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mortality suggest, however, that the populations are considerably larger than they appeared to be in the 1980s, and that some recruitment to the adult age classes has occurred in most or all years of the last decade (see below). Population sizes might range from a few tens of thousands to the low hundreds of thousands (USFWS 2001) but still are much lower than they were originally. Aside from the decline in abundance over the long term, other indications of problems within the sucker populations include absence of spawning at a number of sites historically used for spawning, apparent increase in mass mortality of adults (“fish kills”), and weak recruitment in most years (USFWS 2001).

The water quality of Upper Klamath Lake has changed substantially over the past several decades. The lake appears to have been eutrophic (rich in nutrients and supporting high abundances of suspended algae) prior to any anthropogenic influence (Kann 1998). Mobilization of phosphorus from agriculture and other nonpoint sources (Walker 2001), appears, however, to have pushed the lake into an exaggerated state of eutrophication that involves algal blooms reaching or approaching the theoretical maximum abundances. In addition, algal populations now are strongly dominated by the single blue-green algal species Aphanizomenon flos-aquae (cyanobacteria) rather than the diatom taxa that apparently dominated blooms before nutrient enrichment (Kann 1998, Eilers et al. 2001).

Evidence indicates that changes in the water quality of Upper Klamath Lake have increased mass mortality among adult suckers. Under certain conditions, the bottom portion of the water column in the lake develops oxygen depletion, either no oxygen (anoxia) or lower than normal oxygen levels (hypoxia), and accumulates high concentrations of ammonia. Mixture of those bottom waters with the surface waters under the influence of changes in the weather likely causes mass mortality (Vogel et al. 2001). Although mass mortality has been recorded over the observed history of the lake, its frequency appears to have increased (Perkins et al. 2000). Major incidents were recorded for 1995, 1996, and 1997; low dissolved oxygen appears to have been the direct cause of mortality in these years (Perkins et al. 2000).

Impaired water quality also might stress fry through high pH in surface waters resulting from high rates of photosynthesis, although exposures to the highest pH probably are too brief to cause mortality (Saiki et al. 1999). In addition, the present trophic state of the lake potentially poses a threat of mortality in winter, when anoxia can occur under the ice if oxygen demand is high. Although not yet observed, winter mortality could occur in the future (Welch and Burke 2001).

Factors of concern other than water quality include the presence of exotic species capable of inducing types of predation and competition that are

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