The growth and mortality of the brown meagre, _{∞}=43.8 cm, k=0.145 y^{–1}, t_{0}=–4.88 y for males and TL_{∞}=50.1 cm, k=0.105 y^{–1}, t_{0}=–5.71 y for females. The instantaneous total (Z) and natural (M) mortalities were estimated for each sex, resulting in an exploitation ratio (E=F/Z) of 0.43 for males and 0.46 for females.

Se investigó el crecimiento y la mortalidad de _{∞}=43.8 cm, k=0.145 y^{–1}, t_{0}=-4.88 y para los machos y TL_{∞}=50.1 cm, k=0.105 y^{–1}, t_{0}=–5.71 y para las hembras. La mortalidad instantánea total (Z) y natural (M) se estimó para cada sexo, dando como resultado una tasa de explotación (E=F/Z) de 0.43 para los machos y de 0.46 para las hembras.

The brown meagre,

In the northern Mediterranean Sea,

In the southwestern Mediterranean Sea, the Sciaenidae family is mainly exploited by the small-scale artisanal fisheries on the Tunisian coasts and is recorded in the local fisheries statistics (in 2012, total production was 54 t) (

The age determination and growth rate are determinant for the estimation and explanation of the stock fluctuations and for application of management strategies. For the brown meagre, age and growth have already been studied in Maltese waters (

The aim of the present study was to update the data relative to age and growth parameters of the brown meagre of the Gulf of Tunis by using thin cross-sections. Further, length-length (fish and otolith radii) and length-weight relationships, as well as mortalities (natural, M, and total, Z), were studied and the respective coefficients were estimated. Finally, an idea of the status of the investigated stock was derived by comparing the current exploitation ratio (E=F/Z) with the values conventionally related to a full exploitation. This parameter is indispensable for a rational management of fisheries, which is the only guarantee for a sustainable and responsible exploitation of a fishery resource.

The study was performed in the Gulf of Tunis (central Mediterranean Sea, 37°N 10°30'E). The brown meagre is caught by commercial fishing boats using artisanal fishing gears (mostly gillnets and trammel nets) and trawl, at depths ranging from 20 to 100 m, according to information provided by experienced fishermen who know the fishing area well. A total of 276 individuals were randomly collected on a monthly basis from October 2008 to September 2011. For each fish, the total length (TL) was measured to the nearest millimetre and the total weight (TW) to the nearest gram. The sex (male, female or undetermined) was determined by macroscopic observation of the gonads. Sagittal otoliths were removed, cleaned and stored dry in labelled envelopes for later treatment and examination.

The sex-ratio (males:females) for the entire sample was estimated. A chi-square test was used to detect differences in the sex-ratio of the sampled fish. The Mann-Whitney U test was applied to test the differences between sexes according to the TL (cm) and TW (g) (

The relation between length and weight was described by the power function TW=aTL^{b}. The regression parameters a, b and the coefficient of determination (r^{2}) were estimated for the whole population and for each sex by least square linear regression, after log transformation of both variables. According to

Before estimating the age, two preliminary steps were applied to choose the calcified structure used and to validate the periodicity of observed growth increments. Comparing the results between the otoliths and the scales, in old fishes age is often underestimated or impossible to estimate when studied by scalimetry. Therefore, sagittal otoliths are the most common and widely used calcified structure. They appear to be highly suitable for age estimation as they are metabolically inert, so any material deposited remains unaltered and cannot be resorbed (

Otoliths of

The association of one opaque zone and one translucent zone was considered an annulus. The path of the growth marks was counted from the nucleus towards the tip of the inner face next to the sulcus (_{n}) and to the proximal margin of the otolith (R_{o}) to the nearest 0.01 mm.

The periodicity of opaque zone formation was validated by the marginal increment analysis (_{0}-R_{n}/ R_{n}-R_{n-1}, where R_{n }and R_{n-1} are the radii of the ultimate and penultimate annuli, respectively. The Kruskal-Wallis test was used to test the homogeneity of MIR among months followed by a Student-Newman-Keuls (SNK) non-parametric test after null hypothesis rejection. Correlation between MIR and temperature was tested using the Spearman rank correlation coefficient (r_{s}) (

To assess ageing precision, the growth increment was counted independently by two readers without prior knowledge of fish length or sex. The variability of the within-reader age estimates was calculated with the absolute percent error (APE) (

$${\text{APE}}_{\text{j}}\text{(\%)=100}\frac{\text{1}}{\text{R}}{\displaystyle {\sum}_{\text{i=1}}^{\text{R}}\frac{\left|\text{Xij+Xj}\right|}{\text{Xj}}}$$, |

where R is the number of times each fish is aged, Xij the i(th) ageing of the j(th) fish and X the mean age calculated for the j(th) fish. Only interpretations with an agreement between two readings were used in the successive elaborations.

The von Bertalanffy growth function is the most commonly used to describe fish growth. It was fitted to the observed length-at-age data as follow:

$$T{L}_{t}=T{L}_{\infty}\left(1-{e}^{-k(t-{t}_{0})}\right)$$ |

where TL_{t} is the total length (cm) at age t, TL_{∞ }is the asymptotic total length (cm), k is the growth curvature (years^{–1}), t is the age (years) and t_{0 }is the theoretical age at which length is zero. These parameters were estimated using Fishparm, a computer program based on a nonlinear model with an algorithm for least-square estimation (

Separate analyses were done for males and females. The growth curves were compared between sexes using the Hotteling test (

Catch curves were constructed by plotting the log of the number of fishes caught against their age class. Only age classes fully recruited to the sampling gear (i.e. the age classes in the descending limb of the catch curve) were used to estimate total mortality (Z). Z was estimated by the slope of the linear regression through the descending right limb of the catch curve (_{max}^{–0.916}, where t_{max} is the maximum age (

A total of two hundreds and 76 specimens composed of 121 males, 117 females and 38 specimens with undetermined sex were sampled. The sex-ratio of 1: 0.97 was not significantly different from the1: 1 ratio (χ^{2}=0.7, n=238).

Males ranged in size from 17.6 to 43.3 cm and females from 19.1 to 49.2 cm. The mean TL of males (TL=28.98±0.52 cm) was not significantly different from that of females (TL=30.73±0.62 cm) (Mann-Whitney U test, n=238, p=0.06). Fish of undetermined sex were ranged between 15.3 and 26.3 cm (TL=20.04±0.50 cm) (

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

Regions | Gears used | References | Total length (cm) | Parameters of length-weight relationships | ||||||
---|---|---|---|---|---|---|---|---|---|---|

N (sex) | min | max | mean (±se) | b | se (b) | r² | Type of allometry | |||

East Adriatic Sea | Beach seine, gill and trammel nets | 26 C | 18.1 | 41.2 | 33.07±0.99 | 3.048 | 0.121 | 0.96 | I | |

Gulf of Tunis | Gill nets, long lines and trawl | 238 M | 10.7 | 36.6 | - | 3.016 | 0.98 | I | ||

343 F | 10.4 | 41.2 | - | 2.975 | 0.99 | I | ||||

Western Mediterranean | Long lines, trammel nets and beam trawling | 233 C | 14.2 | 58.2 | 32.81±0.41 | 3.254 | 0.136 | 0.95 | A+ | |

Northern Aegean Sea | Gill and trammel nets | 24 C | 13.9 | 29.8 | - | 3.230 | 0.089 | 0.98 | A+ | |

Northwestern Adriatic Sea | Trammel nets | 532 C | 13 | 50 | - | 3.200 | 0.017 | 0.98 | A+ | |

Balearic Coasts | Trammel nets, spearfishing | 160 C | 25.2 | 53.1 | - | 3.322 | 0.96 | A+ | ||

63 M | - | - | 3.425 | 0.97 | A+ | |||||

78 F | - | - | 3.298 | 0.95 | A+ | |||||

Gulf of Tunis | Gill, trammel nets and trawl | Present study | 277 C | 15.3 | 49.2 | 28.55±0.41 | 3.147 | 0.022 | 0.98 | A+ |

121 M | 17.6 | 43.3 | 28.98±0.52 | 3.093 | 0.039 | 0.98 | A+ | |||

117 F | 19.1 | 49.2 | 30.73±0.62 | 3.176 | 0.04 | 0.98 | A+ |

Growth increments were counted on otolith cross-sections (

The marginal increment analysis showed considerable individual variation (_{s}=0.12, p<0.05).

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

The observed age of fish ranged from 1 to 31 years (

Age groups | ||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

Males | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 11 | 13 | 15 | 16 | 18 | 20 | 22 | ||

TL mean±se | 23.82±0.33 | 27.69 ±0.24 | 30.92 ±0.08 | 32.11 ±0.19 | 34.40 ±0.13 | 35.32 ±0.13 | 36.02 ±0.09 | 36.50 ±0.10 | 37.10 | 37.70 | 39.90 ±0.80 | 41.70 | 41.90 | 42.6 | 42.70 | 43.30 | ||

Range | 18.2-26 | 26.1-30.2 | 30.5-31.5 | 31.5-33.1 | 34-34.7 | 34.9-35.6 | 35.7-36.2 | 36.4-36.6 | 38.1-41.3 | |||||||||

N | 36 | 29 | 12 | 8 | 6 | 5 | 5 | 2 | 1 | 1 | 3 | 1 | 1 | 1 | 1 | 1 | ||

Females | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 16 | 18 | 31 | |

TL mean±se | 23.02 ±0.32 | 28.16 ±0.28 | 31.46 ±0.12 | 33.32 ±0.13 | 34.62 ±0.09 | 36.03 ±0.18 | 36.80 ±0.10 | 37.52 ±0.15 | 38.63 ±0.38 | 40.10 ±0.21 | 40.55 ±0.15 | 41.63 ±0.50 | 43.30 ±0.30 | 43.80 | 44.10 | 47.60 | 49.20 | |

Range | 19.1-25.9 | 26-30.8 | 30.8-32 | 33-33.8 | 34.4-34.9 | 35.3-36.6 | 37.7-37.9 | 37.1-38 | 38-39.3 | 39.7-40.4 | 40.4-40.7 | 41.1-42.6 | 43-43.6 | |||||

N | 31 | 30 | 11 | 6 | 5 | 6 | 2 | 5 | 3 | 3 | 2 | 3 | 2 | 1 | 1 | 1 | 1 |

The growth curves of age-at-observed length data were well described through von Bertalanffy fits (^{2} test=10.07, p<0.05).

The von Bertalanffy relationships between age and length, derived from the assumed annual periodicity of the growth increments, were described by the growth parameters: TL_{∞}=43.83 cm, k=0.145 year^{–1}, t_{0}=–4.88 years for males and TL_{∞}=50.10 cm, k=0.105 year^{–}^{1}, t_{0}=–5.71 years for females.

Total mortality, corresponding to the slope of the descending limb of the catch curves, was Z=0.51 year^{–1} for males and Z=0.41 year^{–1} for females (^{-1} for males and M=0.22 year^{–1} for females. Calculation of fishing mortality gave F=0.22 year^{–1} for males and F=0.19 year^{–1} for females. With the values of M and F known, the exploitation ratios were computed as E=0.43 for males and E=0.46 for females.

This study showed no significant difference between the length-weight relationships of males and females of

The large size and thickness of the otoliths in Sciaenids species do not allow a direct reading on the surface of the whole otolith under reflected or transmitted light. The dense calcium carbonate deposition reduces light transmission, making it almost impossible to distinguish hyaline and opaque zones (

The low values of APE found in this study, like those determined by

Marginal increment analysis confirmed the deposition periodicity of growth increments, showing an annual pattern. It also proved that the opaque zone was deposited between March and June. This period is identical to that observed for the formation of the annulus on the scales of the brown meagre in the same area (

The examination of otolith cross-sections showed that the maximum age observed in the Gulf of Tunis was 31 years for a female of 49.2 cm. A very similar result of the same age was found for a female of 49.7 cm, poached in the marine reserve of Scandola, protected since 2014 (

Regions | References | Methods | Sex | TL_{∞} (cm) |
k (/year) | t_{0} (years) |
n | TL (cm) | Age max |
---|---|---|---|---|---|---|---|---|---|

Northern Tunisian coasts | Burnt broken otoliths | Males | 46.70 | 0.224 | –0.120 | - | 12.4-44.5 | 15 | |

Females | 53.90 | 0.190 | –0.002 | - | 12.5-52.5 | 21 | |||

Gulf of Tunis | Whole scales | Males | 45.00 | 0.225 | –0.817 | 394 | 13.6-44.4 | 9 | |

Females | 53.70 | 0.186 | –0.828 | 484 | 13.4-49.6 | 9 | |||

Maltese coasts | Sectioned otoliths | Males | 42.30 | 0.145 | –5.765 | 129 | 30.5-43.0 | 17 | |

Females | 47.60 | 0.116 | –6.132 | 51 | 31.1-48.5 | 26 | |||

Sea of Marmara | Sectioned otoliths | Combined sexes | 69.80 | 0.418 | –1.075 | 921 | - | 21 | |

Northwestern Adriatic | Sectioned otoliths | Males | 44.90 | 0.268 | –2.168 | 128 | 16.5-48 | 19 | |

Females | 47.20 | 0.279 | –1.823 | 118 | 20-49.7 | 16 | |||

Southeastern Black Sea | Polished whole otoliths | Combined sexes | 51.14 | 0.270 | –0.930 | 329 | - | 18 | |

Gulf of Tunis | Present study | Sectioned otoliths | Males | 43.83 | 0.145 | –4.880 | 113 | 17.6-43.3 | 22 |

Females | 50.10 | 0.105 | –5.710 | 113 | 19.1-49.2 | 31 |

In the growth parameters of the von Bertalanffy model obtained, the theoretical maximal length value is close to the observed largest size. _{0} were reported in Maltese waters (

The comparison of the growth parameter estimates of _{∞} were lower than those found previously in the same area for both sexes (

These variations in growth performance between different regions are probably the result of the following factors:

(1) Variations in environmental conditions (such as temperature and food availability) among sampled areas.

(2) Different size distributions (probably caused by different types of sampling gear). In fact, growth model estimates are greatly affected by the lack of very young or old individuals and the unequal abundance of size classes (Mouine-Oueslati et al. 2015 ).

Distribution difference between sexes when there was sexual dimorphism as for this species.

(4) Mortality, which may affect the variability of the maximum length and has a relatively large and consistent impact on the size of structure of communities (Blanchard et al. 2005 ).

(5) A combination of the above factors.

_{opt}=0.4M. (F>F_{opt}=0.12 for males and F>F_{opt}=0.09 for females). Estimates for females are considered more relevant for population modelling, because aspects of female population dynamics, including age-specific and total life-time fecundity and mortality, determine the potential increase in population numbers (_{opt}). On the Tunisian coasts, females reached ages of 31 years, which is the maximum age recorded in the Mediterranean Sea. The studied life history parameters of the population of the brown meagre of the Gulf of Tunis suggest that the species is long-lived, is slow growing and slow-maturing, and has a low natural mortality. Because of all these previous characteristic,

The results obtained from the current study provide an actualization of practical biologically related parameters for stock assessment and management of the brown meagre of the Gulf of Tunis. Regular follow-up of life history traits of this species are necessary for its optimal management on the Tunisian coasts.