Spatial distribution patterns and population structure of the bluemouth Helicolenus dactylopterus (Delaroche, 1809) in the southern Bay of Biscay. Frande, E.(1); Robledo, R.(1); Aja, A.(2); Quelle, P.(3); Preciado, I.(3); Blanco, M.(3); Punzón, A.(3) 1- Museo Marítimo del Cantábrico, Santander, Spain. 2- Consejería de medio rural, pesca y alimentación, Santander, Spain. 3- Instituto Español de Oceanografía de Santander., Spain. ISOBAY XV International Symposium on Oceanography of the Bay of Biscay 22 – 24 June 2016. Bilbao, Spain. Introduction Results Frequency of occurrence (FO) and mean biomass by haul of H. dactylopterus was calculated each year. An increasing trend in FO along the whole historical series was observed, except for the period 1986-1992 (Fig1), while in biomass the increasing trend began in 2000 (Fig 2). Variations in Center of Gravity (CoG) with depth showed an increasing pattern towards deeper areas (Fig 5). We also found geographical changes in CoG, both in adults and juveniles, with higher biomass in the westernmost part of the study area (Fig 3 y 4). Juveniles seemed to be concentrated near Finisterre Cape.  The scorpionfish Helicolenus dactylopterus (Delaroche, 1809) is a deep water fish species belonging to the family Sebastidae. It is an abundant fish species in the Atlantic Ocean with a bathymetric distribution ranging between 100 and 500 m depth. It is also a commercial species targeted by demersal fisheries in the NE Atlantic. Many studies have been conducted on its morphology, growth and aging (Abecasis et al, 2006; Consoli et al, 2010). Despite the increasing economic value in the southern Bay of Biscay (Spanish continental shelf) in the last 15 years (Rodríguez, 2013) its spatial distribution is virtually unknown. Henk et al, 1996 observed changes in the spatial distribution of the species . Some spatial variations have also been observed in our study area which makes the study of the spatio-temporal distribution of this species an interesting approach to search for some past and futures spatial patterns, as well as analyse possible causes of those changes observed. Fig 1. Evolution of the frequency of occurrence of H. dactylopterus during the whole historical series of trawl surveys (1983-2015). Fig 2. Evolution of the biomass of H. dactylopterus during the whole historical series of trawl surveys (1983-2015). Geographical distribution maps in biomass (kg) of H. dactylopterus (2006-2015) Fig 3. Center of gravity in biomass of H. dactylopterus during the historial series (1983-2015). Fig 4. Evolution of the center of gravity in biomass of juveniles (<9cm) of H. dactylopterus during the historial series (1983-2015). Materials and methods Data come from IBTS bottom trawl survey carried out by the IEO (Instituto Español de Oceanografía) every autumn since 1983. A baca 44/60 bottom otter trawl with a mesh size of 10 mm in the cod end was used. Horizontal opening was of 18.9 m and vertical opening of 2.5 m. The sampling unit was made up of 30-min hauls during daytime at a speed of 3 knots. Bottom trawl gear was monitored using a Scanmar net control system.A stratified sampling scheme was used with 5 geographical sectors and 3 depth strata (70-120 m, 121-200 m and 201-500 m). From each haul, number of specimens and total biomass were collected. Length structure was also recorded. The evolution of biomass and frequency of occurrence along the historical series was analysed. We also tested changes in depth (CoG) along the historical series using a linear model. All data were tested for normality. Trends were considered significant for p-value <0.05. Fig 5. Evolution of the center of gravity with depht of H. dactylopterus during the historial series (1983-2015). Conclusions H. dactylopterus is becoming an increasingly common species according to the results of the evolution of biomass and FO in the last decades. The significant decrease found from 1986 to 1992 could be attributed to a low recruitment period. The peak observed in biomass in 2001, was attributed to high biomass of juveniles which would mean a good recruitment period. The nursery area was observed in a small area near Finisterre Cape, where juveniles concentrated every year. On the other hand, changes observed in CoG towards deeper waters could be related to an increasing fishing pressure although a more detailed study should be carried out to establish direct relationships between both factors. References 1 Abecasis D., Costa A. R., Pereira J. G. & Pinho M. R. (2006) “Age and growth of bluemouth, Helicolenus dactylopterus (Delaroche, 1809) from the Azores”. Fisheries Research, 79, 148-154. 2 Henk j. L. H., John R. G. H., Trevor W.B. (1996) “An invasion of the North Sea By blue-mouth, Helicolenus dactylopterus (Pisces, Scorpaenidae)”. ICES Journal of Marine Science, 53(5): 874-877. 3 Rodríguez R. (2013) “Population structure of the bluemouth, Helicolenus dactylopterus (Teleostei: Sebastidae), in the Northeast Atlantic and Mediterranean using geometric morphometric techniques”. 4 Consoli P., Battaglia P., Castriota L., Esposito V., Romeo T. and Andaloro F. (2010) “Age, growth and feeding habits of the bluemouth rockfish, Helicolenus dactylopterus (Delaroche 1809) in the central Mediterranean (southern Tyrrhenian Sea)”. J. appl. Ichthyol. 26, 583-591. Acknowledgments This study was supported by IEO through the ERDEM Project (Evaluación de Recursos Demersales and by MMC.