This study presents the catch composition and biodiversity of Mersin Bay, northeastern Mediterranean, which is an important fishing ground for demersal trawls. A total of 182 hauls were performed on board a commercial trawler on the commercial fishing grounds between 15 September 2009 and 15 April 2013. The monthly changes in species composition were analyzed by conducting cluster analysis based on catch per unit effort (CPUE), nonmetric multidimensional scaling (nMDS), and correlation vectors, which were used to evaluate the discrete groups. Shannon-Wiener index (H'), its evenness (J') component, and the Pielou evenness index were calculated to clarify temporal monthly changes. One hundred and thirty-five species belonging to 10 classes, 26 orders, and 71 families were identified. CPUE values were 1558 (individual/h) and 23.96 (kg/h), respectively. While the most abundant species were Mullus barbatus in terms of IRI and CPUE (kg/h), Equulites klunzingeri had the highest CPUE (N/h). Four clear clusters of the months were observed. The highest diversity was observed in November, with H' = 3.28 and J' = 0.74 index values. The results highlighted the differences of catch composition between fishing months.