ORONO, Maine -- The discovery of a type of slow growing sea urchin that never attains legal size for harvesting in Maine’s coastal waters has been reported by a team of scientists led by Robert Vadas, marine biologist at the University of Maine. If the finding is supported by further research, it suggests that harvesting legal size urchins could cause a shift in the urchin population toward a non-harvestable stock.
The report was published in the scientific journal Ecological Monographs in February. Vadas is a professor in the UMaine Department of Biological Sciences and School of Marine Sciences. Co-authors include Barry D. Smith of the Canadian Wildlife Service, Brian Beal of the University of Maine at Machias and Tim Dowling, a former UMaine graduate student now of Tenants Harbor. The Island Institute of Rockland assisted by providing ship time.
Worldwide, the U.N. Food and Agriculture Organization reports that fishermen harvested about 262 million pounds of urchins in 1999.
In Maine, the urchin harvest is down considerably from its 1993 peak but still brings in significant revenue. In 2000, harvesters landed about 12 million pounds of urchins worth more than $17 million. The 1993 record of 41 million pounds was worth about $26 million, according to the Maine Department of Marine Resources. Vadas and his colleagues began planning the research in the mid-1990s. "Our original idea was to use a new technique that I had brought back from Norway for determining the age of sea urchins," says Vadas. "We wanted to understand basic aspects of urchin biology, such as their life expectancy, how fast they grow and when they reach reproductive age. Finding two separate groups within the population was a real surprise to us."
The scientists collected urchins from carefully selected spots in two locations along the Maine coast, the Schoodic Peninsula just east of Ellsworth and Allen Island southwest of Port Clyde. After tallying the ages, size and other characteristics of the samples, Smith developed and applied a statistical model that sorts out groups within a population. He found that the urchins at Allen Island fell within one of two groups, a fast growing variety that attained legal size and a slow growing variety that did not. Members of both groups were of similar ages.
In contrast, only a single fast growing group was identified in the Schoodic sample. Those urchins had similar growth and size characteristics across a range of ages.
"At this point we don't know the cause of this division into slow and fast growing groups," says Smith. "We have two competing hypotheses that need to be tested. This division could be related to genetics or to demographic factors in this urchin population."
The researchers explored a variety of possible explanations for the difference in the two groups of Allen Island urchins. They considered factors such as genetics, gender, food availability and urchin settlement patterns. They determined that the small, slow growing urchins shared the same habitat with a fast growing population.
"This sort of finding isn’t unusual in marine populations," says Vadas. "It can be caused by environmental factors or predation. We know that in some areas, flounder, crabs and other animals prey on urchins. However, that doesn’t seem to be what’s happening here."
To help scientists understand why two subgroups exist, Vadas and Irving Kornfield of the UMaine School of Marine Sciences, are proposing new research on urchin genetics. In addition, Vadas is continuing to study the urchins’ growth and environment.
"If the cause has a genetic basis," says Vadas, "then scientists will have to change their models of population growth. Regulators will have to decide how to maintain a commercially viable population, perhaps through re-seeding with fast growing urchins or putting an upper limit on urchin harvesting to protect the broodstock."
Beal points out that if researchers find slow and fast growing populations at other locations, urchin harvesters and regulators may have to adopt new practices to ensure a stable commercial harvest.
"Because size is closely related to urchin fecundity (amount of eggs a female produces), a population of small urchins may not reproduce at a rate necessary to keep urchin populations vigorous," says Beal.
"The ultimate response might be for fishermen to agree to set aside small areas along the coast where harvesting would be off limits so that animals in those ‘spawner sanctuaries’ would have an opportunity to reproduce annually."
Funding for the study came from the Maine Sea Grant College Program, the Maine Department of Marine Resources and the Maine Agricultural and Forestry Experiment Station.
Journal
Ecological Monographs