Expanding populations of purple sea urchins ( Strongylocentrotus purpuratus ) have decimated many kelp beds along the US West Coast, resulting in expansive “urchin barrens”. Sea urchins harvested from barrens are typically devoid of gonad tissue and have little to no market value. Promoting gonad development within barren-sourced urchins through supplemental feeding in land-based tanks is one strategy to regain market value. Pacific dulse (Devaleraea mollis ) is a nutritious red macroalgae commercially produced in land-based tanks that may serve as both a nutritious food source and an in situ biofilter in dulse-urchin co-culture systems.
A series of experiments were conducted in c entral and s outhern Oregon to quantify the rate that purple sea urchins consume Pacific dulse, the rate of gonad growth when fed a mono-algal diet of Pacific dulse, and the effect of sea urchin stocking density on gonad growth and survival. Results were then used to estimate the cost of purple urchin gonad enhancement in a simple small-farm model a ssuming different levels of Pacific dulse productivity due to seasonal availability of ambient photosynthetically active radiation (PAR; 7.5 to 40.0 mol photon m-2 d-1 ).
Purple urchins readily consumed Pacific dulse, with size-specific feed consumption rates ranging from 6% to 2% BW d-1 for 30 g to 160 g animals, respectively. A mono-algal diet of Pacific dulse supported rapid gonad growth in both semi-commercial and research-scale trials. Gonad production, however, was significantly affected by urchin stocking density (p<0.01), with gonad index increasing from a base-line of 2.5% BW to 12.5% BW in the highest density treatment and 18.5% BW in the lowest density treatment (Figure 1). Predicted co-culture production costs were strongly affected by dulse productivity with break-even prices ranging from below $5 to over $14 urchin-1 depending on ambient light levels.
Gonad enhancement of barren-sourced purple urchins was successful in land-based tanks when fed a mono-algal diet of Pacific dulse. While biologically feasible, t hese results demonstrate the importance of considering seasonal variation in dulse productivity when assessing farm profitability . Alternate feed sources should be considered during periods of low solar radiation.