Maps show link between warmer water, lobster habitat

First published in Landings, April, 2015.

It’s a lot of work, picking a good place to live. You have to think of so many factors: property taxes, schools for the kids, the distance to your job. But for a juvenile lobster, finding a place to live is pretty simple. When it comes time to settle on the bottom a young lobster looks for four key factors: a rocky area at the right depth, with the proper salinity and temperature.

habitatMarine biologists have determined that for these youngsters, the ideal conditions are a depth generally less than 50 feet (15 meters), temperatures between 50 and 64.5o F. (10 and 18o C), and salinity in the range of 15-31 parts per trillion.

Kisei Tanaka, a doctoral student at the University of Maine, has produced maps showing the range of suitable lobster habitat along the Maine coast and how the amount of that habitat has changed over time. Drawing on 34 years of temperature and salinity data (1978 to 2012), he has created a suitability index for both juvenile and adult lobsters along the coast, which shows optimal environmental conditions ranked from 0 (very unsuitable habitat) to 1 (very suitable habitat). Tanaka has produced these maps for both the spring and the fall seasons because, as he said, “The lobsters have different needs in the spring and the fall.”

habitat2Tanaka’s habitat suitability index maps show a distinct change over time in the amount of inshore area available during the spring season in which young lobsters could thrive. Between 1980 and 1989, the amount of prime lobster habitat began to expand in the midcoast area, specifically around Penobscot Bay, as well as in pockets of southern Maine. In the offshore waters, no change occurred. In the 1990s, that expansion crept east along the coast, with the areas off Deer Isle and Winter Harbor showing marked expansion. During the last decade, the expansion of suitable area continued, most notably throughout Downeast Maine. During the fall season, however, Tanaka’s data do not indicate as distinct an expansion of suitable habitat.

What has caused the spread of good lobster habitat? “Temperature and salinity have changed due to climate change,” Tanaka said. He added that his maps could also be constructed to reflect the habitat needs for other benthic species, such as scallops and sea urchins. But, Tanaka cautioned, the maps are just a reflection of the data used. “[The data] need to be fine-tuned by fishermen and resource managers,” he said.