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The Problem Northern pikeminnow are an important predator of juvenile salmonids within the Columbia and Snake rivers of the Columbia River Basin. These predators, along with others such as smallmouth bass and walleye, are a major source of mortality for threatened and endangered stocks of salmonids that are migrating toward the ocean. A considerable amount of research has been conducted to understand the
In 1990 a biological control program was initiated by the Bonneville Power Administration and regional fishery agencies to directly remove large numbers of northern pikeminnow. Over 1.2 million northern pikeminnow have been removed from the mainstem rivers, mainly through a bounty fishery. This program costs over $3 million per year, so there is concern about its effectiveness and how it might be better managed to produce improvements in the survival of juvenile salmonids. Large scale patterns are one area of interest with respect to this program, and the general effects of fish predators. Two large-scale factors that might be influencing predator growth rates, and thus predation losses, are temperature differences throughout the river system and the increased abundance of American shad. American shad Alosa sapidissima from the Hudson River (New York) were introduced into the Sacramento River (California) in 1871, and were first observed in the Columbia River in 1876. American shad returns to the Columbia River increased greatly between 1960 and 1990, and recently 2-4 million adults have been counted per year at Bonneville Dam (river kilometer 235). The total return
Water temperatures also differ throughout the river system, especially since the construction of many large hydroelectric dams that have slowed the movement of water and allow it to warm more than it did historically. Warmer water temperatures in the lower Columbia River, for example, could now be causing faster growth of many fish species, including predators of juvenile salmonids. Objectives The objectives of this research is to evaluate the potential effects of American shad and water temperature on the growth and predation impacts of northern pikeminnow in the Columbia and Snake rivers. Large-scale patterns in growth rates, diet, and predation are being examined. Such information may be useful to managers of the system in modifying the predator bounty program, or through other tactics that might control predation losses. Methodology
The USGS portion of the evaluation will be related to the trophic dynamics of major predators and the mechanisms of predator-prey interactions. Existing predation models and a bioenergetics model will be used to assist in the evaluation. We will use field, laboratory, and models to investigate mechanisms that may be responsible for differences in growth rates of northern pikeminnow, which produce significant differences in population structure. Field sampling includes collection of predators for diet analyses and sampling the number of juvenile American shad that pass dams. Fieldwork to collect northern pikeminnow guts for diet analysis from upriver and downriver locations was conducted during the fall of 2000 and 2001. Specific hypotheses that will be examined concern temperature and the role of American shad. Highlights and Key Findings The growth rates of norther pikeminnow differs between the lower Columbia River, the lower Columbia River reservoirs, and reservoirs in the lower Snake River. In general, these predators tend to grow faster in the lower reaches of this river system than in the Snake River, although temperatures are not greatly different. Faster growth rates lead to a larger size at a given age in the lower river. Northern pikeminnow in the lower Columbia River consume a fairly high percentage of juvenile American shad during the late summer and fall period - American shad may contribute as much as 90% of the diet at certain times and places. The energy value of juvenile American shad was measured and it is quite high for fish species, and is much higher than invertebrate types of prey such as crayfish. Thus, American shad could be a source of energy available to the predators that was not available prior to the invasion of shad during the last 50 years. Where Are We Headed In 2003 Current work is ongoing to test the specific hypotheses about temperature and diet differences in the river system. Previous bioenergetics modeling suggests that juvenile shad in the diet of northern pikeminnow can significantly increase growth rate during the fall months. The caloric value of juvenile shad and other dietary components has been determined and further bioenergetics modeling is underway to estimate northern pikeminnow caloric intake, growth, and reproductive potential under upriver and downriver fall thermal regimes and diets. The product of this research will be a report to the Bonneville Power Administration in spring 2003. Project Contact Alec Maule Email: alec_maule@usgs.gov Publications |
