Stocking fish in ecosystems has many benefits and, among others, can provide a unique tool for maintaining ecological balance in managed water bodies. In the Sea of ??Galilee, Israel, a lake that has a water residence time of 9 to 10 years , and like many similar closed ecosystems, phosphorus is the limiting factor for the development of algae, that govern water quality. Fish have a nutritional requirement for phosphorus, and as such can play a role in managing the ecological balance, through the consumption of the phosphorous containing algae and debris. Fish contain about 7 kg of phosphorus per ton. Therefore, one of the most effective tools for lowering the amount of phosphorus in a closed ecosystem such as the Sea of Galilee is to manage its fishery regime by removing adult fish from the lake. Presently, t he total amount of phosphorus remov ed annually from the lake through fisheries is about 3.5% of the average annual phosphorus entering the lake (100 tons per year).
Fishery management in the Sea of ??Galilee is performed by stocking fingerlings of several species. Mullet sp. are a principal group in the stocking program of the Sea of ??Galilee. For decades, every year about one million mullet fingerlings weighing 0.2 grams are being stocked into the lake. Traditionally, these fingerlings would be captured from the wild, on the shores of the Mediterranean. In recent years Israeli commercial hatcheries continuously supply mullet fingerlings, as an alternative to capturing fry from the wild.
As part of the MERC project on stocking hatchery mullet fingerlings we conducted a long learning process to improve stocking procedures of grey mullet (Mugil cephalus ) by using Israeli hatchery sourced grey mullet fingerlings. During 2018 & 2019 90,000 mullet fingerlings , produced by Israeli hatcheries, were transferred to the Ginosar research station. Following a rapid acclimation to fresh water, the fish were divided equally into two rearing systems differing in their water temperature regime: 1) A heat ed and controlled temperature of 260C , 2) An ambient temperature of 170C.
After four months, as a result of the difference in temperature, fingerlings were classified into two size categories (in two consecutive years) : 2018 large (6.25 g’) and small (3.02 g’) & 2019 large (13.53 g’) and small 6.57 g’). Fingerlings were individually tagged using code wire tags (CWT) and were stocked in to the lake, representing four stocking treatments representing: fingerlings size (small vs large) and time of the day (morning and night stocking) . The stocking of all 4 treatments was conducted at three different occasions, 7 days apart, allowing three replications every year. During the July, 2021, the first tagged adult grey mullets, with an average weight of 1,123 grams, were caught in the fishery in the Sea of Galilee. Further collection of tagged fish and identification of tags of the different treatments is under process. This information provides for the first time accurate growth rate data of the grey mullet in the sea of Galilee on an individual fish basis.