106
J. A. C. NICOL
ground, and colour is more important than brightness ; net colour
becomes less important at twilight (Kanda and Koike, 1958a, b ; Kanda
et al., 1958a, b ; Breder, 1959). Fishermen have observed that pilcha&
avoid nets in " phosphorescent " waters at night; they call this
phenomenon " brining ". The luminescence seemingly reveals the
presence of the nets to the fish (Wilcocks, 1883).
Vision is the main factor governing the formation and maintenanm
of fish shoals. In general, as shown by numerous laboratory experiments
and by observations in nature, shoals break up when the illumination
falls below a certain level (Imamura, 1953 ; Breder, 1959). In aquaria,
the schooling of mackerel ceases in darkness and after blinding
(Schlaifer, 1942). The threshold level for schooling in the Pacific sardine
(Sardinops caeruleu) is well below 0.01 ft c (Loukashkin and Grant,
1959) ; in Menidia, Hepsitia, etc. schooling ceases at 0.05 ft c (Shaw,
1961). A check was kept on cod shoals in deep water (100 m) of the
Barents Sea by means of echo sounder and it was observed that the
shoals dispersed at sunset and reformed at sunrise (Ellis, 1956).
Much work has been done on the photo-responses of young salmon,
especially in relation to their migratory movements. Juvenile salmon
move downstream to salt water either as fry or smolts, and this
movement is largely nocturnal. Various species of Pacific salmon
exhibit considerable differences in their photo-responses, and Hoar
(1958) has traced an evolutionary sequence among them on the bllsis
of behaviour to light and other factors. Katadromous progress is
associated with nocturnal activity : coho fry are active by day and by
night, and their downstream progression at night is probably the
r e d t of moro cliHpluactmont with than again& tho current. Pink
salmon, cxemplifying tho other oxtrorne, bocomo inteneivsly aotive at
night and rise to the surface ; on losing visual contact with the bottom,
they swim rapidly with the current, and their exit from the river takm
place rapidly. Coho, on transformation to smolt, show increased
nocturnal activity and strong concealing behaviour (Hoar, 1953 ; Hoar
et al., 1957; McDonald, 1960). Hoar (1958) has pointed out that tho
photo-reactions of young salmon depend both upon absolute intensitiea
and on rates of intensity-change; the onset of nocturnal migration
coincides with rapid decline of light-intensity. The movements of
juvenile rainbow trout in and out of a freshwater lake, and of brook
trout to and from salt water similarly aro controlled by fall and rim of
light-intensity (Smith and Saunders, 1958 ; McDonald, 1900 ; Northcote, 1962). Migration of silver eels downstream is also largely nocturnal (Deelder, 1954).
Correlating field-observcttion wit h retinomotor changes in juvenile
J. A. C. NICOL
ground, and colour is more important than brightness ; net colour
becomes less important at twilight (Kanda and Koike, 1958a, b ; Kanda
et al., 1958a, b ; Breder, 1959). Fishermen have observed that pilcha&
avoid nets in " phosphorescent " waters at night; they call this
phenomenon " brining ". The luminescence seemingly reveals the
presence of the nets to the fish (Wilcocks, 1883).
Vision is the main factor governing the formation and maintenanm
of fish shoals. In general, as shown by numerous laboratory experiments
and by observations in nature, shoals break up when the illumination
falls below a certain level (Imamura, 1953 ; Breder, 1959). In aquaria,
the schooling of mackerel ceases in darkness and after blinding
(Schlaifer, 1942). The threshold level for schooling in the Pacific sardine
(Sardinops caeruleu) is well below 0.01 ft c (Loukashkin and Grant,
1959) ; in Menidia, Hepsitia, etc. schooling ceases at 0.05 ft c (Shaw,
1961). A check was kept on cod shoals in deep water (100 m) of the
Barents Sea by means of echo sounder and it was observed that the
shoals dispersed at sunset and reformed at sunrise (Ellis, 1956).
Much work has been done on the photo-responses of young salmon,
especially in relation to their migratory movements. Juvenile salmon
move downstream to salt water either as fry or smolts, and this
movement is largely nocturnal. Various species of Pacific salmon
exhibit considerable differences in their photo-responses, and Hoar
(1958) has traced an evolutionary sequence among them on the bllsis
of behaviour to light and other factors. Katadromous progress is
associated with nocturnal activity : coho fry are active by day and by
night, and their downstream progression at night is probably the
r e d t of moro cliHpluactmont with than again& tho current. Pink
salmon, cxemplifying tho other oxtrorne, bocomo inteneivsly aotive at
night and rise to the surface ; on losing visual contact with the bottom,
they swim rapidly with the current, and their exit from the river takm
place rapidly. Coho, on transformation to smolt, show increased
nocturnal activity and strong concealing behaviour (Hoar, 1953 ; Hoar
et al., 1957; McDonald, 1960). Hoar (1958) has pointed out that tho
photo-reactions of young salmon depend both upon absolute intensitiea
and on rates of intensity-change; the onset of nocturnal migration
coincides with rapid decline of light-intensity. The movements of
juvenile rainbow trout in and out of a freshwater lake, and of brook
trout to and from salt water similarly aro controlled by fall and rim of
light-intensity (Smith and Saunders, 1958 ; McDonald, 1900 ; Northcote, 1962). Migration of silver eels downstream is also largely nocturnal (Deelder, 1954).
Correlating field-observcttion wit h retinomotor changes in juvenile