Estimating Juvenile Bluefin Tuna Abundance through Aerial Surveys in the Northwestern Mediterranean

Since 2000, Ifremer has conducted yearly aerial surveys during late summer-early fall on juvenile Atlantic bluefin tuna (ABFT) in the Northwestern Mediterranean in order to calculate a fishery-independent index of abundance. This scientific survey provides crucial information for stock assessment, in particular because juvenile ABFT are no longer targeted by most fisheries since 2007 due to an increase in the minimum landing size. Results clearly indicate an increase in the number of detected ABFT juveniles schools during the recent years, probably in response to recent management regulations. However, sighting numbers vary significantly within years and even though surveys are conducted under perfect weather conditions, surveys with no tuna sightings are common. Such variations are assumed to be related to environmental conditions (e.g. changes in the wind forcing, the region's biological productivity, the position and strength of the thermocline and mesoscale structures such as fronts) that are currently not considered by the index. The present study presents a modelling approach that integrates such potential effects in the calculation of the abundance index and further proposes a comparison of strip- and line-transect procedures.