A biological study of Durvillaea antarctica (Chamisso) Hariot and D ...

157
7.5 MORTJ.l.LITY
(a)
Mortality of D. antaPotica:
Mortali ty refers to the loss of pli3.nts from o. marked r;Clluple.
Mortality bei...--ween censuses was calculated by expressing >che number of
plants lost over the interim as a percentage of the total nlmilier of
plants pre.sent on the first of the bl0 censuse s, At each census,
marked plants were segregated into several size classe~. Mortality
in each was measured between censuses. The size distribution of
the marked samples was kept fairly constant throughout the course
of the study by periodically tagging additional small specimens.
Mor~ality
at Tautwcu:
Mortality of D. antarctica at Tautuku over 26 months between
October 1972 and December 1974 was 48%. As shown in Fig. 7.15
smallest and largest specimens had the highest mortalities; 64%
(n==B7) of the small plants, and 71% (n=24) of the largest plants t·/ere
lost. These mortalities were disproportionately higher than the
fraction that these plants formed in the sample. The smallest and
largest size classes comprised respectively, 31% and B% of the sample,
yet they formed 43t and 15% of total mortality. In general, the
life expectancy of D. antanatiaa plants at Tautuku increased with
size until they reached an ovelCall length of approximately 3 m and a sti
diameter of 25-30 rom I but decreased thereafter. ~1e mortality
curve was roughly "U" shaped (Fig. 7.16)0
M.ortali ty of D. antarctioa at Tau tuku. varied with season (Pig.
Mortality was highest in late winter and early spring
(July-September). and lowest during the summer. Presumably this W,lS
mainly attributable to seasonality in wave force, that: '\'las much
stronger during winter than in the summer (Fig.2.S).
Mortality Cl't Ohau Point:
of the 343 plants tagged at Oahu Point by September 1972,
189 (55%) were missing after 26 months (November 1974). MOl' .-tality
at Ohau Point was thus s higher ·than at 'l'autuku. In most.
instances (as ",as also the case at Taut.wm) plants were lost when
their holdfasts ... Jere c1eta.ched. Only about 10% IIlere lost by their
stipes splitting or tearing out of holdfastso Vandals cnt 4."1 %