We investigated the relationship, at local level, between fitness (measured as fish condition) of Pomastoschistus marmoratus and environmental variables of shallow areas in the Mar Menor lagoon (Spain, Mediterranean Sea): water temperature (degrees C), water salinity (parts per thousand), depth (cm), submerged vegetation cover (%), submerged vegetation volume, substrate size, substrate heterogeneity, fish species richness, potential competitor fish species abundance, potential competitor fish species biomass, P. marmoratus abundance and P. marmoratus biomass. The mass-length relationships were used to test differences in fish condition between nine sampling sites. The ecological variable that accounted for most of the variation in condition was the abundance of potential competitor fish species, wich was related to interspecific fish interactions. The condition of P. marmoratus populations may be a good indicator of fish density interactions in coastal lagoons and could be considered when such populations are subjected to recovery plans or any other management programmes.