Seaweed Resource Science

Globally Recognized Resource Science

Resource Description

Ascophyllum nodosum (Rockweed) is a brown seaweed that grows from the Arctic Circle to New York State in North America and in a wide range of wave exposures on stable substrata (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 Maritime Provinces where it forms extensive beds.

Shoots of this seaweed arise from a holdfast and develop a complex structure of dichotomous and lateral branching. The plant is hermaphrodite (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.

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 (Cousens 1984).

Population Dynamic

Since 1986, Acadian Seaplants Limited has maintained a long-term research and monitoring program to study the population dynamics of the Ascophyllum nodosum resource in Atlantic Canada and most recently in Maine, USA. This program has optimized the management strategies for this resource in these regions and will continue to seek ways to improve it further.

Biomass Distribution

The first step toward the proper management of any marine resource is to know its biomass distribution. In Atlantic Canada, Acadian Seaplants has determined this parameter for the Ascophyllum resource by a combination of remote sensing and ground truthing techniques.

The initial action 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). Next, a number is assigned to each individual bed along the shoreline.

After all the beds have been identified along the shoreline, the aerial photos are scanned and transferred into a computer where the surface of the rockweed bed is measured using specific image analysis software.

After aerial photography comes the process of ground truthing. An assessment team physically visits each of the beds, previously identified in the aerial photographs, and transects are set across the beds. Along these transects, several samples are taken to obtain information on cover, plant density, length and biomass. Additional information includes the ability to harvest, type of substrata, 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 measurements from the computer.

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 created for each individual bed with all of its biological and physical information. Each one of these beds is now a management unit. Acadian Seaplants has defined 3,566 management units in its licensed territory.

Rate of Growth and 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.

Within our research and monitoring program, we maintain 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 typically 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 - 55% of the total biomass. This information is consistent with previous production studies in the region (Atlantic Canadian and Maine), (Cousens 1984; Vadas et al. 2004).

Experimental studies carried out during the past 15 years have shown that the harvest with our specially-designed cutter rake increases the growth rate of the plants (Ugarte et al. 2006). This is due to the removal of old branches of the plant from the canopy, thus allowing light to reach the suppressed branches. In calm areas where these old branches are not removed frequently from the canopy, the shaded branches could be in growth-suppressed conditions for 10 or more years.

Harvest Mortality

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 tool. The unique rake 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 carefully monitoring harvest mortality since 1995 and a significant improvement has been achieved.

Harvest mortality is produced mainly by poorly-maintained tools and by harvesting in areas with weak or loose substrata. Consequently, the company set a strict control policy on the condition of the harvesting tool. A program consisting of a weekly blade exchange was established to ensure the harvesters maintain their rakes in optimal condition. In addition, sensitive areas with loose substrata 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 2.6% in 2009.

In 2004, a study was carried out to evaluate the real impact of this detachment on the A. nodosum population of southern New Brunswick (Ugarte 2010). The structure of harvested A. nodosum clumps with associated holdfast material was analyzed and compared to non-harvested clumps from the same harvest area. Results showed that when a rake strips a clump, it only detaches 17.4% of the holdfast surface, leaving 36.8% of the plant biomass and 80.3% of the shoot density intact. An analysis of storm-cast material from the same study area showed a similar effect in the clump structure, although the incidence of holdfast in the detached biomass could be as high as 30%. Due to the high biomass detached each year by coastal storms in New Brunswick, their impact on the A. nodosum resource is 21 times higher than the annual harvest.

When one considers that only 50% of the total seaweed resource is harvestable and that the conservative exploitation rate within that harvestable portion varies between 17% and 25%, the potential mortality rate of Ascophyllum population due to the harvest would vary around 0.78% and 1.45%, respectively. This is a very small proportion when compared to the effect of natural events (such as storms), where annual mortality can reach 9%.

Reproductive Period

The period of gamete release by Ascophyllum nodosum in the Maritime region 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.



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 Nova Scotia and New Brunswick 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.

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 substrata are available. Firm evidence of this is the total establishment of rockweed plants on all breakwater 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

The ecosystem 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, 1996; Santelices and Ojeda 1984; Vasquez, 1989), consequently, these seaweeds cannot be exploited under the concept of single species resource sustainability. Ascophyllum plays an important role in the Bay of Fundy ecosystem as it provides habitat for the prey of some waterfowl (Hamilton 1997). Also, at least 22 species of fish (seven of commercial significance) are known to be associated with Ascophyllum in parts of their life cycle (Rangeley 1994, Rangeley and Kramer 1998). Rarely in a fishery are provisions made to protect the surrounding habitat and to control ecological impact of the gear. Acadian Seaplants is proud that it has developed such an approach to the Rockweed Fishery.

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 for this approach is to either make no significant changes in habitat structure or to keep impact short term and within limits that could be mitigated. Through a joint effort between Acadian Seaplants Limited and provincial, federal and state government, this goal has been achieved. The degree, extent and duration of change in the habitat architecture 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 harvesting is permitted to protect waterfowl species (Ugarte and Sharp 2001).


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