A total of 13.927 individuals of Lophius gastrophysus landed by the shrimp fishing fleet at a fishing port of Niteroi, state of Rio de Janeiro, were measured. The fish were landed separately in two commercial categories: small (individuals less than 1 kg) and large (over 1 kg). Two sampling periods were distinguished: from March 2004 to February 2005 (first period) and from March 2005 to February 2006 (second period). The length-class structure was completely dominated by small-sized individuals, even in the “large” category, over the entire sampling period, and showed the large catch of young individuals. It is concluded that this fleet impacts heavily on the stock of L. gastrophysus, because it acts basically on the population stratum of juveniles, because of the poor selectivity of the trawl nets used and the area of the continental shelf where the fleet fishes. We recommend the implementation of measures to reduce this catch, so as not to compromise the development of a sustainable fishery for L. gastrophysus.