Development of scalable coastal and offshore kelp farming for marine biomass production

Abstract

The US DOE/ARPA-E MARINER program funded a 4-year project to determine an optimal way to grow kelps in large, nearshore and offshore arrays for the eventual purpose of biofuel production with the goal of keeping the cost below $80 USD per dry metric ton of kelp. This project specifically looked at how Saccharina latissima can be grown in the Gulf of Alaska to reach that goal. There were three major aspects of the research: (1) optimize nursery production and seeding lines for outplanting; (2) design an economical, modular outplanting structure; and (3) develop methods to efficiently harvest the product. Farm designs were based on catenary structures and the use of spreader bars with variable spacing of grow-lines and line types. The spacing of the grow-lines makes a difference in the yield. Grow-line spacing of ≥1.5 m showed about a 50% increase in production (kg m−1). There was no statistical difference in the growth of Saccharina latissima whether in the middle or the outside of the array, but the line type and perhaps line thickness can make a difference in yield. Sagging caused by the weight of the mature fronds resulted in lower growth at depth. Various harvesting approaches for mature kelps were tested by collaborating farmers. One promising innovation is the use of large bags with mesh for temporarily holding the freshly harvested fronds in seawater. Although the weight of the fronds on the grow-lines causes the lines to sink, the bags packed with the harvested fronds float, allowing for temporary storage before loading to a vessel heading to port and processing. Another advance in harvesting is a specially built harvest vessel, the Harvest Buddy, allowing a more mechanized and faster way to harvest. A techno-economic assessment (TEA) using our data has pointed to solutions to reach the goal of $80 USD per dry metric ton of kelp.