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Abstract The coastal lagoons of the Tramandaí River basin are dynamic ecosystems characterized by high biodiversity. They provide important ecosystem services, such as water supply for human consumption, industry, agriculture, animal husbandry, leisure activities, tourism and fishing. Constant increases in the human population and the use and occupation of the land around the lagoons has brought growing demands for their resources, resulting in increased pressure that compromises these ecosystems. Understanding how biological populations respond to these anthropogenic pressures is essential. This study examined whether the degree of urbanization around 23 lagoons in the Tramandaí River basin influences the size and biomass of the characid fish species Diapoma alburnum. Specimens were collected between 2009 and 2012 using standardized drag nets. All specimens were measured for standard length to determine average length (size) per lagoon, while all fish collected per lagoon were weighed together to determine average biomass per lagoon by dividing by the number of specimens. Urbanization around the lagoons was measured using satellite images representing artificial nocturnal light as a proxy for urbanization. Nocturnal light intensity was measured within 1 km, 3 km, and 5 km buffers around each lagoon. The resulting urbanization index ranged 6 – 44% for the 5 km buffer, from 3 – 55% for the 3 km buffer and 1 – 65% for the 1 km buffer. Regression analyses showed a significant positive relationship with increasing urbanization around lagoons for D. alburnum average biomass in the 3 km and 5 km buffers and for D. alburnum average length in the 3 km buffer. Although urbanization around the lagoons is not fully established, the results indicate its impact on the size and biomass of D. alburnum.