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Habitat Selection Response of Anchovy and Sardine

regimes accounting for food availability in this area. First, the energy available for photosynthesis, controlling the growth of
presence of BSW input, as reflected in low BL and UML salinity phytoplankton and thus critical also for fisheries and carbon
values, low UML thickness and higher CDOM, CHL sat , POC dynamics. It is often used to determine the euphotic depth in the
values. Secondly, the formation of a weak coastal upwelling, ocean (defined as the depth of 1% of the surface radiation), taking
corresponding to shallow and less stratified waters (lower PED), is into account light attenuation and absorption. However, partic-
also related with higher CHL sat and POC. ularly high values of PAR are known to cause a significant
Furthermore, the two species differentiate their behavior decrease in cell abundance [46]. It is clear that the common trends
concerning the effect of temperature in the upper water layer. In in habitat selection of anchovy and sardine, observed in two
the North Aegean Sea both species prefer higher UML ecosystems with marked differences in hydrodynamics, are driven
temperatures whereas the behavior is different in the Strait of by the oligotrophic character of the two regions. Practically, we
Sicily where sardine seems to select warmer waters and anchovy identified a preference of both species for areas where shallow
colder ones. Moreover, in the Strait of Sicily, both species show an waters over the continental shelf meet suitable conditions for
avoidance behavior for lower UML salinity values (,37.4) photosynthesis levels; such areas coincide with different circulation
corresponding to the values characterizing the AIS core [44]. patterns that enhance productivity and subsequently food avail-
Concerning BL salinity, different behavior of both species is ability. Both anchovy and sardine select areas where the ‘‘ocean
observed in the two areas. In the North Aegean Sea both species triad’’ hypothesis is generally met (i.e. enrichment, concentration
generally prefer salinity less than 38. In the Strait of Sicily anchovy and retention processes sensu Bakun [15]). Stratification, retention
selects a BL salinity range with values slightly higher than sardine, and plankton production have been proposed as controlling
even though both species avoid higher values measured in such factors for anchovy spawning (or spawning success) in the Bay of
area. Sardine selects lower KE values, corresponding to more Biscay [47]. Furthermore, the differences between the two species
coastal waters (lower depths), and avoids KE values higher than can be attributed to their life cycle and the sampling period
2 22
200 cm s , indicating strong currents. A wider depth range targeting the optimization of spawning for anchovy and the
selected by anchovy in the Strait of Sicily is associated also to KE selection of suitable juvenile grounds for sardine. Thus, sardine
range of intermediate values, showing at the same time a general selects the coastal, shallower and warmer waters of both areas, in
tolerance behavior for a large range of current speeds. In the agreement with previous findings [4,27,48]. In the case of
North Aegean Sea, it is only anchovy that clearly shows a selection anchovy, our study highlighted a clear selection for areas
behavior for lower UML salinity values, associated with water presenting moderate upwelling or downwelling features like the
masses influenced by the BSW and/or rivers outflow. Species’ periphery of anticyclones i.e. the Samothraki gyre in North
behavior in terms of PED also reflects local circulation conditions. Aegean Sea, the periphery of the AIS in the Strait of Sicily,
Anchovy differentiates its behavior, selecting less stratified waters enhancing frontal formation and retention for mesozooplankton,
in the Strait of Sicily, possibly associated with moderate upwelling eggs and larvae [34,42,49].
conditions, whereas selects well stratified waters in the North Extensive research effort has been made to understand the
Aegean Sea. Sardine seems to select less stratified waters or factors determining anchovy and sardine habitat in upwelling
otherwise moderate upwelling conditions in both study areas. areas where fish habitat expands or shrinks, largely determined by
A clear difference between the two study areas, in terms of the intensification of the upwelling. In southern African waters
habitat selection, is singled out also by the quotient curves of anchovy spawns from midshelf to the offshore area of the shelf,
MADT values. In particular, both species in the Strait of Sicily within waters of intermediate temperature (17–21uC) and high
show an avoidance behavior for higher (.6 cm) MADT values salinity [9,50,51] clearly separating its niche from adult sardine
corresponding to the area nearest the AIS path. Practically, [52]. In south Australia anchovies spawn in productive waters
–3
although both species show a general preference for relatively (Chl-a concentration up to 4.5 mg m ) and temperatures ranging
shallower waters over the continental shelf, the AIS path acts like a from 15.5 to 23.5uC but anchovies tend to expand their
physical barrier limiting their spatial distribution as both species distribution to shelf waters only when sardine abundance is low
tend to avoid higher current speeds and higher MADT. Often the [53,54]. In the California Current, Northern anchovy spawns all
AIS flows very close to the coast and leads to the formation of a year round and spawning usually is observed nearshore (up to
dense front that entraps mesozooplankton concentrations, main- 100 nmi from the coast), in waters with low temperatures (12–
taining high food availability within coastal areas [34,45]. MADT 18uC), high salinity (33.5–33.75) and high primary production
values in the Strait of Sicily are well correlated with UML [55,56]. The Peruvian anchovy or anchoveta (Engraulis ringens)
thickness (r = 0.5; p,0.05), probably due to the effects of upwelling distribution is generally restricted to cold coastal water masses
phenomena or movement of the Modified Atlantic Water causing characterized by high productivity and large plankton [57].
frontal structures along the southern Sicilian coast. In the North Anchovies habitat in the Humboldt System is strongly related to
Aegean Sea there is not a clear preference/avoidance behavior in the strength and the duration of El Nino events in this area. The
terms of MADT. Instead, anchovy seems to select a range of northern and central stocks spawn on the continental shelf (up to
values corresponding to moderate downwelling processes. Such the 100 m isobath), over a wide temperature range (14–21uC), in
values often characterize the peripheral part of areas in the areas characterized by intense upwelling and high primary
margins of anti-cyclonic formations like the Samothraki gyre, production [58,59]. Anchovies also spawn in the fjords of southern
typically found in Thracian plateau associated with BSW input Chile (42u –47uS), areas characterized by highly stratified, stable
[6,22]. waters that favor larval growth and survival [60].
Concerning PAR, a common selection behavior for values less Although the ‘‘ocean triad’’ hypothesis was initially observed in
than 55 Einstein m 22 day 21 was observed for both species at both upwelling areas, it seems well applicable also to coastal areas
study areas but it was found significant only in North Aegean Sea. where physical processes that increase productivity, are considered
In this region, the spatial distribution of this level of PAR values mostly responsible for the spatial organisation of plankton
coincides with areas associated with high Chl sat concentrations like concentration and subsequently drive the spatial distribution
river outflows and the location of the permanent anticyclone patterns of anchovy and sardine [34,45,61]. However, the strength
formed at the Thracian Sea plateau. PAR is indicative of the solar and the extent of such oceanographic regimes are adjusted to the

PLOS ONE | www.plosone.org 15 July 2014 | Volume 9 | Issue 7 | e101498