Canadian Atlantic Fisheries
Scientific Advisory Committee
CAFSAC Advisory Document 86/23
Advice on the Harvesting of Southwest Nova Scotian Kelp Resources
At its meeting of April 29, 1986, CAFSAC considered a strategy for harvesting kelp (Laminaria
sop.) off southwestern Nova Scotia.
Harvesting of kelp was active between 1942 and 1949 in southwestern Nova Scotia. Landings
ranged from 1,000 to 3,000 t annually but sporadic harvests since 1949 did not exceed 500 t. In 1985,
industry proposed to harvest 10,000-15,000 t of kelp by 1988.
Resource
The kelp resource is estimated at 150,000-200,000 t composed of L. longicruris and L. digitata
in a 1.5:1.0 ratio. Maximum kelp biomass (20-60 t/ha) occurs between 0-5 m depth on boulder
substrate. The population numbers are dominated by plants below 1 m in length but most of the
biomass (60-97%) is contributed by plants over 1 m.
Plant growth occurs primarily at the base of the blade and older tissue moves along the blade as
the tip is eroded. Maximum linear growth (1.8 cm/d) occurs during summer months in both species. In
sites exposed to wave action there is no net growth of either species from October to April. Natural
mortality of L. longicruris over 50 cm is 95-100% per annum versus 40-50% for L. digitata.
Recruitment which results from the settlement of widely dispersed spores is continuous but peaks in
the fall months.
The annual productivity of these species is reported to be near the highest in the plant
kingdom. Production estimates are based upon the rates of natural mortality of whole plants during
the year plus the production rate of blade tissue. Production/biomass ratios i.e. the 'turnover' of
plant tissue in southwestern Nova Scotia were calculated at 3.2 for L. longicruris and 1.3 for
L. digitata.
Harvest
The method proposed to harvest the kelp is to use a dragrake similar in design to that used in
the 1942-49 harvest. The dragrake selectively removes the larger (71m) L. longicruris plants.
The rake harvests the entire plants by breaking the stipe, tearing the holdfast free from the
substrate, or tearing the holdfast away with substrate attached. The dragrake itself disrupts the
substrate for the initial 3-4 m of the tow but thereafter only when the holdfast is torn free with
substrate attached.
Harvesting or artificial denudation in 0-4 m water depth stimulates a recruitment pulse.
However, the denudation of kelp species in low density (less than 2 plants per square meter)
populations in water depths greater than 10 m did not enhance recruitment. In this deep water
habitat, the recruitment of microscopic L. longicruris stages was directly correlated (r = .98) with
spore production. No similar correlation was shown for L. digitata. Although the data suggest that
the removal of mature plants may adversely affect recruitment, spore dispersion from residual biomass
and adjacent unharvested biomass can provide recruitment at least 6 km from the source. Areas that
have been grazed heavily by sea urchins so that only small relic or refuge populations have remained
have been successfully repopulated within 2 years of release from grazing pressure. Harvested or
artificially denuded kelp stands recover original biomass, population structure and reproductive
capacity within 2 years.
2
Management strategies
a)
Season
Gear conflict between fixed lobster gear and mobile dragrake operations will restrict kelp
harvesting to June-November. The summer-fall harvest will begin at peak biomass.
b)
Reproduction
To protect the reproductive potential of kelp beds, a harvest should not extend beyond
.5 km from a spore source and a buffer zone of mature populations equivalent to the harvested
area should remain abutting the harvest zone.
c)
Reharvest
An 18-month fallow period is suggested between harvests (scenario 1 and 3b in Figure 1) to
allow recovery of population structure (e.g. proportion of mature plants, size of plants and
species composition) provided the buffer zones are not harvested. Alternatively, all zones,
(scenario 2 and 3a) may be harvested on a 4 year cycle with 2 year intervals between harvesting
of adjacent areas. In order that harvesting can be carried out annually, this latter
alternative would require parts of the overall harvest area to be designated for an even year
harvest cycle and the remainder to be on an odd year harvest cycle.
d)
Level of harvest
No specific limits on annual harvest are suggested. A standing crop of 150,000-200,000 t
and a turnover of 2-3 times annually will not be affected substantially by a 10,000-15,000 t
harvest.
e)
Multispecies
There is much discussion of kelp-lobster interactions in the literature, but there are no
conclusive studies or arguments linking lobster abundance or productivity with kelp cover.
However, the effects of the harvesting process on lobster catchability and abundance were not
examined.
3
A)
z\—■
0 4 C5 -
•
eb.
„.
ore51: ■;!,?
• • /////////
//
V
/////////
■. I/////////7
,/////////).
r// /// ///./.1-"
HARVEST 1986,1988
HARVEST 1986,1990
HARVEST 1987,1989
.5 km
HARVEST 1988,1992
NO HARVEST
FORESHORE
.5 km
Scenario 1. An extensive subtidal
shoal.
-..._.-- LIMIT OF LAMINARIA
Scenario 3. A foreshore bed greater
than 1 km long and less
than 0.5 km wide.
A) All areas of the bed
harvested, but a 4-yr
rotation is required to
maintain adjacent spore
sources.
0
d0- 0 -,:-.) -cr:=7,
-.,
0 .c7-4D-o.
- 0 'P•a.°
■ •"'.o.
. .' °••
0. • ..;(;)•
.0
' •--C'
- 0 ,—0
•---) ...
cD'c
0. 0a •o.w ., -°•
-
/////////•
....
.'
4
1k4
.
////////....
•
/////////'
/////,
SPORE SOURCE, NO HARVEST
.5km
*Ai
'Ato!t:
-
• ••
////////:
/ // / //// /
/// //// /7., •
///////..
/
/
:-
/7.
//
///
HARVEST 1986
'
-
P
0. 0 0
..
//
///
cfq
*0
0
.
, 1990
.5 km
77-17
/7/.
/7/.
/7/.
HARVEST 1986,1988
FORESHORE
HARVEST 1988 , 1992
LIMIT OF LAMINARIA
FORESHORE
Scenario 2. Small kelp beds not
being 0.5 km in any
dimension but in close
proximity.
Scenario 3. B) One portion of the bed
left unharvested permanently.
Figure 1. Management scenarios for kelp harvesting in southwestern Nova Scotia within
the restrictions: 1) maximum distance of harvest 0.5 km from a spore source
with a mature biomass equivalent to the harvested biomass; and 2) 2-yr
fallow period between harvests.