The Green Sea Urchin in Maine, Fishery and Biology - a general summary by Stanley Chenoweth, DMR
This information was written in 1994
The green sea urchin has recently become one of Maine's most valuable resources. Urchin roe is a delicacy in Europe, Japan, and in some of the ethnic markets of the northeastern US, but it is the Japanese market that has provided the stimulus to the rapid development of this fishery. The success of the fishery is due to an abundant resource, a favorable value between the yen and the dollar, and the development of harvesting and processing techniques that suit the quality that is demanded by the Japanese market. Can the market be maintained, given the distance and the demand for product quality? What fishing practices will best sustain the resource? What are the ecological implications of removing large quantities of urchins from the bottom habitat along the coast? These are questions that we will face over the next few years as this new fishery tests the limits of the resource.
Maine Sea Urchin Landings and Value
There was a small urchin fishery in Maine for many years, but prior to 1985 the urchin harvest was limited to about 45 metric tons, which supplied ethnic markets in Boston and New York. Beginning in 1987, landings began to rise dramatically until by 1992 they reached almost 12000 metric tons (over 26 million pounds). The value of the landings rose from less than $50, 000 prior to 1987 to almost 15 million dollars in 1990. Preliminary landings data for 1993 indicate that around 15000 metric tons of urchins were landed.
Urchin Landings by County 1987-1992
Since 1987 most of the urchins have been landed in Cumberland County, however, most of the processing capacity is also in Cumberland County so the numbers may include urchins taken in adjacent counties. In 1992 the landings in Cumberland County decreased by 54% and the landings in Waldo, Hancock and Washington Counties increased 44%, 71% and 44% respectively. Some of the fishing effort may have shifted to eastern Maine as the urchin beds to the west were fished down.
Urchins with the best roe quality are found in relatively shallow, subtidal waters. The dense concen trations are usually found down to about 40 feet. The most common means of harvesting them are with small bottom drags or by scuba diving, although traps, air lifts or water pumps can also be used. Scuba is most commonly used in the western part of the coast where tidal currents are less of a problem than in the east. A dive team usually consists of one or more divers and an attendant boat and operator. The divers pick the ur chins off the bottom, place them in a mesh bag and the boat operator hauls the bag up and empties it into a bin on deck. The person on deck continually checks for roe quality.
Urchins are harvested with a drag in areas where diving is not feasible. In general, draggers can produce a greater total volume, but can also produce urchins that are damaged and of lower quality. A small, light weight drag used on a short, five minute tow will minimize damage. For information on drag specifications contact Bruce Chamberlain at 667-4595. The quality of sea urchin roe is highly variable and depends largely on the season and the type of habitat in which the urchins are found. Kelp is the preferred food and urchins that feed on kelp produce the highest quality roe. Successful harvesting is not just a matter of going out and grabbing urchins off the bottom. Most of the high quality urchins are found in what is called a "feed line", which is a narrow strip of bottom just below a kelp bed. The ideal method of harvesting is to remove the quality urchins from the feed line and allow others to move into the prime feeding area and develop quality roe for subsequent harvesting. Marketable urchins can be found year around, but most urchins spawn in the early spring. During November to March is when the reproductive tissue is prime for commercial harvest, which is before the roe actually develops.
Urchin spines are delicate, so care has to be taken not to break them when harvesting, particularly with a drag. The spines are part of the skeleton and a broken spine allows salt water to enter the shell and damage the membrane that contains the roe.
The number of people that harvest urchins has increased dramatically since 1987. It is a tempting source of income because the fishing can be done in relatively shallow water with basic diving equipment. The price is also attractive, and to date the market seems able to absorb all quality urchins that can be caught. Until January, 1991 the exact number of fishermen harvesting urchins could only be approximated because only a general commercial fishing license was required. Beginning on January 1, 1992, a special license from the Department of Marine Resources was required to fish for sea urchins. Either a handfishing license or a boat license could be purchased for $89. In 1992 there were 807 handfishing licenses and 232 boat licenses issued. In 1993 there were 1439 handfishing and 568 boat licenses. On January 1, 1994, a law passed by the Maine legislature pertaining to the harvest of sea urchins came into effect. The law established three types of urchin licenses, costing $89 and available to residents only:
- A handfishing license allows taking sea urchins by hand, an urchin boat license allows dragging for sea urchins, and a boat tender license allows a person to operate a boat as a platform for hand harvesting.
Processing and the Market
The Japanese have a great appetite for "uni", which is the orange-yellow reproductive tissue from sea urchins and is sold fresh at premium prices at Tokyo's fish market auctions. Harvests locally in Japan cannot supply the demand, so the Japanese import urchin roe from all over the world. Since the 1970's there has been an active market in California and British Columbia, where several species are targeted, the large red urchin, the purple urchin and the green urchin. As the west coast populations were fished down, the search turned to the Atlantic coast, where the green sea urchin is very abundant in Maine and the Canadian maritimes.
The key word in the urchin roe market is quality. This is not just a general euphemism, but relates to a very specific set of criteria - freshness, correct color, correct weight and product preparation. The Maine market did not get off the ground until Japanese representatives came to Maine to explain what is required to satisfy their market. Even now, many of the Maine processors employ Japanese to supervise production.
Roe is graded according to color and can be very specific to particular buyers. Roe color ranges from bright yellow to orange to gray, but light yellows are considered the highest grade followed by light orange. Different parts of the market in Japan prefer different shades of yellow. A color and volume card was prepared by the Maine Cooperative Extension Service Program and the Maine Department of Marine Resources to aid buyers, dealers and harvesters in discussing the quality of urchin roe.
The weight of the roe is measured in relation to the weight of the whole urchin. The best weight ratio ranges from 10-20%, but a premium price is often paid for the higher ratios. Generally, 10% is the minimum acceptable, although in recent years, as demand has increased, urchins with very low ratios have been accepted by some dealers. A 10% ratio means that it takes 1000 lbs of urchins to produce 100 lbs of product.
There are about 56 locations along the Maine coast that buy urchins, but most of the urchins end up at only a few major dealers, most of them in the Portland area. There are two methods of processing urchins, one is to ship whole urchins air freight to Japan and the other is to extract the roe at the plant and-then fly it to Japan. The former has the advantage of simplicity with minimum labor and handling. The latter has the advantage of lower air freight costs, but requires careful, labor intensive processing and greater risk of product rejection in Japan. As processing technology has developed, many of the larger dealers are processing the urchins here in Maine and shipping the roe to Japan.
Stock Status and Management
At present there are no scientifically derived estimates of the size of the urchin population on the Maine coast or what the harvest levels should be to optimize the yield of urchin roe. Not surprisingly, opinions range from a vast untapped resource to its imminent depletion. In theory it would seem that the market demand for only sexually mature urchins from prime feeding areas might lead to overharvest of marketable urchins in some areas, but not to the depletion of the population. If non-marketable urchins are allowed to survive, there should be adequate recruitment to the marketable population.
Unfortunately, this may not be the case. The industry in the past two years has undergone a dramatic expansion. Expanded processing facilities have in- creased the demand for urchins and the number of urchin harvesters. Urchins are being landed in large quantities, in many cases without regard to quality. There is real concern that, in the rush to get in on the action, we may over-harvest and damage what might have been a healthy, sustainable resource. The law implemented by the Maine legislature on January 1, 1994 pertaining to the harvest of sea urchins set the first limits on the fishery.
The limits include:
• Harvesting season.
The fishery is closed from May 15 to August 15. • Legal size.
A diameter (longest diameter) no less than 2 inches, exclusive of spines. Includes a 10% tolerance.
• Night harvesting.
Night harvesting is prohibited. • Sea urchins and lobsters.
Sea urchin harvesters can not possess lobsters when fishing for urchins.
The green sea urchin is an echinoderm, which means it belongs to a group of radially symmetrical, invertebrate animals that includes starfish, sand dollars and sea cucumbers. There are several urchin species distributed in boreal waters around the world, but only the green sea urchin, which has one of the longest scientific names (Strongylocentrotus droebachiensis), is found in the Gulf of Maine. Urchins are common from intertidal pools to depths of 80-90 feet, but they are most abundant in the shallow, subtidal zone on rocky, gravelly or shelly bottoms, where kelp and other marine algae are their principal food.
Urchins from the Outside
The urchin has a round, calcareous shell made up of interlocking plates. The shell, technically the "test", is flattened at the poles, one pole contains a mouth where food goes in and the other contains the anus where waste comes out.
The mouth is actually a complicated arrangement of muscles and teeth called "Aristotle's lantern" which allows the urchin to bite, chew, grasp and grind. Around the teeth is a relatively soft, peristomal membrane and is the weak link in the urchins armored defense. Predators often attempt to roll the urchin over and 'attack it through that soft tissue.
Looking at a shell that has been cleaned you can see a five-fold star-like arrangement (the ambulacral areas) which correspond to the five legs of a starfish. Out of these arnbulacral areas extend tube shaped, flexible balloons called "tube feet." They are operated hydraulically and used for locomotion, chemical and touch sensing, oxygen absorption, catching food and cleaning the shell. The shell is also equipped with many movable spines which provide locomotion and defense against predators.
Urchins from the Inside
The inside of the urchin is mostly gonad, which lies within the shell like five orange sections. This is the roe that is so highly prized. The gonad is essentially a nutrient storage organ. When food is abundant, the nutrients not needed for immediate body metabolism are stored in special cells in the gonad. When food is scarce the stored nutrients are used for energy. In the former case, the gonad can swell to about 25% of the total weight of the urchin, while in the latter it can shrink significantly.
It is the seasonal cycle of gonad development that is most significant for the commercial fishery. As the spawning season approaches, which is during the winter in the Gulf of Maine, the contents of the gonads are full of stored nutrient material, just prior to being converted to reproductive cells. At this time the commercial quality of the gonad is highest. If the urchin has been feeding successfully the gonad will be large, the water content low and it will have a yellow to orange color. If the urchin has not had proper food, the nutrients that would have gone to gamete production will have been used for energy and the gonad will be small and the color brown or gray. At spawning (usually April), the weight of the gonad reaches maximum, and the contents are converted to reproductive cells. At this time the quality of the roe begins to go down as the color of the gonad changes and the water content increases. The gonad is emptied rapidly during spawning and by summer the urchin begins to feed heavily to replenish it with nutrient cells. During this period the commercial quality of the roe is low.
The other prominent internal feature is the gut, which winds around inside the shell and consists of esophagus, stomach, and intestine. Food passes through these organs, where the digestible material is either converted to energy or stored in the gonads for future use and the indigestible material is formed into pellets and removed through the anus.
Making more urchins
The sexes are separate in sea urchins and reproduction is accomplished by shedding eggs (which can number up to 2,000,000 for one female) and sperm in the water. At peak ripeness the gonads leak sex products into the water which can be sensed by other urchins. This appears to stimulate simultaneous spawning and is a means of insuring that fertilization is accomplished most efficiently. Spawn out is rapid and the exact timing will vary from place to place.
After fertilization the young are on their own. The eggs move up in the water column where they develop into odd-shaped "pluteus" larvae that bear no resemblance to the adult. This stage lasts from 4-6 weeks while they seek suitable bottom. When the larvae settle they quickly change to small juvenile sea urchins only a millimeter or two in diameter and begin to feed in the same manner as adults.
The egg and larval stages are very sensitive to temperature, bacterial action and pollutants. The eggs do not develop well in temperatures above 50'F but they are able to survive in waters just below 32'F, which accounts for their limited distribution south of Cape Cod and their prevalence as far north as the arctic. Likewise, waters with a heavy load of organic material can limit their survival by stimulating bacterial growth and the early stages are so sensitive to marine pollutants that they can be used as biological indicators.
The juvenile stage lasts for a couple of years, during which time the young urchins are preyed upon by many natural enemies. Lobsters, crabs, bottom feeding fishes and birds all consider juvenile urchins a delicacy. As the urchins grow older, some of these predators lose interest and mortality from predation is lower. In areas of open bottom, where dense aggregations of urchins are feeding, predation does not appear to have a severe impact on the urchin populations.
How they Grow
Urchins grow in diameter by expanding the plates of the shell. Growth is largely dependent on the amount of environmental stress an urchin encounters, particularly the limitation on amounts and kinds of food, so there can be great variation in size at a given age between urchins in different habitats. Growth is generally seasonal and therefore the amount of calcareous material laid down on the plates of the shell will vary seasonally and might be used as an indicator of age, but to date it has been difficult to age the green sea urchin.
There is no good data on the age and growth of urchins in the Gulf of Maine. Based on what is known from other areas, an urchin probably matures sexually in its third year of life at a diameter of about 1 to 1-1/ 2 inches. Marketable size begins at 2 inches, so that probably urchins are able to spawn at least once before capture, but this is far from certain. Investigation of the age of sexual maturity and the age at which the urchins enter the fishery should be of high priority for future research.
How they Feed
Green sea urchins can either browse or scavenge plant or animal material. They are capable of eating almost anything from macro algae to lobster bait. They can destroy the heads of lobster traps by scraping off the algae and they can scrape holes in algae covered aluminum cans. They are efficient feeders, so when they move in dense concentrations they are able to consume large quantities of food and can clean the bottom of much of its plant life.
Urchins have strong diet preferences and what they find to eat has an great effect on their growth and gonad development. The preferred food is kelps of the Genus Laminaria. Urchins are attracted to kelp beds through water-borne chemical stimuli and those found along the periphery of the bed will grow the fastest and yield the greatest gonad to weight ratios. Other algal food, in declining order of importance, include members of the Genus Chondrus, Corallina, Ascophyllum, and Agarum. In areas where macroalgae does not grow, the urchins will eat benthic diatoms and small invertebrates, such as ascidians, polychaetes and young mussels. Experiments have shown that urchins respond rapidly to changes in diet, so that growth and gonad development can be greatly increased by feeding a preferred food to urchins that have been previously consuming a less desirable diet.
Urchins and the Bottom Community
Urchins have a profound effect on the bottom community. In the rocky subtidal zone of the Gulf of Maine, the algal community and its associatedinvertebrate animals are structured to a large extent by the grazing -activity @of urchins.
The rocky subtidal zone along the Maine coast is dominated by what is called a "crustose coralline community", which is characterized by relatively low plant production. The cor0ine algae which dominate this bottom have a low profile and hard, crust-like appearance which gives the bottom a barren look, but studies have shown that the animal community is quite diverse and is made up mostly of crustaceans, mollusks, polychaete worms and echinoderms. The green sea urchin is the dominant invertebrate in this community and is able to maintain its dominance by voraciously consuming algae and small invertebrate animals.
Kelp beds along the Maine coast, characterized by high plant production, are limited, largely because the high density of urchins keeps them grazed down. Where urchin numbers are high they form feeding "fronts" which can decimate a bed of kelp. Once the bed is gone the urchin population turns to other less desirable food, such as benthic diatoms, thereby maintaining the population density, but at a reduced rate of growth and gonad development. The kelp beds that are found on the coast are in areas where urchins cannot reach, either in particular refuge areas or in the surf zone near the low tide mark. In contrast, there is another type of bottom community which is more common in the Canadian maritimes, particularly the Atlantic coast of Nova Scotia. It is the kelp forest, which is characterized by high primary productivity dominated by the brown kelp Laminaria longicruris. The kelp provides an ideal habitat for a very diverse animal community and is considered to be prime habitat for juvenile lobsters, shellfish, and some species of juvenile fish. The urchin population within the kelp forest is sparse, widely dispersed, and does not severely limit the production of algae.
There has been much discussion in scientific circles about the mechanism that causes urchin populations to dominate the bottom community in some areas while remaining a minor part of a kelp dominated community in others. Studies in the Canadian maritimes have shown that macro algae quickly recolonize urchin-free areas if there are algal beds close by to provide spores, and some west coast studies have suggested that heavy juvenile mortality on urchins in the kelp forests may be the mechanism that holds down the urchin population, but the question is far from answered.
Considering the magnitude of the Maine urchin fishery, the effect of removals on the bottom community invites speculation. Are urchin dominated communities a natural state along the Maine coast, or are they the result of some imbalance in the ecosystem? What happens when the urchins are removed from an urchin dominated community? At present there are not many answers to these questions because the benthic community is complex and there has not been enough controlled, scientific observation of urchin-free bottom specific to the coast of Maine. Events in the Canadian maritimes, such as the classic case of the destruction and resurgence of the Nova Scotian kelp forests, may or may not apply to the coastal Maine ecosystem, but there is no doubt that some change in the bottom community will occur if urchins are removed to a point where density and recruitment become very low.
There is a considerable literature on the green sea urchin. For those who wish to do their own research on the subject and draw their own conclusions, I have included a partial listing. These include the articles used in this paper and also others that I felt would be of general interest.
Berstein, B.B., K.H. Mann. 198 1. Changes in the nearshore ecosystem of the Atlantic coast of Nova Scotia, 1968-198 1. NAFO SCR Doc. 81/ix/134.
Berstein, B.B., B.E. Williams [and others]. 1981. Formation of destructive grazing fronts by the sea urchin, Strongylocentrotus droebachiensis: the role of predators in modifying aggregation and feeding behavior. Mar. Biol. 63:39-49.
Briscoe, C., K.P. Sebans. 1988. Omnivory in Strongylocentrotus droebachiensis (Muller) (Echinodennata:Echinoidea): predation on subtidal mussels. 1. Exp. Mar. Biol. Ecol. 115:1-24.
Chapman, A.R.O. 1981. Stability of seaurchin dominated barren grounds following destructive grazing of kelpin St. Margaret's Bay, eastern Canada. Mar. Biol 62:307-31 1.
Chapman, A.R.O., C.R. Johnson. 1990. Disturbance and organization of macroalgal assemblages in the northwest Atlantic. Hydrobiologica. 192:77121.
Duggins, D.O. 1981. Sea urchins and kelp: the effects of short term change in urchin diet. Limnol. Oceanogr. 26:391-393.
Evans, P.D., K.H. Mann. 1977. Selection of prey by American lobsters (Homarus americanus) when offered a choice between sea urchins and crabs. J. Fish. Res. Bd. Can.; 34(ti):2203-2207.
Fletcher, G.L., V.A. Pepper [and others]. 1974. A study of the biology of the Newfoundland sea urchin with emphasis on aspects important to the development of a fishery. Mar. Sci. Res. Lab. Mem. Univ. Nfld. Tech. Rep.; Tech. Rep. I 1, pp 1-41.
Harris, L.G. 198 1. Studies of community succession in a sea urchin barrens area following sea urchin removal. Am. Zool. 21(4):1019.
Hinimelman, J.H. 1978. Reproductive cycle of the green sea urchin, Strongylocentrotus droebachiensis. Can. J. Zool. 56:1828-1836.
Himmelman, J.H. 1980. The role of the green sea urchin, Strongylocentrotus droebachiensis, in the rocky subtidal region of Newfoundland. Canada: Can. Tech. Rep. Fish. Aquat. Sci.; Rep. No. 954, pp 92-119.
Himmelman, J.H., Y. Lavergne [and others]. 1983. Sea urchins in the St. Lawrence estuary, Canada. Their abundance, size structure and suability for commercial exploitation. Can. J. Fish. Aquat. Sci. 40(4):474-486.
Himmelman, J.H., D.H. Steele. 1971. Foods and predators of the green sea urchin, Strongylocentrotus droebachiensis, in Newfound- land waters. Mar. Biol. 9:315-322.
Keats, D.W. 199 1. Refugial Laminaria abundance and reduction in urchin grazing in communities in the north-west Atlantic. J. Mar. Biol. Ass. U.K. 71(l):867 876.
Keats, D.W., D.H. Steele [and others). 1984.
Depth-dependent reproductive output of the green sea urchin, Strongylocentrotus droebachiensis (O.F. Muller) in relation to the nature and availability of food. J. Exp. Mar. Biol. Ecol. 80(l):77-92. 973.
Keats, D.W., D.H. Steele [and others]. 1983. Food relations and short-term aquaculture potential of the green sea urchin Strongylocentrotus droebachiensis, in Newfoundland. Nfld: Mar. Sci. Res. Lab. Mem. Univ. Nfld. Techn. Rep.; Rep. No. 24.
Kramer, D.E., D.M.A. Nordin. 1976. Studies on the handling and processing of sea urchin roe. 1. Fresh product. Can. Fish. Mar. Serv. Tech. Rep. Vancouver, B.C.: Can. Dep. Fish. Oceansi Rep. No. 870.
Lang, C., K.H. Mann. 1976. Changes in sea urchin populations after the destruction of kelp beds. Mar. Biol. 36:321-326.
I-Vson, B.R. 1977. Preferential feeding and the growth absorption efficiency and gonad development of the sea urchin, Strongylocentrotus droebachiensis, Muller, in Maine. [MS Thesis]. Orono, Me. Univ. of Maine.
Larson, B.R., R.L. Vadas [and others]. 1980. Feeding and nutritional ecology of the sea urchin, Strongylocentrotus droebachiensis, in Maine, USA. Mar. Biol. (Berl.); 59(l):49-62.
MacKay, A.A. 1976. The sea urchin roe industry on New Brunswick's Bay of Fundy coast. Marine Research Associates, Contractor to New Brunswick Dep. -Fish. Fredricton, N.B.; Ref. NB75- 1.
Mann, K.H. 1977. Destruction of kelp beds by sea urchins: a cyclical phenomena or irreversible degradation? Heigo. Wiss. Meers. 30:455-467.
Mann, K.H., L.C. Wright [and others]. 1984. Responses of the sea urchin Strongylocentrotus droebachiensis (O. F. Muller) to waterborne stimuli from potential predators and potential food algae. J. Exp. Mar. Biol. Ecol. 49:233-244.
Miller, R.J. 1985. Succession in sea urchin and seaweed abundance in Nova Scotia, Canada. Mar. Biol. 84:275-286.
Mottet, M.G. 1979. The fishery biology of sea urchins in the Family Strongylocentrotus droebachiensis. Wash. Dep. Fish., Shellf. Div. Tech. Rep.; Tech. Rep. No. 20.
Ojeda, F.P., J.H. Dearborn. 1989. Community structure of macroinvertebrates inhabiting the rocky subtidal zone in the Gulf of Maine: seasonal and bathymetric distribution. Mar. Ecol. Prog. Ser.; 57:147-161.
Pringle, J.D., EJ. Sharp [and others], Eds. 1980. (Dep. Fish. Oceans Can.). Proceedings of the Workshop on the Relationship Between Sea Urchin Grazing and Commercial Plant/Animal Harvesting. Halifax, N.S.: Can. Dep. Fish. Ocean; Can. Tech. Rep. Fish. Aquat. Sci. No.954.
Rowley, R.J. 1989. Settlement and retention of sea urchins (Strongylocentrotus spp) in a sea urchin barren ground and kelp bed: are populations regulated by settlement or post settlement process. Mar. Biol.; 100:485-495.
Sebens, K.P. 1985a. The ecology of the rocky subtidal zone, a diversity of species on the limited-space of subtidal rocks. Am. Sci.; 73:548-557.
Steneck, R.S. 1986. The ecology of coralline algal crusts: convergent patterns and adaptive strategies. Ann. Rev. Ecol. Syst.; 17:273-303.
Stephens, R.F. 1972. Studies on the development of the sea urchin Strongylocentrotus droebachiensis. 1. Ecology and normal development. Biol. Bull.; 142:132-144.
Swan, E. 1961. Some observations on the growth rate of sea urchins in the genus Strongylocentrotus. Biol. Bull.; 120:420-427. Vadas, R.L. 1977. Preferential feeding: an optimization strategy of sea urchins. Ecol. Monogr. 47:337-371.
Wharton, W.G., K.H. Mann. 198 1. Relationship between destructive grazing by the sea urchin Strongylocentrotus droebachiensis and the abundance of American lobster Homarus americanus on the Atlantic coast of Nova Scotia, Canada. Can. J. Fish. Aquat. Sci.; 38(11): 1339-1349.
Witman, J.D. 1985. Refuges, biological disturbance, and rocky subtidal community structure in New England. Ecol. Monogr.;
Witman, J.D. 1987. Subtidal coexistence: storms, grazing mutualism, and the zonation of kelps and mussels. Ecol. Monogr.; 57:167-