Implications for Juvenile Sucker Health


Between 2007 and 2009, microcystin concentrations in the large (> 63 µm) particulate (cell assoicated) fraction measured in water column samples were elevated, which suggests a potential threat of microcystin exposure to fish and other wildlife ingesting particulates in Upper Klamath Lake. Poor water quality (extremely low dissolved oxygen concentrations, high pH, and elevated ammonia concentrations) has been proposed as a contributing factor for occasional periods of high mortality and overall population decline in Lost River and shortnose suckers (Martin and Saiki, 1999; Saiki and others, 1999). However, sharp decreases in age-0 catch rates between August and September over the last 5 years (Bottcher and Burdick, 2010), combined with low catches of age-1 and older suckers (Hendrixson and others, 2007; Terwilliger and others, 2008) indicate that juvenile suckers suffer widespread, episodic mortality during the first 2 years of life. The mechanism for this mortality is not known, but as many as one-half of juvenile suckers collected from Upper Klamath Lake in 2007 (VanderKooi and others, 2010) and 14 percent of juveniles collected from the central and northern regions of the lake in 2009 (n = 36; C. Ottinger, U.S. Geological Survey, unpub. data, 2010) exhibited liver or kidney damage consistent with microcystin ingestion. Furthermore, the disappearance of juvenile suckers from trap net catches has occurred 2 to 3 weeks after peak concentrations of microcystins have been measured (Burdick and others, 2009; Bottcher and Burdick, 2010). 


Observational field studies and tissue analyses have documented the effects of microcystin exposure in whitefish (Ernst and others, 2001), tilapia (Magalhaes and others, 2001), and flounder (Sipia and others, 2001), but few studies have evaluated the primary mechanisms of cyanotoxin exposure in natural environments, oral ingestion (by feeding), and passage of dissolved toxins across gill membranes (Malbrouck and Kestemont, 2006). Recently, a preliminary risk assessment of shortnose and Lost River sucker microcystin exposure was conducted to determine the daily dry weight dosage per kilogram of live fish weight (Malbrouck and Kestemont, 2006) based on microcystin concentrations measured in 2007 (when the highest concentrations of particulate fraction microcystins were observed). If these fish consume 1 percent of their body weight per day of (dried) toxigenic cyanobacteria producing the most toxic microcystin isomer, microcystin-LR (these factors are currently unknown), at the peak microcystin concentration observed in this study (1.42 µg/mg dry weight; measured in 2007), a fish weighing 1 kg would receive a dose of 14,150 (µg/kg)/d (K. Echols, U.S. Geological Survey, written commun., 2007). This value is approximately 25 times the LC50 intra-peritoneal dose (the concentration required for 50 percent mortality) of microcystin-LR (550 µg/kg) for carp (Rabergh and others, 1991) and trout (Tencalla and others, 1994). Juvenile suckers (and larvae) may be more susceptible to toxins than adults (fish weighing closer to 1 kg) because of their larger surface area-to-volume ratios, higher metabolic rates, and the vulnerability of their key developmental processes. However, more work is needed to understand the dietary intake of toxigenic species in these fish, the concentration (or concentration range) of ingested microcystins that damage liver tissue and promote mortality in juvenile suckers, and if sufficient microcystin concentrations to cause the tissue damage or mortality rates observed are present within the materials these fish consume in Upper Klamath Lake.


First posted May 30, 2012