Total mortality rates of the barndoor skate, Dipturus laevis, from the Gulf of Maine and Georges Bank, United States, 1963–2005

The barndoor skate (Dipturus laevis) population in the offshore waters of the Northeast United States was reported to be possibly on the brink of extinction in 1998. Managers were faced with the task of assessing a population with limited life history information and survey data that contained only a few animals a year. One of the key pieces of information to assess the threat to the species is the current mortality rate. Unfortunately, estimating this parameter through classical approaches proved difficult. In this study we develop variants of commonly used methodologies to estimate total mortality. We present two methods: a length based estimator and one based on the catch rates of recruits and adults. The approaches provided similar results suggesting that mortality rates were very high (0.89–1.0 year−1) in the late 1960s. Recently, and at approximately the same time as the large area closures on Georges Bank, the total mortality rate appears to have been reduced to very low levels (around 0.23 year−1).