9713 - Laserfiche WebLink

Home Browse Search

CONCLUSIONS Evidence has been presented for the past four years that the LCR population of HBC appears to have undergone a decline since the early 1990s. This is most apparent in the spring abundance estimates. All four spring point abundance estimates for HBC ~ 150 mm from the years 2001 through 2004 have been less than those provided by Douglas and Marsh (1996) during spring 1992 (although not all have been significantly less). Contrary to the spring abundance trends, the abundance estimates obtained during the fall since 2000 for HBC ~ 150 have not shown a decline compared to estimates in the 1990s. The spring abundance estimates are presumably more inclusive of the portion of the population that migrates between the mainstem Colorado River and the LCR for spawning activities, whereas the fall abundance estimates are presumably more representative of fish that reside year round (or over-winter) in the LCR. Because of this, it could be hypothesized that the decline in HBC abundance since the early 1990s.may be in the portion of fish that migrate for spawning activities, and that this decline is being manifested in the observed declines in the spring spawning abundance in the LCR. It might also be hypothesized that an abundance of roughly 1,000 to 3,000 HBC ~ 150 mm may be representative of the year round carrying capacity for HBC in the LCR (see Figure 22). Finally, the fall abundance of HBC ~ 200 mm since 2001 has remained low (i.e., < 1000 fish), providing evidence that the numbers of these fish residing year round in the LCR is low, and that carrying capacity in the LCR alone for these larger fish may be lower than is desired for recovery, purposes. If, as Douglas and Marsh (1996) hypothesized may be occurring, HBC are undergoing an alteration of life history and becoming more of a resident LCR population (with an attendant decline in the migrating portion of the population), then strategies to ensure the survivorship of annual cohorts and for providing carrying capacity in the mainstem may become increasingly more important for maintaining this dwindling population, particularly since single cohort survivorship appears to already have the capacity to visibly influence the abundance of fish ~ 150 mm. 29