This report presents all registered catches of pelagic megafauna by the Dutch pelagic fleet in the Mauritanian Exclusive Economic Zone during the years 1999-2003. ‘By-catches’ incidentally include large species, notably cetaceans, sea turtles, sharks, rays, and some large pelagic fish such as swordfish and ocean sunfish. All observations were made in the framework of different projects of the Netherlands Institute for Fisheries Research (RIVO B.V.) in the area, financed by the Dutch Ministry of Agriculture, Nature Management, and Fisheries (LNV) and partly by the Redersvereniging voor Zeevisserij, part of the Pelagic Freezer-Trawler Association (PFA): “Assessment of Sardinella and other small pelagics in West Africa” (313-1230001), “Application of remote sensing data to analyse the distribution and recruitment of sardinella” (313-1230002), and “Preventing by-catches of protected or endangered species in the pelagic trawl fishery in West Africa” (313-1230003). The sampling procedures of incidentally caught pelagic megafauna on board of Dutch pelagic freezer-trawlers in Mauritania throughout the period 1999-2003 are evaluated in this report. The collected raw data on this by-catch of large species is whenever possible validated with an observation factor, shortcomings are discussed, and recommendations for improvements are given. This report presents raw data and explicitly avoids extrapolations up to mission or even fleet level.