ABSTRACT
This study focuses on 1] determining and quantifying the underlying demographic changes and mechanisms that are associated with the Green sea urchin’s (Strongylocentrotus droebachiensis) reduction of kelp forests, by studying their demographic plasticity under a range of conditions from dense (“climax”) kelp forest to decimated (“barren ground”) areas; 2] examining the extent of kelp decimation in particular or characteristic biotopes (e.g, by relating urchin population dynamics to kelp age-structure/productivity and environmental attributes such as bottom substrate and exposure to wave action). The annual instantaneous rate of total mortality (Z) varied from about 0.5 to 1.5, . annual finite rates of 40–78%. Growth rates from relatively high density populations, in moderately sheltered areas indicate that annual test-increments were < 10 mm during the first years and declined markedly with age. Growth performance indices (Moreau et al., 1986), based on fitting the von Bertalanffy growth function, varied from about 0.50 to 0.95. The minimum size at first maturity (test diameter) was relatively constant at about 29 mm, while 50% maturity ogives varied from 40 to 50 mm. Age at maturity varied inversely with growth rate, but mean values were about 4–5 years. The nematode Echinomermella matsi did not significantly affect the test-growth rates of infected urchins, but nevertheless virtually eliminated gonad maturation. Multivariate analyses of urchin density and size related to biotope showed a tendency for “climax” kelp vegetation and large body size and relatively low densities of urchins being found in the outer-lying and more exposed coastal areas, while “barren grounds” dominated by high urchin densities and small body size were more typical of the inner coastal areas.
