Harvesting of Laminaria longicruris in Nova Scotia
Sizes of plants removed
Laminaria species exist in many of the cool and cold water parts of
the world but the major species which is commercially harvested is L. hyperborea.
The other large Laminaria which has been harvested is L. longicruris,
particularly on the Atlantic coast of Canada (Nova Scotia and Newfoundland). This species
can grow up to 12 m long but is commonly 3-5 m in length. It forms dense forests below
low-water mark, with the holdfasts always submerged, but with such long plants the fronds
can be present on the water's surface at low tide. In Nova Scotia it occurs in the depth
range 4-18 m (Mann, 1972a) often in association with L. digitata. In
south-west Nova Scotia during 1979, over 1,000 tonnes wet weight of L. longicruris
were harvested for the production of alginates (Pringle & Sharp, 1980) using dragnet
harvesting - although this is a reduction on a larger commercial harvest taken in the
1940s (Chapman, 1987). Dragnets harvest whole plants in the larger size ranges.
Dragnetting causes bottom perturbation particularly at the beginning of each tow, when the
net is in contact with the bottom. Large boulders may be displaced and rocks of up to 7 kg
in weight and still attached to holdfasts are removed from the habitat. The optimum
sustainable yield of harvest from the kelp population may remove more kelp plants than the
minimum cover required for the survival of associated fauna e.g. the lobster Homarus
americanus (Breen & Mann, 1976).
Effects of harvesting on sporophyte recruitment
In an un-harvested mixed population of L. longicruris and L.
digitata the mean population density of L. longicruris remained constant at
1.2 plants m-2 and the population density of L. digitata was
around 3.2 plants m-2 (Chapman, 1984). In this population L. longicruris
produced 9 x 109 and L. digitata produced 20 x 109
spores m-2 yr-l, of which about 9 x 106 and 1 x 106
sporelings m-2 yr-l respectively were recruited (Chapman, 1984).
From these millions of recruits only 1 sporeling m-2 yr-l of L.
longicruris and 2 m-2 yr-l of L. digitata grew to
visible size. Removal of macroscopic kelp plants had no effect on the recruitment of the
visible stages to the population (Chapman, 1984).
Effects of harvesting on sporophyte growth
After harvesting of L. longicruris and its understorey of L.
digitata, faster growth rates of the remaining sporophytes were detected than in
the un-harvested area (Smith, 1986). The standing crop had recovered to pre-harvest levels
after only one year (Smith, 1986) but it was considered unlikely that the kelp bed had
retained its former diversity and population structure. Following the harvest, L.
longicruris was initially more abundant than L. digitata but this
pattern altered over the next 3 to 5 years as the proportion of L. digitata
Effects of grazing on post-harvesting recovery
If kelp beds are destroyed or partially destroyed by harvesting, then
grazing sea urchins such as S. droebachiensis will not allow regeneration and
recruitment of the kelp population. Much experimental work has been undertaken in St.
Margaret's Bay, Nova Scotia. It is thought that the predators of the urchins such as
lobsters, crabs or fish use the kelp forest for cover and protection. A reduction in
predator pressure due to harvesting kelp or catching lobsters will permit urchin densities
to increase to the point where they form aggregations and graze destructively on Laminaria
forming barrens (Bernstein et al., 1981). These barrens become the new
community (Breen & Mann, 1976) with urchins able to survive by feeding on algal
sporelings and detritus (Warner, 1984; Chapman, 1987).
Effect of kelp harvesting on lobster catches
Mann (1977) used fisheries statistics to show that a slow decline in
lobster and crab stocks was caused by kelp depletion resulting from overgrazing. This did
not fit with normal population patterns. Miller (1985b) used historical reports to show
that these depletions of lobster and crab stocks have happened before in this century,
associated with kelp bed loss and urchin population increases, and suggested that these
slow fluctuations may be part of a long term cyclic succession. Miller (1985b) pointed out
that there was a periodic mass mortality of sea urchins in Nova Scotia which enabled the
slow regeneration of the Laminaria forest. This was confirmed by Novaczek &
McLachlan (1986) who showed that the pattern of recovery from barrens was dependent on the
substratum depth, since kelp initially re-colonised the shallows but deeper depths had not
recovered during the time of the study.
Where urchins were removed through artificial (Lawrence, 1975) or
natural means through urchin pathogens (Miller, 1985c), L. longicruris and L.
digitata quickly re-colonised the area. Miller (1985c) suggested that these
virulent, species-specific pathogens may provide an effective natural urchin control
within kelp beds, achieved by seeding with urchins infected in the laboratory. Similar
methods of germ warfare have been attempted with rabbits in terrestrial forest areas
(Myxymatosis) with mixed and uncontrollable results.
Some of the early ideas about the interactions between predator and
grazer populations have been questioned. The aggregation of urchins grazing along the edge
of the kelp beds has been shown to be purely a feeding response (Vadas et al.,
1986) although such aggregations were originally believed to be defensive reactions to
predators (Bernstein et al., 1981). Miller (1985a) also maintains that data on
stomach contents for lobsters, crabs and fish show that these populations cannot maintain
a control over sea urchin populations. Evidence points to the sea otter as being the
controlling factor in other temperate sea urchin species. This has certainly been shown to
be the case in Alaskan kelp forests (Dayton, 1975).