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crustaceans, isopods, and schizopods.
Among these latter Meganyctiphanes norvegica and Euphausia Kronii
Brandt (so diffused in the Mediterranean as to influence notably in many
places the total quantity of macroplanktonj Jespersen) have a special
importance; shoals of these traverse the Strait of Messina in the autumn
and winter and the tuna appear so glutted on them that under these
conditions they let themselves be easily approached and harpooned by the
fishermen.

It appears therefore likely that these and other forms, widely
distributed in the Atlantic, forming regular shoals, can actually
influence by their fluctuations and migrations from the Atlantic to
the Mediterranean the routes, the concentrations, and the distribution
of the tuna in the latter sea.

I note the analogy with Thunnus alalonga , which, in the Atlantic,
is distributed in relation to the shoals of an amphipod, Buthemisto
bispinosa Boek (Joubin and Roule; Le Danois)o

The planktonic feeding of the tuna probably also facilitates its
exodus from the Mediterranean and contributes to its enormous range,
which would be very restricted if its food habits were strictly linked
to certain pelagic fishes, divided into races of limited geographic
habitat, as for example the Clupeidi or the Scombri of the Mediterranean,
which are distinct from those of the Atlantico

It further appears that the tuna does not pursue indefinitely the
same schools of fish. In the Strait of Messina it gives chase successively
to the fishes which are passing through there, in autumn to the young
"
anchovies, in winter to the eels, and then to the costardelle" ( Scombresox ),
etc.

ll) In regard to the preceding, we can ask whether the fluctuations
in the balance of tuna between the Atlantic and the Mediterranean may be
such as to affect the production of the tuna fisheries. But this discus-
sion involves the whole complicated study of the fluctuations in the catch
of the fisheries and of their causes. It must not be forgotten that the
tuna traps catch fish in spawning condition arid that therefore we should
expect to find the production of the fishery influenced especially by
oceanographic conditions, both local and general, of a physical and
chemical nature.

More than by the abundance of tuna present at a given moment in the
Mediterranean (which reflects also, at a distance in time, the conditions
under which the generations of tuna in different years develop), the
catch of the tuna trap fisheries is influenced by the greater or lesser
degree in which the tuna come close in to the coast and by the geographical
dispersions which occur in the areas of maturation.

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