Page 2 - Habitat_Selection_2014

Habitat Selection Response of Anchovy and Sardine

highlight differences in the habitat selection behavior of anchovy We state that no specific permissions were required for the
and sardine in two areas: the Strait of Sicily and the North Aegean surveyed area both in the Strait of Sicily and in the North Aegean
Sea. The Strait of Sicily (Fig. 1a) is characterized by a complex Sea, since the involved Institutes (IAMC-CNR and HCMR) were
circulation, since it is the area connecting the two main basins of encharged by their Ministries to carry out the data collection in the
the Mediterranean Sea. The Modified Atlantic Water (MAW), framework of European Data Collection Framework (DCF -
fresher and warmer, flows in the upper layer towards the eastern Reg.Ce. Nu 199/2008, Nu 665/2008 and Commission Decision
Mediterranean basin whereas the Levantine Intermediate Water Nu 949/2008).
(LIW), saltier and colder, moves in the opposite direction [17,18]. We confirm that the field studies did not involve endangered or
The general surface circulation pattern is locally controlled by the protected species.
motion of the MAW, which bifurcates in the Atlantic Ionian Biological samples (anchovies and sardines) were collected by
Stream (AIS), a meandering surface current flowing towards the means of a pelagic trawl net during the surveys at sea, with the
Ionian Sea, and the Atlantic Tunisian Current (ATC) flowing main aim of evaluating species composition and size classes
southward [17,19]. During summer, the Atlantic water advected distribution. Due to the low resistance of small pelagic fishes to
by the AIS to the south of Sicily is warmer than its north catch with net, the specimens used for the study were already dead
surrounding waters at the same depth. The AIS motion produces a when coming on board within the trawling net.
cyclonic vortex over the Adventure Bank (Adventure Bank Vortex
– ABV) and an almost permanent upwelling along the southern 1. Acoustic data collection
shore of the island [18]. The North Aegean Sea (Fig. 1b) is Acoustic sampling was performed by means of scientific split-
characterized by high hydrological complexity mostly related to beam echosounders working at 38 kHz and calibrated following
the Black Sea water (BSW) which enters the Aegean Sea through standard techniques (Foote et al., 1987). Acoustic data were
the Dardanelles Strait. The overall circulation is mainly deter- recorded at a speed of 8–10 nmi h 21 . Minimum sampling depth
mined by the presence of the Limnos-Imvros stream (LIS), which varied between 10 to 20 m depending on the area. The size of the
carries waters of Black Sea origin onto the Samothraki plateau Elementary Distance Sampling Unit (EDSU) was one nautical
[20], generating a permanent anticyclonic system. The outflow of mile (nmi, 1.852 km). We considered as anchovy/sardine presence
BSW (salinity ,30) enhances local productivity, and its advection any school or echo assigned to anchovy/sardine either by echo
in the North Aegean Sea induces high hydrological and biological trace classification or attributed to anchovy/sardine based on the
complexity [21,22]. This is further enhanced by the presence of a catch composition of identification hauls [28]. Midwater pelagic
series of large rivers flowing out into semi-closed basins like trawl sampling was carried out in order to identify and verify
Thermaikos Gulf, enhancing the productivity locally. anchovy and sardine echo traces and length frequency.
Acoustic data analysis was performed using the Myriax
In these areas, acoustic data acquisition for small pelagics has 2
been conducted since 1998 [23] and 1995 [6], respectively. Echoview software. Anchovy and sardine density (t/nmi ) for
each EDSU was evaluated by merging the biological and acoustic
Surveys took place in both areas during summer, which
data, based on the nearest haul method [28].
corresponds to the peak of the spawning period for anchovy
[10,11,24,25] and the recruitment period for sardine [26]. The study area in the Strait of Sicily comprises the continental
shelf along the southern coast of Sicily (Fig. 1c). The echosurvey
Previous studies in the basin have focused on habitat suitability
sampling strategy adopted parallel transects characterized by
modeling for these species, using presence/absence information
and determining the areas with suitable conditions to support 5 nmi inter-transect distance (Fig. 1c and Table 1). The
investigated bathymetric range was 20–300 m.
anchovy and sardine presence [1,4,27]. In this work we analyze
In the North Aegean Sea acoustic surveys were carried out
the environmental preference and processes driving the habitat along predetermined parallel transects with 10 nmi inter-transect
selection of anchovy and sardine populations by taking into
distance in open areas, whereas zigzagged transects were sampled
account the spatial distribution of species density in relation to
inside gulfs (Fig. 1d and Table 1).
environmental patterns. Specifically, we focus on high density
values, seeking for similarities or differences in the ‘‘mean’’
2. Environmental dataset
behavioral patterns for each species. Two kinds of variables were
Satellite and in situ measurements were used in this study to infer
selected for this study: one group of factors related to or being
proxies of the trophic status of the water column (e.g. chlorophyll the environmental conditions in the study areas and to investigate
the habitat suitability of the considered species. Satellite dataset
and particulate organic carbon) and a group of variables which
are largely used in fishery community due to their consistent
describe the hydrographic regimes in each area (e.g. temperature, space-time coverage and their ability to highlight specific ocean
salinity, potential energy deficit and kinetic energy).
processes. Furthermore, ETOPO1 bathymetric data were used to
associate bottom depth values at each observation. The ETOPO1
Materials and Methods
dataset, provided by NOAA (http://www.ngdc.noaa.gov/mgg/
The study was carried out on data collected in two sea areas global/global.html), is built by merging a number of global and
without any restriction. We received the necessary permissions for regional datasets, integrating land topography and ocean bathym-
working at sea in National waters by the Italian and Greek Coastal etry over a 1 arc-minute grid.
Guards for the Strait of Sicily and the North Aegean Sea 2.1 In situ measurements. In the Strait of Sicily water
column parameters were collected during co-occurring hydrolog-
respectively.
The study areas are: ical surveys carried out on board the R/V ‘‘Urania’’ covering the
same area as the echosurveys. The sampling strategy (Fig. 1e) was
– in the Strait of Sicily between 35uN and 38uN latitudes and based on a regular grid with 4 nmi mesh size in the coastal sector
between 11uE and 16uE longitudes; and 12 nmi mesh size in offshore waters. Salinity, temperature and
pressure profiles, collected from surface down to bottom or 600 m
– in the North Aegean Sea between 38uN and 41uN latitudes and
between 22uE and 27uE longitudes. (maximum depth) by means of SBE911 plus CTD probe (Sea-Bird
Inc.), were processed using Seasoft-Win32 software following the
PLOS ONE | www.plosone.org 2 July 2014 | Volume 9 | Issue 7 | e101498