Fishes 



Overview 



The inescapable conclu- 

 sion from the data pre- 

 sented in this section is that within historical 

 time, native fish communities have undergone 

 significant and adverse changes. These changes 

 generally tend toward reduced distributions, 

 lowered diversity, and increased numbers of 

 species considered rare. These changes have 

 been more inclusive and more dramatic in the 

 arid western regions where there are primarily 

 endemic (native) species, but similar, though 

 more subtle changes, have occurred throughout 

 the country. These trends are the same whether 

 one focuses on faunas (Johnson; Stames; and 

 Walsh et al., this section) or on populations or 

 genetic variation within a single species 

 (Mamell; Miller et al.; and Philipp and 

 Claussen, this section). Changes in fish commu- 

 nities may be indicative of the overall health of 

 an aquatic system; some species have narrow 

 habitat requirements. 



The fact that fish populations have changed 

 over historical time should not come as any 

 great surprise. We have massively modified fish 

 habitat through the very water demands that 

 define our society (domestic, agricultural, and 

 industrial water supplies; waste disposal; power 

 generation; transportation; and flood protec- 

 tion). All of these activities have resulted in 



controlling or modifying the flow or degrading 

 the quality of natural waters. In addition, almost 

 all contaminants ultimately find their way into 

 the aquatic system. Species of fishes that have 

 evolved under the selection pressures imposed 

 by natural cycles have often been unable to 

 adapt to the changes imposed on them as a 

 result of human activities. 



Physical and chemical changes in their habi- 

 tats are not the only stresses that fishes have 

 encountered over time. Through fish manage- 

 ment programs, the aquarium trade, and acci- 

 dental releases, many aquatic species have been 

 introduced to new areas far beyond their native 

 ranges. Although these introductions were often 

 done with the best of intentions, they have 

 sometimes subjected native fish species to new 

 competitors, predators, and disease agents that 

 they were ill-equipped to withstand. 



The data presented by Philipp and Claussen 

 (this section) further suggest that managed fish 

 populations (hatchery-stocked populations) 

 have a lower genetic diversity than unmanaged 

 populations. In other words, theoretically, the 

 smaller the gene pool, the less likely a species 

 may be able to adapt to changing environmental 

 conditions. 



It appears unlikely that the forces that have 

 led to these changes in our fish fauna will lessen 



Science Editor 



O. Eugene Maughan 



National Biological 



Service 



Arizona Cooperative Fish 



and Wildlife Research Unit 



University of Arizona 



Tucson, AZ 85721 



