Aspects of the biology and length-based assessment of the chub mackerel scomber colias (scombridae) stock off the coast of Ghana

Abstract

The biology and population dynamics of the chub mackerel Scomber colias (Gmelin 1789), one of four small pelagic fish of high economic and nutritional value in Ghana waters, were studied from February 2016 to July 2017 to obtain information for management of its fishery. Total length of the fish in the artisanal landings ranged between 13.7 cm and 40.7 cm with two modal length classes. The von Bertalanffy growth parameters estimated using the ELEFAN method were L∞ = 42.5 cm TL and K = 0.28 yr-1, and the mortality parameters were M = 0.70 yr-1, F = 0.52 yr-1 and Z = 1.22 yr-1. Monthly analysis of growth marks in the otoliths indicated deposition of an incremental (opaque) zone in August, which happens to be the peak upwelling period in Ghana waters. The species had a maturity size (Lm50) of 25.6 cm TL, maturity age (tm) of 2-3 years, and longevity (tmax) of 10 years. Asynchronous egg development and batch spawning was suggested by the presence of two distinct size groups (0.4 mm and 0.8 mm) of ova in mature ovaries. A mean ( s. e) absolute fecundity of 48,238 ± 2834 eggs and relative fecundity of 240  10 eggs g-1 was estimated for the fish. Spawning was estimated to occur between February and September with peaks in March-April and July-August. Fish were recruited into the stock throughout the year with two peak recruitment periods in April and August. Food of the chub mackerel comprised a wide range of invertebrates with fish, mainly anchovies (Engraulis encrasicolus), as the preferred item. Comparison of the current exploitation rate (Ecurr = 0.43) and optimum exploitation rate (E50 = 0.39) suggests that the chub mackerel stock may be overexploited, as generally assessed for the small pelagic fishery in the country. The status of the chub mackerel in trawl by-catch, and proximate analysis of the fish is also discussed.