Innovative Incorporation of Green Sea Urchins on Seaweed Farms: Optimizing Uni Yields and Flavor Profiles for Harvest and Sale

Initiate Project: Project final agreement was not executed until May 9th, 2024, 2 months after planned start date.
Year 1 Progress Report: Project Period Activities 5/9/24 to 1/15/25.
Year 2 Progress Report: Project Period Activities 1/15/25- 1/15/26
End of project: 5/2026

Obj. 1: Our first objective was to determine the patterns of reproduction and spawning at our farm and to correlate patterns with environmental variables. This is important for understanding the reproductive cycles and some of the variables contributing to these cycles, to better predict natural cycles to optimize timing of enhancement and harvest.

Reproductive Cycles
:  Determine reproductive cycles through monthly urchin sampling.

Sampling Data 2024 and 2025
Nearly two full sampling seasons have been conducted on farmed urchins, from September of 2024 to November of 2025, collecting test diameter, total weight, uni weight, and uni observations. Gonadal index (GI) calculated by, in wet weight: [(gonad wt / total wt)*100], is a measure of the proportion of gonad weight to total body weight, where a GI index of 10-15% or higher is generally considered commercially acceptable.

Average gonad index increased through the fall and winter, from September until January, then decreased after February to lows in June and July, with increases occurring again the following fall and winter (Fig. 1). Gonad proportions in farmed samples were high, with maximum values from 10.7% (2024) to 26.9% (2025) in September, 20.3% (2024) to 26.3% (2025) in October, and over 30% in Nov, Jan, and Feb 2024 and Nov 2025 (Table 1). These values are higher than reported values of wild or enhanced green urchins, which typically reach 20-25% at their peak. Average GI values were also higher in 2025 than 2024 in September, October, and November, which may be a result of a more consistent feeding schedule earlier in the summer in 2025.

Seawater temperatures are monitored with Hobo dataloggers on urchin cages. The data loggers collect water temperature and light intensity, but light sensors on loggers are quickly fouled and do not provide reliable light data. Average water temperatures are shown alongside gonad index in Fig. 1.

Wild vs Farmed Urchins

Wild harvested sea urchins from the Downeast area were sampled around the same time period as the farmed urchins in two instances, January of 2025, and November of 2025, to compare gonadal index. In both cases, farmed urchin gonadal index was significantly higher than wild urchins (Figure 2) . (January 2025: two-tailed P value 0.0426, farmed mean 27.183, SD 6.658, SEM 2.718, N 6, wild mean 17.2, SD 1.99, SEM 1.146, N 3; November 2025:two-tailed P value 0.0073, farmed mean 25.9, SD 3.578, SEM 1.352, N 7, Wild mean 18.233, SD 0.833, SEM 0.481, N 3).

Test Diameter and Gonad Index
There is a slight positive relationship between test diameter and gonad index (figure 3). Results are similar to Beal’s in 1997, where they found gonad index was independent of size above 55mm for urchins sampled from the northeast coast of Maine (Vadas & Beal, 1999).

Uni Quality
Farmed urchin samples were analyzed for uni quality and yield by both cracking in half with an urchin tool, and by cutting away the bottom half of the shells with shears. We adopted the removal of the bottom half of the shell for comparison purposes. Once shells were opened, the urchin was cleansed of internal organs with tweezers, rinsed with freshwater, soaked briefly in an seawater ice bath, and photographed (figure 4). Uni was removed and weighed, tasted, and analyzed for color and texture. All samples in fall and winter were of a yellow, yellow-orange, or orange color, ranging from bright orange to brown-orange, with pleasant flavors that were described as buttery, sweet, vegetal, briny, and creamy, with aftertastes of kelp, lobster, and clams, with occasional instances of astringency, iodine, or bitterness. Despite overall patterns in gonad index over time, there is a high degree of variability between individual urchins in uni flavor and texture, though nearly all uni had pleasant flavors and would be marketable.

Uni was not tasted in June or July due to poor quality and very low Gonad Index, however, small amounts of eggs and sperm were released, and when mixed, formed some halos observed under the microscope, indicating that there is some level of reproductive potential occurring year round.

In 2025, there were a few instances of one or more lobes in an urchin being gray, shrunken, or discolored with a black or purplish tint (figure 5). Male uni was observed to be releasing milt during sampling nearly every month sampled, but with increasing occurrences from November on. Reproductive male uni was still edible and the flavor was consistent with other samples.

Obj. 2: Our second main objective was to develop a feeding regime that will ensure consistent roe at harvest. By providing a steady diet of optimized seaweed throughout the year, we expect that our farmed urchins will be consistent in roe quality and quantity, creating a new and superior product in contrast to the highly variable and inconsistent wild harvested urchins.

Urchin Cage Design
Upon inspection of existing urchin nets on farm at project initiation, it was discovered that all of the lantern nets in use were damaged, allowing significant numbers of animals to escape. Sea urchins chew holes in the mesh fabric, and the heavy bio-fouling on the mesh weakens and damages the velcro and wire frames. It was therefore critical to secure urchins immediately by moving into new enclosures. The lantern nets formerly used were no longer available for sale, and the only available lantern nets were too small for our purposes, so we had to develop our own cage systems. Several different urchin cage designs were built and tested, utilizing different types of surface mesh and cage openings. All cages were based on a coated metal wire cylinder that is covered in a plastic mesh. We tried polypropylene deer netting (18mm mesh opening) and rigid plastic mesh (13mm, 9mm, 4mm), and built cages to hang vertically with shelves, and horizontally with and without shelves. We suspended cages from the center and from the sides. The final design is a horizontally suspended cage, with a flap on the end that opens for feeding and sampling of urchins. Each cage is suspended from a horizontal longline at about 10 feet depth in winter and 20 feet depth in summer, and held with one hardshell buoy at the surface. The new cages are easy to bring onto the boat for feeding, don't collapse, are resistant to urchin chewing, and foul less than lantern nets.

Biofouling on Cages
Macroalgal fouling on cages is heaviest in spring, usually a mixture of sugar kelps, red turf algae, green ulva, and red, brown, and green filamentous algae, as well as hydroids. Cages have to be scraped down on the outside with the edge of a knife to clear, and can be fed to urchins. Moving into summer, the dominant fouling organism is the skeleton shrimp, thickly coating lines and cages. This usually starts to increase in June, to a high in September/October, and then decreases to almost none in the winter. To control, we sprayed cages with either white vinegar or freshwater, being careful to remove or protect the urchins from the spray. Spraying and brushing cages with a rigid deck brush reduced shrimp infestation on lines and cages. No or few sea squirts were observed on cages, however, suggesting that high loads of skeleton shrimp may prevent the settlement or development of sea squirts.

Lumpfish, Scallops, and other Visitors (figure 6)
Lumpfish is a common visitor to the farmed seaweed lines, and have been observed to be swimming around the outside of the urchin cages. They also make their way into the cages when they’re smaller, and have been found to be residing inside cages with urchins. We have added lumpfish to cages to test if the lumpfish would reduce the skeleton shrimp infestations the cages suffer in the summer months. The lumpfish have resided in cages successfully for 8 months or more, and seem to help with shrimp on the insides of the cages.
Lantern cages had previously captured natural scallop spat in the area, so we have been cultivating these scallops with our sea urchins. When we moved all the urchins over to rigid cages, the scallops were all moved together in one cage. High amounts of fouling in the form of barnacles and blue mussels were impeding the scallops, so some sea urchins were returned to the cohabitate with the scallops and act as cleaners to the shells.
We have found a small hake living in one of the urchin cages.

Mortality
There is always some degree of mortality to be expected in urchin cages, as we often see empty or broken shells when feeding the cages. However, this number seems to be relatively low, under 10%. We have not been able to estimate mortality due to the losses we had through cage damage and escapes, but hope to determine annual expected loss in the future with a more secured inventory.

Urchin Inventory
We collected initial data on urchin size and density as we moved all urchins into new numbered culture cages to create a full inventory, for a total of over 6,000 urchins in 40 cages in year 1. All cages were fed by filling with sugar kelp primarily, with smaller amounts of other available seaweeds. Urchin size and growth were measured by sampling of at least 10 urchins from each cage by taking photos, and using calipers to determine test diameter. In Year 2, many of the new cages had been chewed open at the seams by urchins, resulting in more escapes. All of the cages had to be re-configured again with wire instead of twine, and a new full inventory was conducted and updated. We hope to update this inventory towards the end of the project to determine growth rates.

Feeding Regime
While we initially were able to offer a diet of optimal mixed farmed sugar kelp and horsetail kelp, it was difficult to provide an even mix consistently through the season based on availability and quality. At the end of the summer, the sugar and horsetail kelps can become so biofouled with bryozoans and other organisms that it becomes mostly unusable, especially in the case of the horsetail kelp. However, lines that were left through the summer started to show new growth in the fall and were harvestable for urchin feed through the year (figure 7). There was a blue mussel seed set on the lines and plants in late summer/early fall, which provided another source of feed for the urchins. While urchins don’t seem to feed on skeleton shrimp, they were able to utilize the fouled kelp and consumed all that was placed in their cages. A freshwater dip can reduce the density of skeleton shrimp on the kelp, and we employed this method several times to provide cleaner feed. These seaweed lines provided critical feed for urchins through the summer and fall. It will be essential to improve feed availability throughout the year to produce enough food to supply urchins.

Seaweed Cultivation
Sugar kelp and horsetail kelp were cultivated on longlines on the farm, and a smaller amount of dulse was cultivated on specialized suspended gear. While there is plenty of high quality kelp available for feeding from March-July, it is more difficult to obtain sufficient food the rest of the year. We tested growing tumble cultured dulse in cages at sea to reduce nursery costs of production, but growth was slow and by the summer, the plants were encrusted by bryozoans.

Obj 3: Our third objective is to test specific seaweed species to determine unique flavor profiles to customize finishing diets. This information will help us determine enhancement strategies. If particular seaweed species lend specific desirable flavor profiles, we will better be able to control flavor at harvest.
Seaweed Feed Production 
Sugar kelp and horsetail kelp can be grown in abundance on longlines on our seaweed farm, as described in obj. 2. Dulse, a red seaweed, is morphologically much smaller than the kelps, with lower yields on longline systems. To ensure we have enough dulse for feeding experiments, we cultivate dulse year round in tanks in our land based facility. We have set up dedicated dulse cultivation systems to optimize growth in tanks, at 10-12C, with filtered seawater, aeration for movement, light and 12:12 photoperiod, and regular seawater changes. While we have been cultivating the same strains of dulse in our nursery for several years, the scaling of production has been limiting due to the size of the culture container. We hope to test larger cultivation tanks to improve production.

While fresh seaweed is preferred, there might be periods of low availability. We will test alternative forms by feeding fresh, dried, and frozen feed to urchins in tanks in our facility and monitoring feed consumption over time. We have done some testing with dried and frozen and experiments are still underway.

Dulse, Dulse and Kelp, and all Kelp Cages- Initiated dulse feeding on several instances, but supply was inconsistent so we were not able to utilize dulse for long term feeding trials. Instead, to determine finishing potential, we dedicated one cage to feeding harvestable urchins dulse, starting in 9/24/24, along with a cage with dulse and kelp (50/50), next to a cage of just kelp. After 2 months, urchins were sampled and photographed (figure 8). The dulse fed cage exhibited the best uni, with an average gonad index of 28.4%, and bright orange color, good texture, and a flavor that was sweet with an essence of red algae. Flavor feeding trials are ongoing.

Obj 4: Our fourth objective will develop harvest, post harvest handling, and marketing strategies for farmed sea urchin.

Packing, Shipping, Marketing
We tested two different types of plastic bag for holding live urchins, an oxygen permeable bag utilized for live fish and a square bottom thick plastic bag. One live urchin with a few pieces of seaweed (rockweed and dulse) was placed in each bag with about .5 liter of seawater, tied at the top, and held in the refrigerator for several days. After 6 days, both urchins were still alive, though water was leaking through the permeable bag, and the water had a purplish hue. Both urchins were returned back to the holding tank after the 6 day trial.
Rigid plastic containers were trialed in the refrigerator, in the same manner as described for the plastic bag trial, and held successfully in for 6 days, when they were returned alive to the holding tank.
We listed live sea urchins for sale on our website, and have had several orders in 2025. We visited a distributor in Boston, and continue to work on sales and distribution opportunities.