Impact of invalid biological assumptions and misapplication of maturity criteria on size-at-maturity estimates for American lobster

Size at maturity is an important tool for fishery management of the American lobster Homarus americanus, but many of the size-at-maturity assessments conducted to date have been confounded by errors of biological fact, misinterpretations of results, and misapplications of maturity assessment criteria. The result has been considerable uncertainty about the magnitude of egg production for various stocks of the American lobster, an undesirable situation for fishery managers. Size-at-maturity studies require an in-depth understanding of the biology of molting and reproduction. The investigator must first determine the physiological state of the captured lobster and then estimate when molting or spawning will occur. Conclusions can then be drawn regarding the maturity of the animal. Three elements are important in the design of maturity studies: the criteria used to discriminate between immature and mature lobsters, the time of year the study is conducted, and the sizes and numbers of lobsters sampled. Maturity assessments should be conducted in the late spring, prior to the onset of the spawning and egg hatching seasons, when almost all mature lobsters are either ovigerous or preovigerous. Cement gland development should be used initially to separate preovigerous (mature) lobsters from those of uncertain reproductive status. Those of uncertain status can then be classified on the basis of ovary stage. If lobster ovaries are at stage 1 or 2, the lobsters are immature and will molt to a larger size before spawning. If lobster ovaries are at stage 3 or 4a, the lobsters are immature if their oviducts are clear and mature if the distal portion of their oviducts is yellow. Lobsters with ovaries at stages 4b to 6 are mature and should spawn within a few weeks. Once fishery biologists adopt a standard sampling protocol and begin applying validated maturity assessment criteria, size-at-maturity estimates will become comparable, reliable, and repeatable.