Commercial Fisheries News - February 2001

Juvenile lobster abundance

Postlarval lobsters that settle to the bottom grow to become adult lobsters harvested in the fishery. It makes sense for us to try to monitor the abundance of recently settled and early juvenile lobsters so that we can detect declines in lobster abundance before we see declines in catch.

Some newspapers have recently reported downward trends in abundance of larval and early juvenile lobsters. Data collected by scientists at The Lobster Conservancy (TLC) do not support these claims. A critical comparison of the methods used to measure abundance by those who reported declines vs. those used by TLC may explain the discrepancies.

Measures of abundance are based on numbers per unit area. Simple counts cannot be compared to other counts. Some reliable, objective, fixed effort must be included. The most acceptable is number per unit of area, which, in my case, is number of lobsters per meter squared. Choosing the relevant times and places to sample are critical to understanding changes in lobster abundance over time. Plausible explanations for such differences include when and where abundance was measured.

Reports of declines in the abundance of recently settled lobsters were based on an index developed through data gathered by researchers surveying in September of each year.

In contrast, TLC scientists take monthly samples year-round, which offers a more complete picture of settlement throughout the year. Although TLC data shows a strong seasonal cycle, the peak abundance of recently settled lobsters generally occurs in October, not September. Therefore, declines in September abundance may not mean that lobsters failed to settle in a given year.

Where abundance is measured is also important. Researchers are not covering enough sea floor to have an adequate picture of what is going on throughout the range of the lobster stocks. This shortcoming is being remedied by several efforts including TLC’s Volunteer-Based Juvenile Lobster Monitoring Program.

In addition to year-round sampling by TLC scientists in Friendship and Harpswell, ME, TLC has recruited community volunteers who measure juvenile lobster abundance at 24 sites distributed throughout the Gulf of Maine from May through October of each year.

How lobsters make use of the locations where they settle is also of interest. Repeated captures of tagged individuals in Harpswell have shown that some juvenile lobsters continue to live in the area where they settled for the first four years of life. Although tagged lobsters have been recaptured during all months of the year, most juveniles undertake seasonal migrations, usually disappearing in November and returning in May.

While providing much needed data on lobster abundance, volunteers participating in the juvenile lobster monitoring program are becoming stewards of coastal habitats. All lobster nurseries identified thus far occur close to shore where the impacts of human activities are greatest. Recognizing and monitoring lobster nursery habitats puts local citizen scientists in a position to better protect the interests of their lobstering communities.

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