Hatchery development for aquaculture of the southern bull kelp Durvillaea potatorum (Fucales): gamete release, fertilisation rate, and juvenile growth

Abstract Durvillaea spp. (Fucales) occur on wave-exposed coasts in parts of the southern hemisphere. In Tasmania, Australia, a small but economically valuable industry (> US$1.5 million GVP) harvests beach-cast Durvillaea potatorum , primarily for alginates and liquid plant biostimulants. Currently, demand for D. potatorum biomass exceeds supply, prompting interest in aquaculture to sustainably increase production. However, fundamental hatchery methods required for its cultivation are not well understood. We conducted a series of experiments to determine optimal conditions for gamete release, fertilisation, and juvenile growth in a hatchery setting. Releasing gametes for 10 min at 14 °C produced consistently high egg densities (mean ± SE: 12,720 ± 1020 cm −2 tissue) and fertilisation rates peaked (89 ± 4%) when reproductive tissue was held overnight and gametes were released at 14 °C. Moreover, ~ 50% of male and female gametes were released within 10 min and > 90% within one hour. Three growth experiments showed that: (i) the highest growth rate was found by first holding the samples in the dark overnight at 14 °C and then using a gamete-release temperature of 18 °C, with juveniles reaching a mean length of 1.2 mm ± 0.1 SE and width of 0.29 mm ± 0.02 SE after 60 days at 14 °C and a light level of 35 µmol photons m −2 s −1 (12 h light: 12 h dark photoperiod); (ii) an irradiance level of 95 µmol photons m −2 s −1 for maximise growth of juveniles; and (iii) nutrient medium and temperature influenced growth, with the best performance in F/2 medium at 15 °C after 30 days (mean length 0.9 ± 0.2 mm; width 0.38 ± 0.01 mm). Growth of juvenile D. potatorum was poor in media containing ammonium (NH 4 + ), such as Provasoli’s Enriched Seawater (PES) and seawater enriched with N and P, at similar concentrations to PES, suggesting NH 4 + toxicity for early life stages at high concentrations. These results represent a significant step toward establishing effective hatchery methods for Durvillaea spp . and other Fucales more broadly, supporting future at-sea cultivation.