Establishing a baseline for evaluating changes in body condition and population dynamics of sardine (Sardinops sagax) in the southern Benguela ecosystem

Abstract

Standard weight (Ws) equations were developed for sardine (Sardinops sagax) in the southern Benguela ecosystem and used to provide fixed baselines for calculating relative weight (Wr) indices. The method was applied to a database of sardine length and weight measurements collected from South African coastal waters from 1953 to 2010. Statistical properties of the standard weight equations were investigated, as well as their application to sardine. Four quadratic standard-weight equations were established, using medians, means and first and third quartiles of log10-transformed weight measurements (Y) per log10-transformed caudal length class (X). A model using medians (Y=−1.825+2.679X+0.224X2) is proposed as the preferred baseline for Ws values for southern Benguela sardine, and is used to calculate Wr values over time and space. The Wr indices showed periods and areas of varied fish condition, with Wr ranging from 50 to 150 for individual fish and 97 to 106 for annual averages. The mean annual Wr index was low during the 1950s and increased to peak levels during the 1970s and 1980s, after which it showed a steady decline to 2010. There were pronounced spatial differences in sardine Wr, with both the lowest and the highest mean values seen in sardine from the upper and lower west coast, respectively, and fish from the southwest and south coast showing little difference in mean Wr. Possible causes of these temporal and spatial dynamics include environmental and ecological factors. Using sardine as an example of an exploited fish species in a changing ecosystem, we show how the Wr index could indicate the status of the population's body condition. Body condition is an important factor in fish reproduction and a proxy for habitat quality in an ecosystem. Additional information on ecosystem status could be obtained when the index is applied to other populations and species.