Fluorochrome Marking of Out-planted Green Sea Urchins, Strongylocentrotus droebachiensis, for Sea Ranching and Restocking Programs in the Gulf of Maine, USA

Marking calcified structures with fluorochromes is done in a variety of vertebrate andinvertebrate species to tag individuals for growth, population,and ecological studies. Here, wedescribe the use of the fluorochrome tetracycline to identify hatchery reared green sea urchinsreleased on-bottom onto two aquaculture leases known as Job andSloop, located in the Gulf ofMaine, USA. This was done to examine the viability of sea ranching and stock enhancement bylooking at recovery rates and growth of reseeded juveniles overthe course of two years. 21,000hatchery reared green sea urchin juveniles (Strongylocentrotusdroebachiensis) were markedwith the fluorochrome tetracycline when they were at 10 - 20 mmtest diameter, and releasedonto 400m2study areas located at each lease. Juveniles from the same hatchery cohort weresimultaneously reared in a land-based recirculating aquaculturesystem so that sea ranching couldbe compared with tank farming. The release areas were surveyedby SCUBA divers at 3-5 monthintervals for over two years. Urchins were collected from the field, measured, and dissected toremove the jaw structures, which were then examined with fluorescence microscopy. Tetracyclinefluorescence was detected for up to 27 months post-release in recaptured urchins. Numbers ofrecaptured marked urchins fluctuated overtime, causing large variability in population survivalestimates for each site at each sample interval. Size measurements of recaptured urchins showeda decline in average test diameter at the Job site, but at theSloop site average test diameterincreased during the two year study. Green sea urchins from thesame hatchery cohort rearedin a land-based tank system had significantly better growth thanthose recaptured from eitherlease site. Environmental factors, rather than genetic factors(hatchery source), were likely thecause of the size differences observed between hatchery seed recaptured from the lease sites andthose reared in tank culture. Site factors may have resulted insize dependant mortality and/orout-migration of larger urchins. One of the limitations of themark/recapture approach with seaurchins is that dive surveys need to expand over time to account for urchin movement away fromthe release area. Given the high cost of such efforts, this maynot be practical or cost effective.Because the marked jaw structures were internally located, it was not possible to identify markedsea urchins in the field, and the animals had to besacrificed for laboratory analysis. Recentadvances in fluorochrome marking and visualization could allowfield identification of markedurchins. This would enhance the ability of resource managers toevaluate restocking programs inthe Gulf of Maine, as well as to assign provenance or ownershipof sea ranched urchins.