Can harvesting regulations drive ecological community change? Effects of limpet (Cellana sandwicensis) size and density on intertidal algal communities

Background/Question/Methods

Despite debates regarding the extent and severity of species declines, there is overwhelming consensus that human impacts on marine ecosystems have decreased individual abundance and altered the size structure of harvested species worldwide. However, few studies have examined the concomitant changes in the communities in which these fisheries species reside.  In this study, we experimentally simulate two commonly used fisheries management strategies (size limits and total allowable catch or bag limits) to investigate how they impact ecological community structure when applied in a controlled setting.  Our focal species, yellowfoot ‘opihi (Cellana sandwicensis) is the most prized of three congeneric limpet species endemic to Hawai‘i.  All three species are harvested commercially, recreationally, and for subsistence, and are currently regulated with a minimum size limit. 

To determine the different impacts that regulations of size versus overall catch may have on communities, we manipulated size (15mm, 45mm) and density (20/m², 40/m²) of yellowfoot ‘opihi in a fully factorial manner using 36 field enclosures (0.25m²) located along the North Kona coastline of Hawai‘i Island. We evaluated algal community changes using monthly photo quadrats, and tracked growth (through changes in shell aperture area) and mortality rates of tagged individuals within each treatment for three months.

Results/Conclusions

Preliminary results after 2 months suggest both size and density of ‘opihi impact ephemeral algal communities.  We used crustose corraline algae (CCA) percent cover as an initial proxy for algal community change.  In both the 20/m² and 40/m² density treatments for the 45mm ‘opihi, and the 20/m² treatment for the 15mm ‘opihi, CCA cover remained approximately the same.  However, the 15mm ‘opihi at 40/m² treatment was almost completely overgrown by ephemeral algae.  These differences do not appear to be driven by biomass, as indicated by ephemeral algal growth in the 4/m² ‘ophi at 45mm biomass control, but little to no ephemeral algal growth in the 160/m²  ‘opihi at 15mm control.

Preliminary growth rates indicate density dependent growth in both size classes.  For 45mm ‘ophi, the highest mean growth rate was the 4/m² ‘ophi density treatment (4.6mm²/day), decreasing to 2.7mm²/day in the 40/m² treatment.  For 15mm ‘opihi, the highest mean growth rate was the 40/m² treatment (2.9mm²/day), and the lowest growth rate was the 160/m² density treatment (0.5mm²/day). These results suggest that harvesting regulations and strategies may have differential impacts on both harvested species and the ecological communities in which they reside.