Systematic conservation planning is increasingly being used in the marine environment, but the relative paucity of data on marine patterns and process still presents a problem, particularly in developing countries. For example, along the south-eastern component of the Agulhas marine bioregion of South Africa, current data regarding the habitat availability, distribution and abundance of 10 seabream species (Sparidae) were insufficient to design a regional-scale conservation plan. We report on how this data void was filled using information provided by marine resource users, managers and scientists during participatory workshops. Participants described the distribution of reefs, and then scored not only the relative abundance of each of the sparid species, but also benthic biodiversity, by arranging beans into piles on paper matrices. Reef positions were verified both by discussions within individual workshops and by subsequent comparisons of outputs among workshops. Similarly, extensive discussions provided an element of verification of relative abundance and biodiversity data. Although the free-scoring method employed in quantifying relative abundance and biodiversity meant that comparisons among workshops were problematic, this was resolved by assigning data to categories according to a simple five-level abundance index. After compiling results from all workshops, relative abundance data reflected conservation status of each species. Opinions by the participants regarding the reasons for recent declines in fish abundance tallied well with explanations from the literature. Biodiversity data were less comprehensive and more difficult to verify. Nevertheless, because participatory workshops provided relatively accurate data regarding fish abundance, were cost effective to run and generated a good level of buy-in from participants, they represent a valuable new tool for marine conservation planning.