7831

• UCREFRP Catalog Number: 7831
• Author: Dowling, T. E., et al.
• Title: Mitochondrial DNA Variability in the Endangered Razorback Sucker ‎(
• USFW Year: 1996
• Copyright Material: YES

[)oIrling et al.

lower than the minimum of 22.5% (mean 40.1%, maxi-
rpum 71.4%, n = 590 females) calculated from DNFH an-
nual reports for the seven preceding year classes (1982-
1989). Because fertilized ova from several individuals are
pooled, fertilization success and embryo viability from
specific females or matings cannot be determined. There-
fore, it is possible that some individuals or matings con-
tribute little or nothing, whereas others contribute pro-
portionately large numbers of progeny to a given year
class. This becomes especially important when few fe-
males are involved. The observed results must at a mini-
mum involve an interaction between the number of fe-
males and the viability of their offspring.
Whatever the cause, the number of females contribut-
ing progeny varies among year classes. Given that we
have measures of heterozygosity for the broodstock (Ho'
as reflected in our sample from Lake Mohave) and each
hatchery year class (H2), a rough approximation of the
effective number of females (Nfe) contributing to each
year class can be obtained using the following equation:

....!.... = 1-~2'
NE If
Je 0
Application of this equation to each year class provided
Nfcs of 23.8, 32.2, and 6.9 for the 1987, 1989, and 1990
year classes, respectively. Values for the 1987 and 1990
year classes were appreciably smaller than the actual
number used to generate these samples (fable 2).

Genealogical History and Genetic Diversity

The relationship between intrinsic and extrinsic factors
. is confounded further by variation in broodstock com-
position over time. A genealogical history of razorback
suckers at DNFH was reconstituted from Inslee (1982),
Minckley (1983), Minckley et al. (1991), and DNFH an-
nual reports (USFWS 1981-1991). We provide here only
information pertinent to the year classes examined.
There were problems with record keeping. Unmarked
wild (Lake Mohave) broodfish from different years and
some hatchery fish unidentified as to age and origin
were mixed. Numbers and identity of fish spawned may
have been known at the time but were not always docu-
mented. Contributions of individuals to production
were sometimes unrecorded, and the number of ova per
female could have varied over six orders of magnitude.
Therefore, the number of fish that physically spawned
was not always the same as the number of fish from
which ova were obtained. Records were generally only
for numbers of females used, and the number of males
was not typically recorded.
The 1987 hatchery year class was produced by 55
adult females (in part, Table 2), themselves FI progeny
of wild broodfish captured in 1981 (number unknown
but thought to be a few) and 1982 (thought to be many)

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II"

mtDNA Diversity in Razorback Sucker

125

that were spawned in 1981 and/or 1982. Fish produced
in 1989 were derived from the same groups, but wild
fish captured as larvae in 1985 and reared in captivity
may also have contributed. Haplotype diversity and
number of alleles was high in both 1987 and 1989 year
classes (fable 2).
The specific origin of the 1990 year class cannot be as-
certained. Available broodstock included Fls from 1981
and 1982 (likely more than 50%), wild fish captured in
1985 (likely less than 20%), and F2s from the 1987 year
class (known to be =33.5% of the broodstock at that
time). Haplotype diversity, number of alleles, and survi-
vorship of the 1990 year class were considerably lower
than for other year classes (fable 2). Given that specific
origins of individuals used to produce the 1990 year
class are not identifiable, it is possible that F2s were in-
terbred with close relatives (parents, siblings, etc.), re-
ducing genetic variability and perhaps the viability of re-
sulting progeny.

Summary and Conclusions

Decline of the razorback sucker is a function of recruit-
ment failure throughout its range. No verified natural re-
cruits have been found among almost 12,500 fish han-
dled in Lake Mohave, Arizona-Nevada, since 1974 (Marsh
1994). Large numbers of young hatch each year but
soon fall to predation, perhaps mediated by nutritional
constraints soon after yolk absorption. Existing adults all
hatched in the early 1950s, coincident with impound-
ment and prior to prevalence of nonnative predators.
Based on timing of disappearance of other reservoir
stocks in the lower Colorado basin (approximately 40
years after impoundment; Minckley 1983), the Lake Mo-
have population should collapse at any time.
Mitochondrial DNA diversity in the razorback sucker
of Lake Mohave is remarkably high, with an average of
0.68 haplotypes per invidual. The population must there-
fore be comprised of direct descendants of an exceed-
ingly large, diverse, panmictic population that inhabited
the lower Colorado River basin before development.
Only natural recruitment can maintain existing genetic
variability. A population crash will result in significant
loss of diversity, and the possibility seems remote of
solving the recruitment problem before the remaining
population collapses. Our efforts can augment the num-
bers of individuals but can only maintain some portion
of the existing genetic variability. Unfortunately, the
level of potential conservation of genetic variability is in-
verse to the numbers of fish that can reasonably be pro-
duced. Large numbers of razorbacks may be hatchery-
cultured, but a limited number of individuals can be
used as brood stock.
In an attempt to overcome recruitment failure, two ad-
ditional programs have been initiated. One involves re-

Conservation Biology
Volume JO, No.1, February 1996

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