Recruitment patterns and juvenile–adult associations of red sea urchins in three areas of British Columbia

Red sea urchins, Strongylocentrotus franciscanus, form meta-populations which are connected by the dispersal of pelagic larvae. Local adult spawning biomass likely plays only a small role in the local recruitment in terms of larval supply, but larger adult red sea urchins provide protection to conspecific juveniles which are often found sheltered under their spines. Based on survey data collected for the past 15 years in three areas of British Columbia, we used generalized linear models to study the juvenile–adult association, the temporal and spatial variations in recruitment, and the effect of adult population density on recruitment success. The probability for a juvenile red sea urchin to be found under a larger red sea urchin decreased with increasing test diameter (TD) of the juvenile and with decreasing adult density. The probability fell from over 50% for juveniles less than 10 mm TD to almost 0% for juveniles at 40 mm TD. The probability for an adult red sea urchin to provide shelter to conspecific juveniles increased with increasing TD of the adult and with increasing juvenile density, but was generally low (<15%). Recruitment levels varied annually within each area, and differed among three surveyed areas. Adult densities, at least up to certain levels, had a positive impact on the recruitment. Intensive harvesting of red sea urchins in a location likely caused a reduction in the amount of recruitment to the location over the next number of years. Understanding the role of adults in the recruitment process is essential for assessing effectiveness of alternative management strategies for the red sea urchin fishery.

► We model red sea urchin juvenile–adult association and its effect on recruitment.
► There is a higher probability for a smaller juvenile to be sheltered under an adult.
► This sheltering probability increases with increasing adult density.
► Adult density generally has a positive impact on the local recruitment.
► Intensive harvesting likely has a negative impact on the local recruitment.