Biology Question #311

David Cheverie, a 40 year old male from the Internet asks on December 19, 1997,

How do you know what size lobsters should be released instead of taken, in order to ensure the survival of the species?

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The answer

Douglas Pezzack answered on December 19, 1997

This question comes up again and again at fishermen's meetings. The minimum size for retention is designed to allow more animals to survive to the size at which they can reproduce. This is generally larger than the size at which they can legally be caught. In some areas they must survive 2-3 years of fishing pressure before they are large enough to reproduce. Thus the size increase is to better match legal size with size of maturity.

At the present removal rates of 70-80% of the lobster population per year in most lobster fisheries, an increase in minimum size has the greatest benefit. The reason for this can be demonstrated with a bit of math. With 75% removal rates only 25% of the animals which grow into the legal size range are left at the end of the first fishing season. Thus of 1000 animals entering the fishery at 2.5" only 250 will be left at the end of the season. By the end of the second year that would be only 62 and after the third year only 16. It takes 4-5 years to grow from legal size of 2.5" to reach 4" and at that point only 1-4 of the original 1000 remain. Thus a maximum size protects very few animals and if removal rates increase the number gets even smaller. The advantage of a maximum size is that it can result in a pool of mature animals, which can help to buffer against year to year changes in the number of animals entering the fishery and reaching maturity; however to be effective more must reach the maximum size than presently do. A minimum size increase of 1/8" in contrast provides one extra year protection to all lobsters between 2 1/2 and 2 5/8" and the calculated benefits are much higher than for a 4" maximum size. A side benefit to the fishermen is that the lobsters thrown back will grow and come back the next year at a larger size worth more money. On average they increase 10% in length and 50% in weight. A size increase should result in an increase in average size and landed weight putting more money in the fishermen's pocket. On the downside, the maximum size removes the large lobster from the catch which would result in a small loss of landings.

The best solution would be combination of minimum size increase and a maximum size which gives the best of each: a large increase in egg production, an increase in the weight and value of the lobster and a small reserve of larger sizes to help add greater stability to the level of egg production.

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