Fisheries Problems in Impounded Waters of California and the Lower Colorado River

Doubling of the amount of impounded waters in California is in the foreseeable future. Basic climatology dictates winter and spring storage and summer and fall drawdowns, a feature common to far western impoundments. Most of these impoundments will support warm‐water fisheries. Native fresh‐water fish populations in California and the lower Colorado River have not adjusted to the combination of impoundment and predation by introduced warm‐water sport species and often the introduced species have not maintained desirable population structures. The sunfishes frequently stunt and are believed to depress the bass populations. Extremely poor survival of bass‐of‐the‐year is common. An unsatisfactory bass‐of‐the‐year/forage ratio exists in the fall when the young bass turn to a fish diet. Food production is poor in the littoral zone because of water level fluctuation, but plankton production in open waters is usually adequate. Over a period of years, following construction, the reservoirs experience a typical pattern of high initial fishing success, a sharp decrease, and then a gradual rise to a fishery somewhere near half the magnitude of the initial phase. The decrease is believed to be the result of the high initial fertility, characteristic of newly farmed lands, being leached out by repeated fluctuation. The readjustment of the fishery to a lower level is the result of its adjustment to the basic fertility of the basin. Management practices are based upon attempts to modify the decline and to maintain a fishery as close to that of the initial phase as possible. Fertilization, in the farm pond sense, is not practical. The establishment of littoral zone cover crops that perpetuate themselves even though undergoing considerable periods of submersion is suggested. The modification of the use pattern and special construction features are useful but require constant vigilance by the fisheries agencies and the cooperation of construction and operation agencies. Population manipulation, principally through the use of rotenone, is a widely used management tool but is a negative approach that usually necessitates the impoundment being removed from the fishery for varying lengths of time. Modification of population structure by the introduction of the threadfin shad into the lower Colorado River as a forage species has met with apparent high success.