Improving green sea urchin (Strongylocentrotus droebachiensis) hatchery production by determining effective settlement cues and post-settlement conditions

Abstract

Hatchery production methodologies of Northeastern U.S. green sea urchins (Strongylocentrotus droebachiensis) have yielded low settlement success and post-settlement survival (approximately 1–5%) contributing towards high costs for producing juveniles for this emerging aquaculture species. These are likely attributed to hatchery operations needing to optimize settlement cues and post-settlement food supplies. This study examined a range of potential settlement cues using adult conspecifics or a benthic diatom biofilm (Cylindrotheca closterium or Nitzschia sp.) alone, or in combination with seawater conditioned with conspecifics or macroalga (Saccharina latissima or Ulva lactuca). This study also examined whether sea urchin post-settlement survival and growth would improve using a benthic diatom food source (C. closterium or Nitzschia sp.) alone or in combination with an additional macroalga food source (S. latissima or U. lactuca). Both trials were conducted under hatchery ambient (control 12 °C) or elevated (14 °C) temperatures to determine whether temperature elevation could enhance S. droebachiensis settlement success and post-settlement growth. This study also examined what influence biofilm age (1, 4 and 8 weeks) and seawater salinity (26, 30, 33 and 36) had on settlement success to help towards optimizing hatchery methods. This study did not improve post-settlement survival causing it to remain a significant barrier in mass producing juveniles, but it did increase juvenile growth by 30–40% and reduce growth variability when fed the benthic diatom C. closterium in combination with U. lactuca. This combination also served as an adequate settlement cue for this species. The use of a conspecific influenced biofilm substantially improved settlement success from 1 to 5% to 50–80%. Seasonal timing and age (4 weeks) of this biofilm indicated importance in yielding this success which will be location specific. The use of conspecific biofilms for improving post-settlement survival requires further investigation and is worth examining in combination with C. closterium and U. lactuca towards enhancing S. droebachiensis production in the Northeastern U.S. Elevating seawater temperature by 2 °C had no significant effect on settlement, growth, or survival in this study. Salinities of 30–33 were optimal yielding 3–7 times more settlement compared to salinities 27 and 36. This study highlights that settlement trials need to incorporate post-settlement survival measurements for at least 1–2 months to truly reflect the effectiveness of method interventions and consider extending the time before turning seawater flow back on into settlement tanks from 24 to 48 h by up to 72–96 h to facilitate greater settlement success if seawater quality parameters permit it.