Declining female size at onset of maturity in Nephrops norvegicus in long-term surveys (1997–2016)

Abstract

Abstract Data on the demography and reproduction of marine species provide important information for a sustainable fisheries management. We show that the size at onset of maturity in female Nephrops norvegicus has been in decline for over 20 years and has always been above the established minimum landing size (MLS) in the Western Irish Sea. Determining the size at onset of maturity is one important factor to inform an effective MLS, such that individuals can reproduce at least once. The length at which half of females are sexually mature (L50) in the overall population declined by over 12% in two decades, from 23.6 mm in 1997 to 20.6 mm in 2016, while the MLS has remained at 20 mm. While L50 values differ among permanent sampling stations, the decline was observed at all stations. Current practice thus allows immature females to be landed before they reproduce and contribute to recruitment. While it is not always possible to identify the determining factors that drive the decline in SOM, we argue that it is appropriate to recognize this as an indicator of declining system productivity.