This paper describes a systematic approach that combines research survey and commercial landings data to estimate the total catch of a species, including discards, in a commercial trawl fishery. The method is applied to the 1989 Norwegian bottom trawl fishery for cod (Gadus morhua L.) in the Barents Sea and its utility is evaluated. Bottom trawl survey data together with results from cod-end selectivity studies of the regulated mesh size are used to determine the 'expected' commercial catch composition in percent-at-length, thereby simulating the catch composition of a commercial vessel fishing at randomly located stations. The commercially landed fish numbers-at-length are adjusted upwards to reflect these ‘expected' catch levels. The minimum legal market length is then used for a knife-edge estimate of numbers likely to have been discarded. Results indicate a 7.4% increase in the 'estimated' total catch in 1989 over the numbers landed. Of this increase, 732 000 fish or 6.9% of the total catch would have been discarded or not reported as catch. Results are plausible as examined through comparison of 'estimated' catch mean lengths with mean lengths from: 1) the 1989 Norwegian commercial catch of cod sampled through a surveillance program; 2) 1989 standard Norwegian trawl surveys; and 3) a 1989 cooperative trawl survey in the Barents Sea. Comparisons illustrate basic differences in length selection between survey and commercial trawl gear, and effective differences in mean lengths of catches from randomized surveys and commercially directed fisheries.