterns. On the basis of investigation carried out by [5, 6] on size composition and also tagging experiments of horse mackerel caught off the Bulgarian coast, they concluded that in the Black Sea, two subpopulations occur that belong to the small size-type of Trachurus mediterraneus ponticus, the eastern and western ones, respectively. According to the same authors, the Black Sea horse mackerel represents a single population, as the environmental conditions are almost one and the same in the whole inhabited area and there exists no positive evidence for the occurrence of two distinct subpopulations differing substantially in their biological parameters. Other authors using electrophoresis methods assume that no difference at
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