Changes in baseline growth and maturation parameters of Northwest Atlantic porbeagle, Lamna nasus, following heavy exploitation

Abstract

We tested for density-dependent changes in growth and maturation of Northwest Atlantic porbeagle (Lamna nasus) after the population declined by 75%-80% from fishing. Vertebrae and reproductive data collected from the virgin (1961-1966) and exploited (1993-2004) populations were analysed to test for differences in growth rate and age and length at maturity between the time period, using likelihood ratio tests. Beyond the age of 7 years, mean length at age was greater during 1993-1994 than during 1961-1966. Between 1961-1963 and 1999-2001, length at maturity decreased in males (from 8 to 7 years) and females (from 19 to 14 years). An analysis of porbeagle temperature associations indicated that sharks occupied comparable temperature conditions during the mid 1960s and 1990s, ruling out the possibility of temperature-induced growth changes. The observed increase in growth rate and decrease in age at maturity following exploitation support the hypothesis of a compensatory density-dependent growth response.