Page 13 - Habitat_Selection_2014

Habitat Selection Response of Anchovy and Sardine

Figure 8. QI plot related to PCA factor coordinates. Each plot shows the observed QI curve (solid line), its CI (dashed lines), and the frequency
histogram of the considered environmental variable. The dotted line in each plot indicates the value QI = 1. For a correct interpretation factor
variables correlation relative to PCA (Tab. 5) must be considered.
doi:10.1371/journal.pone.0101498.g008
regimes can result into a common habitat selection behavior for 25 m, in both study areas. Less extended upper mixed layers are
small pelagics. associated with the occurrence of upwelling processes, so this
During early summer sardine population in the Mediterranean selection possibly reflects the preference for moderate upwelling in
Sea is a mixture of both juveniles (age 0) and adults (ages 1, 2 and both areas. Selection for higher CDOM index, Chl sat and POC
older). Therefore, summer distribution grounds reflect both concentrations is also pronounced for both species. This common
nurseries and feeding areas. In both study areas, sardine exhibits trend is driven by the high demand for food in oligotrophic
a stronger selective behavior, showing higher densities at shallower environments like the study areas, although the selected values in
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waters (up to 70 m depth in the Strait of Sicily and up to 50 m the Strait of Sicily (Chl sat .0.15 mg/m and POC .52 mg/m )
depth in the North Aegean Sea) whereas anchovy shows selective are smaller than the ones in the North Aegean Sea (Chl sat .
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behavior up to the 100 m isobath. Sardine juveniles’ potential 0.4 mg/m and POC .130 mg/m ). This aspect is connected to
habitat in the Mediterranean is known to be narrow in extent, the different ranges observed for these environmental variables in
generally patchier, and mostly located at inshore waters [27]. the two areas: lower productivity of the southern Sicilian waters
Anyway, higher probability of finding adult sardine during [32] and higher Chl sat and POC values recorded in the North
summer is also identified in waters deeper than 65 m depth [4], Aegean Sea due to inputs of BSW, the extended continental shelf
indicating common bathymetric preferences for both juveniles and and rivers runoff [1,22]. Although the range of values of the
adults in accordance to our findings. This is not the case in the Bay CDOM index is similar in both areas, CDOM largely differen-
of Biscay, where adults and juveniles of European sardine separate tiates in terms of its spatial pattern, with high CDOM values
their niches. Adults distribute offshore over the continental shelf extending over a wide area in North Aegean Sea and limited to
along the shelf break [40], whereas only young fish are found coastal waters in the Strait of Sicily. As an example, such
inshore. differences between the two areas in terms of spatial extension of
In the Mediterranean during summer, anchovy stocks are some investigated variables, highly affecting habitat suitability of
dominated by spawning adults and spawning areas are known to both species, are shown in Fig. 9.
be mainly driven by a depth gradient [1,10,11,22,34,41,42]. In Whilst the QI analysis helped to highlight differences and
particular, in the Strait of Sicily anchovy spawning grounds mostly similarities among areas and species in terms of selection/
occurred within the 100 m isobath [34,43], while in the North tolerance/avoidance for specific ranges of investigated variables,
Aegean Sea Somarakis and Nikolioudakis [22] identified two the PCA analysis results verified that the circulation regimes are
major spawning grounds: the first one located in the eastern area largely responsible for the habitat selection patterns observed in
influenced by an anticyclone, the Samothraki gyre (SG), and the both areas. In the Strait of Sicily the formation of a weak, coastal
second one in the Thermaikos gulf in association to river mouths. upwelling (represented into the lower PED, smaller values of UML
Common selection patterns in the two study areas are observed thickness and shallow waters) is well associated with variables
in the case of BL temperature, UML thickness, POC, CDOM related to higher food availability (like CDOM, CHL sat , POC)
index and Chl sat . Specifically, both species prefer BL temperatures explaining most of area variability. In the North Aegean Sea,
above 15.5uC and waters characterized by UML thickness up to analysis also verified the presence of two separate environmental

PLOS ONE | www.plosone.org 13 July 2014 | Volume 9 | Issue 7 | e101498