Fisheries management

Abstract

Fish are at the top of aquatic food webs impacting on other biota. Removal of fish by fishing, or stocking the lake with desired fish species has the potential to modify fish populations (their size/age structure, abundance and biomass) and various other ecosystem components and thus affect water quality. Analysis of long-termdata onthe catches of commercially important fish species,stocking of fingerlings,lake water levels,and littoral modifications, allowed revealing the key factors affecting the fish populations, fishing pressure, and mass of landed fish. Long-term decreasesin water level negatively affected reproduction and survival of natural/stocked fingerlings of some cyprinids (Mirogrex terraesanctae, Hypophthalamichthys molitrix, Cyprinus carpio) and cichlids(Sarotherodon galilaeus, Oreochromis aureus). In contrast, rapid rises in water level vastly enlarged the areas of inundated terrestrial vegetation and stony/rocky littoral and thus positively affected the reproduction and survival of these fish and their landing after a temporallag. Fishing pressure on reproductively active large cichlidswas enhanced at low water levelsdue to exposing these fish at spawning time (spring and summer) to fishermen. Such overfishing resulted in lowering the number of reproductively capable cichlids below a certain critical level, leading to a decrease in the reproductive capacity of the population and the collapse of its standing stock (i.e. “recruitment overfishing”). The endemic bleak, Mirogrex terraesanctae is the dominant pelagic zooplanktivore of Lake Kinneret. Large water level fluctuationsimpacted its reproduction and determinedthe abundance of this ecologically important species. Biomanipulation, by mean of subsidized harvesting, implemented to suppress its abundance in critical years, was highly controversial, and was stopped in 2006. Of the stocked grey mullets, Mugil cephalus had lower natural mortality rate, higher growth rate, reached larger sizes, and thus had a higher market value than Liza ramada. Still, fingerlings of the second species were more readily available, such that L. ramada was stocked in larger numbers and was more abundant in the commercial catch.We argue that fish stock management in Lake Kinneret should focus on the restoration of native fish populations and their sustainable fishery.