Age and growth studies on Coryphaena hippurus (Linnaeus, 1758) in Maltese waters

Abstract

Age, growth and reproduction of the dolphinfish Coryphaena hippurus Linnaeus, 1758 collected in the period 2004-2010 from the traditional Maltese fish aggregating devices (FAD) and surface longline fisheries were studied. The a and b parameters of the length-weight relationship for fish 11-142 cm fork length (FL) (n = 4042) were determined as a= 0.017 and 0.024 with b = 2.87 and 2.76, for males and females respectively. Annual increments from dorsal spines (n = 47) and scales (n = 73) from dolphinfish > 65cm FL, and daily increments from sagittal otoliths (n = 583) from dolphinfish < 65 cm FL, were estimated; the von Bertalanffy growth model applied to these fish gave the following parameters: Lw= 107.8 cm FL and 120.2 cm FL, K = 1.9 yr -I and 1.56 yf1, for males and females respectively. The maximum age observed was 2 years. Daily growth increments for age-0 dolphinfish of< 65 cm FL (n = 566) gave an average growth rate of 5.1 mm/day (± 0.8 SD). Growth rate was fastest for fish below 35 cm FL. Size at 50% maturity for fish 10-131 cm FL (n = 1376) was 58.9 cm FL and 62.5 cm FL for males and females respectively. The sex ratio for the fish sampled from the FADs fishery (10.5 - 131 cm FL) was 1:1.54 (M:F) which was statistically significantly different from 1: 1; the largest proportion of females was for size classes smaller than 60 cm FL. Overall sex ratio for dolphinfish sampled from the surface longline fishery (91 - 130 cm FL) was not significantly different from 1: 1 (1:0.76 M:F). Monthly gonado-somatic index values (n = 1910) suggest a spawning period from May to October; back-calculated hatch dates from age-0 dolphinfish (n = 518) suggested that dolphinfish spawning occurs from June to September with the highest levels in June. An updated and improved age-length function for dolphinfish from the central Mediterranean based on both adult and juvenile fish and using dorsal spine annuli and daily growth increments is provided, together with additional information on central Mediterranean dolphinfish reproduction. This study is the first one to use dorsal spines to determine the age of adult dolphinfish in years.