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Vulnerability of Native Fishes to Nonnative Interactions <br />— Background <br />Li and Moyle (1999) provided an overview of ecological concepts that are <br />important for understanding the effects of introduced species and Moyle et al. (1986) <br />identified six mechanisms that allow introduced fishes to displace native fishes, <br />especially in habitats that have undergone extensive physical habitat alteration. These <br />mechanisms are summarized below: <br />(1) Competition: nonnative fishes can affect the use of food and space by native fishes <br />by interference (e.g., denying access of another species to limited resources, usually <br />through aggression), and exploitation (e.g, better at utilizing resources, such as a visual <br />feeder in clear water as opposed to a fish adapted to turbid conditions). <br />(2) Predation: all fishes are consumers and most can be predaceous at some life stage. <br />Predation may be difficult to observe in riverine systems, and eggs or larvae are so <br />quickly digested that even stomach contents analyses can be misleading (e.g, see <br />Langhorst and Marsh 1986). <br />(3) Reproductive inhibition: nonnatives can reduce reproductive success of native fishes <br />by a variety of agonistic actions whereby spawning and/or parental care of eggs and <br />young are disrupted. <br />(4) Environmental modification: natural habitat complexity can be changed by fishes that <br />destroy vegetation (grass carp) or create unusual conditions (turbidity; common carp), <br />thereby placing native fishes at risk. <br />(5) Parasites and disease: Asian tapeworm and whirling disease are examples of <br />parasites and diseases introduced by nonnative fishes that have caused declines in <br />native fish populations. <br />(6) Hybridization: altered environments can break down reproductive isolating <br />mechanisms between related native species, but nonnatives that are placed in different <br />environments also pose a threat due to hybridization with native species. <br />The preceding list summarizes the many ways that introduced fishes can <br />adversely affect native fish communities. The interactions can be complex, difficult to <br />predict, and in some cased difficult to imagine. However, it has been universally <br />appreciated by naturalists and morphologists for centuries that animals reduce or <br />21 <br />