Habitat Selection Response of Small Pelagic Fish in Different Environments. Two Examples from the Oligotrophic Mediterranean Sea

Abstract

A number of scientific papers in the last few years singled out the influence of environmental conditions on the spatial distribution of fish species, highlighting the need for the fisheries scientific community to investigate, besides biomass estimates, also the habitat selection of commercially important fish species. The Mediterranean Sea, although generally oligotrophic, is characterized by high habitat variability and represents an ideal study area to investigate the adaptive behavior of small pelagics under different environmental conditions. In this study the habitat selection of European anchovy Engraulis encrasicolus and European sardine Sardina pilchardus is analyzed in two areas of the Mediterranean Sea that largely differentiate in terms of environmental regimes: the Strait of Sicily and the North Aegean Sea. A number of environmental parameters were used to investigate factors influencing anchovy and sardine habitat selection. Acoustic surveys data, collected during the summer period 2002–2010, were used for this purpose. The quotient analysis was used to identify the association between high density values and environmental variables; it was applied to the entire dataset in each area in order to identify similarities or differences in the “mean” spatial behavioral pattern for each species. Principal component analysis was applied to selected environmental variables in order to identify those environmental regimes which drive each of the two ecosystems. The analysis revealed the effect of food availability along with bottom depth selection on the spatial distribution of both species. Furthermore PCA results highlighted that observed selectivity for shallower waters is mainly associated to specific environmental processes that locally increase productivity. The common trends in habitat selection of the two species, as observed in the two regions although they present marked differences in hydrodynamics, seem to be driven by the oligotrophic character of the study areas, highlighting the role of areas where the local environmental regimes meet ‘the ocean triad hypothesis’.

European anchovy (Engraulis encrasicolus) age structure and growth rate in two contrasted areas of the Mediterranean Sea: the paradox of faster growth in oligotrophic seas

“…In the Strait of Sicily, the upper layer circulation is mainly controlled by the movement of the Atlantic Ionian Stream (AIS; Robinson et al, 1999), which induces a permanent coastal upwelling along the southern coast of Sicily especially during summer, when this current is stronger (Bonanno et al, 2014b). An important spatial variability exists in terms of shape, position, and strength of permanent or quasi-permanent sub-basin gyres and their unstable lobes, meander patterns, bifurcation structures and strengths of permanent jets, transient eddies, and filaments, making this area highly dynamic from an oceanographic point of view (Bonanno et al, 2014b).…”

“…An important spatial variability exists in terms of shape, position, and strength of permanent or quasi-permanent sub-basin gyres and their unstable lobes, meander patterns, bifurcation structures and strengths of permanent jets, transient eddies, and filaments, making this area highly dynamic from an oceanographic point of view (Bonanno et al, 2014b). The TY study area is located on the continental shelf along the western Italian coast in two Geographical Sub-Areas (GSA 9 and 10 according to GFCM, 2009).…”

“…The TY study area is located on the continental shelf along the western Italian coast in two Geographical Sub-Areas (GSA 9 and 10 according to GFCM, 2009). The surface circulation is characterised by the presence of a fresher water vein of Atlantic origin (Atlantic Water) entering the Tyrrhenian sea according to seasonal dependent patterns northward directed, before entering the Channel of Corsica (Millot & Taupier-Letage, 2005;Bonanno et al, 2014b). Moreover, the central TY is strongly influenced by the outflow of numerous medium and large-sized rivers (Rinaldi, 2012).…”

“…In the Strait of Sicily (Geographical Sub Area -GSA 16, as defined by the General Fisheries Commission for the Mediterranean; GFCM, 2009), the average annual catches have been estimated at around 5000 tons, while approximately 100 vessels of different types employing different fishing gears are actively exploiting anchovy in this area (Data Collection Framework, years 2007; http://stecf.jrc.ec.europa.eu/reports/dcf-dcr). In the same area, acoustic and ichthyoplankton-based biomass evaluation methods showed that the high interannual variability in biomass is largely controlled by environmental variability Bonanno et al, 2014a).…”

“…In the Strait of Sicily (Geographical Sub Area -GSA 16, as defined by the General Fisheries Commission for the Mediterranean; GFCM, 2009), the average annual catches have been estimated at around 5000 tons, while approximately 100 vessels of different types employing different fishing gears are actively exploiting anchovy in this area (Data Collection Framework, years 2007; http://stecf.jrc.ec.europa.eu/reports/dcf-dcr). In the same area, acoustic and ichthyoplankton-based biomass evaluation methods showed that the high interannual variability in biomass is largely controlled by environmental variability Bonanno et al, 2014a).In the Tyrrhenian and Ligurian seas (GSAs 10 and 9), anchovy and sardine landings in the period [2009][2010][2011][2012] were in the ranges 8000 -12000 tons and 2000 -9000 tons, respectively (IREPA, 2012). Although some acoustic surveys in the Tyrrhenian Sea during the last years highlighted the consistent presence of these resources, the management of small pelagic fishes in this area is impeded by the considerable lack of information independent of the fishery .…”