Page 18 - Sella_M_1929

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Thus the sudden drop in the catch at Sidi-Daud around 1895, which
Roule attributes to heavy rains and to the fresh water carried by the
outlet of the lake of Bizertep situated 50 km to the west^ finds a more
plausible explanation in the analogy of habit with Sardinia,

e) With the method of the analysis of modes worked out by Vercellij
(Atti Congresso Soco It., Progro Scienze^ 1928) there are evident, for
some tuna fisheries, fluctuations which are periodical or at least of
almost constant period, of various magnitudeso Thus the curve of the
catch of the two Sardinian fisheries of Po Scuso and Po Paglia^ taken
together, turns out to be composed of periodic elements of 2^ 3-4, 6,
10, 24, and 100 (?) yearso The relative calculations for this series were
made by Profo Vercelli himself

For Sidi-Daud also, it was possible to show fluctuations of quite
similar periodo

Naturally the study of the cases, in part local, in part more or
less general, is still all to be doneo

Granted that the curve of the catch of certain tuna fisheries may
be entirely broken up into periodic or almost periodic elements, it_ is
possible to make predictions for some of them .

It is a pity that these statistics, with few exceptions, are very
badly kept. The fishery with tuna traps, because of the uniformity of the
means (fishing season, position and dimensions of the constituent nets)
would lend itself better than any other to analyses of this natureo

Meanwhile, in a general way, it is possible to give assurances to
the operators of the tuna fisheries thax the crisis which has vexed the
fisheries of the Mediterranean for some years is certainly transitoryo
Apart from fluctuations of lesser magnitude, we are now at the bottom of
a cycle of a century (or at least a very long period), and once that is
passed over such unfavorable conditions for the fishery will probably
not return for several score yearso

12) The result which is perhaps most interesting in connection with
the method of the hooks, is that which leads to the recognition of the
existence of a vast area frequented by tuna of identical raceo It is
this which permits one to attempt an overall outline of the movements
which the tuna make in connection with reproduction and feedingc

This area embraces the Mediterranean, about half of the North
Atlantic as far as the Azores j, and the North Sea, from about the latitude
of the Tropic or a little farther north to the Arctic Circle at the
other extreme

The tuna is not very common at the Canaries (where the other purely
Atlantic or more tropical species of tuna predominate) and certainly it
does not push much farther south along the ccast of Africao To the north
it is common in the region of Trondhjem in Norway and it has been seen
frequently at Lofoten,
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