19
bags before being transported to the boat. The shell
length (SL) of abalone was measured to the nearest
1 mm with callipers on the boat, and measured animals were released to their original habitats.
The recruitment process during the juvenile
stage of 2008–2010 year classes of abalone was
also investigated with the method of Takami et al.
( 2013 ) near station A which is located in CCA.
Since the distribution of juvenile abalone was too
cryptic and patchy to estimate their density by the
quadrat sampling, the abundance of juveniles’
shell length inferior to 40 mm SL was regularly
monitored by intensive visual searching from
December 2009 to June 2011. In every survey,
the same diver searched for juvenile abalone in
every crevice and overhangs by sometimes overturning boulders and removing large epibiota ,
recording each searching time. Juvenile abalone
were carefully removed from substrata and transferred to a sampling bottle with fi ne tweezers.
The relative abundance of juvenile abalone was
expressed as catch per unit effort (CPUE),
accounting for the number of collected juveniles
per searching time in hours. The shell length of
collected juveniles was measured to the nearest
0.1 mm using a video camera system with an
image analyser connected to a dissecting microscope in the laboratory.
2.3
Data Analysis
Changing in abalone densities were tested by a twoway analysis of variance (ANOVA) using date of
sampling and type of algal community as fi xed
factors. Data were transformed in [log( n + 1)] in
the case of heterogeneous variation.
3
Results and Discussion
After the tsunami, the underwater visibility at the
study site was much lower than that before the
earthquake and tsunami due to sediment suspension. A remarkable increase in the amount of fi ne
sediments was observed under boulders and in
crevices of bedrocks. In stations D, E, and G,
although the remaining holdfasts of E. bicyclis
that lost their fronds were sometimes observed,
the biomass of this alga appeared to be undamaged by the event. More severe disturbance was
obvious in station A which was located in
CCA. Many of the large rocks were cracked and
turned over on the seafl oor and in consequence
bare rocks, which were not covered with any epibiota including crustose coralline algae, were
exposed in many places.
Abalone density obtained by the quantitative
quadrat survey varied signifi cantly among stations and dates of sampling, but there was no signifi cant date by station interaction for the abalone
density by the two-way ANOVA (Table 2 ).
After the tsunami, station A located in CCA
indicated the lowest mean densities. In the other
stations located in KB, obvious changes in the
abalone density were not observed (Fig. 2 ).
There are no clear trends in the size-frequency
distributions of abalone before and after the tsunami. The abalone smaller than 30 mm in shell
length was not sampled by quadrat possibly
because of the relatively low density and patchy
distribution of juveniles (Fig. 3 ). Juveniles were
only detected by intensive visual searching during the recruitment process monitoring which
was conducted near station A.
Table 1 Location and environmental features for each
sampling station
Station Latitude
(N)
Longitude
(E)
Depth
(m)
Dominated
algae
A
38°49.673′ 141°35.995′ 4–5
Crustose
coralline
algae
D
38°49.622′ 141°36.258 4–5
Kelp
E
38°49.573′ 141°36.218′ 4–5
Kelp
G
38°49.717′ 141°36.275′ 3–4
Kelp
Table 2 Two-way ANOVA for densities of abalone
Haliotis discus hannai between sampling dates and habitats at Iwaisaki, Japan
df
MS
F
P
Sampling date
1
0.718
8.562
0.006
Habitats
3
2.806
11.152
0.000
Interaction
3
0.380
1.511
0.231
Residual
32
2.684
Impacts of the 2011 Mega-earthquake and Tsunami on Ezo Abalone Haliotis discus…
bags before being transported to the boat. The shell
length (SL) of abalone was measured to the nearest
1 mm with callipers on the boat, and measured animals were released to their original habitats.
The recruitment process during the juvenile
stage of 2008–2010 year classes of abalone was
also investigated with the method of Takami et al.
( 2013 ) near station A which is located in CCA.
Since the distribution of juvenile abalone was too
cryptic and patchy to estimate their density by the
quadrat sampling, the abundance of juveniles’
shell length inferior to 40 mm SL was regularly
monitored by intensive visual searching from
December 2009 to June 2011. In every survey,
the same diver searched for juvenile abalone in
every crevice and overhangs by sometimes overturning boulders and removing large epibiota ,
recording each searching time. Juvenile abalone
were carefully removed from substrata and transferred to a sampling bottle with fi ne tweezers.
The relative abundance of juvenile abalone was
expressed as catch per unit effort (CPUE),
accounting for the number of collected juveniles
per searching time in hours. The shell length of
collected juveniles was measured to the nearest
0.1 mm using a video camera system with an
image analyser connected to a dissecting microscope in the laboratory.
2.3
Data Analysis
Changing in abalone densities were tested by a twoway analysis of variance (ANOVA) using date of
sampling and type of algal community as fi xed
factors. Data were transformed in [log( n + 1)] in
the case of heterogeneous variation.
3
Results and Discussion
After the tsunami, the underwater visibility at the
study site was much lower than that before the
earthquake and tsunami due to sediment suspension. A remarkable increase in the amount of fi ne
sediments was observed under boulders and in
crevices of bedrocks. In stations D, E, and G,
although the remaining holdfasts of E. bicyclis
that lost their fronds were sometimes observed,
the biomass of this alga appeared to be undamaged by the event. More severe disturbance was
obvious in station A which was located in
CCA. Many of the large rocks were cracked and
turned over on the seafl oor and in consequence
bare rocks, which were not covered with any epibiota including crustose coralline algae, were
exposed in many places.
Abalone density obtained by the quantitative
quadrat survey varied signifi cantly among stations and dates of sampling, but there was no signifi cant date by station interaction for the abalone
density by the two-way ANOVA (Table 2 ).
After the tsunami, station A located in CCA
indicated the lowest mean densities. In the other
stations located in KB, obvious changes in the
abalone density were not observed (Fig. 2 ).
There are no clear trends in the size-frequency
distributions of abalone before and after the tsunami. The abalone smaller than 30 mm in shell
length was not sampled by quadrat possibly
because of the relatively low density and patchy
distribution of juveniles (Fig. 3 ). Juveniles were
only detected by intensive visual searching during the recruitment process monitoring which
was conducted near station A.
Table 1 Location and environmental features for each
sampling station
Station Latitude
(N)
Longitude
(E)
Depth
(m)
Dominated
algae
A
38°49.673′ 141°35.995′ 4–5
Crustose
coralline
algae
D
38°49.622′ 141°36.258 4–5
Kelp
E
38°49.573′ 141°36.218′ 4–5
Kelp
G
38°49.717′ 141°36.275′ 3–4
Kelp
Table 2 Two-way ANOVA for densities of abalone
Haliotis discus hannai between sampling dates and habitats at Iwaisaki, Japan
df
MS
F
P
Sampling date
1
0.718
8.562
0.006
Habitats
3
2.806
11.152
0.000
Interaction
3
0.380
1.511
0.231
Residual
32
2.684
Impacts of the 2011 Mega-earthquake and Tsunami on Ezo Abalone Haliotis discus…