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10 glochidia are very sensitive to ammonia. 12, 30, 52 Predation Let us not forget that, for some animals, a freshwater mussel is a tasty meal. It is not uncommon to find shell that has been collected by some predator, cracked open and eaten. There may only be one or two or there may be dozens heaped into a pile called a "midden" . Drought and dewatering It would seem to be rather intuitive that a relatively immobile species would experience heavy mortality by dewatering (Duh!). But the impacts depend on whether this is a total dewatering or low flows (or low water levels). In streams, a totally dry stream means almost total mortality. This is the primary impact. But there are secondary impacts to low flows which are mainly due to low oxygen levels and high temperatures. As long as there is some flowing water, these secondary impacts appear to be minimal. But when flow ceases, there may be heavy mortality even if the stream is not totally dry. In regulated rivers that routinely experience extremely low flows, the impacts are due to high biological oxygen demand and high temperatures. If the low flows affect only short sections of stream, recovery will depend on whether barriers exist to prevent re- colonization. 16 I have walked the beds of several impoundments that have been drained and have found that there will be no mussels in the deepest portions of these impoundments. It has also been my experience that you will seldom find mussels in the fluctuation zone. In the flood control reservoirs, the annual fluctuation is low, maybe a foot or so. In irrigation reservoirs this zone can be dozens of feet. In any case, you seldom find mussels in this zone except for juveniles. (Juveniles drop off of fish in shallow waters in early summer, only to get stranded later in the year. This is most noticeable in irrigation reservoirs that are drained every fall.) The greatest numbers of mussels will be found in the 6' to 10' of lake bottom just below this zone. Any fluctuation greater than normal (like draining for "rehabilitation") will cause 100% mortality. A pile of mussel shell found on banks of Big Blue River, Gage County