Tracking cod diet preference over a century in the northern Gulf of Maine: historic data and modern analysis

Limited information is available regarding long-term compensation in the food webs of exploited ecosystems. We compared the results of 3 studies that assessed the diets of Atlantic cod from Passamaquoddy Bay, Maine, USA, spanning 100 yr, using diet summary statistics to compare historical and modern datasets. We found that 1965 cod consumed fewer invertebrates than cod caught in 1896, but that cod caught in the period 2005 to 2008 ate diet items more similar to the 1896 fish. Large cod caught in the summer of 1965 primarily ate fish, particularly Atlantic herring, whereas few fish were found in the diets of 2005 cod. Documented changes in diet may reflect changes in technology and resource exploitation that altered the prey field available to cod over time. In the 1890s, the herring and finfish fisheries were widespread and landings were large, but other components of the food web, particularly invertebrates, were comparatively unexploited. Impacts on the benthos of Passamaquoddy Bay increased by the 1960s with the widespread use of heavy dredge and trawl gear. By 2005, most fisheries had abandoned Passamaquoddy Bay, and cod and herring were reduced in numbers. Ecosystem resilience can be seen in the return of modern cod to the 1896 diet, but compensatory dynamics in the food web appear to have favored benthic, then pelagic, and finally a ‘balance’ of diet constituents, but with fewer commercially important finfish in the ecosystem overall. When and where the data exist, 18th and 19th century accounts of fishery activity can be used to put modern trends in perspective.