Initial exploration of available data to estimate sardine recruitment on the south coast

Abstract

Annual May/June survey estimates of sardine recruitment east of Cape Infanta are low in comparison to those west of Cape Infanta and in comparison to the numbers of recruits needed to result in the recent peak in 1+ biomass on the south coast. The assessment model for two sardine stocks (split east and west of Cape Agulhas) consequently estimates the majority of the “south” stock biomass to originate from “west” stock recruits, at least across the turn of the century (de Moor and Butterworth 2013). Although sardine are known to spawn throughout the year, this winter recruit survey is timed to measure the recruits emanating from the postulated spawning peak between September – March (van der Lingen and Hugget 2003). However, in contrast to observations on the west coast of South Africa, high sardine egg concentrations have also been observed in winter on the south coast (van der Lingen et al. 2005). This could potentially give rise to local south coast recruitment, which would not be detected during the winter recruit surveys. This document explores the extent to which winter spawning on the south coast and subsequent successful local recruitment there may have an impact on the “south” stock biomass with the data currently available, and proposes future work to explore this possibility further.