Effects of Population Increase on Cui‐ui Growth and Maturation

Abstract

Cui‐ui Chasmistes cujus is endemic to Pyramid Lake, Nevada. The cui‐ui population declined during much of the 20th century as a result of water diversion and the formation of a shallow and virtually impassable delta at the mouth of the Truckee River, its spawning habitat. The population increased more than 10‐fold to more than 1 million adults after access to the river was restored, creating a period of relatively higher density. This change presented the opportunity to test intraspecific density effects on cui‐ui age and length at maturity and on growth. We also compared the year‐class structure of the adult population before and after improved access. At low density, cui‐ui mean age at maturation was 9.2 years for males and 9.6 for females; at high density, it was significantly higher: 11.8 years for males and 12.0 for females. There was no significant change in mean fork length at maturity related to population increase. Growth patterns differed between high and low density, the low‐density fish growing faster than high‐density fish before their respective mean age of maturity; past their mean age at maturity, high‐density fish grew significantly faster than low‐density fish. Fish in both density periods reached similar lengths by about 19–20 years of age. Year‐class structure for both density periods consisted of strong year‐classes, which predominated the adult population for several years.

Population Dynamics of the Cui‐ui of Pyramid Lake, Nevada: a Potamodromous Catostomid Subject to Failed Reproduction

“…We hypothesized that estimated survival values should be highest in low-flow years when passage into the river is precluded or restrictive (Scoppettone et al 1986(Scoppettone et al , 2000Scoppettone and Rissler 2007). During our study, the six lowest flow years (1990-1992, 2001, 2002, and 2004) had a maximum May flow <15 m 3 /s in the Truckee River, and few Cui-ui entered the river.…”

“…Thorarinsson 1998;Berkeley et al 2004). Age at maturity in Cui-ui is density dependent (Scoppettone and Rissler 2007), and we suspect proliferation of the population (and high density) following a period of disrupted reproduction delays maturity in certain regions and contributes to diversity of age of maturity (Scoppettone and Rissler 2007).…”

“…In years when there was a spawning migration, the number of Cui-ui that passed upstream from MBD has ranged widely and is reflective of changes in population density, improved fish passage over the MBD, improved 854 passage over the Truckee River delta, and the migratory response of Cui-ui to water volume and quality. From 1980 through 1987, fish population and migration density was much lower than after 1993 (Scoppettone and Rissler 2007). In 1993, the first spawning migration since 1987 occurred ( Figure 2), and the adult population increased such that the Marble Bluff Fishway Complex infrastructure was inadequate to pass the increased number of migrants; hence, only a small fraction of the migrants were passed.…”

“…Opercula were prepared and read in the manner described by Scoppettone (1988). Age at maturity was determined by an abrupt slowing of growth reflected on the opercle bone (Scoppettone and Rissler 2007). From 1989 through 2014, we collected a random sample of Cui-ui (100-700 fish) from the prespawning aggregation for age determination; an exception was in 2002 when fish were collected at MBD.…”

“…In that year, there were over a million potential spawners (Scoppettone and Rissler 1995), and based upon the low survival rate in 1993 (about 40%), the majority attempted a spawning migration. However, due to restricted passage over the Truckee River delta following several years of drought and lake level decline along with the inadequacy of the MBD facility to pass what was a dramatic increase in the population (Scoppettone and Rissler 2007), only 4,842 females passed upstream from MBD. Most of these females were young first-time spawners with low fecundity, producing a relatively modest number of larval emigrants entering Pyramid Lake (Scoppettone and Rissler 2012).…”