Recruitment and growth of the sea urchin Strongylocentrotus droebachiensis (Muller) following mass mortalities off Nova Scotia, Canada

Abstract

Recruitment of Strongylocentrotus droebachiensis (Muller) was monitored for four successive years following a mass mortality of this species on a subtidal boulder bottom in St. Margaret's Bay, Nova Scotia. Recently recruited juveniles (1–2 mm diameter) were observed in the fall (September–November) between (1982) and (1984), but not in (1985). Juveniles grew to 6–8 mm diameter at 1-yr post-settlement and 19 mm diameter ((1983) cohort) at 2-yr post-settlement. Individuals > 18 mm diameter had macroscopic gonads in the spring of their third year and spawning probably occurred at 2.75-yr post-settlement. The growth rate of juveniles (3–6 mm initial diameter) maintained in cages with coralline-covered rocks was similar to that of the natural population. Growth and gonad index increased significantly when kelp ( Laminaria longicruris (Pylaie)) was added to cages, suggesting that juveniles were food limited in St. Margaret's Bay. Despite an abundance of kelp and other fleshy algae, they remained cryptic under rocks and were not observed grazing on attached plants. The effects of temperature and food supply on juvenile growth and survival were examined in laboratory experiments using factorial designs. Recently-metamorphosed juveniles (from laboratory cultures), maintained at three temperature regimes (5°C, ambient, 16 °C) and two feeding regimes (no added food, coralline algae added), had similar growth rates (over 9 mth) among treatment combinations. However, growth was asymptotic at 16 °C and ambient temperatures, suggesting food limitation. Survival of these juveniles was higher in treatments with coralline algae, but survival was not affected by temperature. Field-collected juveniles (3–6 mm diameter), maintained at three temperature regimes (5°C, ambient, 16 °C) and three feeding regimes (no added food, coralline algae added, and kelp and coralline algae added), grew only in treatments with kelp added as food. In these treatments, growth rates increased with temperature. However, the growth curve at variable ambient temperature regimes was linear, suggesting seasonal acclimation. Survival of the field-collected juveniles was highest in treatments at 5 °C and/or with added kelp.