du Développement animal, 2092, Tunis, Tunisie.

Age and the growth of the black seabream _{∞}=35.4 cm, k=0.15 y^{–1} and t_{0}=–0.19 y for scales and L_{∞}=38.6 cm, k=0.10 y^{–1} and t_{0}=–0.14 y for otoliths. Parameters estimated from scale and otoliths were significantly similar. However, taking into consideration the lower standard deviations of means for estimates based on otolith readings and the higher variance explained by the regression line fitted to otoliths, the latter seem to be more appropriate for ageing

Se han investigado la edad y el crecimiento de la cántara _{∞}=35.4 cm, k=0.15 año^{–1}, t_{0}=–0.19 año) y los otolitos (L_{∞}=38.6 cm, k=0.10 año^{–1}, t_{0}=–0.14 año). Los parámetros estimados a partir de las escamas y los otolitos resultaron significativamente similares. Sin embargo, teniendo en cuenta que las desviaciones estándar de las medias en las estimas son más bajas, y que la varianza explicada es mayor en la regresión ajustada a los otolitos, la lectura de los otolitos parece ser la más apropiada para datar la edad de

The genus

Despite the wide distribution of

Due to low commercial interest of the black seabream on the Tunisian coast, the catch statistics of this species were unfortunately not available. Therefore, no specific management plan has been developed in spite of the vulnerability of this species owing to its sexual pattern characterized by protogynous hermaphroditism (

Estimation of accurate fish age is considered an essential step for age-based assessment of fish population and successful resource management. Because of the lack of basic biological information for the black seabream in Mediterranean, the objective of this study is to provide information on the age and growth of this species in the Gulf of Tunis. First, parameters of length-weight relationship were estimated for males, females and all fish. Second, the reliability of age estimates of

The results of this study can be used as biological input parameters for further evaluation of the black seabream stock in the Gulf of Tunis and lead to improved management

Samples were collected from the commercial trammel and gill net catches of the artisanal fleet in the Gulf of Tunisia from January 2005 to June 2006. A total of 369 black seabream were collected fortnightly at the landing port and stored in iceboxes. In the laboratory, for each fish, the total length (TL) was measured to the nearest millimetre and the total weight (TW) to the nearest gram. Macroscopic examination of gonads was used to determine the sex of individuals as males, females, immatures and hermaphrodites. Sagittal otoliths were extracted from each specimen, cleaned and stored dry in paper envelopes. Scales were sampled from the left side of the body beneath the pectoral fin, cleaned and mounted between glass slides.

Whole otoliths were immersed in a glycerine-ethanol (1:1) solution, to improve the visualization of annual increments, and examined under a compound microscope with reflected light against a dark background. Increments on the whole otoliths comprised two zones: an opaque zone; and a narrower translucent zone, which appears dark with reflected lighting and a black background (

The relationship between total weight (g) and TL (cm) was described by the power function TW=aTL^{b}. The regression parameters a, b and the coefficient of determination (r^{2}) were estimated for both for the whole population and for each sex by least square linear regressions after log transformation of both variables. According to _{0}: slope=3; H_{1}: slope≠3), whereas analyses of covariance (ANCOVA) were employed to detect any significant differences in the linear relationships between sexes (

Each otolith was examined twice by a single reader, with an interval of 3-4 months between readings, and only coincident interpretations were accepted. All counts were performed in random order of fish size and without reference of fish length, sex or the previous reading. The consistency in otoliths readings was quantified by calculating the index of average percentage error (IAPE) (

To validate seasonality of deposition of the opaque and translucent zone, the marginal increment analysis was carried out on the entire otolith and scale samples (

In detail, the change in relative frequency of each edge zone was plotted across months all year round; the cycle frequency of formation of the opaque and translucent zones should equal one year in true annuli (

Back-calculated size of each fish at the time of formation of each annulus was determined by substituting the measurement of each annulus in the body proportional equation (_{th} band is laid down (Li, mm) was calculated according to the formula: (ri/r)^{ν} Lc, where ri is the radius of the _{th} band, r is the radius of otolith or scale at capture, Lc is the length at capture and ν is the constant derived from the power function that describes the relationship between the radius of the calcified structures and TL of fish (

The von Bertalanffy growth model, by far the most commonly used to estimate growth in fishes, was fitted to the back-calculated mean length at age for the whole population by means of Marquardt’s algorithm for non-linear least squares parameter estimation (

Lt=L_{∞} (1− e^{−k (t−t0)}),

where Lt is the length at age t, L_{∞} is the asymptotic length, k is the growth coefficient, and t_{0} is the theoretical age at zero length.

Statistical comparisons of growth equations between structures (otoliths and scales) were conducted using Hotteling tests (

The growth performance index Φ’, expressed as the logarithmic relationship between the von Bertalanffy growth coefficient and asymptotic length, was used to compare growth of

Among the 369 fish individuals examined, 15 were males, 330 were females, 6 were hermaphrodites, and the remaining 18 individuals were immatures with small gonads. The length and weight of the black seabream ranged from 13.4 to 36.6 cm and total weight from 39 to 806 g TW, respectively. TL of males ranged from 16.0 to 36.6 cm and TW from 72 to 806 g. Female TL ranged from 13.4 to 31.6 cm and TW from 39 to 564 g.

The parameters of the linear regression of the weight and length logarithm are provided for each sex and all individuals in

a | b | s.e (b) | n | r^{2} |
t-test | p | |
---|---|---|---|---|---|---|---|

Males | 0.012 | 3.099 | 0.111 | 15 | 0.975 | 8.306 | 0.000 |

Females | 0.012 | 3.098 | 0.025 | 330 | 0.990 | 1.608 | 0.109 |

All fish | 0.012 | 3.100 | 0.021 | 369 | 0.991 | 3.887 | 0.000 |

Otoliths and scales of

The minimum mean scale increment was observed from March to May, suggesting that the annulus formation period occurred in spring (

The relationships between TL and otolith radius (Ro) or scale radius (Rs) was calculated for females, males and the sexes combined. Fish length and radii of the calcified structure were closely correlated (p=0.00;

Back-calculated length-at-age data were fitted to von Bertalanffy growth function, and the parameters of mean (±s.e) L_{∞}, k and t_{0} were estimated to be 35.4 (±5.3) cm, 0.155 (±0.04) and –0.193 (±0.27) year, respectively, for scalimetry and 38.6 (±3.2) cm, 0.100 (±0.02) and –0.142 (±0.12) year, respectively, for otolithometry. Parameters estimated from scale and otoliths were significantly similar (Hotteling T^{2} test, T^{2}=14.28; p>0.05). However, taking into consideration the lower standard deviations of means for estimates based on otolith readings and the higher variance explained by the regression line fitted to otoliths (r^{2}=0.984 and 0.954 for otoliths and scales, respectively), the latter seem to be more appropriate for ageing black seabream.

The analysis of covariance showed no significant difference between males and females in the length-weight relationship for the same range of lengths, although males tended to be slightly heavier than females for the same length. This may be explained by protogyny, because females predominated in smaller size classes and males in larger classes. In fact, the black seabream has been catalogued as a diandric species (

Biogeographic analysis of

Authors | Regions | TL (cm) | Parameters of length-weight relationships | |||||
---|---|---|---|---|---|---|---|---|

n | min | max | a | b | s.e (b) | r^{2} |
||

Portugal | 886 | 19.0 | 37.0 | 9.93 × 10–6 | 3.066 | 0.038 | 0.88 | |

Gulf of Gabes | 109 | 8.3 | 27.0 | 1.17 × 10–5 | 3.061 | - | 0.984 | |

Greece | 53 | 12.6 | 39.6 | 0.01772 | 2.951 | 0.079 | 0.97 | |

Turkey | 46 | 8.2 | 28.7 | 0.019 | 2.87 | 0.151 | 0.891 | |

Western Mediterranean | 86 | 6.4 | 31.5 | 0.016 | 2.995 | 0.074 | 0.994 | |

Present study | Gulf of Tunis | 369 | 13.4 | 36.6 | 0.012 | 3.099 | 0.021 | 0.991 |

Accurate estimation of fish age is considered an essential step for age-based assessment of fish population and successful resource management (

In the current study, age and growth investigations of

The high correlation found between TL and otoliths or scales radius indicates that both hard structures are useful for estimating age and for reconstructing past growth history of fishes by back-calculation (

According to the scalimetric method, the theoretical maximum length, L_{∞}, of _{∞}.

Authors | Regions | Methods | L_{∞} (cm) |
k (/an) | t_{0} (an) |
n | TL (cm) | Φ’ | Age max |
---|---|---|---|---|---|---|---|---|---|

Adriatic | Scales | 47.70 | 0.178 | 0.270 | 745 | 6.2 - 46.5 | 2.61 | 14 | |

Gulf of Gabes | Scales | 35.73 | 0.144 | –1.425 | 109 | 8.3 - 27.0 | 2.26 | 6 | |

Canary Islands | Otoliths | 43.35 | 0.240 | –0.110 | 1272 | 8.0 - 40.0 | 2.65 | ||

Portugal | Scales | 34.48 | 0.210 | –1.220 | 378 | 5.0 - 35.0 | 2.39 | 11 | |

Otoliths | 30.32 | 0.260 | –1.200 | 905 | 4.0 - 40.0 | 2.38 | 13 | ||

Present study | Gulf of Tunis | Scales | 35.35 | 0.155 | –0.193 | 367 | 13.4 - 36.6 | 2.29 | 10 |

Otoliths | 38.61 | 0.100 | –1.142 | 366 | 13.4 - 36.6 | 2.17 | 10 |

The growth rate k is higher in the Atlantic than in the Mediterranean. The lowest growth is recorded on the Tunisian coasts.

The maximum age observed in the present findings was 10 years for a fish of 36.6 cm TL.

The geographical comparison of the growth performance index Φ’ of the black seabream indicates that it shows a minimal variation since it combines several parameters of the equation of von Bertalanffy (

Compared with other sparids, black seabream are slow-growing and short-lived, but fast to mature. These characteristics would imply that the stock can withstand more intense fishing than many other sparids, which reach marketable size long before maturity (

The results from the current study provide practical, biologically related parameters for stock assessment and management proposals of the Mediterranean black seabream, which is a vulnerable species owing to its sexual pattern characterized by protogynous hermaphroditism (