水研センター研報，第32号，75−82，平成22年
Bull. Fish. Res. Agen. No. 32, 75‑82, 2010

Reduction ofbed by a grazer
off Oshika Peninsula: as an example of urchin barren innortheastern coast of Honsu, Japan.
Daisuke MURAOKA＊1

Abstract : The reduction of seaweed beds (Isoyake) has been known nearly all over thecountry in Japan. In northern Japan, reduction of kelp beds has significant consequences on
the local fishing community dependent on the kelp and herbivores such as sea urchins andabalone. Hearing to prefectural governments by Fisheris Agency has shown that the major
factor causing the reduction of kelp beds in northern Japan is sea urchin grazing. As an
example of a receding kelp bed in the cold temperate waters , I show the results of a local
ecological survey of
bed and the sea urchin () andabalone () on the rocky coastal area in Tomarihama of Oshika Peninsula, Miyagi Prefecture (northern Japan). This survey indicated that the high grazing pressureof sea urchins reduced the survival of juvenile
sporophytes near the lower limitof the bed, thus impeding the recruitment of juvenile in the area formerly occupied by adultsporophytes.
Key words : grazing,, sea urchins,degeneration,
kelp beds, monitoring

Introduction

hand,spp. (Kombu in Japanese) beds arevery important not only for food of the herbivores
Seaweed beds play an important role as one of thebut also for human consumption.

spp. isprimary producers in temperate coastal ecosystems;

used as one of the main ingredients for, thethey provide a place for marine animals to spawn

seafood based stock used extensively in Japaneseand grow, and supply food to herbivores such as

traditional food.

sea urchins and abalone (e.g. Dayton, 1985; Steneck

Loss or decline of kelp beds has been reported, 2002). In northern Japan, kelp (Laminariales)

on a worldwide scale. The main reasons haveincludingarebeen considered to be overgrazing by sea urchinsextensively distributed on rocky coastal areas.

,and(Lawrence, 1975; Elner and Vadas, 1990; Sivertsen,Abalone and sea urchins mainly graze kelp, so the2006) and by herbivorous fish (Harrisreduction of kelp beds has significant consequences

It was reported that on the coasts of Japan, 2,194hato the local fishing community dependent on theseof beds ofresources. In particular,

have disappeared in the 13 years after 1978 (Natureandbeds,

which account for approximately 20% of the kelp,and1984).
spp.

Conservation Bureau, 1994).

beds extending along the coastline of Japan (NatureFigure 1 indicates the factors reducing seaweed

Conservation Bureau, 1994), have been recognized

beds along the coast of Japan (Fisheries Agency,

as very important kelp beds for the coastal fisheries2007). This figure shows the major factor inbecause of their high productivity and their highsouthwest Japan is herbivorous fish grazing. On thefood value for sedentary herbivores. On the other

other hand, in northern Japan, the main factor is sea

2010年 月 日受理（Received on July 1. 2010）＊1
Tohoku National Fisheries Research Institute, 3-27-5 Shinhama, Shiogama, Miyagi 985-0001, Japan

Daisuke MURAOKA

Fig. 1. The factors causing reductions of seaweed beds in Japan (Fisheries Agency, 2007)

Fig. 2. The densities (A) and the changes (B) of sea urchins in Japan (Fisheries Agency, 2007)

urchins grazing.

andFigure 2 indicates the densities (A) and the recent

which feed on seaweeds as their main food sources,

trends of abundance (B) of sea urchins in the rocky

the degeneration ofcoastal areas of Japan. It is clear that the densities

consequences to both the ecosystem and the local

remain high (over 5 individuals / m 2 ) and arefishing community dependent on these resources.

increasing in Sanriku Coast, northern Japan.

In this paper, in order to explore the changes andRecently, gradual receding from the offshore sidehas been observed in somebeds has significant

their causes inbeds on the rocky coast

bedsin Tomarihama, the Oshika Peninsula in Miyagi

off the coast of southern Sanriku Coast (Taniguchi,Prefecture (northeastern part of Honshu, Japan) was1996). Because reduction of

selected as the monitoring site and an ecologicalkelp beds directly

affect the harvest of economically high-valued

survey was conducted on

inshore resources in this coast, particularly

herbivoresandand the major. Using

Reduction of

bed by a grazer

off Oshika Peninsula: as an example of urchin barren in northeastern coast of Honsu, Japan.

the data,, the relationship between the extent of thelong measuring rope (fixed line) was connected

coverage of thebetween 0m point and 150m point and used for eachbed and the occurrence

and density of the grazing species was examined

investigation. The following survey was conducted:(Muraoka, 2008).
Ecological Survey
Materials and Methods

In August 1999, July 2000, and February, Mayand July 2001, SCUBA diving surveys were

The rocky coastal area in Tomarihama on theconducted and the number and location of allOshika Peninsula, Miyagi Prefecture (northernjuveniles (< one year old withoutJapan) was set up as the monitoring site

(Fig. 3).

dichotomous branching) and adult (> one year old

At the start of the survey, thebed waswith dichotomous branching) sporophytes growing

distributed in the depth range of 2‑5m.

within 1 m width (i.e. 0.5m either side) of thebeds were also seen at a depth around 2m.

150m-long fixed line. Juvenile and adult sporophytes

From 5 to 10m in depth, crustose coralline algae

were distinguished using the method of Taniguchi

were dominated.

and Kato (1984). At the same time, arm length (theIn 1999, dan buoys were set at the starting point(0m; inside of thelength between top of stipe and base of frond) of allbed) and the terminal

individuals was measured, and the age waspoint (150m; inside the coralline flats) on the bottom

estimated using the method developed by Taniguchiof the sea (Fig. 3). From 1999 through 2001, a 150-m

and Kato (1984).

Fig. 3. The survey area and location of the fixed line at Tomarihama, Miyagi Prefecture.

Daisuke MURAOKA
Ecological Survey ofandrevealing that the major herbivore in the survey

In October of 1999, 2000 and 2001, allandarea was. The total number of, which were observed within thewas different from year to year, 448 individuals in

1m-width of the 150m-long fixed line, were collected

1999, 829 individuals in 2000 and 1400 individuals

by SCUBA diving. They were carried up from thein 2001. Moreover, in terms of the distribution on

bottom of the sea to the boat. On the boat, everyand

was recorded on its

location using the fixed line to pinpoint their exact
location and the test diameter was also recorded.
Furthermore, in the October 2001 survey, the total
weight ofandweremeasured at intervals of 10m on the fixed line.
After measurement, all of

andwas returned to the places where they wereoriginally taken from.
Results
Ecological Survey
Figure 4 shows the distribution of the density ofindividuals (juvenile and adult sporophytes)
at an interval of 10 m along the line for each survey,
and the lower limit ofadult sporophytes.

The densities of adultin the bed ranged

1.08 (July 2000) to 1.80 (February 2001) individuals/m2. At the start of the survey (August, 1999), thebed extended to 64m on the fixed line.
Although the lower limit of adult sporophytes
receded to 61m in July 2000, however the settlementof the juvenile sporophytes could be seen up to 80m.
However, the juvenile sporophytes that settled at60-80m had all disappeared by February of 2001; thelower limit of adult sporophytes was 53m in the May
2001 survey. In the July 2001 survey, the settlementof a large number of juvenile sporophytes (283individuals) was observed at 0-60m on the fixed line.
Yet, no settlement could be confirmed at 60-80m.
Also, only a few isolated macrophytes could beobserved in areas deeper than the lower limit ofbed, thus forming a coralline flat.
andEcological Survey

Fig. 5 shows the density distribution ofandin October of 1999, 2000

and 2001. The two species at the survey site,accounted for 91.1% (1999), 97.3% (2000) and
96.8% (2001) of the total number of individuals,

Fig. 4. Density ofsporophytes ( ■:adult, □：juvenile) recorded at an interval of 10m
within a 1m-width of the 150m-long fixed line (inTomarihama), ; the arrows denote the position oflower limit ofadult sporophytes

Reduction ofbed by a grazer

off Oshika Peninsula: as an example of urchin barren in northeastern coast of Honsu, Japan.

the fixed line, the distribution of the two species

dispersion in the number of individuals by survey

differed between years, with the boundary aroundyear, a large number of small individuals with <4cm

the 50-60m site, which was the lower limit of

test diameter were inhabited the coralline flat thatthespread in areas deeper thanbed. For this reason, whereas
were distributed at a relatively highfrequency on the shallow side (inside thebed), most ofbed (Fig.

6). In October 2001, a relatively larger number ofindividuals were observed within thewere found on the deeper

bed. Also, 86.7% of the individuals withinside (within the coralline flat; Fig. 5). For example,

thethe density ofor larger test diameter. On the other hand, in theinside thebed

2

bed were large individuals with 4cm

was 3.5 individuals /m in October 2001. On thecoralline flat, 66.9% ofother hand, the density inside the coralline flat wasthan 4cm in test diameter.

12.9 individuals /m2.
As a result of surveying the test diameter of, it was found that, although there wasFig. 5. Density of individuals of（ ■ ）and（□）
recorded at an interval of 10
m within a 1m-width of the 150m-long fixed line (inTomarihama)individuals were lessIn the 2001 weight measurement, the total weightof

corresponded to 95.6% of the total

weight of two herbivores (Fig. 7). For

theFig. 6. Density of individuals of( ■ : ≥4cmin diameter; □ : <4cm in diameter) recorded at aninterval of 10m within a 1m-width of the 150m-long
fixed line (in Tomarihama); the arrows denotethe positions of lower limit ofadultsporophytes in each year

Daisuke MURAOKA
for some other reason; i.e., the juvenile sporophytesthat had been generated did not survive to become
adult. Thus, it became clear that the cause of thereceding of the lower limit of thebed

in the monitored area was because the juvenile
sporophytes do not survive to become adult. Thisis supported by the fact that the growth of thejuvenile sporophytes at the 60-80m site,
observed in July of 2000, completely disappearedby February 2001 (Fig. 4).

the major

herbivore in the area occurred mainly from thelower limit of theFig. 7. Biomass of（ ■ ）and（□）
recorded at an interval of 10m within a
1m-width of the 150m-long fixed line in October 2001
(in Tomarihama)

bed to the coralline flat.

Kawamata (1998) suggested that the sweeping actionof kelp fronds in turbulent flow was an effective
mechanism to deter urchin invasion. It might be
one of the reasons why mostfound inside thewere not

bed but on the coralline

flat. The results of the weight measurement, atbiomass of

inside the E. bicyclis bed was

20.06kg /m , while the biomass on the coralline flat
260‑80m where the juvenile sporophytes ofhad disappeared, showed the maximum

was 0.40kg /m . From the weight measurement

biomass (Fig. 7). Based on these findings, it can besurvey ofconducted on the line in Octoberinferred that the receding of the lower limit of theof 2001, the biomass obtained from 60-70m site on

bed in the test sea area has occurred

the fixed rope was 7.2kg, the maximum value, whichin the following way: after thewas followed by 5.0kg, at the 70‑80m site (Fig. 7).

sporophytes growing near the lower limit of theadultbed individuals died of old age, disappeared
Discussion

during storms and/or were grazed, the settled
juvenile sporophytes ofSebens (1985) indicated that colonies of erect algae, which typicallywould mature as adult, were consumed by the highcould be changed into coralline flat by grazing ofgrazing pressure ofsea urchins. The survey in this study revealed thatsurvive to become adult sporophytes. Agatsuma

in the two years from August, 1999 through July,(1997) removed sea urchins from a coralline

2001, the lower limit of theand thus they did not

bed (adultflat on the Hokkaido coast of the Sea of Japan andsporophytes) had receded 11m towards the shoreline

confirmed that seaweeds would colonize there if(Fig. 4). There is also data that, as of 1981 in

the urchin grazing pressure was removed. Vega

Tomarihama, thebed exist from shore to(2005) reported that an abrupt change in the8m in depth (Taniguchi1987). 8m depth in thespatial-temporal patterns of abundance of the black

test area is situated at approximately 120m along thesea urchin

fixed line. If this is the case, the bed has retreatedbenthic grazer in northern Chile, produced local

approximately 67m in 20 years. The disappearance

extinctions and compression of bathymetricof kelp (includingdistribution of) beds has been shown

the most conspicuousandto occur because after the existing individuals die of(Laminariales, Phaeophyceae). On theold age, disappear during storms and/or are grazed,

seaward face of Port Erin (Isle of Man, U. K.)

the recruitment of the next generation of individuals

large algae on the breakwater do not occur ondoes not occur for some reason (Kawamata, 1998).

the lowest 3m. The density of sea urchin

The reason could be that there was no supply of
spores from the parent seaweed in the first place or

was high, but after removal of the sea
urchin, a kelpand otherReduction of

bed by a grazer

off Oshika Peninsula: as an example of urchin barren in northeastern coast of Honsu, Japan.

algae occurred in the lower area (Jones and Kain,ecology: The sea urchin phenomenon in the1967). In California, after the mass mortality due

northwestern Atlantic.

to disease of a localized population of a sea urchin, the formation of kelp

136, 108‑125.

Fisheries Agency, 2007: Isoyake recovery guideline.
Tokyo, 213 pp. (in Japanese)

forest was followed by the rapid seaward expansion

Harris L. G., Ebeling A. W., Laur D. R., and Rowleyof 4 species of brown algae (Pearse and Hines, 1979).

R. J., 1984: Community recovery after storm

These reports show that the grazing pressure of sea

damage: A case of facilitation in primaryurchins directly affects the colonization of kelp beds.

succession.

Most ofinhabiting on the coralline flats

were small individuals, while large individuals were
dominant within the224, 1336‑1338.

Jones N. S., and Kain J. M., 1967: Subtidal algalcolonization following the removal of15, 460‑466.

bed. This finding

is consistent with the behavioral pattern of

Kawamata S., 1998: Effect of wave-induced, which is believed to settle onto a coralline
flat area due to the effect of the settlement inducer
dibromomethane (Taniguchi

oscillatory flow on grazing by a subtidal sea
urchin(A. Agassiz).
224, 31‑48.

1994), and thenmove into kelp beds in order to find food to grow
(Agatsuma and Kawai, 1997; Sano, 1998).

Lawrence J. M. 1975: On the relationships between
marine plants and sea urchins.
13, 213‑286.

In this area, observations were further continued
after 2001, but the expansion ofbeds has

Muraoka D., 2008:

bed coverage

not been verified. Therefore, in order to understand

off Oshika Peninsula, Japan, in relation tothe mechanism ofsporophyte survival andbed fluctuations

more clearly, longer-term observations (includingoceanographic events) are necessary.

20, 845‑851.

abundance.

Nature Conservation Bureau, 1994: The report of themarine biotic environment survey on the 4th

Acknowledgments

national survey on the natural environment. Vol.
2. Algal and sea-grass beds. (in Japanese)

I would like to thank Dr. K. Saitoh, Dr. H. Takami,

Pearse J. S., and Hines A. H., 1979: Expansion of aMr. M. Fukuda, Dr. H. Tamaki and Ms. K. Inomata

central California kelp forest following the mass

for their support and assistance during the course ofmortality of sea urchins.

this study.

51,83‑91.
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