After 16 years under a limited access program with effort controls, the New England groundfish fishery transitioned to a catch share management system in 2010. For much of its earlier management history issues related to fishing capacity were paramount as effort controls were increasingly restrictive to meet biological objectives. As the size of the active fleet declined from over 1,000 vessels from 1994-2001 to less than 400 vessels in 2012 the management concern shifted to fleet diversity. Fleet diversity has been cast in terms of vessels based on characteristics such as size, gear, and region rather than their share in landings or economic value. This lends itself to indices used to measure biodiversity such as richness, and the numbers equivalent of the Simpson’s Index and Shannon Index. The paper describes the management context surrounding the transition from concerns over capacity to fleet diversity and provides estimates of fleet diversity from 1996 to 2012. The results indicate that fleet diversity as measured by the Shannon Index has declined by approximately 35% from 1996 to 2012 yet there were 40 vessel types or “species” that were present in all 17 years. These vessel types accounted for about 85% of active groundfish vessels and over 90% of total groundfish landings in all years. Thus, even though the size of the fleet and overall diversity has declined the “core” groundfish fleet remains.