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54 CHANGE i i t ;~~ 'z t one of the main objections to the introduction of herbivorous fishes (e.g., grass carp, Zill's cichlid) into natural waters, since they may eliminate or change the composition of aquatic plant communities important in the life cycles of fishes already present. Hybridization between two closely related species or subspecies has been a problem primarily when fish are transferred from one drainage system in California to another. The Mojave tui chub is now an endangered subspecies because it has hybridized in most of its natural range with the introduced arroyo chub and the hybrids are almost identical with pure arroyo chubs (Hubbs and Miller, 1942). Results are similar when I.ahonfan cutthroat trout hybridize with introduced rainbow trout. Hybridization with rainbow trout has almost eliminated redband trout as a distinct form. The loss through hybridization of a distinctive population represents an irreplacable aesthetic and scientific loss, as well as the loss of a genetic reservoir of potential future use by man. FISHING Sport fishing and, to a lesser extent, commercial fishing are today major factors shaping the freshwater fish communities of California. Fishing is highly selective for both species and size of fish. Sport fishing is aimed primarily at large carnivorous . r species while freshwater commercial fishing is aimed at large fishes not reserved for sport fishing, such as carp and Sacramento blackfish. If sport fishing removes a large percentage of the fishes at the top of a food chain, the population structure of the species making up the lower links is bound to change. In simple systems, such as farm ponds containing only largemouth bass and bluegill, excessive harvest of the top carnivores (the bass) may irreversibly change the system, unless fishing imbalances are continuously corrected. Thus, harvest of large-sized largemouth bass from a pond may cause a population explosion among their prey, the bluegill. The bluegill in turn may greatly reduce the insect and zooplankton populations needed to support young bass, resulting in fewer bass than before and large numbers of stunted bluegill. Similarily, removal of trout by fishing from streams that also contain squawfish may be partially responsible for the apparent dominance of squawfish in rivers, such as the American and Pit, which once provided excellent trout fishing. In an undisturbed stream, the faster growing rainbow trout can apparently dominate squawfish and keep them out of trout feeding areas (H.I.i, pers. comm.). Removal of trout allows squawfish to exploit the food normally utilized by trout, presumably resulting in bigger and more squawfish. The larger population of big squawfish then presents more of a predatory threat to small trout, making recolonization more difficult. Assuming the foregoing hypothesis is true, questions that need to be answered are: (1) how much fishing pressure can such a trout population take before the situation becomes unbalanced, and (2) how can the balance be restored once it has been upset? In other aquatic systems in California, such as reservoirs and warmwater streams, the effects of fishing on the fish communities is even less well understood than in trout streams, although overfishing of naturally reproducing populations of gamefishes seldom seems to be a problem. If fish populations decline, it is usually the result of environmental change rather than overfishing. One major exception to this rule, ~. . ~-~. .v+~w~-'..R~~aSES",~. -~_ .,. :'.$+i'erg!pn~e»~°?n''Y'~l*"F',~^`"."~. ,.. 4.;~"p~h ~,~, .`r~