FIG. 5-10 Schematic diagram of possible stream modifications 

 affording improved protection and spawning facilities to fish 

 (after Lagler 1952). 



Stone deflector 



Fish management 



Basic to fish management in streams is the con- 

 trol of soil erosion and pollution. In clean, clear 

 streams, both invertebrates and fish can attain high 

 populations through normal reproduction. Artificial 

 propagation and release of reared fish into streams 

 to improve fishing is not necessary except where 

 habitats have been depleted of breeding stock or 

 where the fishing pressure is excessive. The artificial 

 raising and releasing of fish of suitable size for quick- 

 recapture in sport fishing is expensive but sometimes 

 justified in highly populated areas. In most regions 

 the fish manager is better concerned with improving 

 habitats and letting the fish repopulate them to full 

 carrying capacity on their own accord. 



Stream fishes suitable for sport and food are pri- 

 marily those inhabiting the pools rather than the rif- 

 fles. The carrying capacity of streams can sometimes 

 be raised by artificially increasing the number of 

 pools without destroying too many of the riffles, the 

 main source of fish food. The interspersion of ponds 

 along the stream also increases its fertility, since they 

 are the sources of plankton, detritus, and dislodged or 

 escaping organisms. The formation of pools may 

 often be done inexpensively by making simple log 

 dams or deflectors. Occasionally, it may be desirable 

 to haul in gravel from elsewhere to make spawning 

 beds and to provide artificial log or brush shelters 

 (Needham 1938, Lagler 1952). 



There has been a country-wide practice of intro- 

 ducing species of fish into streams where they did 

 not originally occur. The result has been to greatly 

 mix up and modify the fish fauna ; original primitive 

 communities no longer prevail. This is unfortunate 

 for ecological research. The U.S. National Park 

 Service is, however, attempting to preserve a certain 

 number of natural stream areas in their original con- 

 dition, prohibitng fishing therein (Kendeigh 1942a). 



SUMMARY 



Streams contain riffles, sand-, and mud- 

 bottom pools. Inhabitants of the riffles and sand- 

 bottom pools constitute a distinct stream biocies. 

 Mud-bottom pools are inhabited by species from the 

 pond-marsh biocies. Animals adjust to the action of 

 water current by clinging mechanisms, avoidance, 

 or vigorous swimming. They are generally positively 

 rheotactic, and several forms maintain orientation to 

 a particular position in the stream by means of visual 

 landmarks. Segregation to dififerent habitats depends 

 largely on differential response to the substratum ; 

 that is, preference respectively for rock, sand, or 

 mud. Animals occurring in mud-bottom pools are 

 usually negatively rheotactic, or become helpless in 

 strong current. They also have adaptations tolerant 

 of lower oxygen concentrations in the water. Changes 

 in the size of the stream, occasioned by various physi- 

 cal factors, also afifect the responses of animals. 

 Stream animals have apparently evolved from an- 

 cestral types that occupied the quiet waters of lakes 

 and ponds. The life cycles of many stream insects are 

 remarkable for the long duration of immature stages 

 and the brief life of adults. Animals have various 

 adaptations to feed on detritus in the water, on di- 

 atoms, on filamentous algae, or for being carnivorous. 

 Density of individuals, biomass, and productivity of 

 invertebrates are ordinarily less in sand-bottom pools 

 than in either riffles or mud-bottom pools. Clams and 

 game fish, however, inhabit sand-bottom pools. Fish 

 management requires the proper interdigitation of 

 pools and riffles, as well as control of erosion, silting, 

 and pollution. 



58 Habitats, communities, succession 



