Reproduction and movement patterns of female lobsters
Lobsters are unusual for invertebrates because they are very long-lived (potentially more than 60 years). This makes them an interesting model in which to study reproductive senescence, or the decline in reproductive output and ability prior to death. To answer this question we have been going out with the fishermen of Lobster Fishing Area 38 (LFA 38; around Grand Manan) and counting and sampling eggs from females of different sizes. Females develop their eggs internally for a year, then extrude and fertilize them and let them develop externally (under their tails) for another year. Females carrying eggs externally are referred to as “berried”; berried females cannot be landed by the fishery and must be returned to the sea – but we can sample them first!
Since 2008 we have examined more than 1700 berried female lobsters. One of the problems is that we don’t yet have a way to accurately age lobsters, since they moult their hard parts every few years, and we are forced to use size as a relative index of age. Data thus far suggest that some very large (presumably very old) females may be producing fewer eggs than expected. In addition, egg size, and measures of egg quality are lower in very large females (those with carapace length >160 mm) than in other size classes. However, with such a large group of animals it is necessary to sample many individuals over several years in order to see ecologically valid trends – and there is a lot of variation from year to year! We will continue our sampling effort – during the spring, summer and in December as well – to better understand trends in our data before we draw any strong conclusions.
We are also trying to determine where berried female lobsters move to between fall and spring. We know that larger females tend to be found offshore, and smaller ones inshore, and that lobsters slowly migrate out to deeper water as it gets colder. However we would like to see exactly where berried females of different sizes are found. The last time anyone carried out a similar study on berried females was in the 1980’s, and some of those lobsters ended up traveling more than 30 km (some moved several hundreds of kilometers). We would like to see if anything has changed in the 30 years since that study was done because fishing practices have changed significantly, and the ocean is a much different place, with less groundfish, and warmer water temperatures. For the first phase we have put out more than 600 numbered plastic zip ties on berried females and the fishermen of the Bay of Fundy will help us to see there they go when they catch these females in their traps. Once we know movement patterns, we will put out small tags that record depth and temperature to see what the “thermal histories” of berried females are over the year-long egg development period.