142
ANTON F. BRUUN
selection must have been a t work, and probably still is, especially in
A . rostrutu. From the known distribution of the A . rostruta leptocephali it may be concluded that a considerable number fail to reach
the coasts, especially those found north of 40" N. lat. and east of 55" W.
long. But even some of these do reach Europe as mentioned above.
IV. How DOES A . unguilla REACH THE BREEDINU PUCE?
As already mentioned Tucker is of the opinion that A. anguillu
cannot reach the breeding places. He then asserts (Tucker, 1969a,
p. 497) : " The east-coast American migration, by contrast, presents
little difficulty. Along that coast the coastal water and immediately
adjacent slope water are moving in a south-westerly direction (Fig. 3).
Recent Discovery I1 work (Swallow, 1957) has confirmed the hypothesis of Stommel and proved the existence, 150 miles off South Carolina,
of a strong south-going current (up to 84 miles a day) deep under the
Gulf Stream. This system provides a simple, rapid and direct path,
and without demanding any differences of behaviour among the different
eastern groups of American eels. Temperature and salinity gradients
encountered on this route (Sverdrup et ul., 1942) are such that active
migration would reinforce passive transport. The fate of the Mississippi
eels need not concern us.
" We may here meet Bertin's demand (1956) for a migration theory
containing all the observed phenomena of physiology and behaviour."
We do know that the European eel shows positive reactions to
higher temperature, as Tucker (1959a, p. 497) states on the authority
of Bertin (1956). This is well known to all fishermen and biologists who
have studied the eel. The last silver eels are caught in Denmark when
the season with frost starts, but the great majority#are caught during
the dark nights of the interlunar periods of August, September and
October, and male silver eels may start migration as early a8 in the
interlunar periods from June onwards (Otterstr~m, 1948, p. 296). In
the spring the fishermen cannot use baited hooks for catching yellow
eels until the water temperature reaches about 10" C, simply because
the eel does not eat at lower temperatures (Otterstrsm, bc. cit.).
According to Matsui (1952, p. 242) the Japanese eel becomes inactive
and loses its appetite when the water temperature falle below 15O C,
and he advises that the feeding of stocks in eel-ponds should be stopped
at temperatures below 10" C. This is in good agreement with the fact
that twelve of the seventeen species of Anguilh are tropical and
subtropical in distribution, and all of them breed in tropical or subtropical seas,
ANTON F. BRUUN
selection must have been a t work, and probably still is, especially in
A . rostrutu. From the known distribution of the A . rostruta leptocephali it may be concluded that a considerable number fail to reach
the coasts, especially those found north of 40" N. lat. and east of 55" W.
long. But even some of these do reach Europe as mentioned above.
IV. How DOES A . unguilla REACH THE BREEDINU PUCE?
As already mentioned Tucker is of the opinion that A. anguillu
cannot reach the breeding places. He then asserts (Tucker, 1969a,
p. 497) : " The east-coast American migration, by contrast, presents
little difficulty. Along that coast the coastal water and immediately
adjacent slope water are moving in a south-westerly direction (Fig. 3).
Recent Discovery I1 work (Swallow, 1957) has confirmed the hypothesis of Stommel and proved the existence, 150 miles off South Carolina,
of a strong south-going current (up to 84 miles a day) deep under the
Gulf Stream. This system provides a simple, rapid and direct path,
and without demanding any differences of behaviour among the different
eastern groups of American eels. Temperature and salinity gradients
encountered on this route (Sverdrup et ul., 1942) are such that active
migration would reinforce passive transport. The fate of the Mississippi
eels need not concern us.
" We may here meet Bertin's demand (1956) for a migration theory
containing all the observed phenomena of physiology and behaviour."
We do know that the European eel shows positive reactions to
higher temperature, as Tucker (1959a, p. 497) states on the authority
of Bertin (1956). This is well known to all fishermen and biologists who
have studied the eel. The last silver eels are caught in Denmark when
the season with frost starts, but the great majority#are caught during
the dark nights of the interlunar periods of August, September and
October, and male silver eels may start migration as early a8 in the
interlunar periods from June onwards (Otterstr~m, 1948, p. 296). In
the spring the fishermen cannot use baited hooks for catching yellow
eels until the water temperature reaches about 10" C, simply because
the eel does not eat at lower temperatures (Otterstrsm, bc. cit.).
According to Matsui (1952, p. 242) the Japanese eel becomes inactive
and loses its appetite when the water temperature falle below 15O C,
and he advises that the feeding of stocks in eel-ponds should be stopped
at temperatures below 10" C. This is in good agreement with the fact
that twelve of the seventeen species of Anguilh are tropical and
subtropical in distribution, and all of them breed in tropical or subtropical seas,