The current study attempts to examine the impact of changes in seasons on the ichthyofaunal composition of the coastal waters of the Gulf of Mersin, based in the northeastern Mediterranean region. The Gulf of Mersin with its outstanding ecologic and productive environment is a vital habitat to exist for multiple populations of the marine fish. By examining the correlation between seasonal environmental changes and composition of ichthyofauna, this study provides valuable material for research in marine ecology and the drafting of fishing strategies that are sustainable. Fish samples were collected in a sequential manner in spring, summer, autumn and winter, from different coastal sites using standard trawling and gill netting data. Salinity data, mean sea surface temperature, and dissolved oxygen were recorded with the objective of establishing associations with fish assemblages. The sample data were analyzed by the prism of biodiversity indices, multivariate statistical methods, and ecosystem ecology. Analysis revealed significant seasonal variation as well as species and abundance in the fish community. In spring and summer, the greatest species richness and abundance was recorded, correlated to the following factors: warm sea temperatures, increased primary productivity, and the reproductive activities of many fish species. Important species observed during these periods included Sardinella aureate, Somber sombrous, and Trachurus trachurus, suggesting dominance by pelagic and temperature sensitive assemblages. The implication of the results for management of fisheries, formulation of conservation strategies, and protecting marine biodiversity are significant. It is possible to improve the resource management and minimize ecological effects by including seasonal information in regulatory policies. In addition, the findings provide grounds for the establishment of a critical baseline concerning Mediterranean marine biodiversity, the need of which for monitoring future changes of climate and human action is obvious.