|Ph.D Thesis||Department of Biology|
|Supervisors:||Assoc. Prof. Arad Zeev|
|Dr. Katzir Gadi|
In Israel, there are two species of cormorants: The Great Cormorant (Phalacrocorax carbo sinensis) is a migrating bird that overwinters in Israel (16000 individuals) from November to March and returns to Europe for breeding. The smaller Pygmy Cormorant (P. pygmeus) is a resident bird that lives and breeds (400 individuals) in colonies along the Hula, Jordan and the Beit Shean Valleys. The natural sites for both species in Israel have diminished during recent decades due to human activity. As a consequence, intensive fishery and aquaculture sites became their favorite feeding sites and the fish industry reports huge damage to fish yield. This study compares the energy demands and growth rate in captivity of the two species, as a basis for a future solution of the conflict.
Age-related changes in morphometric parameters and in energy demands were measured in captivity throughout ontogenesis. Basal metabolism was measured in the laboratory in fasting, resting birds, Existence metabolism, daily food intake and digestibility were measured in outdoors cages. Morphometric measurements of wintering Great Cormorant corpses, enabled a discriminant analysis between sexes.
The growth rate of Pygmy Cormorants was higher than that of the Great Cormorant in all parameters. Growth rate constant (K) of both species was higher than predicted from the allometric equation, based on the asymptotic body mass of the chicks. In both species, the legs grew faster than any other body part (including body mass), whereas the wings grew at the lowest rate. Male and female Pygmy Cormorants differ in body mass and wing length only, whereas in the Great Cormorant they differ in all morphometric parameters. Bill length, body length and wing length are the most discriminant parameters of sex in the Great Cormorant. The mass specific energy requirements of the Pygmy cormorant are much higher than those of the Great Cormorant, as expected from the size difference. The highest basal metabolism in both species was measured in young chicks (2-3 weeks), and decreased there after, in juveniles and adults. Basal metabolism of adults of both species was higher than predicted from the allometric equation. However, the existence metabolism was lower than predicted for waterbirds and shorebirds. Daily food intake of the adult Pygmy Cormorant (115 g) is higher than predicted from allometric equation for piscivorous birds whereas that of the Great Cormorant (244 g) is lower than predicted.
Based on the above energetic demands, the potential damage to the fish industry by 400 Pygmy Cormorant and by 12000 Great Cormorant that feed in the fish ponds is estimated at 460 tons of fish annually, corresponding to 2.8% of the annual fish yield in Israel.
From this study, it is clear that the two species of cormorants differ in their growth rate and energy demands. Therefore, their ecological impact on waterbodies in Israel is different and thus, a different management policy is necessary. The Pygmy Cormorant, as an extremely vulnerable species, needs a complete protection at the breeding colonies. In some areas, various deterring measures might be combined to prevent the cormorants from fish ponds, while offering some alternative reservoirs for feeding. Although the Great Cormorant is no longer endangered, its treatment should combine advanced management that would take into consideration its specific demands.