144
ANTON F. BRUUN
countercurrent of the Gulf Stream, will have first to head for higher
temperature and salinity, then to dive to abyssal depths with a very
low temperature and salinity, and finally to rise again to the level about
500 m below the surface with water of a higher temperature and
.salinity.
I do not intend to discuss fully how the American and European eels
may find their breeding places, but will only refer to Hasler’s excellent
review (1956). After mentioning his experiments with the minnow
which can smell the difference between water from Georges Bank and
samples from the Sargasso Sea, he writes (p. 461): “ Salinity, gases, or
odors at any one placc at sea xeeni to me to be signals t h a t would be
more meaningful as appropriate (‘UCR for informing the fish it is, for
exaniplc, in a place to spawn, and not for directional orientation. To
help in reaching a distant goal, the cdge (sheer) effcct presumcs that the
fish might follow the border of two water masses, but it leaves unexplainrd what might direct it straigltt ahead or the reverse.” In the case
of the eel he says (p. 462): “ It niight be conceivably stimulated by
diffwcnccs in sheer between the Gulf Stream and the water masses on
each side. But more likely a chemical difference might also serve as
signals, in which case by swimming straight ahead, with the odor of
the Gulf Stream on the left, it could arrive in the vicinity of the
spawning grounds of the Sargasso Sea.”
I would also like to call attention to Waterman’s (1959) paper on
animal navigation in the sea. He reminds us (p. 253) of experiments
showing that aquatic animals can use the Sun and perhaps the Moon
for determining compass directions, as do many terrestrial animals,
and he also mentions the possibility of an addition to the direct effect
of the Sun’s image from the polarization of natural unr1c:rwrrtor illumittation; furthermore that laboratory studies yicldorl g r d cvid~rico tfrnt
the fish’s directional sense is regulated by an intcmial clock mcdrunintn
which compensates for the Fjun’H movcmcnt through thc: Hky an iii many
terrestrial forms.
Mtiiir (1947) has given a detailed analysk of marking experiments on
silver ecls, or as ho very propcrly calls thcm, Wandoraale, migrating
ecls. A total number of 882 were marked nnd no fewrr than 167 or
17.7%, were rrcaptitrcd. Miiiir docs not study the migration speed, but
n fow cctlculations of migrations longer than some 500 km support the
views of Jones (1959) and Deeldcr (1960) of a considerable speed, much
higher than thoso accepted by Tucker. Tucker’s low estimates may
have several reasons; only one may be mentioned here, that the
silver eel does not migrate between sunrise and sunset in shallow
water. In the open Baltic or in the Atlantic Ocean, howcver, the eel
ANTON F. BRUUN
countercurrent of the Gulf Stream, will have first to head for higher
temperature and salinity, then to dive to abyssal depths with a very
low temperature and salinity, and finally to rise again to the level about
500 m below the surface with water of a higher temperature and
.salinity.
I do not intend to discuss fully how the American and European eels
may find their breeding places, but will only refer to Hasler’s excellent
review (1956). After mentioning his experiments with the minnow
which can smell the difference between water from Georges Bank and
samples from the Sargasso Sea, he writes (p. 461): “ Salinity, gases, or
odors at any one placc at sea xeeni to me to be signals t h a t would be
more meaningful as appropriate (‘UCR for informing the fish it is, for
exaniplc, in a place to spawn, and not for directional orientation. To
help in reaching a distant goal, the cdge (sheer) effcct presumcs that the
fish might follow the border of two water masses, but it leaves unexplainrd what might direct it straigltt ahead or the reverse.” In the case
of the eel he says (p. 462): “ It niight be conceivably stimulated by
diffwcnccs in sheer between the Gulf Stream and the water masses on
each side. But more likely a chemical difference might also serve as
signals, in which case by swimming straight ahead, with the odor of
the Gulf Stream on the left, it could arrive in the vicinity of the
spawning grounds of the Sargasso Sea.”
I would also like to call attention to Waterman’s (1959) paper on
animal navigation in the sea. He reminds us (p. 253) of experiments
showing that aquatic animals can use the Sun and perhaps the Moon
for determining compass directions, as do many terrestrial animals,
and he also mentions the possibility of an addition to the direct effect
of the Sun’s image from the polarization of natural unr1c:rwrrtor illumittation; furthermore that laboratory studies yicldorl g r d cvid~rico tfrnt
the fish’s directional sense is regulated by an intcmial clock mcdrunintn
which compensates for the Fjun’H movcmcnt through thc: Hky an iii many
terrestrial forms.
Mtiiir (1947) has given a detailed analysk of marking experiments on
silver ecls, or as ho very propcrly calls thcm, Wandoraale, migrating
ecls. A total number of 882 were marked nnd no fewrr than 167 or
17.7%, were rrcaptitrcd. Miiiir docs not study the migration speed, but
n fow cctlculations of migrations longer than some 500 km support the
views of Jones (1959) and Deeldcr (1960) of a considerable speed, much
higher than thoso accepted by Tucker. Tucker’s low estimates may
have several reasons; only one may be mentioned here, that the
silver eel does not migrate between sunrise and sunset in shallow
water. In the open Baltic or in the Atlantic Ocean, howcver, the eel