Many population parameters of the whale shark such as growth rate, survival probability and migratory routes are still unknown, but large advances in research have been achieved primarily in areas where predictable aggregations occur. Here, results from the first study on whale sharks in Costa Rica are presented. Research was prompted by anecdotal information on aggregations of 13–55 individuals in Golfo Dulce and was submitted for assessment as part of the author's Master thesis in 2007. From 20 December 2006 to 3 March 2007, a total of 47 interviews were conducted in the vicinity of this tropical fjord, and a previously established sampling transect was followed to search for whale sharks and measure surface salinity and temperature. Equipment for sampling water properties in the event of a sighting was held on the research vessel, but no whale sharks were encountered. The maximum number of individuals sighted from 2001–2007 was Nmax=226. To reduce the probability of multiple sightings, a more conservative minimum value (Nmin=190) was calculated from the lowest number of sharks sighted in one observation and the total number of sharks in a month, respectively, and by comparing sizes. Several accounts include juveniles of very small size (2–3.5 m tl). Categorization of sizes and behaviour as well as an approximated position of a sighting enabled the integration of the data in a GIS environment. This system is available in the participating institutions and can be updated with further anecdotal information until more precise research is in place. An illustrated literature review was included to compare global whale shark research efforts with those of Costa Rica. This preliminary and unpublished study served to investigate an unknown whale shark habitat and suggests new null hypotheses for further research. Based on the new information made available through this study, management decisions in the area of the Osa peninsula should include the annual presence of .


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