Demersal chondrichthyans in the western Mediterranean: assemblages and biological parameters of their main species

Abstract

The composition and structure of demersal chondrichthyan assemblages and the biological parameters of their main species were compared in four geographical subareas (GSAs) established by the General Fisheries Commission for the Mediterranean in the western Mediterranean : northern Alboran Sea (GSA01) and Alboran Island (GSA02), Balearic Islands (GSA05) and northern Spain (GSA06), with the first two being considered jointly. Data were obtained from 199 hauls undertaken from May to June 2013 during the Spanish International Bottom Trawl Survey in the Mediterranean (MEDITS). Twenty-five different chondrichthyan species were caught in all GSAs: seven were common to all areas, five appeared only in GSA01, nine appeared only in GSA05 and one appeared only in GSA06. Analysis of community structure (clusters, multidimensional scaling, analysis of similitude) revealed two assemblages related to depth, with very similar bathymetric ranges in all GSAs, namely continental shelf (from 41 to 252 m) and slope (from 284 to 813 m). The highest diversity, biomass and abundance values on the shelf and slope assemblages were recorded in GSA05 and GSA01 respectively. These results highlight the effects of the distinct fishing and oceanographic parameters related to the GSAs. Generally, the biological parameters of the most important species did not show differences between GSAs, which could suggest the existence of a single population in the western Mediterranean.

Molluscs collected with otter trawl in the northern Alboran Sea: main assemblages, spatial distribution and environmental linkage

“…The distribution of the main molluscan assemblages identified on the shelf and upper slope in the northern Alboran Sea is primarily linked to depth, and this study may be interpreted as a preliminary contribution in the detection of further assemblages on the shelf and slope related to sediment types and other environmental and anthropogenic variables. Nevertheless, the general spatial distribution pattern observed in the different analyses for the different groups of species resulted in two main assemblages being identified at depths of 30-200 m (shelf) and 201-800 m (slope), with a further subdivision (201-350 m depth, upper slope assemblages; and 351-800 m depth, middle slope assemblages) of the deeper assemblage identified in some analyses, which is similar to those previously reported for different faunal groups in the western Mediterranean Sea (fish: García-Ruiz et al, 2015;Ramírez-Amaro et al, 2015;decapods: Abelló et al, 1988;cephalopods: Quetglas et al, 2000;González & Sánchez, 2002;megabenthic fauna: Abad et al, 2007). Moreover, the existence of an upper slope assemblage in some of the analyses can be interpreted as a transition zone between shelf and slope assemblages, as detected at a similar depth range in other studies on cephalopods and fish, with the coexistence of species that display a wide bathymetrical distribution from adjacent bathymetrical areas (Quetglas et al, 2000;González & Sánchez, 2002;García-Ruiz, 2012).…”

New morphological and molecular evidence confirm the presence of the Norwegian skate Dipturus nidarosiensis (Storm, 1881) in the Mediterranean Sea and extend its distribution to the western basin

“…Moreover, the deep-water trawl is not well developed in this area, and its western part and Alboran Island showed a low level of fishing exploitation mainly between depths of 500 and 800 m depth (Moranta et al, 2007). Current records allow an improvement of chondrichthyan biodiversity in the Alboran Sea and support the importance of this area for the biodiversity conservation of deep-sea species of these vulnerable fishes, as has been suggested in previous studies (Ramírez-Amaro et al, 2015). Our results represent an important contribution to the knowledge of the distribution of D. nidarosiensis in the Mediterranean Sea.…”

“…The Alboran Sea is characterized by complex bottom topography, where the presence of numerous submarine canyons provides a favorable environmental scenario to the enrichment of deep-water ecosystems (Millot, 1999;Baro et al, 2012). These conditions would be favorable to the presence of D. nidarosiensis, as well as to other deep-water Atlantic chondrichthyan species, such as Galeus atlanticus, also present in this area but unknown in other parts of the Mediterranean (Rey et al, 2010;Ramírez-Amaro et al, 2015).…”

New record of a sharpnose sevengill shark Heptranchias perlo (Elasmobranchii, Hexanchidae) from the Balearic Sea, western Mediterranean Sea

“…Lozano Rey () reported a specimen captured off Motril (Alboran Sea) in 1915. To the best of our knowledge, the nearest occurrences of H. perlo in the Spanish Mediterranean waters were reported from the Alboran Sea by Barrull and Mate () based on one specimen captured in 2000 and by Ramírez () based on two individuals captured during the MEDITS surveys conducted in 1995 and 2002 in that area (GSA01). Therefore, the finding reported in the present paper represents the second record of the species off the Spanish Levantine coasts and the first to include detailed observations of the specimen.…”

“…From the limited data available on H. perlo in the western Mediterranean Sea and particularly in the Balearic Sea, it cannot be stated whether its apparent low abundance is due to a regressive trend of the species or to a naturally low density in this area. However, waters around the Balearic Islands and off the Spanish Levantine coast have been subject to numerous studies during recent decades, including scientific surveys such as the MEDITS programme (Massutí & Moranta, ; Ramírez ) and the monitoring of commercial fisheries, resulting in an almost complete absence of records. Another factor potentially contributing to the scarcity of H. perlo records could be its probable strong‐swimming ability (Compagno, ; Ebert et al ., ).…”