# Evaluation of native macroalgae species of the Southeast U.S. and Caribbean for use in integrated multi-trophic aquaculture (IMTA)

## TL;DR

This study evaluates native seaweed species for use in aquaculture systems to reduce fish waste and improve sustainability.

## Contribution

The study provides new data on how specific macroalgae species can effectively reduce fish effluent nutrients and their nutritional profiles.

## Key findings

- Agardhiella subulata reduced TAN levels in fish effluent below detectable limits at a density of 6.73 kg/m³.
- Caulerpa racemosa had the highest protein content (25.49%), and Ulva lactuca had the highest carbohydrate content (61.48%).
- Ulva lactuca efficiently uptook and incorporated carbon from effluent water.

## Abstract

This study assessed the comparative performance of candidate marine macroalgae species under integrated multi-trophic aquaculture (IMTA) culture conditions in a system culturing yellowtail snapper (Ocyurus chrysurus) at a commercial-scale biomass density. The nutrient-rich effluent water from the marine finfish culture tank supplied replicated macroalgae raceways containing four candidate species of macroalgae (Agardhiella subulata, Caulerpa racemosa, Gracilaria caudata, and Ulva lactuca) native to the Southeast U.S. and Caribbean regions. Water temperature, dissolved oxygen (DO), salinity, alkalinity, pH, phosphate, and total ammonia nitrogen (TAN) readings were sampled daily, and dissolved CO2 was calculated over the course of the trials. Each species of macroalgae was analyzed for protein, fat, fiber, ash, minerals, and metals. Additionally, the carbon and nitrogen content and stable isotope ratios of the nutritionally enriched (i.e., “fortified”) macroalgae (“seaweed”) species were analyzed. Results provide novel insights on macroalgae biomass levels under specific hydraulic retention times (HRT) that allow for the reduction of TAN levels in fish effluent water below detectable levels. Specifically, Agardhiella subulata at a density of 6.73 kg m−3, lowered TAN levels in fish effluent water below detectable limits under a hydraulic residence time (HRT) of 54 min from average starting concentrations of 0.04 mg/L TAN. Compositional analyses indicated Caulerpa racemosa had the highest protein content (25.49%) and Ulva lactuca had the highest carbohydrate content (61.48%) of the macroalgae species tested. Calculated dissolved CO₂ concentrations of effluent water before and after the macroalgae culture tanks and the carbon-to-nitrogen ratio of macroalgae samples indicated that Ulva lactuca up took and incorporated carbon most efficiently. The results of this project provide novel information that will help improve economic and environmental sustainability for existing and prospective marine finfish aquaculture operations throughout the focal regions and allow producers to make well-informed decisions on candidate species of macroalgae for marine IMTA applications.

The online version contains supplementary material available at 10.1007/s10499-026-02441-1.