The potential impact of kelp forest habitat restoration on the gonad production of a sea urchin fished commercially

Background/Question/Methods

Harvesting red sea urchin (Strongylocentrotus franciscanus) for their gonads is one of the most important commercial fisheries in California. However, sea urchins in high densities, typically purple sea urchin (Strongylocentrotus purpuratus) and red sea urchin, can clear expanses of kelp forest, leaving the reef devoid of standing macroalgae resulting in “urchin barrens”. Pilot kelp forest habitat restoration projects (i.e. relocating large numbers of sea urchins from barrens) have been effective, restoring kelp to densities similar to nearby intact kelp forests. To examine the potential of large-scale habitat restoration to positively impact the red urchin fishery along the Palos Verdes Peninsula, the extent of urchin barrens were mapped and measures of urchin density and size structure across habitats were obtained using a standardized comprehensive monitoring protocol. Urchins were also collected from both kelp forest and urchin barren habitat and the amount of gonad tissue produced at a given body size were compared using generalized linear models. Finally, these data were integrated using Monte Carlo simulations to estimate the potential of habitat restoration to increase urchin gonad biomass and offset losses in fishing grounds due to the recent establishment of two MPAs in the region.

Results/Conclusions

Sixty-one hectares of urchin barren were mapped in the study region, representing much of the nearshore rocky reef habitat in some areas. Benthic survey transects in barrens had 5.2 times more red urchin and 11.7 times more purple urchin than those exclusively in kelp forest habitat. Mean size (test diameter) of both red and purple urchin were also around 50% greater in kelp forest habitat. Modeling the relationship between gonad weight and length (test diameter) demonstrated a clear difference in gonad production between habitats. These relationships varied over time, with mean gonad biomass in red urchin peaking in May - 484% greater for red urchin at the legal size limit in kelp forest habitat. When urchin density and size structure are incorporated with the gonad production model, restoration can potentially result in an 883% increase in urchin gonad biomass per unit area restored. If all 36 ha of urchin barren habitat mapped outside of the 2 new MPAs in the study region were restored, the increase in available gonad biomass could potentially offset 52% of the total gonad biomass which is now unavailable to the fishery in the 109 ha of rocky reef habitat within the two MPAs.