Top-down control of lobster in the Gulf of Maine: insights from local ecological knowledge and research surveys

Abstract

American lobster Homarus americanus landings in the Gulf of Maine have been steadily increasing since the 1980s. As a result, lobsters now support one of the most important fisheries in the USA and Canada. One hypothesis for this pattern is that lobsters have been released from predation as groundfish stocks declined, expanding both in abundance and habitat. Lobster habitat is typically rocky substrate of the inshore region, which is difficult to sample. Some long-term fisheries-indepen- dent abundance indices for lobsters and their predators are available for the Gulf of Maine in the USA, but not in Canada. To try and fill those research gaps, we designed a local ecological knowl- edge (LEK) survey. Semi-structured interviews of 42 fishermen in southwest Nova Scotia, Canada, revealed consistent trends of the depletion of large groundfish, particularly Atlantic cod Gadus morhua. Eighty-three percent of fishermen concluded this depletion was the main reason for an observed increase in lobster abundance. They also reported the expansion of lobsters to new habitats and depths. We further show that the proposed top-down control mechanism is independently sup- ported by USA research trawl surveys, which revealed a negative correlation (p < 0.05) between the summed abundance indices of 5 groundfish predators of lobster and lobster abundance (kg tow -1 ) at time lags ranging from 0 to 9 yr. Survey-based diet data also corroborated direct observations by fish- ermen on lobster predation by groundfish. These results suggest that LEK may be a useful supple- mentary tool to investigate the ecosystem effects of fishing, particularly in data-poor situations.