| Seaweed
Resource Science
| Resource
Description
Ascophyllum
nodosum (rockweed) is a brown seaweed that extends
from the Arctic Circle to New Jersey in North America
and in a wide range of wave exposures on stable substrate
(Baarsdeth 1970). Rockweed is replaced or mixed with
other related species (Fucus spp.) in the most
exposed or ice scoured areas (Sharp 1986). Rockweed
has become the most important commercial seaweed in
Canada and it is the dominant perennial seaweed in the
intertidal zone along the Atlantic coastline of the
Maritimes where it forms extensive beds.
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Ascophyllum
bed |
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Ascophyllum
plant
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Shoots
of this seaweed arise from a holdfast and develop
a complex structure of dichotomous and lateral branching.
The plant is dioecious, producing gametes from specialized
structures called receptacles. As the tide rises,
the plant is buoyed by means of gas bladders (vesicles)
on the shoots creating a floating canopy. The majority
of new shoots arise vegetatively from existing basal
holdfast tissues. As the plant grows, its holdfast
begins to coaless with holdfasts of adjacent plants
forming clumps.
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The
high density of branching shoots in a clump and the
distribution of clumps in a bed create a complex habitat
for invertebrates and fishes during the tide cycle.
This is a productive habitat; annual production of vegetative
biomass varies between 35% to 45% depending on wave
exposure (Causens 1984).
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Picture
of submerged bed |
Population
Dynamic
Since
1996, Acadian Seaplants Limited
(ASL) has maintained a long-term research and monitoring program
to study the population dynamic of the resource Ascophyllum
nodosum in Atlantic Canada. This program has optimized
and will continue improving the management strategies for
this resource in this region.
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Biomass
Distribution |
The
first step toward the management of any marine resource is
to know its biomass distribution. In Atlantic Canada, ASL
has determined this parameter for the resource Ascophyllum
nodosum by a combination of remote sensing and ground
truthing techniques.
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| Aerial
photo showing rockweed beds |
This
first step consists of a visual examination of the
shoreline using aerial photographs and the determination
of the Ascophyllum beds. A bed is defined
as a homogenous and continuous geographical unit containing
Ascophyllum. Usually the border of a bed
is defined by a geographical disruption (e.g. a sandy
beach). Then, a number is assigned to each individual
bed along the shoreline.
After
all the beds have been identified along the shoreline,
these aerial photos are scanned and transferred
into a computer were the surface of the rockweed
bed is measured using specific image analysis software.
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Ground
truthing |
In
the field, an assessment team visits each one of the
beds, previously identified in the aerial photos,
and transects are set across the beds. Along these
transects, several samples are taken to obtain information
on cover, plant density, plant length and plant biomass.
Additional information includes the ability to harvest,
substrate type, wave exposure, bed width, slope and
any other particular detail of the bed. Some of these
parameters (e.g. bed width and slope) are used to
calibrate area measurement from the computer.
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After
the ground truthing data are obtained and the computer measurement
of bed areas is completed, an integration of all these parameters
is made. A computer file is then created for each individual
bed with all its biological and physical information. Each
one of these beds is now a management unit. The company has
defined 2,430 management units in its licensed territory.
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Rate
of Growth & Production |
Rates
of growth and production are important population parameters
as they provide information on how the resource grows
and regenerates. This information is then used to determine
adequate exploitation rates.
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| Tagged
plants |
Within
its research and monitoring program, the company
maintains a growth study using tagged plants. These
tags allow us to follow the growth of individual
plants for several years through different ecological
gradients and in harvested and non-harvested beds.
By knowing the rate of growth, the rate of production
is then determined by weighing the biomass generated
in the plants each year.
The
rate of annual growth of harvestable plants (60
cm plus) ranges between 13 and 20 cm, depending
on the degree of water exposure. However, during
a year a plant generates several lateral branches,
each of them contributing to the annual biomass.
The annual production of the resource biomass
in our leases has been estimated to vary between
3.5 kg to 5.5 kg of wet material per square meter,
or between 35% to 55% of the total biomass. This
information is in agreement with previous production
studies in the region (Causens 1984).
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Tagged
harvested plant |
Experimental
studies carried out during the past three years have
shown that the harvest with our cutter rake increases
the growth rate of the plants. This is due to the
removal of old branches of the plant from the canopy,
thus allowing the light to be reached by suppressed
branches. In calm areas where these old branches are
not removed frequently from the canopy, the shaded
branches could be in growing-suppressed conditions
for 10 years or more.
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Harvest
Mortality |
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| Plants
with holdfasts |
The
harvest of Ascophyllum nodosum is a trimming
process where only a portion of the plants (the
longest branches forming the canopy) is harvested
by the hand-cutting rake. This tool is sharpened
on a daily basis to ensure a clean cut of the plants.
Despite these precautions, and for different reasons,
some plants are harvested with holdfasts causing
mortality (harvest mortality). Acadian Seaplants
has been scientifically monitoring the harvest mortality
since 1995 and a significant improvement in the
harvest mortality has been achieved. |
Harvest
mortality is produced mainly by poorly maintained tools
and by harvesting in areas with weak or loose substrate.
Consequently, the company has set a strict control policy
on the condition of the harvesting tool. A program consisting
of a weekly blade exchange has been established to ensure
the harvesters maintain the rake in optimal condition. In
addition, sensitive areas with loose substrate are closed
to the harvest. With all these policies in place, the average
incidence of holdfast in the harvest, in Nova Scotia for
example, has been reduced from 18% in 1995 to 3.9% in 2002.
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Storm
cast events increase holdfast incidents |
Considering
that only 50% of the total resource is harvested and
that the exploitation rate in that harvested proportion
varies between 17% to 25%, the potential mortality rate
of Ascophyllum population due to the harvest
would vary around 0.78% and 1.45%. This is a very small
proportion when compared to the effect of natural events
(such as storms), where annual mortality could reach
9%. |
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Reproductive
Period |
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| Plant
showing reproductive organs |
The
period of gamete release by Ascophyllum nodosum
in the Maritime shores is between mid-May and early-June.
During this period, the biomass allocated to the production
of reproductive organs could be as high as 30% of
the total plant weight. The harvest does not interfere
with the reproductive process as the former only starts
after the reproductive structures are gone.
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Recruitment |
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Rockweed
recruits |
Recruitment
of Ascophyllum nodosum by zygotes has been
characterized as a highly stochastic process and relatively
unsuccessful in maintaining the populations (Vadas 1986).
If this were the case, the Ascophyllum population
of Southern New Brunswick and Nova Scotia would have
evidenced some cumulative effects of such a high natural
mortality. On the contrary, there is no register of
large disruptions on the resource according to aerial
photo analyses conducted since 1978.
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| Breakwater |
An
ongoing study by Acadian Seaplants Limited
on the recruitment of Ascophyllum in the
Bay of Fundy shows that recruitment can occur each
year in this region if new stable substrate is provided.
Firm evidence of this is the total establishment of
rockweed plants on all breakwaters built along the
Bay of Fundy shore at different interval periods.
Therefore, the high resilience of rockweed to natural
mortality may indicate that sexual reproduction would
be playing a more important role in the recruitment
process than previously thought. |
Habitat Impact
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Rockweed
and fish in their natural habitat |
The
ecosystem aspect is critical for marine plants, especially
large fucoids and kelps, which have been recognized
as both a resource and a habitat (Foster and Barilotti,
1990; Santelices and Ojeda, 1984; Santelices, 1996;
Vásquez, 1989), consequently, these seaweeds
cannot be exploited under the concept of single species
resource sustainability. Ascophyllum has an
important role in the Fundy ecosystem as it provides
habitat for the prey of some waterfowl (Hamilton 1997).
Also, at least 22 species of fish (7 of commercial importance)
are known to be associated with Ascophyllum
in parts of their life cycle (Rangeley 1994, Rangeley
and Kramer 1995). Rarely in a fishery are provisions
made to protect the surrounding habitat and to control
ecological impact of the gear. Since there is no fishery
where all the necessary biological information is available
to develop a zero risk management plan; the recommendation
is to apply a precautionary approach. The goal of this
approach is to either make no significant changes in
habitat structure or to keep impacts short term and
within limits that could be mitigated. Through a joint
effort between Acadian Seaplants Limited
and provincial and federal governments, this goal has
been achieved. The degree, extent and duration of change
in the habitat structure is being controlled through
a conservative exploitation rate, minimum cutting height,
controlled incidence of holdfast removal, and using
area based management at a high level of resolution.
In addition, there are exclusion zones and protected
areas where no harvest is allowed to protect waterfowl
species (Ugarte and Sharp 2001). |
References
Baardseth
E. 1970. Synopsis of biological data on knobbed wrack Ascophyllum
nodosum. Fao Fisheries Synopsis, 38, Rev. 1:
41 pp.
Cousens
R. 1984. Estimation of annual production by the intertidal
brown alga Ascophyllum nodosum (L.) Le jolis. Botanica
Marina, 27: 217-227.
Foster
M.S. & Barilotti D.C. 1990. An approach to determining
the ecological effects of seaweed harvesting: a summary. Proceedings
of the International Seaweed Symposium, 13: 15-16
Hamilton
D.J. 1997. Community consequences of habitat use and predation
by common eiders in the intertidal zone of Passamaquoddy Bay.
Ph. D., University of Guelph. 211 pp.
Rangeley
R.W. 1994. Habitat selection in juvenile Pollock, Pollachious
virens: Behavioral Responses to Changing Habitat availability.
Ph.D., McGill University (Montreal). 179 pp.
Rangeley
R.W. & Kramer D.L. 1998. Density-dependent antipredator
tactics and habitat selection in juvenile pollock. Ecology
79(3): 943-952.
Santelices
B. 1996. Seaweed research and utilization in Chile: moving
into a new phase. Hydrobiologia, 326/327: 1-14.
Santelices
B. & Ojeda F.P. 1984. Effects of canopy removal on the
understory algal community structure of coastal forest of
Macrocystis pyrifera from Southern South America.
Marine Ecology Progress Series, 14: 165-173.
Sharp
G.J. 1986. Ascophyllum nodosum and its harvesting
in Eastern Canada. In: Case studies of seven commercial seaweed
resources. FAO Technical Report, 281:3-46.
Ugarte,
R. & G. Sharp. 2001. A new approach to seaweed management
in Eastern Canada:
The case of Ascophyllum nodosum. Cah. Biol. Mar.42:
63:70
Vásquez,
J.A. 1989. Estructura y organización de huirales de
Lessonia trabeculata. Ph. D. Thesis. Facultad de
Ciencias, Universidad de Chile: 261 pp.
Vadas
L.V., Wesley A. Wright, S.L. Miller. Recruitment of Ascophyllum
nodosum: wave action as source of mortality. Mar. Ecol.
Prog. Ser. 61: 263-272.
Research
To
view or download Abstracts or the entire published scientific
seaweed research paper in PDF format, please click here:
ACADIAN
SEAPLANTS LIMITED
30 Brown Avenue, Dartmouth,
Nova Scotia, Canada, B3B 1X8
Telephone:(902)468-2840 · Fax:(902)468-3474
E-mail:info@acadian.ca
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