Defining and finding the profit for seaweed secondary extensive aquaculture

Seaweed aquaculture is expanding globally, yet small-scale producers, such as those emerging in the United States, face challenges in achieving economic viability. We introduce and formalize the concept of secondary extensive aquaculture (SEA): the non-target cultivation, harvest, and use of extractive species (e.g., seaweeds) that naturally recruit and grow on existing aquaculture infrastructure designed for a different primary species (e.g., oysters). To explore the economic viability of SEA, we employed a scenario-based bioeconomic modeling approach, coupling logistic seaweed growth and sustainable harvest with a simple profit function. The model was parameterized with literature-derived ranges for seaweed growth and carrying capacity, as well as data from a 2025 pilot harvest on a 65 ha oyster farm in California. Using simulations, we evaluated key ecological and economic combinations under which seaweed SEA may offset the estimated 5-10% production losses associated with typical, unharvested biofouling. We showed that profitability increased with farm size, growing season length, and intrinsic growth rate ( r ) of seaweed, with viable scenarios typically requiring > 50-100 ha and > 60 growing days under conservative market assumptions ($400 t -1 wet weight). Intrinsic growth rate exerted a stronger influence on profitability than carrying capacity ( K ), indicating that faster-growing seaweed species (e.g., Ulva spp., Gracilaria spp., and Pyropia spp.) are more suitable candidates for SEA. In fact, modal ecological conditions on a 65 ha farm may offset biofouling costs. Labor intensity, however, emerged as a major bottleneck, with pilot-informed labor estimates (~140-200 hrs. t -1 including processing) far exceeding the ca. 10 hrs. t -1 needed to breakeven at minimum wage, but could be achieved with realistic improved efficiency. Although our pilot SEA yields fell near the lower bound of global seaweed aquaculture production, they remain within plausible commercial ranges. Overall, SEA represents a promising complementary strategy for diversifying revenue streams and enhancing circularity in aquaculture systems, but its realization will depend on sufficient scale, favorable biological productivity, labor efficiency improvements, and supportive policy frameworks.