282
MAKO‘PO OMORI
Bergin 1 u w v ~ has bcen rcvwecl suaoessfidly from the egg to the
fifth poxt-larvial s t q e under laboratory conditions (Omori, 1971a).
‘rhc larvae showed eonsiderahle tdertance to salinity changes, but the
mortality wits signifioant1.v inc*rewscd at temperatures above 25°C and
below 18°C. The growth rate of tho larval stages was 0.21 mm/day at
23°C and O.l(i mmlduy at 20°C. The result of the experiments suggostod that the critical period or t,he peak of physiological weakness
in A. lucens exists soon ttftw they begin to feed on phytoplankton in
the protozoeal stages mid t lliat retardation in the development and
growth due to lower temperirturcs IciLds to prolongation of this critical
period.
R e d c s ticirtjh from niitural abuses such ns rinfavourable temperature
w d Itack of food, that rcsulting from predation must be considerable
during 1 k r li~rval lifc: of A%Jrf/in l u r ~ n s i l l nature. n a t s obtained from
field smi plitigs t L n d from lil boriat,ory experiments suggest that the
mortalit,v in the post-lnrval sttkges cannot be so great as that in the
earliest stages. The death of post-lrrrvae during the autumn and
wint,or is rnorc prohrildy CiLrisetI 1 ) ) ~ lack of food and by predation, whiIe
the tLdiiltns ciic chiefly irs tlic rcsult, of thc fishing or from their poor state
of health :tftrer spawning. T)ctailrtl analysis of the mortality in each
phase of life is being purwod at proscnt. There is some tentative evidence to suggwt, that, the atnolint of food available in the autumn
causes fl ucturbtion in the niortdity of post-larvae and adolescents during
the severe winter month (Omori, unpublished). On the whole, however,
the greilteat factor affecting fluctuations in the abundance of X. lucens
appears to be t emperature and the environmental conditions caused
by tempcwtmv cliiring the breeding season. Omori et al. (1973) showed
that, thcre is itti tapparcnt close corrdation between the fluctuation of
catrhcs of 8. lzicens from different ycvnclasses and the water temperature ut around 50 m in the bay from June to August. There is also some
iiidictttioii to suggest that thc mortality of Acetes juponiczcs is greatly
influenctd by the temperaturc during the breeding season (Ikematsu,
1957).
From records of fishery statistics, Tanaka and Kawai (1967)
used the methods of population dynamics to attempt to estimate the
mortality of rS’mgh lucens. According to their study, 2-8 x lo9 individuals of Sergin produce 2-8 x 1012 eggs in the sea during a spawning
srttson; the number of recruits in the fishing ground is 40-200 x 109
individuals in November, 15-70 x lo9 in January, and 10-30 x log
in April respectively. The natural mortality coefficient is 0.324 per
month. I h w e some comment on their estimate of the population.
but it will be discussed elsewhere.
MAKO‘PO OMORI
Bergin 1 u w v ~ has bcen rcvwecl suaoessfidly from the egg to the
fifth poxt-larvial s t q e under laboratory conditions (Omori, 1971a).
‘rhc larvae showed eonsiderahle tdertance to salinity changes, but the
mortality wits signifioant1.v inc*rewscd at temperatures above 25°C and
below 18°C. The growth rate of tho larval stages was 0.21 mm/day at
23°C and O.l(i mmlduy at 20°C. The result of the experiments suggostod that the critical period or t,he peak of physiological weakness
in A. lucens exists soon ttftw they begin to feed on phytoplankton in
the protozoeal stages mid t lliat retardation in the development and
growth due to lower temperirturcs IciLds to prolongation of this critical
period.
R e d c s ticirtjh from niitural abuses such ns rinfavourable temperature
w d Itack of food, that rcsulting from predation must be considerable
during 1 k r li~rval lifc: of A%Jrf/in l u r ~ n s i l l nature. n a t s obtained from
field smi plitigs t L n d from lil boriat,ory experiments suggest that the
mortalit,v in the post-lnrval sttkges cannot be so great as that in the
earliest stages. The death of post-lrrrvae during the autumn and
wint,or is rnorc prohrildy CiLrisetI 1 ) ) ~ lack of food and by predation, whiIe
the tLdiiltns ciic chiefly irs tlic rcsult, of thc fishing or from their poor state
of health :tftrer spawning. T)ctailrtl analysis of the mortality in each
phase of life is being purwod at proscnt. There is some tentative evidence to suggwt, that, the atnolint of food available in the autumn
causes fl ucturbtion in the niortdity of post-larvae and adolescents during
the severe winter month (Omori, unpublished). On the whole, however,
the greilteat factor affecting fluctuations in the abundance of X. lucens
appears to be t emperature and the environmental conditions caused
by tempcwtmv cliiring the breeding season. Omori et al. (1973) showed
that, thcre is itti tapparcnt close corrdation between the fluctuation of
catrhcs of 8. lzicens from different ycvnclasses and the water temperature ut around 50 m in the bay from June to August. There is also some
iiidictttioii to suggest that thc mortality of Acetes juponiczcs is greatly
influenctd by the temperaturc during the breeding season (Ikematsu,
1957).
From records of fishery statistics, Tanaka and Kawai (1967)
used the methods of population dynamics to attempt to estimate the
mortality of rS’mgh lucens. According to their study, 2-8 x lo9 individuals of Sergin produce 2-8 x 1012 eggs in the sea during a spawning
srttson; the number of recruits in the fishing ground is 40-200 x 109
individuals in November, 15-70 x lo9 in January, and 10-30 x log
in April respectively. The natural mortality coefficient is 0.324 per
month. I h w e some comment on their estimate of the population.
but it will be discussed elsewhere.