The role of food limitation in lobster population dynamics in coastal Maine, United States, and New Brunswick, Canada

Lobster (Homarus americanus) landings in Maine, United States have increased steadily over the past two decades to levels that are consistently more than triple the 40-year (1950 to 1990) annual average of c. 9000 MT. Meanwhile, the use of herring (Chipea harengus) as bait has increased fourfold since the 1970s, and is currently subsidising lobster productivity in mid-coast Maine by augmenting the diet and growth of large juvenile and young adult lobsters before they recruit to the fishery. We investigated whether herring bait in the eastern portions of the Gulf of Maine is also subsidising these lobster populations. In 2004, we compared stomach contents, tissue production via stable isotope ratios, and growth rates of lobsters from seasonally open (Cutler, Maine, United States) and closed (Dipper Harbour, New Brunswick, Canada) fishing areas in the spring, summer, and autumn. Contrary to previous results in mid-coast Maine, lobsters from seasonally closed sites at Dipper Harbour outgrew those from fished sites around Cutler. Yet examination of the diet of lobsters revealed several differences between open and closed sites that did not exist at sites in mid-coast Maine. For instance, lobsters at Dipper Harbour consumed markedly more bivalves than those at Cutler. Furthermore, quantification of trap densities and the number of licensed fishers in mid-coast versus eastern Maine determined that fishing effort was greatly reduced in eastern Maine. Thus, our results indicate that herring bait subsidies are not important in eastern portions of the Gulf of Maine, but the availability of natural prey seemingly limits the growth of lobsters in eastern Maine. Moreover, our study suggests that bottom-up forcing (i.e., food limitation) can have important consequences for lobster population dynamics and the productivity of lobster fisheries.