Age,growth and length-weight relationship of the white skate,Rostroraja alba(Linnaeus,1758)(Chondrichthyans:Rajidae),from the Gulf of Gabes(Tunisia,Central Mediterranean)

Objective:To investigate the length distribution,sex ratio,length-weight relationship,age and growth of Rostroraja alba in the Gulf of Gabes.Methods:This study estimated age and growth of the white skate,Rostroraja alba by counting vertebral band pairs from 112 specimens taken by commercial fisheries during 2006-2009 from the Gulf of Gabes(Southern Tunisia,Central Mediterranean Sea).Results:This is the first and only known information in regards to the age and growth of this species in Tunisia.Vertebra diameter was strongly correlated with total length and age,which were expressed by linear or cubic regression equations.The oldest female in this study was 35 years and 160 cm,whereas the oldest male was 32 years and 150 cm.The von Bertalanffy growth parameters were TL_(∞)=(202.26±3.40)cm,K=(0.040±0.038)/year and t_(0)=(1.84±0.01)10^(-1)/year for females and TL_(∞)=(198.60±3.61)cm,K=(0.060±0.076)/year and t_(0)=(1.28±0.04)/year for males.Conclusions:This study would be an effective tool for fishery biologists,managers and conservationists to initiate management strategies and regulations for the sustainable conservation of the remaining stocks of this species in the Gulf of Gabes(Southern Tunisia)ecosystem.

Age,growth and length-weight relationship of the rough skate,Raja radula(Linnaeus,1758)(Chondrichthyans:Rajidae),from the Gulf of Gabes(Tunisia,Central Mediterranean)

Objective:To investigate the length distribution,sex ratio,length-weight relationship,age and growth of Raja radula in the Gulf of Gabes.Methods:The age and growth characteristic of the rough skate(Raja radula)inhabiting in the Gulf of Gabes(Central Mediterranean Sea)was determined.A total of 1250 specimens were sampled.According to the marginal increment band its growth was annual.Results:The growth parameters were derived by using the von-Bertalanffy method and were separately evaluated as follows:TL∞=97.2 cm,K=0.15 y^(-1),t_(o)=0.35,and W_(∞)=5.28 kg,and TL∞=76.4 cm,K=0.22 y^(-1),to=0.16,and W∞=3.77 kg,for females and males respetively.The maximum age was 12 years for females and 10 years for males.Conclusions:Results from this research will provide a starting point to develop a management plan for the rough skate in the Gulf of Gabes.Further investigations are necessary to quantify the impact of the existing regulations on the population dynamics and recruitment patterns of this species in the region.

Distribution and morphometric characters of the Mediterranean longnosed skate,Dipturus oxyrinchus(Chondrichthyans:Rajidae)in the Gulf of Gabes(Tunisia,Central Mediterranean)

Objective:To investigate biometrics of Dipturus oxyrinchus(D.oxyrinchus),including length-weight relationships,total length(TL),disc width(DW),distance measured of snout tip to eye,width of the mouth,distance from snout tip to first dorsal,and the numeric characters in the Gulf of Gabes.Methods:Monthly sampling was conducted commercial bottom-trawl during December 2006 and January 2007,operating at a depth of 80-185 m.The TL,DW,distance of snout tip to eye,width of the mouth,distance from snout tip to first dorsal were measured to the nearest 0.1 cm and the weight was measured to the nearest 0.1 g.The parameters a and b were estimated by the least squares regression method.Results:A total of 520 D.oxyrinchus was ranged from 16.5 to 105.0 cm in TL and 30-5300 g in total mass were analyzed during this study.The overall sex ratio was significantly different from the expected value of 1:16(df=1,χ^(2)=3.38,P<0.05),and there was significant difference in the TL(t-test=16.53,df=520,P<0.05).There was significant difference in the regression slopes between sexes for the TL and DW relationship(t-test=17.53,df=520,P<0.05).The b value for the TL/weight relationship for males D.oxyrinchus sampled in this study(3.01)was lower than that for female(3.23).Morphometric characters were strongly positively correlated for TL.Conclusions:This study reported the first description of biometric relationships for D.oxyrinchus,which would be useful for the sustainable conservation of this rear fishery in the Gulf of Gabes and also form the basis for their inclusion in a regional security programme.