the population biology of the thornback ray, raja clavata in caernarfon bay, north wales

Abstract

amid concern about the status of skates and rays in welsh waters the present work set out to provide up-to-date information on the population biology of raja clavata in caernarfon bay. the von bertalanffy growth parameters were l_∞=100.9 cm, k=0.18 y⁻¹ and t₀=−0.95 y for males; and l_∞=117.6 cm, k=0.16 y⁻¹ and t₀=−0.7 y for females. male r. clavata reached 50% maturity at 58.8 cm and 3.9 y, which was smaller and younger than females (l₅₀ and a₅₀ were 70.5 cm and 5.3 y, respectively). although previously over-exploited, the present estimate of total mortality (z) was 0.48–0.49 y⁻¹ which, in terms of replacement mortality (calculated as 0.7 y⁻¹), suggests no immediate prospect of recruitment failure for r. clavata in caernarfon bay. the potential for the recovery of a depleted population is discussed in relation to the allee effect.