7194

• UCREFRP Catalog Number: 7194
• Author: Nesler, T. P.
• Title: Aquatic Nongame Research - 1985-86 Squawfish-Humpback Studies.
• USFW Year: 1986.
• USFW - Doc Type: Fort Collins, Colorado.
• Copyright Material: NO

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degree-days above 18C have been discussed as key factors. This is further
supported by evidence from hatchery operations. Both Toney (1974) and
Hamman (1981) indicate temperatures of 20-22C appeared important.
While this evidence of the importance of a threshold temperature is
substantial, another perspective is .forwarded here. The basis for water
temperature as a potential spawning cue in Haynes et ala (1984, 1985) was
based on the calculation of spawning dates from the larval age-growth
formulae. Since the earlier Haynes report, these equations have undergone
some modification, which in turn have modified the predicted spawning
dates. The revised data in Haynes et ala (1985) indicate that, while
water temperatures of 20C remain associated with spawning activity, some
spawning activity occurred when temperatures in the Yampa River were less
than 20C. This appears especially so in 1985, when the mean temperature
during the peak spawning period was only 16.1 C (Tyus and Jones 1985).
Bye (1984) stated that sexual development and spawning in cyprinids is
modulated both by temperature and photoperiod, although temperature is
predominant in most species. Stacey (1984) further clarified this by
stating that temperature and photoperiod interact with the endogenous
reproductive cycle to ensure gonads are mature at the appropriate season,
but ovulation, spermiation, and gamete release require other stimuli such
as current velocity, chemical water quality or pheromone release
stimulated by aggregation. Scott (1979, cited in Bye 1984) suggested that
any recurring environmental variable within the sensory competence of an
organism can act as a timing cue. In discussions of environmental cues
leading to ovulation and spawning, Stacey (1984) highlighted differences
between salmonids and cyprinids. This is particularly relevant since the
spawning ecology of the Colorado squawfish represents a cyprinid
reproductive strategy that incorporates salmonid attributes. In
salmonids, ovulation occurs as a consequence of completed ovarian
development; and is analogous to spontaneous ovulation. The apparent
insensitivity of the salmonid ovulatory mechanism to exogenous factors of
immediate relevance to spawning may be related to both their
characteristic reproductive migratory behavior and extended time of
postovulatory oocyte viability (Stacey 1984). This translates to the use
of spawning migration as a reproductive tactic to synchronize gonadal
maturation and ovulation with suitable temperatures and flows, proper
substrate quality, absence of predators, etc. In cyprinids, ovulation
occurs rapidly in response to specific e};ogenous .factors that are relevant
to reproductive success (Stacey 1984).
In reference to Colorado squawfish, the hypothesis may be proposed
that increasing water temperatures act to prepare the adults for spawning
through gonadal maturation and also act as a stimulus for migratory
behavior, while the fluctuation associated with river flows near baseline
acts as the cue that initiates peak spawning activity. In the context of
Stacey (1984), observations from Hamman (1981) indicated that Colorado
squawfish spawning in hatchery conditions required injection with
gonadatropin at temperatures of 18C, but occurred spontaneously at
temperatures of 20C and above. Observations in three consecutive years
that some spawning by Colorado squawfish occurred in a setting of river
temperatures below this threshold of spontaneous ovulation support the
flow cue hypothesis. It should be noted that "peak spawning activity" is
used in the above hypothesis, and refers to participation in spawning by a
significant percentage of the adult spawners. Similar to what was