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<br />. <br /> <br />. <br /> <br />1 <br /> <br />DRAFT <br />Colorado River Programmatic Biological Opinion <br />Endangered Fish Population Status Assessment <br />February 12. 1999 <br /> <br />Introduction <br /> <br />A practical approach is needed for determining the status of or trends in <br />endangered fish populations and whether that status/trend is related to <br />historic or new water development projects, This information will be used to <br />determine if the Service should (al reinitiate consultation on water <br />development projects that have been consulted on by the Service or (b) extend <br />the water depletion limit for the upper Colorado River above the 15-Mile <br />Reach. The determination of status and trends should utilize the best <br />available scientific information and expert opinion of knowledgeable <br />biologist, <br /> <br />Assumptions/Limitations <br /> <br />There are a number of practical limitations to our ability to detect changes <br />in the status of endangered fish populations in a timely and accurate manner: <br /> <br />o Colorado pikeminnow (formerly Colorado squawfish): <br />-Adult Colorado pikeminnow can be sampled relatively effectively <br />and reliable populations estimates developed. Estimates of the <br />size of the adult Colorado pikeminnow population in the Colorado <br />River subbasin will be made every 3-5 years. <br />-Colorado pikeminnow spawning success is monitored annually by <br />sampling for young of the year fish in the fall. The abundance of <br />YOY fish varies greatly from year-to-year. <br />-It takes a long time to determine if a year class of fish will <br />successfully contribute to a population because Colorado <br />pikeminnow do not reproduce in the wild until age 6-7. <br />-Estimates of the numbers of juvenile Colorado pikeminnow (age 2- <br />5) are unreliable because these life stages cannot be sampled <br />effectively. <br />-The age-class structure of the population can be estimated based <br />on length-frequency data. However. older year classes are harder <br />to differentiate by this method as growth rates decrease with age. <br />