Research Article Mediterranean Marine Science Indexed in WoS (Web of Science, ISI Thomson) and SCOPUS The journal is available on line at http://www.medit-mar-sc.net DOI: http://dx.doi.org/10.12681/mms.706 Age, Growth and Reproduction of Coryphaena hippurus (Linnaeus, 1758) in Maltese Waters, Central Mediterranean M. GATT1, M. DIMECH1 and P.J. SCHEMBRI1 1 Department of Biology, University of Malta, Msida MSD 2080, Malta Corresponding author: [email protected] Handling Editor: Kostas Stergiou Received: 24 November 2014; Accepted: 22 January 2015; Published on line: 27 April 2015. Abstract Age, growth and reproduction of the dolphinfishCoryphaena hippurus Linnaeus, 1758 collected from the Central Mediterranean in the period 2004-2010 by 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 = 0.018 and 0.022 with b = 2.85 and 2.79, for males and females respectively. The counting of annual increments from dorsal spines of >65 cm FL dolphinfish at X25 magnification (n = 47) permitting an age reading resolution in years, and the counting of daily increments from sagittal otoliths of <65 cm FL dolphinfish at X400 magnification (n = 583) permitting an age reading resolution in days, were estimated; the von Bertalanffy growth model applied to these fish gave the following parameters:L∞ = 107.8 cm FL and 120.2 cm FL, and K = 1.9 yr-1 and 1.56 yr-1, for males and females respectively. The maximum age observed was 2 years. 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 FAD fishery (10.5-131 cm FL) was 1:1.54 (M:F) and for dolphinfish sampled from the surface longline fishery (91-130 cm FL) was 1:0.76 (M:F). Back-calculated hatch dates from age-0 dolphinfish (n = 518) suggested that spawning occurs from June to September with the highest levels in June. This study is the first to use dorsal spines to determine the age of adult dolphinfish. Keywords: Coryphaena hippurus, Malta, age determination, von Bertalanffy, growth model, otoliths, dorsal spines. Introduction recently become more common due to a greater yield of catch. During the day, artificial trolling lures are used to Dolphinfish are members of the family Coryphae- lure any fish that may be present around the FADs. Night nidae which only includes one genus with two species: fishing involves localisation of the shoals close to the Coryphaena hippurus (Linnaeus, 1758) and Coryphae- FADs by means of powerful searchlights. If the number na equiselis (Linnaeus, 1758). Dolphinfish are found in of fish makes the fishing operation worthwhile, the boat tropical and sub-tropical surface waters of the Atlantic, Indian and Pacific oceans (Gibbs & Collette, 1959). makes a complete circle around the shoal. Historical land- Coryphaena hippurus occurs in the Mediterranean ings of dolphinfish in relation to the landings of two other from May - December, when the surface water temper- major targeted species by the Maltese fisheries sector are atures reach >16-18oC (Massuti & Morales-Nin, 1995). shown in Figure 2. In the sea around the Maltese Islands, adult dolphinfish Several dolphinfish age and growth studies have (63-117 cm FL) are mainly caught as by-catch from the shown that these fish are short lived, fast growing spe- swordfish and bluefin tuna longline fisheries in spring and cies (Beardsley, 1967; Rose & Hassler, 1968) and their summer. Juvenile fish (14-70 cm FL) are exploited by sur- maximum longevity has been estimated at four years. rounding net (lampara & purse seine) in Malta, Majorca, Longevity of dolphinfish from Florida was estimated to Sicily and Tunisia from late August to December. The be four years, but only 2% of the population was found Maltese “kannizzati” fish aggregating device (FAD) fish- to be older than two years (Beardsley, 1967), and only ery targeting dolphinfish is a traditional one in Malta. The 4% in North Carolina (Rose & Hassler, 1968). The maxi- FADs consist of a float which is anchored by a heavy piece mum lifespan of the Southern Caribbean dolphinfish of limestone (minimum of 70 kg) with tied palm fronds as stock does not appear to exceed 18 months, and few indi- shown in Figure 1. The net used in the FAD fishery is a viduals of the North Caribbean stock live longer than two modified purse seine, in view of the absence of pursing years (Oxenford & Hunte, 1999). Dolphinfish of 55cm rings. Fishermen fish these anchored FADs either during fork length (FL) read from otoliths and caught off Ma- the day or the night. Fishing the floats during the night has jorca were reported to be 176 days old and specimens of 334 Medit. Mar. Sci., 16/2, 2015, 334-345 Fig. 1: FADs setup (Adapted from Farrugia Randon, 1995). Fig. 2: Comparison of dolphinfish, swordfish and bluefin tuna landings (Data supplied by the Agriculture and Fisheries Regulation Department, MRRA) dolphinfish caught off the Canary Islands within a range of 76 to 103 cm FL were found to be two and three years for these ages of Florida and North Carolina dolphinfish. old from age reading of scales (Beardsley, 1967; Oxen- Dolphinfish, on average reach theirL ∞ in a year or two ford & Hunte, 1983; Uchiyama & Burch, 1986, Massuti and have high values of K. The K value for Mediterrane- & Morales-Nin, 1995). The maximum lifespan reported an dolphinfish was estimated to be 1.9 by Massuti et al. from scales for these fish in the Mediterranean was of (1999). This leads to steep curves in the von Bertalanffy three years, with fish at this age class ranging from 92 - growth curve. The asymptotic length (theoretical maxi- 120 cm FL (Massuti et al., 1999). mum length) of Florida dolphinfish is of 1.89 m and 1.53 Age and growth rates for dolphinfish have been de- m FL for males and females respectively, according to termined by means of scale annuli, daily growth incre- Beardsley (1967). The estimated maximum weight of 58.4 ments in otoliths (Oxenford & Hunte, 1983; Uchiyama kg is however much higher than the maximum weight of & Burch, 1986) and modal progression analysis (Wang, 46 kg reported for this species (Florida sportsman, 1979). 1979; Murray, 1985). Beardsley (1967) reported that a There is general agreement that dolphinfish in the juvenile dolphinfish specimen caught from Florida and western central Atlantic and Mediterranean reach sexual reared in captivity grew from 450 g to 16 kg in one year maturity in the first year of life, and that females reach and Schekter (1972) reported a growth rate of 4.3 kg maturity at a smaller size, but at similar age to males (from 0.7 to about 5 kg) in 30 days. In Barbados, dolphin- (Massuti & Morales-Nin, 1997; Oxenford, 1999). In fish may reach lengths of over 80 cm FL in 5.5 months a study by Potoschi et al. (1999), it was found that the and over one meter in less than one year (Oxenford & sampled dolphinfish belonging to the 0 age group had a Hunte, 1986). In Hawaii, they also attain a length of over sex ratio of 2:1 (females : males) whereas for older age one meter at the end of the first year (Uchiyama & Burch, groups the sex ratio was 1:1, indicating that the sex ratio 1986). By applying the length-weight regression of Rose varies with age class. The results obtained were in con- & Hassler (1968) to these data, this would correspond to cordance with those registered by Massuti & Morales- a mass of about 8 kg in one year. Growth rates presented Nin (1997) who reported a 1:1 ratio in adults sampled in by Benetti et al. (1994), for captive dolphinfish of 4.93 the western Mediterranean Sea. kg and 75.8 cm FL in 9.5 months were lower, yet among The present work provides biological information for the fastest recorded for bony fish reared in captivity. The Central Mediterranean dolphinfish, including length and absolute growth rates in terms of length of wild and cap- weight relationships, calculation of the condition fac- tive dolphinfish vary between 0.1 - 0.58 cm.d-1 for the tor, sex ratios, spawning period, and length at maturity, first year of life (Oxenford & Hunte, 1983). together with growth parameters and respective growth From the von Bertalanffy growth functions, Sch- rates from age readings of otoliths, scales and spines. wenke & Buckel (2008), reported faster growth rates for Such biological information is crucial for determining dolphinfish from the Gulf of Mexico and the Caribbean, the degree of exploitation that a fish stock can sustain than those from Florida, North Carolina and the Mediter- without suffering a population decline. Full-scale stock ranean. Mediterranean dolphinfish were reported as hav- assessments have never been carried out for this species ing slightly smaller sizes at age 2 and 3 compared to size in the Mediterranean, although a preliminary stock as- Medit. Mar. Sci., 16/2, 2015, 334-345 335 sessment was attempted by the General Fisheries Com- technique of Secor et al. (1991), and were cleaned and mission for the Mediterranean (2004). More extensive mounted in cold mounting resin which was then poly- and updated biological information needs to be obtained merised at 60oC for 24 hrs. The resulting blocks were for this fish so as to better calibrate population models for sectioned with a Buehler Isomet low speed saw and then future assessments.