AGE-DEPENDENT AND SIZE-DEPENDENT GROWTH AND MORTALITY IN A POPULATION OF FUCUS-DISTICHUS

A Fucus distichus L. emend. Powell population at False Creek, Vancouver, Canada, showed seasonal variations in plant mean length and growth rates. Mean length was greater in winter (4.2 to 5.3 cm) and lower in summers of 1986 and 1987 (2.7 to 4.3 cm). Absolute growth rates showed a significantly opposite trend, being higher in spring and summer (0.24 to 1.17 cm mo-1) and lower in fall to winter (-0.5 to 0.4 cm mo-1). Spearman rank-order correlation indicated that, with some exceptions, monthly patterns of change in plant mean length for plants of different ages were significantly correlated, but growth rates were not significantly correlated among ages. Patterns of change in length among cohorts were significantly correlated, but those of growth rates were not, suggesting a low dependence of growth rate on age. Growth rate was significantly correlated (Spearman rank-order correlation) with plant length, and the patterns of change in growth rates among plants of different size classes were also significantly correlated (Pearson's correlation), suggesting that growth rate is strongly size-dependent. A seasonal pattern became more obvious only if plants were grouped by size rather than by age. Plants from different cohorts generally had constant rates of mortality. Mortality was generally higher for younger and smaller plants during winter and spring, and for older and larger plants during summer. Older plants also exhibited higher mortality in the fall. Log linear analysis indicated that the effects of both size (length) and age on growth rate, with or without the effect of mortality, and on mortality considered alone, were significant, However, when the contribution of size was known, the effect of age on mortality became insignificant. Size is shown by association analyses to be a better predictor of growth and mortality than age.