Effect of prepared and macroalgal diets and seed stock source on somatic growth of juvenile green sea urchins (Strongylocentrotus droebachiensis)

Populations of sea urchins, harvested for their gonads, are in decline worldwide and so scientific research is now focusing on full life cycle grow-out. The objective of this study was to compare the growth rates of juvenile (initial test diameter: 4.5-13.7 mm) green sea urchins that were held in laboratory tanks and fed a variety of diets. Two separate experiments were conducted. The first compared juvenile growth on seven different diets: 1) prepared diet, 2) Porphyra purpurea, 3) Palmaria palmata, 4) Ulva linza, 5) a mixture of Ulvaria obscura and Ulva lactuca, 6) Laminaria saccharina collected from an Atlantic salmon aquaculture site, and 7) L. saccharina collected from a site relatively distant from salmon aquaculture operations. The latter two treatments were chosen in order to determine if the nutrient regime in which an alga was grown could affect juvenile growth rate. A second experiment compared the growth rates of wild-caught and hatchery-reared juvenile green sea urchins fed either a prepared diet or fresh kelp (L. saccharina) collected from a site relatively distant from salmon aquaculture. Test diameter and whole wet weight measurements were taken monthly for a period of 16 months. In the first experiment, feed type significantly affected somatic growth rate, the overall best growth being supported by Porphyra purpurea (mean growth rate ±SE: 0.059±0.001 mm d−1) and the prepared diet (0.056±0.001 mm d−1). L. saccharina collected from the site distant from salmon aquaculture operations (0.019±0.001 mm d−1) produced significantly slower growth than any other feeding treatment including L. saccharina collected from a salmon aquaculture site (0.047±0.001 mm d−1). In the second experiment, feed type also significantly affected growth with the prepared diet (0.058±0.001 mm d−1) supporting better growth than L. saccharina (0.027±0.001 mm d−1). Seed stock source also affected overall growth with the hatchery-reared individuals displaying significantly greater growth (0.045±0.002 mm d−1) than wild-caught individuals (0.040±0.002 mm d−1). Identifying which diets and sources of juvenile sea urchins optimize growth may help aquaculture operations produce market-size sea urchins in a minimum amount of time, thus maximizing profitability.