Recovery following experimental harvesting ofLaminaria longicruris andL. digitata in southwestern Nova Scotia

Abstract

Laminaria population variables and understory community composition were monitored just prior to, and for two summers following, a September 1980 experimental total harvest of L. longicruris De la Pylaie and L. digitata (L.) Lamouroux within two plots in Lobster Bay, Nova Scotia. Both plots, distinguished mainly by depth, were characterized by high Laminaria standing crop and no recent history of extensive sea urchin grazing. Within the shallower plot (2-3 m below MSL), recovery could not be assessed thoroughly due to ice damage, but within the deeper plot (3--4m below MSL), L. longicruris regrew cropped biomass and attained maximum observed abundance within one year. Both Laminaria species required two years to mature to pre-harvest population characteristics. Survivorship of 0-1 year old and mature populations of both species was generally low (0--67 % per year); however, the higher maximum life expectancy of L. digitata (> 4 years vs 2 years) can result in that species persisting to the disadvantage of L. longicruris. Analysis of understory community composition for both harvested plots and their adjacent controls weakly distinguished the harvested plots one summer after harvesting from all others. It is doubtful the distinction is attributable to harvesting and in neither site was there evidence of a critical change in the understory community. Management implications for the commercial harvest of the brown alga Laminaria are discussed.

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

“…This feature may explain some of the year-to-year variation in standing crop for unharvested populations of L. longicruris and L. digitata (Table 1). The location of this study is subject to storms and ice scouring during winter causing plant mortahties (Smith 1985). This study predicts increased success of L. longicruris following a disturbance because of reduced standing crop and plant densities, as observed for Lessonia nigrescen.…”

“…Alternatively, canopy removal has also resulted in patterns of succession that do not immediately result in increased recruitment by the species removed , Santelices & Ojeda 1984a, and papers cited therein). This does not appear to be the pattern for this system (Smith 1985).…”

“…13.9 cm (n = 1200) for the harvest and control plots at 3 to 4 m below mean sea level (MSL), the depth range relevant to this study. The composition of the seaweed community at this site was described in Smith (1985).…”

“…Table 2 for summary of survivorship parameters ' For Laminana digitata the data do not suggest that control and harvest populations should be considered separately, therefore the data reported are for the control and harvest populations combined From Fig. 3; X = number of days since monitoring began; Y = percentage survivorship C Based on arguments in Smith (1985) Laminaria growth enhancement by canopy removal Table 3 variables introduced in Smith (1985). Daily net growth rates are mean rates between sampling dates.…”

Productivity and life history of Laminaria longicruris at its southern limit in the Western Atlantic Ocean

“…Although Parke (1948) and Reynolds (1974) have reported life-spans of up to 3 yr in individuals, Parke (1948) demonstrated that the average life expectancy for L. saccharina in Great Britain was 2 yr. Chapman (1984) and Smith (1985) reported that the longevity of L. longicruris in Nova Scotia, Canada, was 25 and 20 mo, respectively. Populations of L. saccharina from LIS (Brinkhuis et al 1983) and intertidal populations of the same species from British Columbia, Canada, (Druehl & Hsiao 1977) were described as being annuals.…”