THE BREEDING OF THE NORTH ATLANTIC FRESHWATER-EELS
145
may find its preferred layer of illumination and migrate continuously
day and night.
Besides discussing his own results in comparison with all earlier
experiments, Maar pays special attention to the question whether the
silver eel is guided by some sense of direction, and his answer is convincingly in the positive ; the migration is essentially directed towards
the west as seen in the open Baltic, “ in der Eigentlichen Ostsee hat sie
im freien Wasser immer eine westliche Richtung gehabt, mit einer
gewissen Tendenz nach dem Suden ” (Maar, 1947, p. 33). Naturally the
migrating eel from the Baltic must at some point turn to the north
for some time to get out of Danish waters, but here salinity, temperature, currents and, perhaps, the smell of oceanic water may interfere
with the directional sense.
We would like to see many more experiments carried out on the silver
eel, either in aquaria or by marking experiments and other observations
in free waters. But with the evidence a t hand it is not too difficult to
see a pattern of the migration leading to the Sargasso Sea. A directional
sense towards the west with a deviation towards south ; a preference
for a small amount of dim light, which is indicated by the beheviour of
the migrating fish and by the change of visual pigment in the eye from
a mixture of rhodopsin and porphyropsin in the yellow eel to deep-sea
rhodopsinin the silver eels (Wald, 1958, p. 1487, with further references) ;
a preference for a temperature of about 15°C and the hydrostatic
pressure at 500 m ; these factors combined can be found only in the
Sargasso Sea, which in addition may attract the eel by its smell long
before it reaches there.
The olfactory sense of the apodal fishes is well known to be highly
developed, even to the extent that Gosline (1959, p. 161) asserts : “ As
to sensory systems, the eels have undoubtedly the largest olfactory
surface relative to body size of any fishes, and their sense of smell is
notorious.” By “ eels ” Gosline in this connection means apodal fishes
in general.
V. WHY HAVE No MIGRATING EELS BEEN CAVQIIT IN THE
STRAIT OF GIBRALTAR?
Tucker (1959a, p. 496) writes: “The expected passage of the vast
Rlediterranean populations through the sharp focus provided by the
Strait of Gibraltar has never been witnessed, despite much study ”;
Jones (1959, p. 1281) replies to this: “ It is no cause for astonishment
that eels are not caught in the Strait of Gibraltar, for no commercial
fishing gear in use there can be expected to catch eels.” Then Tucker
145
may find its preferred layer of illumination and migrate continuously
day and night.
Besides discussing his own results in comparison with all earlier
experiments, Maar pays special attention to the question whether the
silver eel is guided by some sense of direction, and his answer is convincingly in the positive ; the migration is essentially directed towards
the west as seen in the open Baltic, “ in der Eigentlichen Ostsee hat sie
im freien Wasser immer eine westliche Richtung gehabt, mit einer
gewissen Tendenz nach dem Suden ” (Maar, 1947, p. 33). Naturally the
migrating eel from the Baltic must at some point turn to the north
for some time to get out of Danish waters, but here salinity, temperature, currents and, perhaps, the smell of oceanic water may interfere
with the directional sense.
We would like to see many more experiments carried out on the silver
eel, either in aquaria or by marking experiments and other observations
in free waters. But with the evidence a t hand it is not too difficult to
see a pattern of the migration leading to the Sargasso Sea. A directional
sense towards the west with a deviation towards south ; a preference
for a small amount of dim light, which is indicated by the beheviour of
the migrating fish and by the change of visual pigment in the eye from
a mixture of rhodopsin and porphyropsin in the yellow eel to deep-sea
rhodopsinin the silver eels (Wald, 1958, p. 1487, with further references) ;
a preference for a temperature of about 15°C and the hydrostatic
pressure at 500 m ; these factors combined can be found only in the
Sargasso Sea, which in addition may attract the eel by its smell long
before it reaches there.
The olfactory sense of the apodal fishes is well known to be highly
developed, even to the extent that Gosline (1959, p. 161) asserts : “ As
to sensory systems, the eels have undoubtedly the largest olfactory
surface relative to body size of any fishes, and their sense of smell is
notorious.” By “ eels ” Gosline in this connection means apodal fishes
in general.
V. WHY HAVE No MIGRATING EELS BEEN CAVQIIT IN THE
STRAIT OF GIBRALTAR?
Tucker (1959a, p. 496) writes: “The expected passage of the vast
Rlediterranean populations through the sharp focus provided by the
Strait of Gibraltar has never been witnessed, despite much study ”;
Jones (1959, p. 1281) replies to this: “ It is no cause for astonishment
that eels are not caught in the Strait of Gibraltar, for no commercial
fishing gear in use there can be expected to catch eels.” Then Tucker