Biological Opinions - Bureau of Reclamation

Figure 7.2. Adult spawning populations of suckers in UKL have consistently declined since at least 2001, as
estimated by two approaches using mark-recapture models in Program MARK (from Hewitt et al. 2012).
The number of spawning female LRS in UKL has declined by 60 to 80 percent between 2002 and 2010.
Recent LRS and SNS size distribution trends reveal that the adult spawning populations within
UKL are comprised mostly of similar age, relatively old individuals. Since the late 1990s,
median lengths of populations of SNS have increased by approximately 0.16 in (4 millimeters
[mm]) per year and 0.35 to 0.47 in (9 to 12 mm) per year for the LRS (Hewitt et al. 2012). If
younger individuals (which are typically smaller) were frequently joining the population the
median length would remain stable, suggesting that recruitment of new adults is minimal to
nonexistent. Most adult suckers currently in UKL are believed to be the result of spawning that
occurred in the early 1990s (Janney et al. 2008). These fish are now approximately 20 years of
age, and are well beyond the average life span of 12 years for the SNS and equal to that of 20
years for the LRS. Even though viable eggs and larvae are produced each year, a bottleneck
during subsequent life stages causes a lack of recruitment of new adults into UKL sucker
populations, which continue to exist only because of their long life. However, this trend is
especially untenable for the SNS, and, without substantial recruitment in the next decade, the
population will be so small that it is unlikely to persist.
58