Macroalgal aquaculture is a priority in the blue economy for its potential to enhance local economies, provide food security, and develop nutrition and biotechnical products. Estuarine systems such as Humboldt Bay offer a more protected environment for kelp aquaculture in comparison to more exposed environments and may provide more stable nutrient levels due to coastal proximity. However, they can also experience more extreme fluctuations in salinity and temperature. This project aims to establish a robust cultivation protocol for the ribbon kelp Alaria marginata by comparing single and multiline systems along a depth gradient in an embayment. Insights from the first year and comparative analyses informed the second year’s strategy, aiming to maximize biomass production and optimize cultivation protocol within the best growing season.
A. marginata sporophylls were collected from local populations and spawned in a controlled environment. Spores were settled on Matsuura Cremona twine spooled around 5-cm PVC to seed farm lines. Two farm systems were used: 1) a single-long-line system, where there is a single 107 m length of grow line attached to buoys, and 2) a multi-long-line system, where five 61 m grow lines were spaced 0.76 m apart using a 3.05 m aluminum spreader bar along a depth gradient from one end of the line to the other. The multi-long-line system allows the assessment of growth of more biomass in a smaller area while the single-line system evaluates optimal growth depth. Biweekly measurements for blade length and width were taken randomly along the length of each grow-line. Water quality measurements were also taken. Kruskal-Wallis and Dunn post-hoc statistical analyses were utilized to look at statistical differences in kelp lengths.
In the first and third growing seasons (July to September 2024 and June to August 2025), mortality occurred over time. Temperatures of 18 °C, a maximum of 0.1 nitrates ppm, and a maximum of 1.0 phosphates ppm likely limited growth and survival. In the second grow season (December 2024 to April 2025), successful harvest data showed that A. marginata required line depth shallower than 1 meter and wide spacing between lines for optimal growth. This was concluded as the difference in kelp length at the shallowest points were longer than those at the deeper points of the line (p=6.059e-13).