Re-assessing the growth parameters for the razor clam, Ensis siliqua, from Scottish electrofishing grounds using external shell marks and oxygen isotope validation

Abstract

Landings of razor clams by global capture fisheries have increased by around 15 times since the turn of the century. Harvesting techniques range from hand-picking and use of brine solutions to force the clams to emerge, to more intensive mechanical and hydraulic dredging. In Scotland, electrofishing for the pod razor (Ensis siliqua) is presently the focus of a government run scientific trial. Growth estimates are an important component of fisheries management contributing to yield-per-recruit and natural mortality estimation. However, growth rate estimates should be periodically re-evaluated, particularly when harvesting practices have changed. Interpretation of external shell growth increments alongside validation by oxygen isotope analyses was used to generate length-at-age data for E. siliqua collected from five sites within the Scottish electrofishing scientific trial. Von Bertalanffy growth curves were then fitted to these data using a hierarchical Bayesian approach. Across all five sites the mean of the asymptotic length (L_∞) was estimated to be 191 mm and the mean of the Brody coefficient (k) estimated at 0.27 y⁻¹. The 95 % credible intervals for L_∞ were from 182 to 206 mm, and for k were between 0.17 and 0.33 y⁻¹. These new estimates are consistent with previous published results from Scottish sites suggesting that recent changes to harvesting practices have not led to substantial changes in E. siliqua growth rates.