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Fish Surve~~s The abundance and diversity of the fish community can also serve as an excellent environmental indicator. Surprisingly, relatively few studies have examined the influence of imperviousness on fish communities in headwater streams.. The results of one study is illus- trated in;Figurc 6. Four similar subwatersheds in the Maryland Piedmont were sampled for the number of fish species present. As the level. of watershed imper- viousness increased, the number of fish species col- lecteddropped. Two sensitive species (trout and sculpin) were lost as imperviousness increased from !0 to l2% and four more were lost when impervious cover in- creased to 25%. Significantly, only two species re- mained in the fish community at 55% imperviousness. Sensitive species, defined as those with a strong depen- dence on the substrate for feeding and/or spawning, showed a more precipitous decline. Klein'° found a similar relationship between fish diversity and water- shed imperviousness in several dozen headwater streams in the Maryland Piedmont. Salmonid fish species (trout and salmon) and anadromous fish species appear to be most negatively impacted by imperviousness. Trout have stringent temperature and habitat requirements, and seldom are present in mid-Atlantic watersheds where impervious- ness exceeds 15%." Declines in trout spawning suc- cess are evident above 10% imperviousness." In the Pacific Northwest, Luchetti and Feurstenburc'x sel- dom found sensitive coho salmon in watersheds hc- yond 10 or IS%, imperviousness. Booth and Rcinclt' noted that most urban stream reaches had poor quality fish habitat when imperviousness exceeded 8 to 12%n. Fish species that migrate from the ocean to spawn in freshwater creeks are also very susccptihlc to im- pacts of urbanization such as fish barriers, pollution, flow chances, and other factors. Forexample, Limburg and Schmidt" discovered that the density of anadro- mous fish eggs and larvae declined sharply after a 10% imperviousness threshold was surpassed in 16 subwatersheds draining into the Hudson River. The influence of imperviousness on other urban water resources Several other studies point to the strong influence of imperviousness on other important aquatic systems such as shellfish beds and wetlands. Figure 6: Fish diversity as a function of watershed imperviousness in four subwatersheds in the Maryland Piedmont (Scheeler and Galli, 1992) Number of Species 14 12 70 8 6 4 2 0 C Sensitive Species ®Total No. of Species Even relatively low levels of urban development yield high levels of bacteria, derived from urban runoff or failinc septic systems. These consistently high bac- terial counts often result in the closure of shellfish beds in coastal waters and it is not surprising, that most closed shellfish beds are in close proximity to urban areas. Indeed, it may be difficult to prevent shellfish closure when more than one septic drain field is present per seven acres-a very low urban density.' Although it is widely believed that urban runoff accounts for r0 Numbers in brackets represent levels of watershed impervlouarteaa. Sources: 7) MdDNR, 1983 2) ftdDNR, 798b 3) tCpRB, f989 106 ~ Tlfi~~~a~.-tar~;i°.Liu7~~Ja~1~~,~1f~1_~-~ ~°K°~J 4,~~C~ j ~:i ~.; o a~-~ Good Hope Trib. Nursery Run' Hollywood Br. Wheaton Branch (Paint Branch) (NW Branch) (paint. Branch) (Sligo Creek)