L’écologie de l’oursin

L’oursin vert est le plus répandu des échinodermes. Il se trouve dans les océans Pacifique et Atlantique, de l’Arctique à l’Antarctique. Sur la Côte est, il se trouve dans des régions situées aussi loin au sud que Cape Cod et les mers de New Jersey.

La population d’oursins verts n’a pas toujours été si abondante le long des côtes de la Nouvelle-Écosse qu’elle l’a été au début des années 1990. Dans les années 1980, une amibe que l’on lie à des températures marines élevées était responsable d’une diminution très importante de la population des oursins. En 1993 et en 1995, la même cause a réduit à nouveau la population des oursins.

Plusieurs facteurs, qui d’ailleurs ne sont pas tous répertoriés, sont à l’origine de ces fluctuations. Par contre, l’arrivée de l’homme, au début des années 1990, en tant que prédateur important est déterminante. De plus, l’argent que peut rapporter la pêche à l’oursin à de quoi inquiéter. Afin de s’assurer que les oursins ne seront pas rayés de la carte, de nouvelles recherches sont nécessaires.

The Not-So-Lowly Green Sea Urchin

 

Chris Pearce,
Ross Island Salmon Ltd.
October 1998

 

t.gif (259 bytes)he green sea urchin (burdened with one of the longest scientific names in the animal kingdom -
Strongylocentrotus droebachiensis) belongs to a group of marine invertebrates called the Echinoderms. The organisms belonging to this group, which includes sand dollars, sea stars (or starfish), brittle stars, and sea cucumbers, often bear spines or tubercles on their body surface giving them a spiny or warty appearance, hence the name Echinoderm (Latin for "spiny skin").

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Larval Urchin
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urchin2.gif (12767 bytes)
(photo: Christopher Pearce, Ross Island Salmon Ltd.)

The green sea urchin is one of the most widely distributed of all Echinoderms. It has a circumpolar distribution which extends into the Arctic regions of both the Atlantic and Pacific Oceans. On the east coast of North America, it regularly occurs as far south as Cape Cod and in deeper waters to New Jersey. On the Pacific coast, its distribution ranges southwards to Puget Sound. It commonly inhabits the rocky subtidal zone from the low-tide mark down to a depth of 1200 m, but also occurs intertidally in tide pools.

The green sea urchin primarily grazes on seaweeds (kelp being its preferred food source), but will also consume a wide variety of organisms including mussels, sand dollars, barnacles, whelks, periwinkles, sponges, bryozoans, dead fish, and - when hungry enough - other sea urchins. In turn, green sea urchins are eaten by a variety of predators including lobsters, crabs, sea stars, ocean pout, cunner, Atlantic wolfish, and humans. Green sea urchins are external spawners, meaning they release their gametes into the water column where the eggs are fertilized by the sperm. The sexes are separate. The resulting larva (termed an "echinopluteus") undergoes development planktonically for a period of one to several months before settling on the sea floor and metamorphosing into the adult form. Reproduction occurs on an annual cycle with spawning occurring in the spring, generally between February and May, but sometimes as late as June.

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Adult Sea Urchin
Shell
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urchins.jpg (4560 bytes)
(photo: NBEN-RENB)

On the east coast of Nova Scotia, the green sea urchin is the dominant grazer of seaweeds or macroalgae in rocky subtidal environments. Like many other sea urchins, the green sea urchin plays a key role in determining the structure, diversity, and distribution of subtidal macroalgal communities. Where urchins occur at high density, destructive grazing can produce habitats devoid of seaweeds. These areas may be termed "barren grounds", "coralline flats", or "Isoyake areas". When sea urchins are removed from these sites, either manually or by disease, the reduction in grazing pressure often results in the development of highly productive kelp forests. These kelp beds provide shelter for a wide variety of marine organisms (e.g. fish, lobsters, crabs, sea stars, bivalves, gastropods, bryozoans) and the habitat is typically much more diverse than barren grounds. Hence, sea urchins are one of the principal factors controlling habitat diversity in the rocky subtidal environment.

In the past, large-scale transitions between kelp beds and barren grounds have occurred off Nova Scotia in association with large fluctuations in sea urchin abundances. A population explosion of green sea urchins in the late 1960's led to the widespread destruction of kelp forests and the formation of barren grounds along much of Nova Scotia’s Atlantic coast. Mass mortalities of urchins in the early 1980's, however, allowed kelp beds to re-establish. This widespread die-off of sea urchins was a result of a pathogenic amoeba which appears to be associated with unusually high seawater temperatures in the late summer or early autumn. By the early 1990's, dense populations of sea urchins had re-occupied the shallow subtidal zone along much of the coast - presumably due to migration of adult sea urchins from deeper water which serves as a thermal refuge from the disease - and kelp beds were once again on the decline. Recurrences of the amoebic disease caused localized urchin mortalities in 1993 and widespread mass mortalities in 1995. As in the early 1980's, these recent outbreaks were associated with high seawater temperatures. Current evidence suggests that these disease outbreaks may be linked with large-scale meteorologic and oceanographic events such as hurricanes/tropical storms and warm-core rings (circular pockets of warm water). These processes may play a role in triggering disease outbreaks by transporting the amoeba from other habitats [if the amoeba is an exotic species (a point yet to be proven)] and/or by generating environmental conditions favorable for disease proliferation.

urchin1.jpg (29981 bytes)
(photo: Robert Scheibling, Dalhousie University)
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Urchin Barrens Ground
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The green sea urchin population explosion in the 1960's has been attributed to a decline in predation by the American lobster. However, a number of studies have questioned the ability of predators such as lobsters to effect large-scale changes in adult sea urchin densities and the role of predation in determining population outbreaks remains controversial. Predation may be more important in regulating recruitment of juveniles to adult populations, although little is known about predators of very young urchins. It has been hypothesized that unusually high seawater temperatures in the spring may cause population explosions of sea urchins by increasing the rate of larval urchin development in the plankton. This would decrease larval losses due to predation resulting in increased juvenile recruitment. Currently, however, this hypothesis remains untested.

One predator, that has more recently appeared on the scene, does have the ability to effectively control sea urchin abundance. Humans fish green sea urchins for their reproductive organs, termed roe or uni. The main market for sea urchin roe is Japan, which accounts for over 90% of the world's sea urchin consumption. While humans have been commercially exploiting the green sea urchin along both coasts of North America since the 1950's, harvests prior to 1990 were relatively small and sporadic. Escalating market demand, coupled with a stable high product price and the recent collapse of groundfish stocks in eastern Canada, has renewed local interest in green sea urchin harvesting. The sea urchin fishery in New Brunswick and Nova Scotia has expanded rapidly in the 1990's. Final landed values for green sea urchins in Nova Scotia have grown from 131 metric tonnes worth $162,000 in 1993 to over 1,099 metric tonnes worth $3,385,000 in 1996 - an 8-fold increase in landings and 21-fold in monetary value in a mere three years. Similarly, New Brunswick landings have escalated from 95 metric tonnes worth $86,000 in 1990 to 1,896 metric tonnes worth $3,994,000 in 1996 a 20 fold increase in landings and a 46 fold increase in dollar value over a period of only six years.

While these landings appear very promising there is a real threat of potential over-harvesting and destruction of future sea urchin populations. While fishery quotas have been set, there is very little scientific research to base them on. Recently, Nova Scotia landings have levelled off and have even fallen slightly, from 1,141 metric tonnes in 1995 to 1,099 metric tonnes in 1996. There is preliminary data to suggest that New Brunswick landings have also levelled off or decreased in the last year. Further scientific research examining population numbers, larval abundance, and annual recruitment patterns is required if we hope to avert a collapse of wild urchin stocks. Boom-bust sea urchin fisheries have already occurred in both California and Maine due to over-harvesting. Will eastern Canada be next?