TliE I3IOLO(IY OP PELAGIC! SHRIMPS 1N THE OCEAN
265
considsrablc. Their swarms art’ most probably maintained to increase
availability for c.opulation.
An ext~nrordinarily lwge population of Sergia lucens is maintained
throughout the ycw in tho tiorthcrii tind wostorn parts of Suruga Bay.
The locality of the shoals vtwks dcprntling upon the stage of growth.
The snialler swarms in u shoal tcnd to bo compoRed of individuals of the
same size group. Observation indicated that eurlier post-larvae are
Licparatmd from adolescents and adults, and their swarms are usually
found offshore. The adolescents swarm more or less widely both in
shallow, innhorc arem and in rather offshore area. The swarms of
adults ure rnostly located on or around the 200 m isobath near the
mouths of rivers (Fig. It?). The shoal area on any one night is usually
very localized, at best 8 km long and 2 km wide, lying along the
cottst. The bottom slopes steeply from close to the shore. The flow
of water from the land causes a strong internal mixing of the salt
wedge, and the area contains rich nutrients which promoh the production of the food of 8. lucene. The bottom contour is complicated
and the sea-floor is covered with fine mud. Among shoaling areae,
the environmental conditions for 8. lucene must be best near the mouth
of the River Fuji. Although tho detailed relationship between environmental conditions and swarming behaviour remuins to be studied, the
largest shoal is uaiially observed in this area. When the population
den8ity rcachen a11 upper limit, in the shoal areas, the remaining animals
scatter in adjacent waters.
The Bwarms may have been more discrete than aa indicated by the
samples. The size and population density of the swarms of Sergia
Z~ucen!ns appear to become largest in early post-larval stages. The
density of adults in the swarms is grcatcst in early summer, that is,
just prior to or during the early spawning season. Similarly, in South
India the Itirgent gregarious swarms of Acetee erythraew Nobili and
A . ~erruldue johni Nataraj appear in the coastal waters during their
~pawniiig nctmon from January to April (Nataraj, 1947). Thew phenomena arc’ 110 doubt the same as the pre-Hpawning aggregations of
euphausiids which have been noticed by many workers for e long time
und tho o(*ourrcnces are probably related to physiological changes, such
88 copulation ~ n d
egg production, within the species themselves.
The amoiint, of light penetration in the sea also appears to affect
tho shoaling behaviour of the shrimps. In the surface layers very
noticmble swartns of Sergia lueena usually persist during the fht few
hours of darkness and at or around moonrise, moonset and just prior
to sunriw, suggesting that visual rues and illuminations stimuli are
iinportant .
265
considsrablc. Their swarms art’ most probably maintained to increase
availability for c.opulation.
An ext~nrordinarily lwge population of Sergia lucens is maintained
throughout the ycw in tho tiorthcrii tind wostorn parts of Suruga Bay.
The locality of the shoals vtwks dcprntling upon the stage of growth.
The snialler swarms in u shoal tcnd to bo compoRed of individuals of the
same size group. Observation indicated that eurlier post-larvae are
Licparatmd from adolescents and adults, and their swarms are usually
found offshore. The adolescents swarm more or less widely both in
shallow, innhorc arem and in rather offshore area. The swarms of
adults ure rnostly located on or around the 200 m isobath near the
mouths of rivers (Fig. It?). The shoal area on any one night is usually
very localized, at best 8 km long and 2 km wide, lying along the
cottst. The bottom slopes steeply from close to the shore. The flow
of water from the land causes a strong internal mixing of the salt
wedge, and the area contains rich nutrients which promoh the production of the food of 8. lucene. The bottom contour is complicated
and the sea-floor is covered with fine mud. Among shoaling areae,
the environmental conditions for 8. lucene must be best near the mouth
of the River Fuji. Although tho detailed relationship between environmental conditions and swarming behaviour remuins to be studied, the
largest shoal is uaiially observed in this area. When the population
den8ity rcachen a11 upper limit, in the shoal areas, the remaining animals
scatter in adjacent waters.
The Bwarms may have been more discrete than aa indicated by the
samples. The size and population density of the swarms of Sergia
Z~ucen!ns appear to become largest in early post-larval stages. The
density of adults in the swarms is grcatcst in early summer, that is,
just prior to or during the early spawning season. Similarly, in South
India the Itirgent gregarious swarms of Acetee erythraew Nobili and
A . ~erruldue johni Nataraj appear in the coastal waters during their
~pawniiig nctmon from January to April (Nataraj, 1947). Thew phenomena arc’ 110 doubt the same as the pre-Hpawning aggregations of
euphausiids which have been noticed by many workers for e long time
und tho o(*ourrcnces are probably related to physiological changes, such
88 copulation ~ n d
egg production, within the species themselves.
The amoiint, of light penetration in the sea also appears to affect
tho shoaling behaviour of the shrimps. In the surface layers very
noticmble swartns of Sergia lueena usually persist during the fht few
hours of darkness and at or around moonrise, moonset and just prior
to sunriw, suggesting that visual rues and illuminations stimuli are
iinportant .