Effects on fisheries and waterbirds of raising water levels at Kerkini Reservoir, a Ramsar site in northern greece

Abstract

ABSTRACT/A new, higher dam was installed at Kerkini Reservoir in 1982, causing habitat and landscape disruption. A decrease in the area of grassland and shallow water areas, the rapid disappearance of reedbeds, the appearance of beds of Nymphaea, and the disappearance of half the forest area were all observed between 1982 and 1991. With the new hydrological regime, a lacustrine system was created, with an extensive, rather deep (4-8 m), pelagic zone favorable for the development of coarse fish species throughout the year. After 1982, an increase in fishing effort and a change in the relative abundance of fish species in the catch, including the disappearance of eels and wels, were observed. The impact of the rise in the water level of breeding aquatic birds led to a general decline in species typical of marshy habitats in favor of species preferring deeper open water habitats. A decrease was recorded in bird species that feed largely on invertebrates and to a lesser extent fish (e.g., glossy ibis) and that require extensive shallow feeding areas. There was a decline in geese, whose nests were regularly flooded, and a major increase in piscivorous birds, particularly diving birds (e.g., cormorants), which prefer deeper open water and benefitted directly from the large increase in coarse fish biomass. The disappearance of birds breeding in flooded meadows (e.g., black-winged stilts) and of those restricted to reedbeds (e.g., marsh harrier) occurred from 1983. Over the same period, the changes in populations of wintering birds at Kerkini were different from those occurring in other wetlands in northern Greece. The changes recorded in the populations of wintering birds at Kerkini did not therefore result from overall regional trends but from the major habitat modifications that occurred to this wetland. As for breeding birds, strictly piscivorous species increased greatly as a result of the increased availability of fish, but also due to the appearance of many suitable night roosting sites (flooded trees) and to the great increase in the area of open water greater than 2 m deep. Today, Kerkini has become the most important breeding site in Greece for a majority of colonial waterbirds. In contrast, wintering shorebirds practically disappeared. The many changes recorded in the status of breeding and wintering birds at Kerkini can mostly be explained by the changes that occurred in the functioning of the ecosystem and in the habitat structure following the inauguration of the new hydrological regime. These changes did not all occur at the same time: some were immediate and others required a delay before they could be detected.The loss and degradation of natural wetlands is continuing throughout the Mediterranean region, despite the efforts of many conservation organizations (Britton and Crivelli 1992, Psilovikos 1992). Meanwhile, because of the acute problem of water shortage in the Mediterranean region, many artificial wetlands

Fish are not safe from great cormorants in turbid water

“…Further, fish biomass could be affected by a variety of factors other than great cormorant predation, such as local eutrophication due to varying agricultural practices around the different lakes, varying water level management, human fishing pressure or fishing pressure by other piscivorous predators. For instance, there is some evidence that dam construction and dredging along the Strymon River might have also contributed to a decrease in local fish abundance over recent years (Crivelli et al 1995b). However, such decreases only concern the carp Cyprinus carpio population, which is targeted by fishermen but not by cormorants because these fish are too large to be swallowed (Grémillet et al 2006).…”

“…At the Prespa lakes, great cormorants use different places for breeding according to years, such as cliffs along Megali Prespa, ground within the reedbed in association with pelicans or in juniper trees (alive or dead) on rocky islands (Catsadorakis et al 1996). At Kerkini, great cormorants breed exclusively in trees (alive or dead) located in the flooded forest (Crivelli et al 1995b). …”

The political culture of poaching: a case study from northern Greece

“…Ecological changes have also had a serious impact on many of the 300 local bird species (Crivelli et al 1995). By contrast, numbers of Great Cormorants grew dramatically since 1990 from 500 pairs to 3500 pairs in 2002.…”

Plant-water regime management in a wetland: consequences for a floating vegetation-nesting bird, whiskered tern Chlidonias hybridus

“…It provides the major needs of the species: extensive beds of floating vegetation, unique at the national level (see the importance of the vegetation-dominated Kerkini reservoir for Chlidonias spp. in Greece, Crivelli et al 1995), abundant food resources (Carpentier et al 2002) and multiple protection status. These suitable conditions have probably favoured the recent development of a whiskered tern population of international significance for the conservation of the species in Lake Grand-Lieu.…”