The American lobster (Homarus americanus) lives in the cold waters of the Northeastern US and Canada, including Long Island Sound, and range offshore as far south as Virginia. In US waters three separate lobster stocks are recognized: the Gulf of Maine where they are very abundant, Georges Bank where populations are stable or increasing, and southern New England where abundance has dramatically declined in the last decade.
Lobsters are social, territorial crustaceans that live in a variety of different habitats, preferring areas that have a rocky or firm mud bottom they can burrow into. Studies have shown that inshore lobsters like those in the Sound have localized movements, while offshore lobsters seasonally migrate from as far as the edge of the continental shelf to inshore waters to spawn in late spring and summer.
Because most of the Sound’s lobster population does not migrate, the standardized catch of lobster from the CTDEEP Long Island Sound Trawl Survey is used to assess the relative abundance of the local stock. The trawl index is a more accurate representation of abundance than lobster landings because landings are influenced by how each fisherman sets their gear, how often they fish, and socio-economic factors such as market price.
The above graph shows the trend of lobster abundance in the fall (note that no data are available for 2010). The spring index for American lobster has also been declining for 12 out of the past 13 years (1998-2011) and both indices have remained below the time-series average for the past nine years (2003-2011). One of the factors implicated in the lobster population decline is increased summer water temperatures in Long Island Sound.
The above graph depicts data from CTDEEP’s fall trawl survey.
The survey uses geometric mean instead of arithmetic mean to find the most frequently observed number of fish collected per tow. In a natural environment such as Long Island Sound fish have a “patchy” distribution, i.e. some areas will have a very high abundance of fish, and other areas very low abundance or no fish at all. An arithmetic mean can be easily biased by unusually high or low values so that it doesn’t reflect the true center of a data set. The geometric mean minimizes the effects of very high or low values using a log transformation and is a better average for this type of biological data.