New data on incidental captures of Leatherback (Dermochelys coriacea Vandelli, 1761) obtained by the IEO on board observer program in longline Spanish fisheries targeting tuna and tuna-like species in the western Mediterranean and Gulf of Cadiz (NE Atlantic) J.A. Camiñas, S. García, J.M. Ortiz de Urbina, E. Zas de Uña, E. Camacho, M. Teles, J.C. Báez & D. Macías Instituto Español de Oceanografía (IEO). Centro Oceanográfico de Málaga, Puerto Pesquero de Fuengirola, Málaga. España www.ieo.es Conservation of highly migratory marine species needs coordinated efforts among experts and authorities supported on the best available scientific information. Leatherback (Dermochelys coriacea V.) has the wider geographical distribution including the regions of the Gulf of Cadiz and Western Mediterranean. Specimens in these areas are usually belonging to two different Regional Management Units (RMU, Wallace et al., 2010): Northwest Atlantic or Southeast Atlantic RMU. Leatherbacks are abundant in the Gulf of Cadiz appearing currently stranded as effect of fishery bycatch (Camiñas & Gonzalez de Vega, 1997) and other activities. The geographical distribution of Mediterranean Leatherback records include the whole basin (Camiñas, 1998; Casale et al., 2003). The Gulf of Cadiz and Western Mediterranean gather important fisheries potentially dangerous for the species that is considered globally vulnerable (A2bd, according to MTSG-IUCN criteria). Between 1999 and 2012, 8 leatherback were observed as by-catch of Spanish longline fishery in the Western Mediterranean, which contrasts with the 3940 loggerhead observed in the same period (Ratio= 0.00203). The surface longline targeting bluefin tuna (LLJAP) concentrated half of the observed leatherbacks catches, while the higher incidence of loggerhead by-catch took place in the longline targeting albacore (LLALB). These differences are explained mainly by the abundance of each species in the study area, but also by the specific depths and habitat preferences and the different depths explored by each fishing gear. In the Gulf of Cadiz (2 turtles observed) the sea turtle bycaught took place in the traditional longline targeting swordfish (LLHB). However, the ratio between leatherback versus number of loggerhead (21 turtles) was 0.095, greater than the ratio observed from the Mediterranean. These differences could be explained by the neritic behavior of the loggerhead in Gulf of Cadiz and more pelagic distribution in western Mediterranean specimens. The neritic behaviour could avoid overlapping with the longline fishing depth. Distribution of fishing effort (blue dots) observed from Spanish longline fleet between 1999 to 2012 YEAR GEAR LAT. LON. N. 1999 LLHB 37,467 0,767 1 2002 LLJAP 38,533 2,183 37,983 1,617 2 2003 38,400 1,000 2010 LLALB 38,555 2,838 3 2012 35,869 -8,205 35,672 -8,759 Distribution (blue dots) of Leatherback (D. coriacea) bycatch observed, over the Loggerhead (C. caretta) bycatch density, from 1999 to 2012 Data corresponding the western Mediterranean (orange) and Atlantic (green) Leatherback bycaught Camiñas, J. A. (1998). Is the leatherback (Dermochelys coriacea Vandelli, 1761) a permanent species in the Mediterranean Sea? Rapp. Comm. int. Mer Médit. 35, 388-389. Camiñas & Gonzalez de Vega, 1997. Presencia y mortalidad de la tortuga laúd (Dermochelys coriacea V.) en el Golgo de Cádiz (SW de España). Actas del 2º Simposio sobre el margen continetal Ibero-Atlántico. Cádiz, 17-20 Octubre 1997. Casale P., Nicolosi, P., Freggi D., Turchetto M and R. Argano, 2003. Leatherback turtles (Dermochelys coriacea) in Italy and in the Mediterranean basin, Herpe. Journ Vol., 13 pp. 135-139 Wallace BP, DiMatteo AD, Hurley BJ, Finkbeiner EM, Bolten AB, et al. (2010) Regional Management Units for Marine Turtles: A Novel Framework for Prioritizing Conservation and Research across Multiple Scales. PLoS ONE 5(12): e15465. doi:10.1371/journal.pone.0015465 The program of board observers on commercial longline fleet has been supported by different IEO projects. Particularly funded by GPM12-13 (IEO), GPM-4 program (IEO) and PNDB (EU-IEO).