A new study by the Israel Nature and Parks Authority is debunking a long-held theory by fishermen and Agriculture Ministry fishery experts that a bird known as the great cormorant is responsible for the worrisome disappearance of fish in Lake Kinneret.
According to reports by fishermen, the quantity of fish in the lake has declined from more than 2,000 tons a year two decades ago to a few hundred tons in recent years. Experts ascribed the decline largely to eating habits of the great cormorant, an avian winter guest in this country with a voracious appetite for the bounty of the lake.
Counts made by the Parks Authority show that between 13,000 and 23,000 cormorants land here every year (last winter's count was 14,000 ), with a few thousand settling down in the Kinneret area. Their favorite meal is St. Peter's fish (tilapia ), which also happens to be one of the most economically valuable species to the fishermen.
Thanks to the discovery in undigested food regurgitated by the cormorants of certain crystals called otoliths found in the inner ears of fish - each species of which has its own unique kind - Authority scientist Yifat Artzi was able to show that cormorants have been getting a bad rap in terms of the declining fish population. Her calculations show that, at most, the birds are responsible for eating only some 38 tons of St. Peter's fish - not enough to explain the decline.
Authority scientists were also able to cross-check the quantities of fish with the area closest to the water where the birds spend the night. This showed no connection between the decline in the fish and the presence of the birds. Moreover, in 2010, which saw record numbers of the cormorants, the quantity of St. Peter's fish actually increased.
Parks Authority experts say that cormorant dietary habits are but one piece in the puzzle. Attention must now turn to other reasons for the decline in fish - first and foremost the sharp drops in the water level of the lake over the years and the impact on its ecology.
Among the effects that should be checked from the lower water level are the death of eggs exposed when the water recedes, and changes in the food chain that do not allow the fish to thrive.
Another cause for the fall in fish numbers could be overfishing in the past. Yet another reason for the decline, according to a recent article by Prof. Moshe Gofen of Tel-Hai Academic College, who has been studying the lake for many years, is a disease that has struck the St. Peter's fish.
The study appeared in this month's issue of the Hebrew journal Ecology and Environment.