predominantly the myctophids. Like most of the
pelagic and bathypelagic cephalopods, 1. coindetii is basically a fish predator. although its diet
includes a sizeable quantity of crustaceans and
sometimes also cephalopods. Young individuals
exhibit less alimentary selectivity, but it appears
that, with increasing size, the preference moves
ever more toward fish.
Discussion and Conclusion
For an adequate understanding of pelagic trophic webs, various of problems must be investigated thoroughly. For example, it is necessary to
understand the relationship between large surface predators and mesopelagic fishes that
migrate at night into surface layers, the connection between these two important trophic food
chains, and the distribution of available energy
in the environment. The structure of the socalled epipelagic zone is well known, and is dominated during the day by tunas. swordfishes and
sharks. These feed on a wide variety of smaller
fishes, pelagic cephalopods and, to a lesser
degree, on crustaceans (euphausiids). Tunas
(Thunnus thyn n us, T. albacares, T. obesus, T.
alalunga) pursue their prey very rapidly, performing true migrations in search of adequate
food sources only during the day and in the
upper layers of water. Sharks generally occupy
the same trophic level, except for the very large
species that can occupy a higher trophic level,
assuming the role of fmal predators. Legand et al.
(1972) stated that the organisms found in the
stomachs of tunas inhabiting tropical regions
were very different from those caught with the
IKMT
(Isaacs-Kidd
Midwater
Trawl).
Cephalopods were more numerous and larger in
the stomachs than in the nets, in contrast to crustaceans which were numerous in the nets but virtually absent in the stomachs. Epipelagic fishes,
which are rapid swimmers and thus not easily
caught with pelagic nets, constitute the main
resource of food for tunas, with a frequency percentage of 60%. A study by Roger (1973) on the
relation between the euphausiids and organisms
that occupy higher trophic levels (Thunnus
alalunga. T. albacares, T. obesus, Alepisaurus
ferox) confirmed that tunas do not feed directly
on euphausiids. Instead, euphausiids constituted
only 10% of the total food ingested by micronektonic epipelagic fishes, which in turn are preyed
Energy Flux in the South Tyrrhenian Deep-sea Ecosystem
205
upon by tunas. These fishes normally measure 30
to 130 mm, are rapid swimmers and, consequently, are rarely caught in pelagic nets. Their
preferred euphausiid prey belong to the genera
Stylocheiron and Nematoscelis. This implies that
epipelagic fishes preyed upon by tunas are able
to utilize only organisms that inhabit sub-surface
waters during the day and ascend toward the surface only at night (Euphausia, Thysanopoda).
The same observations of daytime feeding
were made for tunas. Roger and Grandperrin
(1976) studied the diet of Thunnus alalunga and
T. albacares and confirmed the previous data.
They observed that euphausiids constituted 12%
by volume of the food of micronektonic fishes;
euphausiids were exclusively resident in the
epipelagic zone and in turn formed 600/0 of the
tuna diet. Cephalopods formed 40% of the tuna
diet, whereas mesopelagic fishes that migrated to
the sub-surface zone at night were virtually never
found in tuna stomachs. Also in this case,
euphausiids found in the stomachs of micronektonic fishes were non-migratory epipelagic
species of the genus Stylocheiron. Thus. both
tunas and their prey, micronektonic fishes, feed
during the day. Therefore, large predators in surface waters have a diet composed of two thirds
smaller epipelagic fishes and one third
cephalopods, and do not have a direct trophic
relationship with mesopelagic fishes. Hence, the
trophic chain "Tunas c) Prey fishes c)
Euphausiids" is based exclusively on epipelagic
energy production (Mann, 1984). Roger and
Grandperrin (1976) maintained that vertically
migrating mesopelagic fishes subtract food, and
thus energy, from the surface layers without
returning the removed portion, since they do not
form part of the epipelagic trophic chain. If this
were so, there would be an important and active
net transport of energy and materials toward
lower depths, with the function of supporting
meso- and bathypelagic production. However,
there is another parallel chain that involves
cephalopods, which form 40% of the tuna diet. It
is important to know the role of cephalopods as a
link that joins the epi- and mesopelagic trophic
chains. This question is tied to the hypothesis that
energy flow to lower layers is partly compensated
by a return towards the surface, by means of a
chain that has as its key organisms the
cephalopods. In fact, recent studies (Marabello,
1994; Marabello et aI., 1996) on the feeding ecology of Todarodes sagittatus in the South
pelagic and bathypelagic cephalopods, 1. coindetii is basically a fish predator. although its diet
includes a sizeable quantity of crustaceans and
sometimes also cephalopods. Young individuals
exhibit less alimentary selectivity, but it appears
that, with increasing size, the preference moves
ever more toward fish.
Discussion and Conclusion
For an adequate understanding of pelagic trophic webs, various of problems must be investigated thoroughly. For example, it is necessary to
understand the relationship between large surface predators and mesopelagic fishes that
migrate at night into surface layers, the connection between these two important trophic food
chains, and the distribution of available energy
in the environment. The structure of the socalled epipelagic zone is well known, and is dominated during the day by tunas. swordfishes and
sharks. These feed on a wide variety of smaller
fishes, pelagic cephalopods and, to a lesser
degree, on crustaceans (euphausiids). Tunas
(Thunnus thyn n us, T. albacares, T. obesus, T.
alalunga) pursue their prey very rapidly, performing true migrations in search of adequate
food sources only during the day and in the
upper layers of water. Sharks generally occupy
the same trophic level, except for the very large
species that can occupy a higher trophic level,
assuming the role of fmal predators. Legand et al.
(1972) stated that the organisms found in the
stomachs of tunas inhabiting tropical regions
were very different from those caught with the
IKMT
(Isaacs-Kidd
Midwater
Trawl).
Cephalopods were more numerous and larger in
the stomachs than in the nets, in contrast to crustaceans which were numerous in the nets but virtually absent in the stomachs. Epipelagic fishes,
which are rapid swimmers and thus not easily
caught with pelagic nets, constitute the main
resource of food for tunas, with a frequency percentage of 60%. A study by Roger (1973) on the
relation between the euphausiids and organisms
that occupy higher trophic levels (Thunnus
alalunga. T. albacares, T. obesus, Alepisaurus
ferox) confirmed that tunas do not feed directly
on euphausiids. Instead, euphausiids constituted
only 10% of the total food ingested by micronektonic epipelagic fishes, which in turn are preyed
Energy Flux in the South Tyrrhenian Deep-sea Ecosystem
205
upon by tunas. These fishes normally measure 30
to 130 mm, are rapid swimmers and, consequently, are rarely caught in pelagic nets. Their
preferred euphausiid prey belong to the genera
Stylocheiron and Nematoscelis. This implies that
epipelagic fishes preyed upon by tunas are able
to utilize only organisms that inhabit sub-surface
waters during the day and ascend toward the surface only at night (Euphausia, Thysanopoda).
The same observations of daytime feeding
were made for tunas. Roger and Grandperrin
(1976) studied the diet of Thunnus alalunga and
T. albacares and confirmed the previous data.
They observed that euphausiids constituted 12%
by volume of the food of micronektonic fishes;
euphausiids were exclusively resident in the
epipelagic zone and in turn formed 600/0 of the
tuna diet. Cephalopods formed 40% of the tuna
diet, whereas mesopelagic fishes that migrated to
the sub-surface zone at night were virtually never
found in tuna stomachs. Also in this case,
euphausiids found in the stomachs of micronektonic fishes were non-migratory epipelagic
species of the genus Stylocheiron. Thus. both
tunas and their prey, micronektonic fishes, feed
during the day. Therefore, large predators in surface waters have a diet composed of two thirds
smaller epipelagic fishes and one third
cephalopods, and do not have a direct trophic
relationship with mesopelagic fishes. Hence, the
trophic chain "Tunas c) Prey fishes c)
Euphausiids" is based exclusively on epipelagic
energy production (Mann, 1984). Roger and
Grandperrin (1976) maintained that vertically
migrating mesopelagic fishes subtract food, and
thus energy, from the surface layers without
returning the removed portion, since they do not
form part of the epipelagic trophic chain. If this
were so, there would be an important and active
net transport of energy and materials toward
lower depths, with the function of supporting
meso- and bathypelagic production. However,
there is another parallel chain that involves
cephalopods, which form 40% of the tuna diet. It
is important to know the role of cephalopods as a
link that joins the epi- and mesopelagic trophic
chains. This question is tied to the hypothesis that
energy flow to lower layers is partly compensated
by a return towards the surface, by means of a
chain that has as its key organisms the
cephalopods. In fact, recent studies (Marabello,
1994; Marabello et aI., 1996) on the feeding ecology of Todarodes sagittatus in the South