7046 (2) - Laserfiche WebLink

Home Browse Search

22 I I I I I I I I I ,I I ,I I I I, I I I I Since analysis of fish muscle tissue was not performed on samples from the Dolores River, it is not known if consumption of these fish represent a human health hazard. 4.3.4 lJiological Attributes 4.3.4.1 Macroinvertebrates. Eleven orders of macroinvertebrates were collected in the Dolores River during the study (Tables 48-53). In 1990, 47.7% of all macroinvertebrates sampled were Diptera, mostly from the family Simuliidae. Ephemeroptera and Trichoptera comprised 22.4% and 20.5% of all invertebrates, respectively. In 1991 invertebrate composition was more evenly distributed among Ephemeroptera (28.7%), Diptera (28.4%), and Trichoptera (28.4%). Invertebrate composition in the San Miguel River in 1990 was dominated by Diptera (926%), most of which were Simuliidae collected from one sample on Trip 1. Invertebrates sampled in the San Miguel in 1991 were primarily Diptera (46.8%), Ephemeroptera (25.5%), and Trichoptera (15%). Longitudinal composition of macroinvertebrates in the Dolores River in 1990-1991 was fairly consistent. In 1990, there was a gradual shift in composition downstream as Trichoptera increased from 2.2% at RM 122.5 to 37.8% at RM 1.3. Conversely, Diptera declined downstream, from 70.4% at RM 122.5 to 32.3% at RM 1.3. The same trend was not evident in 1991 samples. A summary of invertebrate collections prior to this study is presented in Table 54. It was difficult to make meaningful comparisons of historical macroinvertebrate data because of inherent differences in techniques, season, flows, etc. Little information on macroinvertebrates in the Dolores and San Miguel rivers was available before 1980, but what does exist indicates very low species diversity in both systems in the 1970's and 80's. Some insight may be gained by examining the presence or absence of "indicator species", that is specific macroinvertebrate taxa that are known to be pollution tolerant or intolerant. In 1960, pollution intolerant Plecoptera were absent from samples collected in the San Miguel River, but this taxa was present in samples collected in the 1980's and 90's. This evidence supports water chemistry and sediment analysis which indicate a substantial improvement in water quality of the San Miguel River since the 1960's. Pollution-intolerant Trichopterans (Le., Glossosomatidae) were present in both the Dolores and San Miguel rivers in 1990, but were not found historically in samples prior to our study. Biotic Condition Index (BCI) values were calculated for the Dolores and San Miguel rivers in 1991. The BCI is based on mean community tolerance, and is a composite of tolerance of individual taxa which varies in response to intensity of perturbations in the ecosystem. Parameters analyzed in calculating BCI include stream gradient and substrate, total alkalinity, sulfate concentration, and tolerance quotients (TQ) for each macroinvertebrate taxon (USFS 1985). Relative to their own potential, the Dolores (BCI=108) and San Miguel (BCI=56) rivers were rated excellent and fair to poor, respectively. Crayfish (Orceonectes virilis) were abundant in the Dolores River. Crayfish densities increased%in upper portions of the study. Beck (1989) reported mean catch rates of 3.3 crayfish/trap day (24 hours) from RM 76 to RM 101, 20.1 crayfish/trap day from RM 102 to RM 128 below the confluence of Disappointment Creek and 41.5 crayfish/trap day from RM 129 at the confluence to Disappointment Creek to RM 173 near Bradfield Bridge. Data from a similar survey done in 1991 resulted in 5.5, 13.1 and 58.3 crayfish/trap day respectively for the same reaches discussed above (T. Beck Pers. Comm. April 1992).