The Issue
in the Columbia River
Caspian tern colony on East Sand Island
East Sand Island in the Columbia River Estuary is the largest known Caspian tern (Sterna caspia) nesting colony in the world, with about 7,000 breeding pairs (as of 2011). It first became a nesting colony in 1984 after the eastern tip was used as a dredge material disposal site making it suitable for tern nesting. However, at that time it was only colonized for one year when the regrowth of vegetation in 1985 deterred subsequent nesting. By 1986 the colony had relocated on a non-natural dredge material disposal island, Rice Island, about 21 kilometers further up the Columbia River. By 1990 the Rice Island Colony was well established and at 16,000 nesting pairs, about 67% of North America's Pacific Coast population, was the largest Caspian tern Colony in the world [5].
impacts on the Salmon
Between 1997 and 1998 about 73% of the diet of Caspian terns on Rice Island was comprised of juvenile Salmonids (Oncorhynchus spp.), which come through the estuary every year on their outward migration towards the Pacific Ocean. That 73% amounts to an estimated 12.4 million juvenile salmonids, which , at the time, was roughly 12.8% of the out-migrating smolts. Predation from terns, therefore, has a significant impact on the yearly salmonid stocks in the area. Furthermore, over half of the Evolutionarily Significant Units (ESUs - a population of organisms that is considered distinct from all others, even of the same species, for purposes of conserving biodiversity) of salmonid species in the Columbia River Basin are considered threatened or endangered by the U.S. Endangered Species Act (ESA). Because of this, action was taken in 1999 to relocate the Rice Island Colony back to East Sand Island, 21 km closer to the mouth of the river, to encourage foraging in more marine and brackish water with accessibility to more oceanic forage fish, and in doing so, reduce the pressure on the juvenile salmonids [5]. The move was successful in this respect; the proportion on juvenile salmonids in their diet decreased by roughly half and the proportion of forage fish increased accordingly [3].
impacts on the Birds
Colony sizes
Still, East Sand Island remains the largest nesting colony of Caspian terns in the world, and despite the reduction in pressure, these birds still have an impact on the out-migrating salmonid populations. In addition, there are important considerations for the health of the terns. Though these birds are not considered endangered they have have experienced a major decrease in the number of breeding colonies in the Pacific Northwest and likewise across the globe. This has resulted in fewer and larger nesting colonies in order to accommodate the displaced birds [6]. These changes are mainly the result of anthropogenic influence in the form of habitat degradation (mainly human development and disturbance), introduced predators and increased predation and competition from other birds [7]. The reduction in numbers of nesting colony sites and the subsequent increase in colony size makes these birds far more vulnerable to local catastrophic events, such as disease outbreaks, oil spills, predation, weather, human disturbances [6], and a decline in prey species [3].
Therefore, the Caspian Tern Management to Reduce Predation of Juvenile Salmonids in the Columbia River Estuary was developed in 2005 by the USACE, the U.S. Fish and Wildlife Service, and NOAA fisheries for Caspian tern colony restoration, not only for the health of the commercially and recreationally important salmonid species, but also for the general health and protection of the terns themselves.
