In the frame of the “Grey Mullet Stock Enhancement” work package of the MERC – USAID project we used local wild fry grown within experimental freshwater facilities of INSTM.
Small (0.1-0.2g) wild mullets were captured from delta rivers and acclimated at INSTM’s freshwater hatchery where they were maintained in 500 L tanks (1000 to 1200 fry/tank) and were fed compound diet s (8-10% of the biomass) until reaching the required size (4,5 cm) for tagging with CWT. A total of 12370 tagged fish have been released within three lakes (Kasseb , Lebna & Smati ) allowing acceptable stocking densities; containing enough water (country was suffering drought conditions in the last 4 years); and in which there “should” be collaboration opportunities with local fishermen for fish capture. For the seeding program, we selected different treatments applied with replications. Treatments consisted ins hore versus offshore release; no anesthesia versus MS-222 or clove oil anesthesia; oxygen peroxide treatment before release.
Up to date, 80 tagged fish have been obtained from fishermen. Biometry and tag number were identified for each fish. Data were compared by ANOVA. The averaging recovery rates (i.e., number recaptured/number released) between release batches , release lake, and experimental treatment, were Arcsinus (square root (X)) transformed. Only data concerning Kasseb and Lebna lakes (for which we had enough samples) were statistically analyzed.
As expected for mullets coming from the wild, captured tagged fish showed a mixed population consisting in 56% Mugil cephalus and 44% Liza ramada . All biometric indicator (LT, weight & K) appeared higher within samples of Lebna lake compared with Kasseb lake (Table 1). As all the fish came from the same juvenile cohort, this suggests better conditions (thermal, nutritional, … ) in lake of Lebna .
Lebna lake is considered a low depth (max. about 10 m), tempered (near the sea) lake. It has a surface of 650 Ha for 30 M.m3 volume and it has a surrounding region strongly urbanized with intensive agriculture. Contrastingly, Kasseb lake is considered an inland (30 km from the shore) deep lake with much cooler conditions in winter. It has a surface of 430 Ha for 82 M.m3 volume (2.7 folds the volume of Lebna ) and it has a surrounding region weakly urbanized with poorly covered mountains. The highly distinct growth performances are suggesting the necessity of developing a typology of exploited lakes for optimizing priorities of seeding operations.