Surveys are often insufficient to accurately capture the distribution of a species in both space and time. Complementary to the use of research vessel data, platforms of opportunity can be a powerful strategy to monitor species distributions at high temporal and spatial resolution. In this study we use data from commercial fishing vessels, collecting - under the European Union data collection framework - biological data on all species that are caught and subsequently discarded. Using such discard data in combination with a systematic trawl survey, we model the spatial and temporal distribution of juvenile plaice (Pleuronectes platessa) in the central North Sea. There is a clear age-dependent difference between the commercial fishing vessel data and the research vessel data, with age 1 being the dominating age in the survey catches, while age 2 is the dominating age in the discards. The results show how immature plaice, slowly migrate from the nursery areas, westwards into the deeper regions of the North Sea. Also, the results show that during the study period, juvenile plaice gradually moved to deeper waters at an earlier age. Finally we discuss how the framework can be applied to similar opportunistic data to monitor seasonal and inter-annual migration of marine organisms, and to quantify how they may be influenced by biotic and abiotic gradients, such as temperature. (C) 2012 Elsevier B.V. All rights reserved.