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<br /> <br />c.) <br />c) <br />I-' <br />CD <br />l' ) <br />CD <br /> <br />Management Considerations <br />Contaminants were not shown to be a major concern in the San Juan River at this time. It is possible that, <br />historically, contaminants from oil and gas activities in particular, and perhaps other basin activities, led to <br />the decline in razorback sucker and Colorado pikeminnow populations, although there is no evidence to <br />either substantiate or refute such a claim, The extensive information collected during the 7-year research <br />period suggested that contaminants would not limit recovery of the two endangered fish species, <br /> <br />POPULATION SIZE <br /> <br />Introduction <br />When the SJRlP was initiated, it was known that population levels of the two endangered fish species were <br />low, but it was thought that improved reproductive success resulting from altered Navajo Dam flows would <br />result in increased populations, It was also thought that the most likely "biological response" to the flow <br />changes would be increased numbers ofYOY, especially Colorado pikeminnow YOY; hence an emphasis <br />was placed on seining backwater habitats in the fall. Because of this anticipated response, the need to <br />stock either species was not realized until 1997 or later, and thus stocking was not included in the LRP <br />(USFWS 1995) until the end of the 7-year research period, By 1994, no wild razorback sucker had been <br />collected (Ryden 2000a, 2000b), suggesting that this species did not have a population remaining in the <br />San Juan River, and that improved recruitment would not occur with the reoperation of Navajo Dam only, <br /> <br />Numbers ofYOY Colorado pikeminnow did not increase during the 7-yearresearch period (Table 3.9), <br />and this escalated concerns that habitat for young fish was not available, predation on young was too high, <br />or the population was too small to respond to changes in habitat. As discussed above, when YOY were <br />stocked in the river to determine habitat availability and use, the stocking was very successful, suggesting <br />that habitat for YOY and predation on YOY were not limiting, at least at the levels of stocked fish that <br />survived. Two wild juvenile Colorado pikeminnow (363 and 432 mm TL) were captured in the lower San <br />Juan River in 1996, indicating that some recruitment from wild fish still occurred (Ryden 2000a), but at a <br />very low level. Ryden (2000a) reported 17 adult Colorado pikeminnow captures from 1991 through <br />1994, but only one was captured during the period of 1994 to 1997. However, YOY Colorado <br />pikeminnow were found, suggesting adult fish learned to avoid electrofishing rafts. Ryden (2000a) used <br />capture and recapture information between RM 119,0 and RM 136.0 to perform a population estimate <br />resulting in 19 adult Colorado pikeminnow. Based on this information, it appeared that the population of <br />Colorado pikeminnow in the San Juan River was very small and was likely too small to respond to <br />improvements in habitat or other environmental features. <br /> <br />September 2000 <br /> <br />3-57 <br /> <br />Program Evaluation Report <br />