Length-weight relationships (LWRs) of endemic and introduced freshwater fish species in 13 Tunisian reservoirs

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Volume 41, Article 8

[Volume 41, Article 8]

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S. MILI^1*

R. ENNOURI²

M. CHHIBI³

H. LAOUAR⁴

N. ROMDHANE⁵ 

H. MISSAOUI²

¹ Institut Supérieur de Pêche et d’Aquaculture de Bizerte, Unité de recherche: Exploitation des Milieux Aquatiques, Errimel, B.P.15. 7080 Bizerte, Tunisie.

² Institut National des Sciences et Technologies de la Mer (INSTM), Laboratoire Milieu Marin, Centre la Goulette, Tunisie.

³ Faculté des Sciences de Bizerte, 7021 Bizerte, Tunisie.

⁴ Centre Technique d'Aquaculture: 5, rue du Sahel Montfleury, 1009 Tunis, Tunisie.

⁵ Direction Générale de la Pêche et de l’Aquaculture, 30, Rue Alain Savary, 1002 Tunis, Tunisie.

Abstract - A study was conducted during 2013-2015 in 13 Tunisian reservoirs. A total of six fish species belonging to three families were collected using multi-mesh gill nets (EN 14575 amended) for the first time. Fish sampling was made in the most important Tunisian reservoirs for analysis of fish length-weight relationships (LWRs). Analyses included species for which no previous LWR information was available. Ranges of parameters “a” and “b” of the 6 species were from 0.0016 to 0.1774 and 2.859 to 3.260, respectively.

Keywords: Length-weight relationships, freshwater fish, Tunisian reservoirs.

Résumé - Un total de six espèces de poissons dulçaquicoles, appartenant à trois familles, ont été collectées entre 2013 et 2015 dans le but d’analyser leurs relations Taille-poids. L’échantillonnage a été effectué, en utilisant des filets maillants à mailles multi-mailles (EN 14575 modifié), au niveau des retenues de barrages les plus importantes en Tunisie. Cette étude a permis de déterminer les relations tailles-poids pour les espèces ichtyques dulçaquicoles pour lesquelles aucune information antérieure n’existe. Les intervalles de variabilité des paramètres « a » et « b » chez les 6 espèces étudiées sont respectivement de 0,0016 à 0,1774 et de 2,859 à 3,260.

Mots clés: Relation taille-poids, poissons dulçaquicoles, barrages tunisiens.

1. Introduction

Freshwater fish farming is a recent activity in developing Tunisia that began with the experimental stocking of reservoirs with brood stock and fry of freshwater fish. With the majority of reservoirs located in the northern part of the country (Mili et al. 2015), 450 fishermen and 232 boats are currently involved in this activity (DGPA 2015). The fishing activity in Tunisian reservoirs concern especially 9 species: carp (Cyprinus carpio), pike-perch (Sander lucioperca), mullet (Mugil cephalus and Liza ramada), eel (Anguilla Anguilla), catfish (Silurus glanis), roach (Rutilus rutilus), rudd (Scardinius erythrophthalmus), barbell (Luciobarbus callensis) and tilapia (Oreochromis niloticus). Production was around 1034 tons in 2014 and mullet was the most abundant fished species and represents 30% of the total landing (DGPA 2015). Despite numerous cited studies on the biology of freshwater fishes in Tunisian reservoirs (Djemali 2005; Kraim 1994; Mili et al. 2015; Mili et al. 2016; Tlili et al. 2010; Toujani et al. 2000), few detailed information related to length-weight relationships are available. Knowledge of length-weight relationships (LWRs) allow to compare the condition factor and ontogenetic allometric changes of different populations (Froese et al. 2011), and to convert fish length into absolute biomass which is essential in fisheries management.

The present study describes the length-weight relationships (LWRs) for the most abundant fish species in 13 Tunisian man-made lakes (Fig. 1). The data is believed to be the first published reference on LWRs for fishes in Tunisian reservoirs.

2. Material and methods

Fishes are caught using multi-mesh gillnets during fishing surveys between May 2013 and June 2015. The gill nets used for sampling have 8 different mesh sizes ranging between 18 mm to 80 mm. All specimens were identified to species level and validated following FishBase (Froese and Pauly 2013), measured for total length (TL to nearest 1 mm) and weighed (to 0.1g accuracy). The relationships between total length and weight were determined by linear regression. In this study we adopted the regression equation W =aL^b to fit the length-weight relationships (LWRs) where W is the total weight (g), L is the total length (cm), “a” and “b” are regression parameters (Ricker 1973). Additionally, we calculated the 95% confidence limits for “a” and “b”(CL 95%) to determine if the hypothetical value of isometry fell between these limits (Froese 2006).The correlation between W and L is evaluated by the coefficient of determination r². For LWRs with r²<0.95 the regression was repeated after removing outliers (Froese 2006). The model fit to the data was measured by the coefficient of the Pearson r-squared (r²) test. Outliers observed in the log-log plots of all species were excluded from the regression.

The LWRs were analyzed by STATISTICA software Origin version 8.0.

3. Results and discussion

The number of samples, degree of threat, range of standard length and total weight, estimated parameters of length-weight relationships for 6 fish species from 13 Tunisian man-made lakes, the 95% confidence interval (CL) of a and b, as well as the determination coefficient (r²) were listed in Table 1.The allometry was negative, for all species in the 13 reservoirs except for roach form Ghezala and Bekbeka where which was symetric and presented a coefficient of allometry statistically equal to 3. Additionally a positive allometry was detected for mullet (L. ramada) from Sidi Barrak and Seliana and for rudd in Laabid reservoir. The r² values for all species were ranged between 0.868 and 0.994, and b values varied from 2.859 to 3.260. The LWR parameters of the species in previous studies in Fishbase had been compared to our results in order to indicate relationships of body shape between related species.

The application of the Bartlett test associated with t-Student test showed that there is a significant difference between LWRs for each species in all dams except for Cyprinus carpio (Table 2).

LWRs for six species from 13 Tunisian reservoirs are published for the first time in scientific literature as shown in Table 1 in order to create a useful reference for future similar studies. As established by Froese (2006), the range of b values oscillates between 2.5 and 3.5, inside the normal range. LWRs showed a high determination coefficient, indicating the reliability of our results for the estimation of the length-weight relationships. The parameters of the LWR can be used safely within the indicated length range. The differences between LWRs could be explained by the growth phase, genetic discrepancy, the gonad maturity, stomach fullness, the local nutrition conditions and preservation techniques of the captured specimens (Froese 2006; Mousavi-Sabet et al. 2015; Wootton 1998), which were not considered in the present study. Thus, differences in LWRs between the results and other studies could be potentially attributed to the combination of one or more of the factors given above. The comparison of LWRs parameters with previous studies (FishBase) indicates that these species have a negative allometry in the Tunisian reservoirs which differed from the prediction in the European lakes that estimated a positive allometry of LWRs.

This observation confirms that the environmental factors such as temperature have a direct effect on the relative growth of fish. As the dominant species, these fishes are important in maintaining the ecological balance in the Tunisian reservoirs.

4. Conclusion

These results contribute to the knowledge of the species from the most important reservoirs in Tunisia where species had no previous length-weight relationships estimates.

Acknowledgements

The authors thank all the staff of Technical center of Aquaculture (Tunisia) and the General Directorate of Fisheries and Aquaculture (Tunisia). Many thanks to Ahmed Kassab, Becher AMAMMI, Arij ZAATOURI, Dalel KAMEL, Hela SNOUSSI, Rabeb THABET, WissemEdine CHAWALI, Najet ALOUI, Rabeb MHIMDI, Sabri ADOULI, Chiheb BEN GAGA, Lamia CHANABI, Ghaith ZNEIGUI, Faten MECHERGUI and Rabia SAIDENI for their kind help during sampling and collecting.

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