Fishes | |
| Overview | | |
| The inescapable conclusion from the data presented 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; Starnes; and Walsh et al., this section) or on populations or genetic variation within a single species (Marnell; Miller et al.; and Philipp and Claussen, this section). Changes in fish communities may be indicative of the overall health of an aquatic system; some species have narrow habitat requirements. |
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| 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 protection). 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. |
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| Physical and chemical changes in their habitats are not the only stresses that fishes have encountered over time. Through fish management programs, the aquarium trade, and accidental 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. |
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| 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. |
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| It appears unlikely that the forces that have led to these changes in our fish fauna will lessen significantly in the immediate future. Therefore, if we are to preserve the diversity and adaptive potential of our fishes, we must understand much more of their ecology. Vague generalizations about habitat requirements or the results of biotic interactions are no longer enough. We must know quantitatively and exactly how fishes use habitat and how that use changes in the face of biotic pressures. Only when armed with such information are we likely to reduce the current trends among our native fishes. |
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