9556 - Laserfiche WebLink

Home Browse Search

of fish in the 125-150 mm TL size classes into the 2005 population. They further postulated that environmental effects were playing a role, and thought that fish of that size were age-2 or the 2003 year class. We agree that the likely cause was environmental effects but believe the year- class involved was 2004 not 2003. We believe this because smallmouth bass in this area grow to an average size of about 75 mm TL by the end of their first growing season; that size may even be conservative because upstream populations in the Yampa River where water temperatures are cooler and bass hatch later are of similar size. By the time Christopherson et al. (2005) conducted removal sampling in August 2005, the now age-1 bass hatched in 2004 could easily have grown to 125 to 150 mm TL by August 2005 in the almost two full growing seasons; growth rate estimates from otoliths of smallmouth bass collected in the Yampa River just upstream corroborate growth rates postulated here (pers. comm. P. Martinez, Colorado Division of Wildlife). In 2004, smallmouth bass in that age-1 size class were about 41% of all fish in the population susceptible to electrofishing (based on size frequencies in Fig. 1, Christopherson et al. 2005). Near-complete absence of that generally abundant size-class from the 2005 Whirlpool Canyon population could be the difference between abundances expected in 2005, based on 2004 estimates and removal levels. This, of course, assumes that the abundance and removal estimates of Christopherson et al. 2005 are unbiased. We believe those abundance estimates are essentially correct (no differential bias among years) and the decline shown in 2005 is real, because our own 2005 sampling showed a similar decline in relative abundance for smallmouth bass in Whirlpool Canyon of approximately 50%; we also showed the expected and similar reduction in Lodore Canyon. Further, the lower Yampa River smallmouth bass removal sampling effort in 2005 showed that a large year-class of the 125-150 mm TL fish were present: no large floods or turbidity events were documented there in 2004 based on observations during drift net sampling. Thus, high displacement of age-0 smallmouth bass by a flood or floods in the upper Green River in 2004 may be responsible for a significant reduction in larger bass abundance in 2005 and perhaps a lasting effect into the future. This is important because even relatively substantial removal efforts in most portions of the Upper Colorado River Basin (Hawkins 2005, middle Yampa River; Fuller, 2005 in the lower Yampa) rarely show any 29