Changes in population structure of the white grouper Epinephelus aeneus as a result of long-term overexploitation in Senegalese waters

Abstract

In Senegal, a significant decrease in catches indicates that many demersal fish stocks are being overexploited. The white grouper Epinephelus aeneus, locally known as the ‘thiof’, is exploited by both small-scale and industrial fisheries. A 28-year database of E. aeneus catches along the Senegalese coast provided by the Centre for Oceanographic Research of Dakar-Thiaroye, and size at maturity measured in Dakar (Senegal) from monthly samples in 2010, were used to analyse changes in population structure in the area over the past 37 years. Catches from the northern fishing areas were lower than those from the southern fishing areas, and decreased steadily during the period (Kolmogorov–Smirnov test, D = 0.243, p = 0.0002). The individual mean weight of catches decreased from 1974 to 2010 (linear regression, r² = 0.40, n = 37) and only 60% of the individuals were mature. The calculated sizes at maturity were 49 cm total length (TL) for females and 55 cm for males, and the optimal length of capture for a sustainable fishery was 96 cm, but only 0.03% of E. aeneus caught reached this length. Most of the catch consisted of juveniles; the larger reproductive individuals had disappeared. The number of individuals caught decreased significantly between 1974 and 2010 (1974–1983, r² = 0.98, n = 74 674; 1984–1993, r² = 0.95, n = 96 696; 1994–2003, r² = 0.93, n = 12 619; 2004–2010, r² = 0.91, n = 12 887), whereas the length range remained the same (10–110 cm TL). Biological indicators clearly showed that E. aeneus stocks in Senegal are overexploited and the species is now endangered. Immediate active management of fishing pressure is needed, therefore, to maintain E. aeneus populations in the area. Our results suggest a minimum size of

Keywords: biological indicators, fishing pressure, length at maturity, size spectrum, West Africa

African Journal of Marine Science 2013, 35(4): 465–472