American lobster nurseries of southern New England receding in the face of climate change

Historically, southern New England has supported one of the most productive American lobster (Homarus americanus) fisheries of the northeast United States. Recently, the region has seen dramatic declines in lobster populations coincident with a trend of increasingly stressful summer warmth and shell disease. We report significant declines in the abundance, distribution, and size composition of juvenile lobsters that have accompanied the warming trend in Narragansett Bay, Rhode Island, since the first comprehensive survey of lobster nurseries conducted there in 1990. We used diver-based visual surveys and suction sampling in 1990, 2011, and 2012, supplemented by post-larval collectors in 2011 and 2012. In 1990, lobster nurseries extended from the outer coast into the mid-sections of the bay, but by 2011 and 2012 they were largely restricted to the outer coast and deeper water at the mouth of the bay. Among five new study sites selected by the lobster fishing industry for the 2011 and 2012 surveys, the deepest site on the outer coast (15-17 m depth) harboured some of the highest lobster densities in the survey. Separate fixed site hydrographic monitoring at 13 locations in the bay by the Rhode Island Division of Fish and Wildlife recorded an approximately 2.0 degrees C increase in summer surface temperatures over the period, with 2012 being the warmest on record. Additional monitoring of bottom temperatures, dissolved oxygen and pH at our sampling sites in 2011 and 2012 indicated conditions falling below physiological optima for lobsters during summer. The invasion of the Asian shore crab, Hemigrapsus sanguineus, since the 1990s may also be contributing to declines of juvenile lobster shallow zones (<5 m) in this region. Because lobster populations appear increasingly restricted to deeper and outer coastal waters of southern New England, further monitoring of settlement and nursery habitat in deep water is warranted.