Fish biological control of the Israeli National Water Carrier and dual-purpose reservoirs (fish culture/crop irrigation): the Israeli concept

Abstract

The location of Israel in a semi-arid part of the globe and the seasonal rainfalls that are limited to short periods in wintertime (about 50 days of rain with ~650 mm average precipitation in the north of the country), are the main causes of chronic water shortage and frequent droughts. The Israeli Water Authority planned and constructed the National Water Carrier (NWC) to convey water from the sole Israeli source of fresh water, Lake Kinneret in the north, through the country to the Negev Desert (10 mm average precipitation) in the south. The water is transferred through a system that combines pipes (108 inch diameter), canals, tunnels, and reservoirs. Because the water is intended for human consumption, the reservoirs were stocked with a complex of fish for biological control. To support agricultural irrigation, regional local authorities and private fish farms have also constructed dual-purpose reservoirs in various parts of the country. These reservoirs store water for crop irrigation and also serve for fish culture. Some of them are used solely for irrigation, but in some reservoirs, fish are reared as a by-product. About 250 dual-purpose reservoirs of various capacity and surface areas (10-40 ha) have been constructed in Israel in the last two decades. The first fish species, tilapia, Oreochromis aureus (Steindachner), was stocked in reservoirs in the early 1960s, and common carp, Cyprinus carpio L., was stocked in reservoirs in the end of 1960s in an attempt to control phyto- and zooplankton. Although the results were positive, it soon became obvious that these species were prolific and had multiplied extensively, which resulted in overcrowded populations of stunted fishes and caused management difficulties. Consequently, a group of Chinese carp was introduced, namely: the phytoplanktophagic silver carp, Hypophthalmichthys molitrix (Val.), the macrophytophagic grass carp, Ctenopharyngodon idella (Val.), the zooplanktophagic bighead carp, Aristichthys nobilis (Richardson), and the malacophagic black carp, Mylopharyngodon piceus (Richardson). In addition to these four species, predatory (or “police”) fish have been integrated into the fish complex; these include the marine European seabass, Dicentrarchus labrax (L.), or the hybrid striped bass, Morone saxatilis (Walbaum) and white bass, M. chrysops (Rafinesque), or red drum, Sciaenops ocellatus (L.). Red drum is also considered to be a high quality consumer fish. These fish are capable of controlling efficiently the unwanted spawning of carp and tilapia. With these combinations of fishes, various species play specific roles by utilizing specialized feeding niches without competing with other species. Due to the variety of conditions in the reservoirs, the composition of fish species is modified according to individual characteristics of each reservoir (soil, area, depth, source of water, role of the fish, etc.). Adequate stocking of species into reservoirs may positively affect synergism among fish and may have a positive impact on fish growth. Stocking water reservoirs with fish species that cannot reproduce naturally in the water system is advantageous. However, in such cases, the entire process of reproduction, including the incubation of eggs, must be accomplished artificially indoors. Each species is induced to spawn with hormones, namely calibrated (carp) pituitary extract (cCPE) or a synthetic substitute of GnRH, s-GnRHa. Both products were developed in Israel and are presently distributed and available worldwide. The fertilized eggs are incubated to hatching and the young are nursed until they are stocked into the various reservoirs. Many countries prohibit the introduction of exotic fishes so as to protect local fish species from competition. An extensive study of chromosome-set manipulation has been carried out in order to develop techniques to produce sterile triploid grass and black carp. Female triploids are preferred because they have amorphous gonads. Such non-reproducing fishes have export potential for biological control purposes (e.g., black carp to control bilharziasis and grass carp to prevent river blockage by aquatic vegetation), without endangering local fish fauna.

CORRESPONDING AUTHOR:

Shmuel Rothbard, YAFIT Laboratory, Fish Breeding Center, Gan Shmuel 38810, Israel, e-mail: rot–bard@gan.org.il