mitochondrial dna variability in the ... - native fish lab · dexter national fish hatchery and...

Mitochondrial DNA Variability in the Endangered Razorback Sucker (Xyrauchen texanus): Analysis of Hatchery Stocks and Implications for Captive Propagation THOMAS E. DOWLING,* W. L. MINCKLEY,* PAUL C. MARSH,t AND ELLIOTT S. GOLDSTEIN;~ *Department of Zoology, Arizona State University, Tempe, AZ 85287, U.S.A., email [email protected] tCenter for Environmental Studies, Arizona State University, Tempe, AZ 85287, U.S.A. ,Department of Zoology, Arizona State University, Tempe, AZ 85287, U.S.A.

Abstract~ The razorback sucker (Xyrauchen texanus) is a large, long-h'ved catostomid f ish endemic to the Col- orado River drainage o f western North America, endangered because o f recruitment failure. Efforts to preserve the species have emphasized artificial propagation and reintroduction. Given the importance o f main- raining genetic diversity in such a program, we examined mitochondrial DNA diversity in a source population ~ a k e Mohave, Arizona-Nevada) and three hatchery-produced year classes (1987, 1989, 1990). The source contained considerable variation, indicated by high haplotype diversity ( h = 0.97) and a large number o f unique haplotypes (17 in 25 individuals). Diversity also was high in the 1987 ( h = 0.89, 6 haplotypes in 10 individuals) and 1989 hatchery-produced year classes ( h = 0.91, 7 in 11), but significantly lower in the 1990 year class ( h = 0. 71, 4 in 10). Low diversity in the last class was likely because of differences among females in fecundity, viability o f progeny, or both. Because natural populations have collapsed throughout the species" range, we mast identify methods that preserve the most diversity. We examined three potential alternatives: standard hatchery propagation, natural spawning in predatorfree environments, and protective custody of larvae collected f rom the lake with reintroduction after growth to a size likely to survive. The last is the preferred alternative and should be pursued as the most cost-effective option for preserving genetic diversity in the razorback sucker.

Variabilidad del ADN mitocondrial en Xyrauchen texanus, una especie en peligro: Anfdisis de los stocks de las granjas de cr/a y su implicaci6n en la propagaci6n de animales criados en cautiverio

R e s u m e n : Xyrauchen texanus es un pez catostomido longevo enddmico al drenaje del rio Colorado en el oeste de Norte Amdrica, que se encuentra en peligro debldo a fracasos en el recrutamiento. Los esfuerzos para preservar la especie han enfatizado Ia propagaci6n artificial y la reintroducci6n. Dada la importancia que tiene el mantenimiento de la diversidad gendtica en tales programas, hemos examinado la diversidad del ADN mitocondrial en una poblaci6n fuente (lago Mohave, Arizona-Nevada) y e n tres clases anuales producidas en estaciones de crianza (1987, 1989, 1990). La fuente tuvo una considerable variaci6n gendtica evlden- ciada por una alta diversidad haplotfpica (h = 0.97) y un gran nt~mero de haplotipos ~nicos (17 en 25 indi- viduos). La dtversidad tambidn fue alta en las clases anuales producidas en las estaciones de crla de 1987 (h = 0.89, 6 haplotipos en 10 indivlduos) y 1989 (h = 0.91, 7 en 11) pero significativamente menor en la clase de 1990 (h = 0. 71, 4 en 10). La baja diversidad en la fatima clase era esperada debldo a diferenclas en la fecun- didad entre las hembras, la viabilidad de la progenie, o ambas. Dado que las poblaciones naturales se ban colapsado a Io largo y ancho del t~rea de dtstribuci6n, debemos identificar mdtodos que preserven la mayorfa de la diversidad. Nosotros examinamos tres alernativas: la propagaci6n estandard de ias estaciones de cria, la reproducci6n natural en ambientes libres desde predadores y la custodia protectora de larvas recolectadas

Paper submitted December 2, 1993; revised manuscript accepted March 9, 1995.

120

Conservation Biology, Pages 120-127 Volume 10, No. 1. February 1996

Dowling et aL mtONA Diversity in Razorback Sucker 1 2 1

del lago reintroducci6n con, una vez que las mismas alcancen una talla con alta supervtvencta. La altima opci6n es la preferible y deberfa ser puesta en efecto como la opct6n md~s eflciente para preservar la diversidad gen6St~'ca en Xyrauchen texanug

Introduction

Many freshwater fishes native to the North American West are threatened with extinction, largely because of human influences (Minckley & Deacon 1991). Water- management practices have reduced and extensively modified aquatic habitats and their surrounding land- scapes, and normative species, mostly introduced as sport or baitfish, have seriously affected indigenous species through competition, predation, and hybridization. More than 20 native taxa have become extinct in the past cen- tury (Minckley & Douglas 1991).

The plight of the razorback sucker ( X y r a u c h e n texa-

nus) typifies that of many western big-river fishes (Minckley et al. 1991). Historically, this large catostomid was widespread and abundant throughout major rivers of the Colorado River basin, to which it is endemic. Ra- zorback sucker populations seemingly exploded in size in newly constructed reservoirs in the lower Colorado River basin, but recruitment was short-lived. In three in- stances strong initial year class(es) failed to reproduce successfully, and populations persisted _+ 40 years, then essentially disappeared (Minckley 1983). Various human impacts have combined to extirpate the species, and recrui tment failure n o w seems prevalent in streams and reservoirs throughout the species' range CLanigan & Tyus 1989; Minckley et al. 1991; U.S. Fish & Wildlife Ser- vice [USFWS] 1991).

The largest remaining populations are in the upper Green River, Colorado-Utah (est'tmated by Lanigan & Tyus [1989| at fewer than 1000 adults), and Lake Mo- have, Arizona-Nevada (fewer than 23,300 adults in 1993 [Marsh 1994]). Natural populations comprised of scattered individuals elsewhere are too small to obtain reli- able size estimates (McAda & Wydoski 1980; Minckley 1983; Lanigan & Tyus 1989; Marsh & Minckley 1989). Despite more than a decade of active management (re- v iewed in Minckley et al. 1991), its status remained pre- carious, and the razorback sucker was listed federally in 1991 as endangered CLISFWS 1991).

Although razorback suckers continue to spawn and produce larvae in both large rivers and reservoirs (Marsh & Langhorst 1988; Marsh & Minckley 1989; Marsh & Pa- poulias 1989); all populations are otherwise comprised only of adults thought to average 25 to more than 40 years of age. Juveniles are virtually unknown, and recrui tment to spawning stocks is undetected. In response to lack of successful natural recrui tment in razorback suckers and other western fishes, existing hatcheries

were pressed into service to assist in their conservation (Rinnee t al. 1986; Johnson &Jensen 1991).

Concerns for threatened and endangered fishes contrast sharply with those for fishes under more traditional propagation. Sport and commercial fishes are in high demand; therefore, many hatchery stocks of commo n but highly-prized species have existed for some time. Rapid growth, high survivorship, and ease of culture were selected to maximize production, as were other enhance- ments of the product ' s desirability, such as body size, "fighting" qualities, beauty (color), and even palatability. Propagation programs were designed to meet quotas that satisified demand in cost-effective ways, wi th little regard for anything other than the health of broodstock or progeny to ensure consistent production. In contrast, the goal of captive breeding programs for imperiled taxa is to maintain genetic diversity wi thout adaptation to captivity. Consideration of genetic features (such as population structure and effective population size) is therefore critical if reintroduced fish are to retain their ability to respond to changing environments in the wild (Templeton 1990; Echelle 1991; Hedrick & Miller 1992).

Thus, the institutional goals of hatcheries assigned to handle endangered taxa were abruptly changed. Instead of efficient product ion of large quantities of fish, hatchery managers became custodians of an irreplacable re- source temporari ly removed from nature. We collected mitochondrial DNA (mtDNA) restriction-site data f rom the razorback sucker, the analysis of which revealed an example of some problems arising from mixing a production phi losophy for fishes destined for harvest with the conservation phi losophy essential for management of endangered species.

Mitochondrial DNA has been a useful marker for studies of population structure (Avise et al. 1987; Avise 1992) and estimates of genetic diversity (Avise et al. 1988; 1989). It is maternally inherited (Dawid & Blackler 1972; Avise & Lansman 1983; but for exceptions see Satta et al. 1988; Kondo et al. 1990; Hoeh et aL 1991; Gyllensten et al. 1991) and evolves rapidly in many vertebrates (Moritz et al. 1987; see Avise et al. 1992), making it a highly polymorphic character for tracing maternal lineages.

Razorback suckers have been cultured since 1981 at Dexter National Fish Hatchery and Technology Center (DNFH; Johnson 1985; Johnson & Jensen 1991). We contrast mtDNA diversity in hatchery-produced stocks with those of the source population in Lake Mohave to evaluate success in maintaining genetic variability. Re- duction in the mtDNA diversity of a hatchery stock rela-

Conservation Biology Volume 10, No. 1, February 1996

122 mtDNA Diversity in Razorback Sucker Dowling et al.

tive to its source would indicate a change in genetic composition with potentially deleterious effects, especially in a species already subject to other environmental stresses.

Materials and Methods

Description of Source and Hatchery Populations

A broodstock was obtained in 1981-1982 from Lake Mo- have, a mainstem impoundment on the Colorado River created in the early 1950s by closure of Davis Dam. The reservoir consists of an upstream riverine reach ~ 6 0 km long that grades into a wider lower basin ~ 4 8 km long (maximum width 6.4 km). There are no direct tributar- ies to the reservoir that contained razorback sucker populations prior to impoundment, and, based on age structure (McCarthy & Minckley 1987; Minckley et al. 1991), recruitment was negligible after an initial, in s i tu population explosion. At the time of sampling (1987-1989) the source population consisted of approximately 55,000 individuals, presumably representing progeny of residents in this reach of river at the time of impoundment. The population is declining; more recent estimates indicate fewer than 23,300 adult razorback suckers remained in 1993 (Marsh 1994). Although scattered throughout, fish are concentrated in upper parts of the lower basin. They are vulnerable to capture when aggregated near shore for spawning in late autumn through spring, but are not as readily collected at other times when they are presumably dispersed throughout the basin. The species has made up 10-30% of net catches in spawning areas over the past 20 years (Minckley 1983; Minckley et al. 1991).

Procedures for the culture of razorback suckers are described in Toney (1974), Inslee (1982), and Hamman (1985). Adult fish are held in outdoor ponds and trans- ferred to indoor holding facilities in spring for spawning. Ovulation is induced by intramuscular injection of human chorionic gonadotropin (up to three 220 IU per kg injections at 24-hour intervals). When necessary to in- duce or maintain milt production, males are injected with 660 IU common carp (Cypr inus carpio) pituitary extract. Eggs are stripped manually, fertilized, and incu- bated in hatchery trays. Production per female (fecundity) is determined gravimetrically. Embryo viability (used to estimate expected hatch) is assessed 48 hours after fertilization. Actual larval production is measured by water displacement after hatching is complete.

Isolation and Analysis of mtDNA

Genetic variation in razorback suckers was character- ized by isolation and restriction endonuclease analysis of mtDNA from samples derived from two sources: gamete samples (ova or milt) from 25 individuals from Lake Mo-

have (captured in 1987-1989), and tissues (heart, liver, gonad where available) from 31 juveniles representing three year-classes produced at DNFH (1987 and 1990, n -- 10; 1989, n = 11) and sampled in 1990. Mitochondrial DNA was isolated by either phenol-chloroform extrac- tion (Chapman & Powers 1984) or equilibrium-density ultracentrifugation (Dowling et al. 1990). The former was used initially on a small number of egg samples, and the latter was used to purify intact, circular mtDNA from the remaining samples.

The following restriction enzymes were used to char- acterize mtDNAs: BanI (GGPyPuCC), BstEII (GGTNACC), H i n t (GANTC), HinPI (GCGC), MboI (GATC), NheI (GCTAGC), ScrFI (CCNGG), and TaqI (TCAG). The re- suiting cleavage fragments were end-labeled using all four ¢t32P-dNTPs, electrophoretically separated on 1.0- 1.5% agarose and 4.0% polyacrylamide gels, and visual- ized by autoradiography (Dowling et al. 1990). Distinc- tive restriction-fragment patterns for a specific enzyme were identified by acronym, which may include a letter and prime ( ') mark. Acronyms were assigned by order of discovery, not pattern similarity. The composi te haplotype for each individual is identified by an eight-

Table 1. Haplotypic identification and distribution among hatchery year classes of razorback sucker and their source population, ~ e Mohave, Arizona-Nevada.

Lake Haplotype* Mohave 1987 1989 1990

1. AAAABA'AA 3 2. ABADA'AAA 3 3. BCBBCA'AA 1 4. ACAEA'A'BA 2 5. ACAAA'A'BA 1 6. ACEA'HA'AC 2 7. ACFABEAA 1 8. EEAABA'BA 1 9. FCAAAA'BA 1

10. ACAAA,~kA 1 11. ADAADCAA 1 12. ACAAFDAA 1 13. BFA'AKA'AA 1 14. BCAFIA'AA 2 15. ACAGJA'BA 1 16. ACDCFBAB 2 17. ACAA'HA'AC 1 18. JCIAAA'AA 19. DAAABA'AA 20. JCIALA'AA 21. IGAFMA'AA 22. ACGFAA'AA 23. GAAABA'AA 24. JCHAAA'AA 25. JCAAAAAA 26. BCAAAA'AA 27. BHABIA'AD

2 1

5

1

2 3 2 1 1

1 Total 25 10 11 10

*The composite haplo~/pe is defined by the following enzymes (in order): MboL HinPI, HinfI, TaqI, ScrFL NheL BstEIL BanI.

Conservation Biofog 3, Volume 10, No. 1. February 1996

Dowling et al. mtDNA Diversity in Razorback Sucker 123

acronym code describing the pattern for each enzyme (Table 1).

Statistical Analysis

Levels of genetic diversity were assessed by direct count of haplotypes (alleles) and estimates of haploty~ic diversity. Number of haplotypes was corrected for sample size. Haplotype diversity (/~) was defined by Nei and Tajima (1981) as

n(1 - •x 2 )

b - - n - 1 '

where n is sample size and x t is f requency of the i th haplotype in the sample. This value is frequently used to assess levels of mtDNA diversity within a sample of individuals (Avise 1992) and provides a measure of the relative contribution of each haplotype. High values (maximum = 1.0) are found when each genotype is equally represented in the sample; low values (minimum = 0.0) occur when specific genotypes are over-represented.

The significance of differences in /~ values among source and hatchery samples was tested by random resampling (modification by T. E. Dowling of the com- puter program HAPLOID written by Weir [1990]). A dis. tribution of diversity values was generated by drawing 1000 samples of 10 individuals f rom the source population represented by our sample of 25 individuals f rom Lake Mohave and calculating diversity for each. Sam- piing of an individual was followed by its immediate replacement, so an individual could be selected several t imes or not at all in each replication. Mean diversity values for each set of 1000 resamples and its 95% confidence interval were obtained f rom the distribution of diversity values (Weir 1990).

The effect of broodstock size on/~ and the number of alleles (A) was also examined by compute r simulation. The program was modified to sample a highly diverse source population (from 2 to 100 individuals, each possessing a different haplotype) to generate 10,000 progeny. Progeny haplotypes were obtained by randomly sampling parental haplotypes (with replacement as described above). From this model hatchery stock 1000 samples of 10 individuals were removed; haplotype fre- quencies and h were calculated each time. Mean diversity values and number of alleles and 95% confidence limits were calculated as described above.

Results and D i s c u s s i o n

mtDNA Variation in the Source and Hatchery Populations

Restriction endonuclease analysis of mtDNA from razorback suckers with eight enzymes produced ~ 1 6 0 fragments. Analysis of all 56 individuals (25 wild and 31

hatchery fish) yielded 27 haplotypes (Table 1). All changes in fragment pat tern resulted from restriction site gains and losses, with no evident variation attributable to length differences.

The 25 wild-caught individuals from Lake Mohave exhibited 17 haplotypes (Table 1). The two most co mmo n were in three individuals each. Most (11 of 17) were unique to an individual. Diversity was high (/~ = 0.97) compared with that of other vertebrates (Avise et al. 1989) and consistent with the high fecundity (~2000 ova/cm standard length; Minckley et al. 1991) and large population size (Marsh 1994).

Analysis of subsamples f rom three DNFH year classes yielded results in Tables 1 and 2. Unique haplotypes (not found in the Lake Mohave sample) were present in all year classes but were more c o m m o n in 1987 and 1989 (four and six unique haplotypes, respectively) than in 1990 (one unique haplotype). The 1987 and 1989 year classes possessed levels of variation--relative number of haplotypes and diversity es t imates- -comparable to those of the source (Lake Mohave), whereas the 1990 year class possessed fewer haplotypes and a considerably lower diversity value.

Statistical comparison of /~ values from hatchery year classes with the source population was provided by resampling the Lake Mohave sample (n = 25). The mean diversity value of the 1000 resamples was 0.91, with the 95% confidence interval ranging f rom 0.80 to 0.98. Only the 1990 year class (/~ -- 0.71 ) was significantly different from the source population, with only one of the 1000 resamples exhibiting a lower estimate (Fig. 1).

This means of testing for significant differences in diversity be tween the source and hatchery populations is conservative because our representative sample f rom the source was relatively small (n = 25). This reflects the fact that even if the source populat ion is maximally diverse, the finite number of parents forces an uppe r limit on/~. An increased source size decreases the prob-

Table 2. Characteristics of hatcher}, samples of razorback suckers and their source (Lake Mohave). IIIIIIIIII

Lake Hatchery f i s h

Characteris t ic M o h a v e 1987 1989 1900

Sample size 25 10 11 10 Number of haplotypes 17 6 7 4 Haplotypes per individual 0.68 0.60 0.64 0.40 Diversity 0.97 0.89 0.91 0.71 Number of females 11,650 a 55 14 17 Nff ? 23.8 32.2 6.9 Fecundity c 0.84 a 0.84 1.00 0.94 Survivorship (%) 0 49 46 11

aAssumes I:I sex ratio. Effective number o f females.

CNumber o f eggs per female (X 105). aBased on 20 ripe females collected in 1983-1984 (Minckley et aL 1991).


124 mtONA Divers#y in Razorback Sucker Dowling et al.

250

200

.~ 150

o 100 5 Z

50

1.0 ' 80

0.9 60

< 0 . 8

"~" 0.7 40 ~

0 .6 -~

o.si.,,l,r ..h ,o 90 0 . 4 1 - , , , , , , , , , , 0

^ 0 20 40 60 80 100 h (X 100) Broodstock size

Figure 1. Distribution of diversity values generated from 1000 bootstrap replicates from our sample of razorback suckers collected in Lake Mohave, Arizona- Nevada. Estimates for each replicate were calculated from subsamples of 10 individuals.

Figure 2. Results of simulation depicting effects of varying broodstock size on haplotype diversity ( h ) and number of alleles (A). Broodstock sizes of 2, 5, 10, 15, 20, 25, 50, and 100 individuals were used.

ability of resampling the same individual, thereby de- creasing variance in distribution of expected/3 values. If subsamples were drawn from a larger source population, the diversity values even for the 1987 and 1989 year classes would be significantly different from those of the source.

Although our estimate of diversity measures the relative frequency of haplotypes within a sample, it does not provide an accurate indicator of change in the genetic composition of hatchery stocks relative to natural populations (see Allendorf & Ryman 1987 for a similar conclu- sion relative to allozyme diversity). In our simulations we examined the behavior of/3 in high-diversity organ- isms by resampling from a broodstock in which every individual possessed a unique haplotype. When progeny were derived from broodstocks of different size, mean h values for progeny populations increased rapidly with increased founder size, producing a curve that leveled near /3 ~0.95 and broodstock sizes near 20 individuals (Fig. 2). Therefore, in an organism with high diversity-- each individual possessing a unique haplotype--/3 is relatively insensitive to broodstock size because the difference in diversity values from stocks generated by random use of ova from 15-20 females will not be significantly lower than those produced from 100 females.

The number of haplotypes (A) was also examined in these simulations, and the increase in A was directly cor- related with increase in broodstock size (Fig. 2). Given that the progeny of 100 females will clearly possess more different genotypes than those produced from 15- 20, /3 misrepresents levels of variation maintained in the broodstock, whereas A provides a much better indicator of levels of genetic diversity.

The simulation results are consistent with our empiri- cal data. Year classes in 1987 and 1989 exhibited similar diversity values despite substantial differences in number of females used to generate each (Table 2). Thus, variation in diversity was not due to the number of females used (17 in 1990 versus 55 and 14 for the 1987 and 1989 year classes, respectively'). Given the relative insensitivity of /3 to broodstock size, reduced diversity in the 1990 year class indicates the difficulty in consis- tently producing stocks that mimic a source population.

Factors Reducing Diversity in Hatchery Stocks

Reduction in diversity could be caused by extrinsic factors (hatchery effects) or intrinsic factors (differences in fecundity/viability among females and their progeny), or both. We cannot assess the role of extrinsic factors. They should be consistent, however, because in this example personnel, methods, and equipment at DNFH were relatively constant from year to year.

The observed reduction in diversity might be ex- plained by over-representation of some females in the 1990 year class. Initital comparison of the average number of ova per female recorded in DNFH annual reports indicates no difference between the 1990 year class (N0.94 × 105 eggs/female, n = 17) and 1982-1989 year classes (~0.83 × 105 eggs/female, n = 590). These statistics have little meaning, however, because large fish have more ova than small ones, and the unrecorded sizes of individual broodfish likely varied from 40 to 60 cm standard length.

On the other hand, survival from fertilized ova to swim-up was only 11.1% in 1990 (n = 17), substantially


Dowling et al. mtDNA Diversity in Razorback Sucker 125

lower than the minimum of 22.5% (mean 40.1%, maxi- m u m 71.4%, n = 590 females) calculated f rom DNFH annual reports for the seven preceding year classes (1982- 1989). Because fertilized ova f rom several individuals are pooled, fertilization success and embryo viability f rom specific females or matings cannot be determined. There- fore, it is possible that some individuals or matings contribute little or nothing, whereas others contribute pro- portionately large numbers of progeny to a given year class. This becomes especially important when few females are involved. The observed results must at a mini- m u m involve an interaction be tween the number of females and the viability of their offspring.

Whatever the cause, the number of females contributing progeny varies among year classes. Given that we have measures of heterozygosity for the broodstock (H0, as reflected in our sample f rom Lake Mohave) and each hatchery year class (H2), a rough approximation of the effective number of females (Xfe) contributing to each year class can be obtained using the following equation:

1 _ 1_ H~H~. Nfe

Application of this equation to each year class provided Nfes 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 (Table 2).

Genealogical History and Genetic Diversity

The relationship be tween intrinsic and extrinsic factors is confounded further by variation in broodstock composition over time. A genealogical history of razorback suckers at DNFH was reconsti tuted f rom Inslee (1982), Minckley (1983), Minckley et al. (1991), and DNFH annual reports (USFWS 1981-1991). We provide here only information pert inent to the year classes examined.

There were problems with record keeping. Unmarked wild (Lake Mohave) broodfish f rom 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 product ion 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 F 1 progeny of wild broodfish captured in 1981 (number unknown but thought to be a few) and 1982 (thought to be many)

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 (Table 2).

The specific origin of the 1990 year class cannot be as- certained. Available broodstock included FlS f rom 1981 and 1982 (likely more than 50%), wild fish captured in 1985 (likely less than 20%), and F2s f rom the 1987 year class (known to be ~33.5% of the broodstock at that time). Haplotype diversity, number of alleles, and survivorship of the 1990 year class were considerably lower than for other year classes (Table 2). Given that specific origins of individuals used to produce the 1990 year class are not identifiable, it is possible that Fzs were in- terbred with close relatives (parents, siblings, etc.), reducing genetic variability and perhaps the viability of re- sulting progeny.

Summary and Conclusions

Decline of the razorback sucker is a t r ac t ion of recruitment 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 impoundment 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 therefore 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 populat ion crash will result in significant loss of diversity, and the possibility seems remote of solving the recruitment p rob lem before the remaining population collapses. Our efforts can augment the numbers of individuals but can only maintain some port ion 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 produced. Large numbers of razorbacks may be hatchery- cultured, but a limited number of individuals can be used as broodstock.

In an at tempt to overcome recruitment failure, two ad- ditional programs have been initiated. One involves re-

Conservation Biology Volume 10, No, 1, February 1996

126 mtDNA Dimi ty in Razorback Sucker Dowling et al.

straining adults to spawn naturally in protected, isolated bays whe re larvae can survive and g row to suitable size in a predator-free e n v i r o n m e n t (Marsh & Langhorst 1988; Minckley et al. 1991); the o ther involves capture and transfer of reservoir-spawned larvae into p ro tec ted bays (Marsh, unpubl i shed data). Although natural spawning of adults in p ro tec ted backwaters will yield fewer

fish per year than hatchery product ion , con t inua t ion of such a program for several years us ing different adults each year wou ld al low for ma in t enance of h igher levels of genet ic variability. Even fewer total p rogeny per year are available by col lect ing wi ld-spawned larvae and rearing t hem in pro tec ted sites, yet this me thod will almost certainly preserve the most variability.

We strongly r e c o m m e n d the last op t ion be emphasized and advise that such a program, already initiated, be pu r sued in earnest. We fear, however , that hesi ta t ion or in t rans igence may prec lude the direct capture and

husbandry of wild larvae in n u m b e r s that can assure ma in t enance of genet ic variation. W h e n the old, wild in-

dividuals beg in to die, wh ich will be soon, it may be too late; there is s t rong ev idence that the popu la t ion is already in prec ip i tous decl ine (Marsh 1994). We advise even more strongly to resist the t empta t ion to use k n o w n and established methods, such as ha tchery cul-

ture, to quickly p roduce large quant i t ies of individuals to replace the exist ing Lake Mohave stock. Popula t ion size can clearly be main ta ined through stocking hatchery-cultured fish bu t at a substantial cost to innate genet ic variability and wi th u n k n o w n future results.

On the other hand, w e do not advocate an end to the hatchery program because it is imperat ive that a large, diverse broods tock be main ta ined in case o ther efforts fail. The presen t broods tock at DNFH should be re- p laced wi th wild-caught individuals, however , and product ion should be des igned to main ta in d o c u m e n t e d genet ic variability in offspring p roduced for any future purpose . Progeny of the cur ren t broods tock should no t be released to replace the exist ing Lake Mohave population. The bay-culture program should also be c o n t i n u e d and expanded , if for no o ther reason than to provide ad- dit ional habitat where bo th bay-culture and wild-caught larvae can grow. Rearing should be geographical ly as near Lake Mohave as possible to minimize expense and

fish losses th rough handling. The safest course w o u l d be to keep all three op t ions open , if economics al low and if dedicat ion and mandate are sufficient incentives, so that if one me thod fails the others may be a t tempted.

Acknowledgments

We thank M. E. Douglas, B. L. Jensen , S. Johnson , D. D. Oakey, B: Williams, J. H. Will iamson, and innumerab le o ther colleagues and s tudents for assistance in col lect ing or providing specinlens; B. L. Jensen for hatchery records;

M. E. Douglas and D. D. Oakey for technical assistance; B. D. DeMarais, A. A. EcheUe, and P. W. Hedrick for dis- cuss ion and critical review of the manuscr ipt . Federally and state-listed species we re ob ta ined u n d e r appropr ia te permits, those from Arizona u n d e r Federal Endangered Species subpermi t n u m b e r PRT-676811. This work was suppor ted by Cooperat ive Agreement 0 - F C - 4 0 - 0 9 5 3 0 (to T. E. Dowling and W. L. Minckley) f rom the U. S. Bu- reau of Reclamation and the U. S. Fish and Wildlife Ser- vice (to P. C. Marsh and W. L. Hinck ley) .

Literature Cited

Allendorf, F. W., and N. Ryman. 1987. Genetic management of hatchery stocks. Pages 141-159 in N. Ryman and F. Utter, editors. Popu- lation genetics and fishery management. University of Washington Press, Seattle.

Avise, J. C. 1992. Molecular population structure and the biogeo- graphic history of a regional fauna: a case history with lessons for conservation biology. Oikos 63:62-76.

Avise, J. C., and R. A. Lansman. 1983. Polymorphism of mitochondrial DNA in populations of higher animals. Pages 165-190 in M. Nei and R. K. Koehn, editors. Evolution of genes and proteins. Sinauer Associates, Sunderland, Massachusetts.

Avise, J. C., J. Arnold, R. M. Ball, E. Bermingham, T. Lamb, J. E. Neigel, C. A. Reeb, and N. C. Saunders. 1987. Intraspecific phylogeogra- phy: The mitochondrial DNA bridge between population genetics and systematics. Annual Review of Ecology and Systematics 18: 489-522.

Avise, J. C., R. M. Ball, and J. Amoid. 1988. Current versus historical population sizes in vertebrate species with high gene flow: a comparison based on mitochondrial DNA lineages and inbreeding the- ory for neutral mutations. Molecular Biology and Evolution 5:331- 344.

Avise, J. C., B. W. Bowen, and T. Lamb. 1989. DNA fingerprints from hypervariable mitochondriai genotypes. Molecular Biology and Evolution 6:258-269.

Avise, J. C., B. W. Bowen, T. Lamb, A. B. Meylan, and E. Bermingham. 1992. Mitochondrial DNA evolution at a turtle's pace: Evidence for low genetic variability and reduced microevolutionary rate in testu- dines. Molecular Biology and Evolution 9:457-473.

Chapman, R. W., and D. A. Powers. 1984. A method for rapid isolation of mtDNA from fishes. Technical report MD-SG-TS-84-05. Mary- land Sea Grant, College Park, Maryland.

Dawid, I. B, and A. W. Blaclder. 1972. Maternal and cytoplasmic inheritance of mitochondrial DNA in Xenopus. Developmental Biology 29:152-161.

Dowling, T. E., C. Moritz, andJ. D. Palmer. 1990. Nucleic acids II: Re- striction site analysis. Pages 250-317 in D. M. Hillis and C. Moritz, editors. Molecular systematics. Sinauer Associates, Sunderland, Massachusetts.

Echelle, A. A. 1991. Conservation genetics and genic diversity in freshwater fishes of western North America. Pages 141-153 in W. L. Minckley and J. E. Deacon, editors. Battle against extinction: Native fish management in the American West. University of Arizona Press, Tucson.

Gyllensten, U., D. Wharton, A. Josefsson, and A. C. Wilson. 1991. Pa- ternal inheritance of mitochondrial DNA in mice. Nature 352:255- 257.

Hamman, R. 1985. Induced spawning of hatchery-reared razorback sucker. Progressive Fish-Culturist 47:187-189.

Hedrick, P. W., and P. Miller. 1992. Conservation genetics: techniques and fundamentals. Ecological Applications 2:30-46.

Hoeh, W. R., K. Blakey, and W. M. Brown. 1991. Heteroplasmy sug-


Oowling et al. mtDNA Diversi~ in Razorback Sucker 127

gests limited biparental inheritance of Mytilus mitochondrial DNA. Science 251:1488-1490.

Inslee, T. D. 1982. Culture of endangered Colorado River fishes. Section III. Spawning of razorback suckers. Pages 145-147 in W. H. Miller, J. J. Valentine, D. L. Archer, H. M. Tyus, R. A. Valdez, and L. R. Kaeding, editors. Part 3. Colorado River fishery project, final report contracted studies. U.S. Fish and Wildlife Service, Salt Lake City.

Johnson, J. E. 1985. Reintroducing the natives: razorback sucker. Pro- ceedings of the Desert Fishes Council 13(1981):73-79.

Johnson, J. E., and B. L. Jensen. 1991~ Hatcheries for endangered freshwater fishes. Pages 199-217 in W. L. Minckley and J. E. Deacon, editors. Battle against extinction: Native fish management in the American West. University of Arizona Press, Tucson.

Kondo, R., Y. U. Satta, E. T. Matsuura, H. Ishiwa, N. Takahata, and S. I. Chigusa. 1990. Incomplete maternal inheritance transmission of mitochondrial DNA in Drosophila. Genetics 126:657-663.

Lanigan, S. H., and H. M. Tyus. 1989. Population size and status of the razorback sucker in the Green River basin, Utah and Colorado. North American Journal of Fisheries Management 9:68-73.

Marsh, P. C~ 1994. Abundance, movements , and status of adult razorback sucker, Xyrauchen texanus, in Lake Mohave, Arizona and Ne- vada. Proceedings o f the Desert Fishes Council 25(1993):35-36.

Marsh, P. C., and D. Langhorst. 1988. Feeding and fate of wild larval razorback suckers. Environmental Biology of Fishes 21:59-67.

Marsh, P. C., and W. L. Minckley. 1989. Observations on recrui tment and ecology of razorback sucker: lower Colorado River, Arizona- California-Nevada. Great Basin Naturalist 49:71-78.

Marsh, P. C., and D. Papoulious. 1989. Ichthyoplankton of Lake Havasu, a Colorado River impoundment , Arizona-California. California Fish and Game 75:68-73.

McAda, C. W., and R. S. Wydoski. 1980. The razorback sucker, Xyrauchen texanus, in the upper Colorado River basin, 1974-76. Technical paper 99. U.S. Fish and Wildlife Service, Fort Collins, Colorado

McCarthy, M. S., and W. L. Mincldey. 1987. Age estimation for razorback sucker (Pisces: Catostomidae) from Lake Mohave, Arizona-Ne- vada. Journal of the Arizona-Nevada Academy of Science 23:87-97.

Minckley, W. L. 1983. Status o f the razorback sucker, Xyrauchen texa- n u s (Abbott), in the lower Colorado River basin. Southwestern Nat- uralist 28:165-187.

Minckley, W. L., and J. E. Deacon. 1991. Battle against extinction: Na- tive fish management in the American West. University of Arizona Press, Tuscon.

Minckley, W. L., and M. E. Douglas. 1991. Discovery and extinction of wes tern fishes: a blink in the eye of geologic time. Pages 7-17 in W. L. Mincldey and J. E. Deacon, editors. Battle against extinction: Native fish managemen t in the American West. University of Ari- zona Press, Tucsoll.

Minckley, W. L., P. C~ Marsh, J. E. Brooks, J. E. Johnson, and B. L. Jensen. 1991. Management toward recovery of the razorback sucker. Pages 303-357 in W. L. Minckley andJ . E. Deacon, editors. Battle against extinction: Native fish management in the American West. University of Arizona Press, Tucson.

Moritz, C., T. E. Dowling, and W. M. Brown. 1987. Evolution of animal mitochondrial DNA: relevance for populat ion biology and systematics. Annual Review of Ecology and Systematics 18:269-292.

Nei, M., and F. Tajima. 1981. DNA polymorphism detectable by restriction endonucleases. Genetics 97:145-163.

Rinne, J. N., J. E. Johnson, B. L. Jensen, A. W. Ruger, and R. Sorenson. 1986. The role of hatcheries in the management and recovery of threatened and endangered fishes. Pages 271-285 in R. R. Stroud, editor. Fish culture in fisheries management . American Fisheries Society, Bethesda, Maryland.

Satta, Y. U., N. Toyohara, C. Ohtaka, Y, Tatsuno, T. K. Watanabe, E. T. Matsuura, S. I. Chigusa, and N. Takahata. 1988. Dubious maternal inheritance of mitochondriat DNA in/9. simulans and evolution of D. mauritiana. Genetical Research 52:1-6.

Templeton, A. R. 1990. The role of genet ics in captive breeding and reintroduction for species conservation. Endangered Species UPDATE 8:14-17.

Toney, D, P. 1974. Observations on the propagat ion and rearing of two endangered fish species in a hatchery environment. Proceed- ings of the Annual Conference of Western Association Fish and Game Commissions 54:252-259.

U.S. Fish and Wildlife Service (B. L. Jensen). 1981-91. Annual narrative reports, FY 1981-FY 1991. U.S. Fish and Wildlife Service, Dexter National Fish Hatchery (and Technology Center, after 1991), Dex- ter, New Mexico.

U.S. Fish and Wildlife Service. 1991. Endangered and threatened wildlife and plants; the razorback sucker (Xyrauchen texanus) determined to be an endangered species. Federal Register 56(205): 54957-54967.

Weir, B. 1990. Genetic data analysis. Sinauer Associates, Sunderland, Massachusetts.


mitochondrial dna variability in the ... - native fish lab · dexter national fish hatchery and...

Documents