CHAPTER 25
Energy Flux in the South Tyrrhenian Deep-sea Ecosystem: Role
of Mesopelagic Fishes and Squids
A. Granata, G. Brancato, O. Sido~ and L. Guglielmo
ABSTRACT
We here review various studies concerning the ecology of mesopelagic organisms in the South
Tyrrhenian Sea and other oceans to direct attention to the importance of mesopelagic animals
within the context of meso- and epi-pelagic food webs. The results of recent studies on the
micronekton in the South Tyrrhenian Sea were examined (Marabello 1994; Guglielmo et al.
1995), together with the relationship between immense populations of vertically migratory fishes and squids and the fixed resident population of more surface and deeper waters. The review
summarizes the present state of knowledge on migratory activity related to trophic purposes of
mesopelagic fishes and on the diet of squids, tunas and other large predators. The conclusions
seem to confirm the hypothesis of a trophic connection between cephalopods and mesopelagic fishes. They also elucidate the fundamental role of cephalopods as a top link in the trophic
chain of mesopelagic fishes. The hypothesis of a trophic chain characterised by cephalopods,
mesopelagic fishes and crustaceans, with cephalopods acting as a connection between surface
and mesopelagic trophic webs, is discussed.
Introduction
The so-called Deep Scattering Layer (DSL) has
been known for some time in all of the major
oceans. During the day. it is located at a depth of
450 down to 750 m, but at night time it splits into
a component that rises towards the surface and
another that remains stationary. The organisms
responsible for this phenomenon are mainly midwater fishes and crustaceans, in addition to
siphonophores (Farquhar 1977; Guglielmo and
Zagami 1985). The DSL is discontinuous since an
incalculable number of organisms moves towards
the surface for trophic purposes (Mann 1984).
Marine organisms are linked to each other by
complex trophic food webs. knowledge of which
is important for understanding energy flows that
are established within pelagic communities. The
trophic dynamics of certain "key groups" delineate several basic features of the functioning of
pelagic ecosystems (Fig.I). Within the micronekton, mesopelagic fishes feeding on zooplankton
are important components in several oceanic
ecosystem models (Hopkins and Baird 1985).
This group is the main component, in terms of
biomass, of the mesopelagk zone (200-1000 m of
depth). Due to their high density and wide diffusion in all oceans, mesopelagic fishes can be considered the most abundant vertebrates on earth
(Mann 1984). Their importance in this context
stems from the fact that each day they perform
large vertical migrations. for trophic purposes,
which make possible the continuous flow of
energy between surface and underlying waters.
At present, the most accepted hypothesis
regarding trophic migrations is that these organisms move lit night to feed in upper layers. where
presumably food can be found in higher concentrations, and then descend to greater depths during the day to avoid predation (Marshall 1960)
(Fig. 2). The daily vertical migrations associated
with feeding have been well documented in different species of mesopelagic fishes (Scotto di
Carlo et al. 1982) and squids (Marabello et al.
1996).
The aim of this study was to address the
problem of what relationship exists between
these immense populations of vertically migraDipartimento di Biologia Animale ed Ecologia Marina, Universita di Messina, Salita Sperone 31, 98166 Messina, Italy
RM. Faranda. 1. Guglielmo, G. Spezie (eds)
Mediterranean EcoS}'litems: Structures and Processes
C Spri.nger-Verlag ltalia 2001
Energy Flux in the South Tyrrhenian Deep-sea Ecosystem: Role
of Mesopelagic Fishes and Squids
A. Granata, G. Brancato, O. Sido~ and L. Guglielmo
ABSTRACT
We here review various studies concerning the ecology of mesopelagic organisms in the South
Tyrrhenian Sea and other oceans to direct attention to the importance of mesopelagic animals
within the context of meso- and epi-pelagic food webs. The results of recent studies on the
micronekton in the South Tyrrhenian Sea were examined (Marabello 1994; Guglielmo et al.
1995), together with the relationship between immense populations of vertically migratory fishes and squids and the fixed resident population of more surface and deeper waters. The review
summarizes the present state of knowledge on migratory activity related to trophic purposes of
mesopelagic fishes and on the diet of squids, tunas and other large predators. The conclusions
seem to confirm the hypothesis of a trophic connection between cephalopods and mesopelagic fishes. They also elucidate the fundamental role of cephalopods as a top link in the trophic
chain of mesopelagic fishes. The hypothesis of a trophic chain characterised by cephalopods,
mesopelagic fishes and crustaceans, with cephalopods acting as a connection between surface
and mesopelagic trophic webs, is discussed.
Introduction
The so-called Deep Scattering Layer (DSL) has
been known for some time in all of the major
oceans. During the day. it is located at a depth of
450 down to 750 m, but at night time it splits into
a component that rises towards the surface and
another that remains stationary. The organisms
responsible for this phenomenon are mainly midwater fishes and crustaceans, in addition to
siphonophores (Farquhar 1977; Guglielmo and
Zagami 1985). The DSL is discontinuous since an
incalculable number of organisms moves towards
the surface for trophic purposes (Mann 1984).
Marine organisms are linked to each other by
complex trophic food webs. knowledge of which
is important for understanding energy flows that
are established within pelagic communities. The
trophic dynamics of certain "key groups" delineate several basic features of the functioning of
pelagic ecosystems (Fig.I). Within the micronekton, mesopelagic fishes feeding on zooplankton
are important components in several oceanic
ecosystem models (Hopkins and Baird 1985).
This group is the main component, in terms of
biomass, of the mesopelagk zone (200-1000 m of
depth). Due to their high density and wide diffusion in all oceans, mesopelagic fishes can be considered the most abundant vertebrates on earth
(Mann 1984). Their importance in this context
stems from the fact that each day they perform
large vertical migrations. for trophic purposes,
which make possible the continuous flow of
energy between surface and underlying waters.
At present, the most accepted hypothesis
regarding trophic migrations is that these organisms move lit night to feed in upper layers. where
presumably food can be found in higher concentrations, and then descend to greater depths during the day to avoid predation (Marshall 1960)
(Fig. 2). The daily vertical migrations associated
with feeding have been well documented in different species of mesopelagic fishes (Scotto di
Carlo et al. 1982) and squids (Marabello et al.
1996).
The aim of this study was to address the
problem of what relationship exists between
these immense populations of vertically migraDipartimento di Biologia Animale ed Ecologia Marina, Universita di Messina, Salita Sperone 31, 98166 Messina, Italy
RM. Faranda. 1. Guglielmo, G. Spezie (eds)
Mediterranean EcoS}'litems: Structures and Processes
C Spri.nger-Verlag ltalia 2001