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.

We sampled urchins from our farm for 18 months, from September of 2024 to March of 2026, 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, then decreased after February to lows in June and July (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 through March 2026 (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 the second year of sampling compared to the first, which is probably the result of a more consistent feeding schedule during the summer of 2025.

Seawater temperatures were monitored with Hobo dataloggers on urchin cages, and additional temperature data was added from the nearby NOAA buoy. 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.

Comparison of Gonad Index of Farmed and Wild Urchins
Wild urchins collected locally by commercial divers were obtained for uni sampling to compare to Springtide’s farmed urchins in Jan and Oct/Nov 2025. Across all test periods, sampled farmed urchins yielded a significantly higher gonad index than wild urchins, with farmed urchins reaching a mean gonad index of 25.01% (±4.77%), peaking at an individual maximum of 35.3%. In comparison, wild urchins yielded a significantly lower mean gonad index of 16.46% (±2.40%), with a maximum observed yield of only 18.9%. Farming practices consistently produced an average gonad volume increase of roughly 8.5 percentage points over wild-harvested urchins. Farmed individuals also demonstrate a much higher ceiling for maximum market quality (35.3% vs 18.9%).

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 3). 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 4). 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 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, and consist of a mixture seaweeds, including sugar kelp, red and green 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 the macroalgae growth. Moving into summer, the dominant fouling organism is the invasive skeleton shrimp, thickly coating lines and cages. This usually starts to increase in June, to a high in September/October, and then decreases in the winter. To control, we sprayed cages with either white vinegar or freshwater, being careful to remove or protect the urchins with a covering of kelp. 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 5)
Lumpfish (Cyclopterus lumpus) are a common wild visitor to farmed seaweed lines, and have been observed to be swimming around the outside of urchin cages, and are sometimes found inside cages as small juveniles. We added small lumpfish to cages to test if they would reduce the skeleton shrimp infestations in the summer months. The lumpfish resided in cages successfully for 8 months or more, and seem to help clean the insides of the cages.

Sea Scallops
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 with no urchins. High amounts of fouling in the form of barnacles and blue mussels were impeding the scallops, so some sea urchins were returned to the cages with the scallops to act as cleaners for the shells. Sea urchins are effective grazers for scallops, and keep shells clean. 

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 variable 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. 

A full inventory of cages was made in 2024 and 2025 (fig 6). Due to cage design failure and large number of escapes, as well as some mortality, sampling, and urchin transfers, the change in cage density from 2024 to 2025 was highly variable. Eliminating cages that showed drastic decreases due to escapes or increases due to transfers, average loss was about 11% per year. New cage counts in 2026 with cage improvements will provide a better estimate of average losses due to natural mortality, but from this initial work, we can estimate average yearly losses of 10-11%.

Cage Densities
Cages from the farm inventory were analyzed for urchin densities per size class. Size classes were split into groups of ten, and sizes were determined using urchin test diameter (mm), measured with calipers on farm. Average densities on farm in 2024 decreased with increasing size, from 509 urchins in the 10-20 mm size class, to 49 for the 60-70 mm size class. (Table 2) The cylindrical cages have a total inside surface area 1.2m2. If the footprint of each sea urchin is calculated as total diameter including spines, a ratio of 1.45 can be added to test diameter, based on an average of measurements of seed urchins from sizes 5-30mm. Based on these calculations, average stocking densities on the farm ranged between 16 and 53% total surface area coverage, and we can utilize these numbers to estimate stocking densities at different levels of inside cage surface area.

Sea Urchin Growth
Test diameter of sea urchin samples from cages were measured in 2024 and 2025. Cages with data from both years and exhibiting positive growth were analyzed for growth patterns (fig 7). On average, the increase in test size was 5.7 mm, with a range of 12.45 to 0.2mm. This data may not represent a true estimate of growth over one year, as there were issues with gear, escapes, and transfers, and should better represent growth once gear and feeding regimes are stabilized. However, several cages exhibited growth of 10-12mm per year, which is the expected average growth rate of green sea urchins. Smaller urchins (under 45mm) grew at a much faster percentage rate compared to larger, more mature urchins (over 50mm).

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 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.

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. Sugar kelp was the feed most consistently available throughout the year. While smaller urchins consumed feed more quickly than larger urchins, in general, a feeding regime of every 2 weeks will keep urchins satiated. This feeding schedule should be followed from spring to late fall to optimize uni production. Ideally, farmed urchins will be ready to market from September through December, as winter weather restricts access to the farm for harvest. Urchins can persist for long periods of time without feed, which gives some leeway to farmers if situations arise that disrupt feeding schedules. 

Biofouling is a major issue for seaweed feed production in the summer months. By the end of summer, the sugar and horsetail kelps can become so fouled 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 during this project started to show new growth in the fall and were utilized for urchin feed through the year (figure 8). There was a blue mussel seed set on the lines in late summer/early fall, which provided another source of feed for the urchins, provided that the mussels are small enough for the urchins to feed on. 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.

Feeding Trials
While fresh seaweed is preferred, there are periods of low availability. A variety of fresh, frozen, and dried seaweeds were fed to large adult urchins and smaller seed size urchins in tanks in our facility and monitored for feed consumption over time. All dried and frozen samples were re-hydrated in seawater for several hours before determining starting weight. Samples were removed every few days and weighed to determine feed rate and preference (figure 9). Adult urchins fed samples of fresh, frozen, and dried dulse preferred the fresh and dried forms over the frozen. Adult urchins fed a variety of dried farmed kelps consumed more Sugar kelp and Digitata than Alaria. When small juvenile seed urchins were fed dried kelps, there seemed to be a higher preference for Alaria, and variable consumption rates for Sugar kelp and Digitata, with all dried kelps consumed at 51-100% after 6-9 days. When juvenile urchins were fed samples of frozen and dried Sugar Kelp and Alaria, there was a clear preference for dried Alaria over frozen, and similar consumption rates for the dried and frozen Sugar kelp. A primary finding across the experiments indicates that processing methods influence palatability, with dried or fresh macroalgae consistently outperforming frozen treatments. The data suggests an age-dependent dietary preference of younger seed urchins for dried Alaria, and strong but variable consumption of sugar kelp and digitata. Adult urchins had higher consumption rates of dried Sugar kelp and Digitata over Alaria. These preliminary results suggest that dried kelps and dulse can be utilized to provide feed for urchins when fresh seaweed is limited, and that dried feed is preferable to frozen.

Kelp and Dulse Feed
While we initiated separate cage trials using dulse and kelp as test feeds for finishing urchins, the seaweed supply was inconsistent so we simplified the approach and fed one cage dulse, one cage sugar kelp, and one cage a 50/50 mix of each, starting in September. After 2 months, urchins were sampled and photographed (figure 10). The cage fed dulse exhibited the best quality 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. While the flavors were somewhat variable between samples from the same cages, there was a distinct improvement in color and quality with the addition of dulse to cages. 

Obj 4: Our fourth objective developed 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 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 offered live urchins for sale on our website, and sold to about 5 customers, 3 of which were pick up orders, and 2 which were shipping orders. We trialed shipping of the live urchins in rigid containers by sending a cooler with ice packs and live urchins to a customer in Minnesota through the US mail. Despite unexpected delays and below freezing weather, the package arrived intact with 4 out of the 6 urchins alive and in good condition after 6 days in the mail. We met with 3 major seafood distributors in New England, who were interested in the live farmed sea urchins, but were curious how they could be marketed differently than the existing wild processed sea urchin roe that is already available and sold from September through March. There was little to no understanding or experience with live sea urchins, as most customers experience uni after it has been processed and placed on a tray or piece of sushi.