The biology of the skates raja wallacei and r.pullopunctata (batoidea: rajidae) on the Agulhas bank, South Africa

Abstract

Aspects of the biology of Raja wallacei and R. pullopunctata were investigated using data collected in 1995 and 1996 from research and commercial trawls on the Agulhas Bank, South Africa. Age and growth parameters were investigated by examination of bands on the vertebral centrum. Estimates of Von Bertalanffy parameters for R. wallacei were L‡ = 405.40 mm disc width (DW), K = 0.27 and t0 = . 0.08 years for males and L‡ = 435.23 mm DW, K = 0.26 and t0 = .0.21 years for females. For R. pullopunctata, the estimates were L‡ = 770.50 mm DW, K = 0.10 and t0 = .2.37 years for males and L‡ = 1 326.75 mm DW, K = 0.05 and
t0 = .2.20 years for females. Growth was significantly different between sexes for both species. The oldest aged R. wallacei and R. pullopunctata were 15 and 18 years respectively. The length at first maturity was approximately 350 mm DW or 7 years of age for R. wallacei and 600 mm DW or 9 years of age for R. pullopunctata. The length-at-50% maturity for R. wallacei was 395 mm DW for males and 400 mm DW for females. R. wallacei fed primarily on benthic teleosts and crustaceans. There were significant differences (p < 0.05) in the diet between research- and commercially caught fish, in terms of percentage frequency of occurrence and volume of prey.