Implications of population dynamics and interspecific competition for harvest management of the seaweed Laminaria

Abstract

Growth rates and population variables of the seaweeds Laminaria longicruris De la Pylaie and L. digitata (Hudson) Lamouroux were monitored from July 1980 until April 1983 within adjacent harvested and control plots in southwestern Nova Scotia, Canada. Faster growth rates within the harvested plot, relative to the control plot, for new recruits of both species were detected only during the first year following a September 1980 total harvest of both species. Since standing crop had nearly recovered to pre-harvest levels after 1 yr, the increased growth rate was attributed to reduced standing crop during the first year post-harvest. Analysis of the relationship between each species' summer canopy biomass and the density of the most recent year-class developing in the sub-canopy at the same time provided evidence for interspecific competition, and provided parameter estimates for the relationship. A stochastic model of this 2-species interaction using reliable data on growth, recruitment, and mortality predicts long-term persistence of both species in the absence of harvesting, or other disturbances. Following total harvest, L. longicruris is initially more abundant than L. digitata, but an increasing canopy of L. digitata diminishes the dominance of L. longicruris over a period of 3 to 5 yr.Annual harvesting of only the canopy of L. longicruris (a practical and perhaps technically preferable strategy) produces only 4 % of the yield for a total annual harvest. Annual harvesting of the canopy of L. longicruris plus L. digitata produces only 11 % of the yield for a total annual harvest. These lower yields result from a large decrease in L. longicruris growth rate with increased density. This feature diminishes the effectiveness of removing L. digitata to reduce competition with faster growing L.longicruris. The need for controlled and monitored experimental harvests to evaluate these predictions is discussed, as is the need for more functional analyses of seaweed systems.

Seasonal Growth of the KelpLaminaria longicrurisin Baie des Chaleurs, Québec, in Relation to Nutrient and Light Availability

“…After removal of the algal canopy, growth rates of the young sporophytes (stipe length between 25 and 50 cm in length) was twice as fast as that measured in the presence of the algal canopy. This is in agreement with what has been obtained from predicted growth rates of L. longicruris within a post-harvest population (Smith 1986), where growth of L. longicruris following harvesting was estimated to be 1.9 times faster relative to plants growing in an unperturbated mature kelp community.…”

Disturbance and organization of macroalgal assemblages in the Northwest Atlantic

“…15-20 m, growing in a more or less continuous band where there is suitable substratum (MacFarlane, 1952;Edelstein et al, 1969;Mann, 1972a;Tremblay & Chapman, 1980;Moore & Miller, 1983;Miller, 1985a;Novaczek & McLachlan, 1986;Scheibling, 1986;. However, with increasing exposure L. longicruris may co-exist with L. digitata in shallow water, and in shallow areas of extreme exposure L. digitata may replace its congener entirely (MacFarlane, 1952;Edelstein et al, 1969;Mann, 1972a;Smith, 1986). A similar pattern occurs in New England (Witman, 1987).…”

Patterns of organizations of intertidal and shallow subtidal vegetation in wave exposed habitats of central Chile

“…The expected outcome of the recolonization following these disturbances should be the settlement of juveniles of Lessonia nigrescens within these openings, maintaining the belt in space and time. It is as yet unknown if a similar phenomenon occurs in populations of other kelp species, but Markham (1973), Dayton (1975a) and Smith (1986) have suggested that the seasonal reproductive pattern of species of Laminaria and Hedophyllum represent adaptive responses to seasonally predictable patches of substratum produced by storms. Thus, the phenomenon seems to be common among kelps.…”