INTRODUCTION
Gilthead seabream, Sparus aurata Linnaeus 1758, inhabits Posidonia beds and sandy bottoms commonly to depths of about 30 m, but adults may occur to 150 m depth. The euryhaline gilthead seabream are sedentary fish, either solitary or in small aggregations, and they often occur in brackish waters of coastal lagoons and estuaries in spring (Bauchot & Hureau, Reference Bauchot, Hureau, Quero, Hureau, Karrer, Post and Saldanha1990). Gilthead seabream is common throughout the Mediterranean Sea, but very rare in the Black Sea. It is also present in the eastern part of the Atlantic Ocean, from Britain to Cape Verde and the Canaries (Bauchot & Hureau, Reference Bauchot, Hureau, Whitehead, Bauchot, Hureau, Nielsen and Tortonese1986).
Gilthead seabreams are captured with traditional bottom trawl nets, coastal purse-seines, bottom set longline and hand lines, and are regularly present to the markets in Turkey. In 2008, the total marine capture and aquaculture productions of gilthead seabream were 1526 t and 31,670 t, respectively, i.e. the captured fish are a little bit more than 4.8% of aquaculture production (TUIK, 2009).
In spite of the fact that culture of the gilthead seabream has benefited from considerable research effort, there is a lack of research on its ecology (i.e. age, growth, reproduction, feeding habits, etc.) in natural environments in the Mediterranean (Lasserre & Labourg, Reference Lasserre and Labourg1974; Arnal et al., Reference Arnal, Alcazar and Ortega1976; Lasserre, Reference Lasserre1976; Kraljevic & Dulcic, Reference Kraljevic and Dulcic1997; Chaoui et al., Reference Chaoui, Kara, Faure and Quignard2006). Furthermore, no information is currently available on the biology and ecology of this species in the Aegean Sea.
This paper presents some basic biological information such as age, growth, length–weight relationship, mortality rates for the adult component of the species, caught during spawning migration in the south-eastern Aegean Sea.
MATERIALS AND METHODS
Gilthead seabream were collected from four coastal purse-seiners (24–26 m LOA, 2 × 400 hp engine) by using purse-seine net (total length 1000 m and stretched mesh size 36 mm) in winter months between November 2006 and January 2007 off Güllük Bay at a depth of about 60 m (Figure 1). Though the purse-seiners usually catch sardines and/or anchovy, some of them are concentrated to catch mature gilthead seabream during their spawning migration to provide the mature fish for the hatcheries in the same area.
Fig. 1. Sampling area. The circle indicates the fishing ground.
Lively caught fish were transferred to the airing tanks on the deck. They were then transported ashore to the fish farm. A total of 476 fish were randomly chosen and bathed in an anaesthetic, 2-phenoxyethanol solution. Anaesthetized fish were measured (fork length (FL) to the nearest ±0.5 mm) and weighed (total weight, to the nearest ±5 g).
Ageing was done by scale reading. Scale samples (from 6 to 8 scales), taken from 332 fish, were removed from the base of the pectoral fin and from the flanks below the dorsal fin. They were cleaned in 5% sodium peroxide and then immersed in glycerol in a black Petri dish, and annuli, defined as opaque and hyaline zones were counted by using a binocular microscope (10×) (Figure 2). Bauchot & Hureau (Reference Bauchot, Hureau, Whitehead, Bauchot, Hureau, Nielsen and Tortonese1986) reported that the spawning season of the gilthead seabream ranges from October to December. So, this period was assumed as birth date.
Fig. 2. Scale of gilthead seabream at 6 years old (45 cm fork length).
The length–weight relationship (LWR) of all fish was estimated based on power regression as W = aLb, where W is the weight (g), L is the fork length (cm), a and b are constants. The Fulton's condition factor (CF) for all fish was calculated according to the equation CF = (W/FL3) × 100.
Non-seasonal growth parameters (L ∞, K, t 0) were estimated with the von Bertalanffy growth (VBG) formula in the FISAT (FAO–ICLARM Stock Assessment Tools) computer program (Gayanilo et al., Reference Gayanilo, Sparre and Pauly1994) using individual lengths-at-age. The von Bertalanffy growth equation for length, L t = L ∞ [1 − e−K(t−t 0)], where L ∞ is the asymptotic length, K the growth curve parameter, and t 0 is the theoretical age when fish length would have been zero, was applied.
Overall growth performance was estimated by the index Φ′ (phi-prime test) (Pauly & Munro, Reference Pauly and Munro1984), Φ′ = logK + 2logL∞.
Natural mortality (M) of gilthead seabream was computed using Pauly's (1980) multiple regression formula: logM = −0.0066 − 0.279 * logL∞ + 0.6543 * logK + 0.4634 * logT, where M is natural mortality in a given stock, and the value of T is the mean annual temperature at the surface (in °C). The average annual temperature (T = 20°C) for Güllük Bay was taken from Özfuçucu et al. (Reference Özfuçucu, Katağan, Tolun, Ergen, Önen, Yılmaz, Dereli, Kırkım, Morkoç, Yüksel and Dinçer2000). Total mortality (Z) was estimated as the linearized catch curve based on age composition data (Sparre & Venema, Reference Sparre and Venema1992). Fishing mortality (F) can be estimated from F = Z – M. Once values of F and M are available, an exploitation ratio (E) can be computed from E = F/Z. The exploitation ratio can be used to assess if a stock is overfished or not, on the assumption that the optimal value of E (Eopt) is about equal to 0.5 (Pauly, Reference Pauly1980).
RESULTS
Length and weight–frequency distribution
The length–frequency distribution of all fish is shown in Figure 3. The length and weight of specimens ranged from 26.5 to 51.5 cm and from 375 to 2600 g, respectively (Table 1).
Fig. 3. Length–frequency distribution for the gilthead seabream in the Aegean Sea.
Table 1. Range, mean with standard error (SE), median and mode of fork length (cm) and body weight (g) for gilthead seabream from the Aegean Sea.
Age and growth
Age groups were varied between 2 and 7, and mean fork lengths (and mean wet weight) were 29.5 cm (497.5 ± 7.6 g), 35.3 ± 0.15 cm (881.9 ± 9.7 g), 39.3 ± 0.14 cm (1200.8 ± 12.0 g), 42.2 ± 0.19 cm (1467.8 ± 24.2 g), 45.6 ± 0.43 cm (1878.6 ± 61.8 g) and 51.5 cm (2600.0 g), respectively (Table 2). Modal age was 3 years (55.1%). The observed lengths of specimens assigned to each group were used to fit the VBG formula (Figure 4). Growth parameters with SE were found as L∞ = 64.97 ± 12.93 cm, K = 0.14 ± 0.07 year−1, t0 = –2.47 ± 1.09 year−1, and index of phi-prime was Φ′= 2.772 ± 0.51.
Fig. 4. Von Bertalanffy growth curve fitted by length-at-age for gilthead seabream.
Table 2. Age–length key for the gilthead seabream in the Aegean Sea based on scale reading.
SE, standard error.
There were no statistical differences between the observed (obs.) and calculated (calc.) mean lengths in all age groups (t-test, P > 0.05) (Table 3).
Table 3. Observed and calculated mean lengths (fork length, cm) of gilthead seabream for each age group.
Length–weight relationship
The LWR equation calculated was: W = 0.0515 × L2.737 (r2 = 0.95) (Figure 5). The mean CF of all fish was estimated as 2.058.
Fig. 5. Length–weight relationship for the gilthead seabream.
Mortality rates
Mortalities (M, F and Z) and exploitation rate (E) of gilthead seabream from the Aegean Sea were 0.34 year−1, 0.77 year−1, 1.11 year−1 and 0.69 year−1, respectively. Total mortality rate (Z) was calculated from linearized catch curve based on age composition data (Figure 6).
Fig. 6. Total mortality of gilthead seabream from linearized catch curve based on age composition data.
DISCUSSION
Adult gilthead seabream were caught by purse-seiners during the spawning migration from November to early January in Güllük Bay. This type of fishery has only been carried out in this area for about 5 years in order to supply mature fish to the hatchery.
Juvenile individuals of this species are more common in lagoons, as reported by Lasserre (Reference Lasserre1976) in the Arcachon basin (France), by Kraljevic & Dulcic (Reference Kraljevic and Dulcic1997) in Mirna Estuary in the Adriatic Sea, by Perçin (Reference Perçin2005) in Homa lagoon in the Aegean Sea (Turkey) and by Chaoui et al. (Reference Chaoui, Kara, Faure and Quignard2006) in Mellah lagoon in north-eastern Algeria. In this study, the majority of fish were adults, suggesting that sampling occurred on the spawning migration within Güllük Bay, and that juveniles seek the nearby Akköy Lagoon and Büyük Menderes estuary environments as nursery areas.
Compared to other areas of the Mediterranean Sea and Atlantic Ocean where gilthead seabream are found, the growth rate in the Aegean Sea is slower than all areas except Mirna. The fastest growth rate was reported for the Mellah lagoon population due to very favourable thermal conditions (temperatures recorded in this area were higher than 15°C (15–30°C) during eight months of the year) (Chaoui et al., Reference Chaoui, Kara, Faure and Quignard2006). In general, the western part of the Mediterranean encompasses faster growth populations than the eastern part. Our results (L∞ = 64.97 cm) support previous estimates for gilthead seabream in the area of an intermediate asymptotic length (Table 4). Our estimate of K is also within the range of those of other populations (Table 4). However, the estimation of t0 = −2.47 in this study may indicate that the smaller gilthead seabreams were not sampled adequately (i.e. the smallest size of the gilthead seabream was only 26.5 cm FL). The differences between sizes-at-ages may arise from different localities such as lagoon, estuary and open sea, and from various hydrographical conditions.
Table 4. Growth parameters (L∞, K and t0) of gilthead seabream from different localities (adapted from Chaoui et al., Reference Chaoui, Kara, Faure and Quignard2006).
(*), phi-prime test, this parameter was calculated from data for other studies.
The b coefficient of the LWR (b = 2.737) indicates allometric growth. This value is close to the Thau's pond and Graveyron, France (Lasserre & Labourg, Reference Lasserre and Labourg1974; Lasserre, Reference Lasserre1976). The low value of b is possibly explained by the winter catches of specimens which had already spawned (after the November–December period). The mean CF (2.058) of gilthead seabream in the Aegean Sea is higher than the Mirna estuary due to the younger population there, i.e. about 72% of gilthead seabream consisted of age 1 fish in Mirna.
The gilthead seabream is a long lived species, and maximum reported age and size are 12 years and 57.5 cm (2500 g) (Kraljevic & Dulcic, Reference Kraljevic and Dulcic1997). The observed maximum length for the Mediterranean is 70 cm total length by Bauchot & Hureau (Reference Bauchot, Hureau, Whitehead, Bauchot, Hureau, Nielsen and Tortonese1986). So, the absence of larger, older fish in our samples may suggest heavy fishing pressure on gilthead seabream in the Aegean Sea. Although all sampled fish were bigger than the minimum landing size (MLS) for gilthead seabream, 15 cm in the Turkish Fishery Regulation Circular (TFRC), the estimate of fishing mortality (F = 0.77) is much higher than natural mortality (M = 0.34), and the exploitation rate (E = 0.69) suggests overfishing. Chaoui et al. (Reference Chaoui, Kara, Faure and Quignard2006) stated that the onset of sexual maturity for gilthead seabream was reached at 32.6 cm, at an average age of 18 months. Thus, the relatively low MLS of gilthead seabream in TFRC does not appear effective in allowing fish to spawn at least once. Kraljevic & Dulcic (Reference Kraljevic and Dulcic1997) reported natural mortality (M = 0.32) almost equal to that of this study. The low natural mortality was associated by the authors with biological features (big, slow-growing and long-lived species), as well as behaviour and ecology of this fish.
In conclusion, adult gilthead seabream represent an important fishery for hatcheries in the south-eastern Aegean region, and this fishery is likely to increase due to demand by fish farms. The resource may not be sustainable, if some regulations (e.g. closed area and/or season, higher MLS, size selectivity, etc.) are not implemented. Further investigations are necessary to quantify the impact of the existing regulations on the population dynamics and recruitment patterns of gilthead seabream in the region.
