Early life-cycle biology and nursery suitability of the southern bull kelp (Durvillaea potatorum)

Abstract The large fucoid alga Durvillaea potatorum (southern bull kelp) has been identified as a suitable species for offshore seaweed aquaculture due to its large size, preference for wave-exposed sites, and high levels of bioproducts. However, there is a limited understanding of its reproduction and early life-cycle biology and a lack of scalable nursery techniques. We conducted experiments assessing: 1) gamete release, fertilisation, survivorship and early growth of D. potatorum juveniles in the nursery across three months during the austral winter, 2) any differences in the density and size of juveniles seeded onto four distinct substrata: synthetic polymer twine, jute, granite, and glass microscope slides, and 3) any differences in the density and size of seeded juveniles between three similar substrata: synthetic polymer twine, synthetic polymer ribbon and polyvinyl alcohol (PVA) twine. Gamete release and fertilisation increased throughout winter (June—August) while survivorship and maximum juvenile length at 35 days post-fertilisation did not differ among months. Synthetic polymer twine was the best substratum amongst the four distinct substrata, having significantly higher densities of juveniles compared to granite and jute after 28 days post-fertilisation and these juveniles also grew to a larger size. Compared to synthetic polymer twine, synthetic polymer ribbon and PVA twine maintained higher densities of juveniles 56 days post-fertilisation. This study identified techniques to release and fertilise gametes and determined suitable twine substrates for nursery-seeding juvenile D. potatorum .