Variation in mesopelagic fish community composition and structure between Mediterranean and Atlantic waters around the Iberian Peninsula

Abstract

Mesopelagic fish populations are characterised by high species richness and abundance, and have been identified as important contributors to the active carbon fluxes in the open ocean. We report variability in communities of mesopelagic fish between five zones around the Iberian Peninsula, i.e. Balears and Alboran in the Mediterranean, and Cadiz, Lisboa and Galicia in the Atlantic. Day and night samples were collected from 7 layers of the water column with a midwater trawl fitted with a multisampler. Temperature and salinity regimes were very different on the Mediterranean and Atlantic sides of the peninsula, with much higher values through the entire water column in the Mediterranean, characterized by a strong pycnocline. The highest productivity was observed off Lisboa, where Chlorophyll a concentrations were two orders of magnitude higher than in any other zone. Samples from the western Mediterranean held 22 fish species, while 67 were found in the Atlantic. The lowest diversity and the highest dominance were observed in Balears, and the highest diversity in Cadiz zone. In all zones, but particularly in those in the Mediterranean, mesopelagic populations were dominated by a high number of small fish with low individual biomasses. The species Benthosema glaciale, Cyclothone pygmaea and Ceratoscopelus maderensis were common in the Mediterranean populations, whereas in the Atlantic, Cyclothone microdon/livida, Valenciennellus tripunctulatus, Ceratoscopelus warmingii and Benthosema suborbitale were the most common species. Temperature and salinity (both at surface and in the mesopelagic zone) were the main environmental factors explaining variability in assemblage composition. A persistent (day-night) deep scattering layer was observed using the vessel-based echosounder in all zones, and was comprised primarily of the gonostomatid Cyclothone spp. Night-time echosounder observations of scattering layers near the surface were observed in Balears, Alboran, Cadiz and Lisboa, where night surface net collections indicated that Myctophidae, Stomiidae and Phosichthyidae migration extended to the upper 100 m. Sternoptychids and the gonostomatid Sigmops elongatus seldom reached the upper 100 m in their night vertical migrations. Night stratified hauls of 30 m resolution carried out in the epipelagic zone showed that abundances maxima of migratory fish coincided with the location of the Chlorophyll a maxima.

Seasonal patterns in the mesopelagic fish community and associated deep scattering layers of an enclosed deep basin

“…All the remaining species were found to migrate. The most abundant species found in the upper layer during the night were C. maderensis, H. benoiti and B. glaciale, in line with what has been previously evidenced in the W. Mediterranean 22,24 . Additionally, although large numbers of individuals of M. punctatum were observed on the sea surface at night (and caught with a landing net; authors' pers.…”

“…The current work presents the results of the rst acoustic survey aiming to study DSLs and their composition, not only in the Gulf of Corinth, but also in the Central Mediterranean Sea. The shes diversity of the mesopelagic zone in the study area is relatively low, as has been previously reported for the Mediterranean 10,15,24 where the presence of many deep-water species inhabiting the adjacent eastern Atlantic seems to be constrained by (i) the relatively shallow strait (Gibraltar) connecting the two Seas and (ii) the high temperature in the deep waters of the Mediterranean compared to the colder Atlantic 22,38 . In addition, no Mediterranean members of the Gonostomatidae and Phosichthyidae families (e.g.…”

“…In total, at least 15 sh species were caught (Trawl: Table 3; Methot frame: Suppl. material Table S1), belonging to the families Myctophidae, Paralepididae, Sternoptychidae and Stomiidae, while the -elsewhere very abundant 24 -families Gonostomatidae and Phosichthyidae were completely absent.…”

“…In the Mediterranean Sea, although there are some works providing information on the biology of mesopelagic shes e.g. [19][20][21] , targeted studies on the mesopelagic sh communities are very limited and mainly refer to the western basin 15,[22][23][24] . In the eastern basin, to our knowledge, the only relevant studies concern ichthyoplankton surveys which provided insights on the species composition in several Greek seas and gulfs 25 and a preliminary survey targeting plankton and (mainly) juvenile sh assemblages of the North Aegean Sea mesopelagic zone 26 .…”

Multiple Approaches to the Trophic Role of Mesopelagic Fish around the Iberian Peninsula

“…13.5 • C and salinity ca. 38.5, below 200 m) [65]. The feeding habits of these species in the western Mediterranean (off the Balearic Islands and the Gulf of Lions) and the northeastern Atlantic (e.g., the northern Mid-Atlantic Ridge, Azores, and the Canary Islands) have been studied through the analysis of stomach contents and stable isotopes at the species and community levels, e.g., [17,42,43,[66][67][68][69][70].…”