A Life-history Model for Peppered Chub, a Broadcast Spawning Cyprinid. Gene R. Wilde and Bart W. Durham

Transcription

1 Wilde and Durham 1 A Life-history Model for Peppered Chub, a Broadcast Spawning Cyprinid Gene R. Wilde and Bart W. Durham Department of Biological Sciences, Mailstop 3131 Texas Tech University Lubbock, Texas, USA Citation: Wilde, G. R., and B. W. Durham. In press. A life-history model for peppered chub, a broadcast spawning cyprinid. Transactions of the American Fisheries Society 000:

2 Wilde and Durham 2 Abstract. We estimated age-specific fecundity and survival rates for peppered chub Macrhybopsis tetranema from the Canadian River, New Mexico and Texas. We used these estimates to construct a life-history matrix model that assumed age-0 survival was related to river discharge. Model predictions agreed well with observed abundance of peppered chub for the six-year period 1996 to Based on the Akaike Information Criterion, this model received greater support from observed catches of peppered chub than did two alternative null models (one null model assumed a static or fixed population and the other assumed a population with constant growth rate over the six-year study period). Elasticity analysis showed peppered chub population growth rate was most sensitive to changes in age-0 survival (elasticity = 0.48) and age-1 fecundity (elasticity = 0.44). We performed sensitivity simulations to determine the effect of parameter uncertainty on observed elasticities. Based on N = 1000 simulations, we found that peppered chub population growth rate was most sensitive to age-0 survival and age-1 fecundity and was robust with respect to uncertainty in our estimates of these parameters. Our model accurately predicts changes in peppered chub abundance, based on river discharge, and provides a mechanistic explanation for previous anecdotal observations that reproductive success of peppered chub is related to river discharge. Streams and rivers of the Great Plains region of central United States have been modified extensively during the past 100 years. Reservoir construction, groundwater withdrawal, and diversion of surface waters for irrigation, in combination, have altered historic patterns in the timing, magnitude, and variability of stream discharge (Williams

3 Wilde and Durham 3 and Wolman 1984; Cross and Moss 1987; Limbird 1993). Associated with these changes have been decreases in distribution and abundance of several obligate riverine species (Cross and Moss 1987; Pigg 1991; Dieterman and Galat 2004). This is best evidenced by the growing number of species considered imperiled by state and federal agencies and conservation organizations. A growing body of literature has emerged as ecologists have attempted to understand changes in the distribution and abundance of Great Plains fishes by relating their presence and abundance to various habitat attributes (Polivka 1999; Scheurer et al. 2003; Dieterman and Galat 2004; Everett et al. 2004; Welker and Scarnecchia 2004), stream discharge (Cross and Moss 1987; Pflieger and Grace 1987; Braaten and Guy 1999; Everett et al. 2004; Durham and Wilde 2006; Falke and Gido 2006), presence of dams (Luttrell et al. 1999), and length of unimpounded river (Platania and Altenbach 1998; Braaten and Guy 1999; Dieterman and Galat 2004). However, these studies are correlative in nature and imply no causative relationships or mechanisms: fish abundance is assumed to increase or decrease in response to changes in the magnitude of various predictor variables. Few studies have explicitly examined the demography of Great Plains fishes (Brown 1986; Rahel and Thel 2004a, b, c), and with the exception of Pigg et al. (1999), Durham (2007), and Durham and Wilde (in press) there has been little attempt to model population dynamics of Great Plains fishes or to identify the life-history stages most at risk. However, there is an increasing recognition of the need for models that relate the population dynamics of fishes, especially imperiled species, to specific aspects of their environments (Oakes et al. 2005; Anderson et al. 2006). In particular, there is a need for models that include feedbacks between physical and chemical aspects of the

4 Wilde and Durham 4 river environment that can describe how population or community viability will respond to changes in the discharge regime (Anderson et al. 2006). Peppered chub Macrhybopsis tetranema historically was widespread in the Arkansas River drainage, but is now restricted to a 218-km stretch of the Canadian River in New Mexico and Texas (Bonner and Wilde 2000) and a 100-km reach of the Ninnescah River and an associated portion of the Arkansas River in Kansas (Luttrell et al. 1999). In many respects, peppered chub is representative of smaller Great Plains cyprinids that have experienced reductions in their distribution and abundance. Peppered chub is short lived, with few individuals reaching their third year of life; the species broadcast spawns semi-buoyant ova into the current (Platania and Altenbach 1998); and reproductive success appears to be related to stream discharge (Starrett 1951; Bottrell et al. 1964; Platania and Altenbach 1998; Durham and Wilde 2006). In this paper, we develop a life-history model for the peppered chub. Because the biology of the peppered chub has been little studied, we based our model on the Leslie matrix, a deterministic age-structured population model that requires only estimates of age-specific survival and fecundity. The Leslie matrix model has a long history of use in natural resource management (Caswell 2001) and has been extensively used in fishery management and conservation (e.g., Jensen 1971; Heppell et al. 1999; Brewster-Geisz and Miller 2000; Gross et al. 2002; Mollet and Cailliet 2002). Because peppered chub is a broadcast spawner and because first-year survival rates of fishes are strongly, and often nonlinearly, related to stream discharge (Emlen et al. 1993; Capra et al. 2003; Lobón-Cervia 2004), we modified the basic Leslie matrix to relate first-year survival of peppered chub to stream discharge. Our model allowed us to assess the potential effects of altered flow

5 Wilde and Durham 5 regimes on abundance and persistence of the species and identify the life-history stages that have the greatest impact on peppered chub population growth rate. Study Area The Canadian River originates in the Sangre de Christo Mountains of northeastern New Mexico and southern Colorado and flows 1,510 km east and southeast through New Mexico and Texas to its confluence with the Arkansas River in eastern Oklahoma (Dolliver 1984; Sublette et al. 1990). Four reservoirs have been constructed on the Canadian River mainstem: Lake Conchas (San Miguel County, NM); Ute Reservoir (Quay County, NM); Lake Meredith (Potter, Moore, and Hutchinson Counties, TX); and Lake Eufaula (Pittsburg, McIntosh, and Haskell Counties, OK). Prior to the construction of these impoundments, the majority of the cumulative annual discharge in the Canadian River primarily was from high intensity, short duration summer rainfall events (Dolliver 1984). Currently, the volume of the river is greatly reduced and is determined by discharges from upstream dams. Between Ute Reservoir and Lake Meredith, the reach of river studied herein, mean annual discharge averaged 17.3 m 3 /s during , the period that preceded construction of Ute Reservoir. Since 1962, mean annual discharge has averaged 8.8 m 3 /s, a 49% reduction. Methods Fish Sampling. We collected peppered chub at three sites on the Canadian River in New Mexico and Texas. From upstream to downstream, sampling sites were located 1) approximately 20 km downstream from Ute Reservoir (Quay County, NM; 35 º N,

6 Wilde and Durham º W); 2) south of Tascosa (Oldham County, TX; 35 º N, 102 º W) at US Highway 385; and 3) north of Amarillo (Hutchinson County, TX; 35 º N, 105 º W) at US Highway 287, upstream from Lake Meredith. A map of the study area is presented in Bonner and Wilde (2000). We collected fish from September 1996 through August Our collections were made once per month from September through April and twice per month during the spawning season, May through August. At each sampling site, on each date, ten to 15 transects, perpendicular to the main channel of the river, were established such that no two transects were located within 20 m of each other. One to five non-overlapping seine hauls were made across each transect, with transects being sampled in a downstreamupstream sequence to minimize impacts on adjacent transects (Matthews and Hill 1979). We sampled fish with a 3.7 x 1.8-m (5-mm mesh) seine. Each seine haul covered 5 m and was made in a downstream direction, with the current, over homogenous habitat (Matthews and Hill 1979). After each seine haul, captured fishes were identified, enumerated, and released. To minimize stress to fish and avoid large concentrations of fish, we sampled only when the river was flowing. After we completed 50 seine hauls at a site, we collected 25 fish from each site, on each date, which were euthanized with MS- 222 and preserved in buffered formalin for laboratory analyses. Because peppered chub undergoes an upstream movement in summer (Bonner 2000), we calculated peppered chub catch-per-effort as the total catch of fish per 50 seine hauls, averaged across the three sampling sites.

7 Wilde and Durham 7 Sex Ratio and Age-specific Fecundity. Peppered chub sex ratio occasionally deviated from a 1:1 ratio at one or more sites, or on some dates. However, across all samples there was no evidence of any systematic departure from a 1:1 female to male ratio (Bonner 2000). We assumed a 1:1 sex ratio in our models. We removed ovaries from five mature female peppered chub collected on each sampling date during September 1996 to August One ovary from each female fish was dissected and all oocytes were counted, and the remaining ovary was used for studies of oocyte development using standard histological techniques (Hinton 1990; Phillip 1993). All oocytes in one longitudinal cross-section of each sampled ovary were classified as being in one of six stages: perinuceolear (immature oocytes), cortical alveolar (oocytes in early stages of maturation), early vitellogenesis (maturing oocytes), late vitellogenesis (mature oocytes), germinal vesicle breakdown (spawning in progress), or post ovulatory (indicating that ova had been released within the last 24 hr [Parrish et al. 1986; Fitzhugh and Hettler 1995]). We observed oocytes in all stages of maturation in individual fish throughout the spawning season, demonstrating that the peppered chub is a fractional spawner (Heins and Rabito 1986; Ali and Kadir 1996; Fernandez-Delgado and Herrera 1994; Lowerre-Barbieri et al. 1996). Consequently, total oocyte counts cannot be used to estimate fecundity (Heins and Rabito 1986; Heins and Baker 1987; Rinchard and Kestemont 1996). Herein, we estimate fecundity of peppered chub as the number of oocytes two the proportion of oocytes that were post ovulatory or in germinal vesicle breakdown. We estimated fecundity separately for age-1 and age-2 fish. Population Age-structure and Age-specific Survival. Bonner (2000) described the

8 Wilde and Durham 8 reproductive ecology of peppered chub in the Canadian River, New Mexico and Texas, during September 1996 through August We used his April and May samples to determine population age-structure of peppered chub because these samples were representative of the population at the start of the spawning season and because age-2 individuals became progressively scarcer from June through July, and senesced by midsummer. Based on visual examination of length-frequency histograms (Anderson and Neumann 1996) of fishes collected in April and May of both years, we determined that peppered chub < 60-mm SL were age 1 and that those > 60-mm SL were age 2. We estimated 397 age-1 and 44 age-2 peppered chub were present in samples collected during these four months. To estimate survival from age 1 to age 2, we used the exponential population growth model, N 2 = N 1 e rt, where N 1 is the number of age-1 individuals, N 2 is the number of age-2 individuals, e is the base of natural logarithms, r is the population growth rate, which is used here as an estimate of annual survival, and t is the time between samples (here, t = 1 year). Although peppered chub can survive into their third year in the laboratory (Wilde unpublished), there was no evidence the species survived its second year in the Canadian River (Bonner 2000). Therefore, we assumed that survival of age-2 fish was nil. We were able to develop estimates of all necessary population parameters except survival from age 0 (ovum) to age 1 to parameterize the projection matrix (de Kroon et al. 1986; Caswell 2001) of a Leslie matrix model for peppered chub. If all elements of the projection matrix are known, the rate of population growth λ can be estimated (Caswell 2001). Alternatively, if the rate of population growth is known, or is assumed to take some value, any one missing element of the projection matrix can be estimated

9 Wilde and Durham 9 (Vaughan and Saila 1976). Therefore, we assumed that the peppered chub population was static, with a rate of population growth λ = 1 (Vélez-Espino et al. 2006) and used the method of Vaughan and Saila (1976) to estimate the survival rate from age 0 to age 1. Life-history Models. Our models are based on modifications of the age-structured Leslie matrix population model and assumes a birth-pulse population with post-breeding census. Because reproductive success of peppered chub is believed to be related to magnitude and variability in stream discharge (Bottrell et al. 1964; Platania and Altenbach 1998; Bonner 2000; Durham and Wilde 2006), we adjusted age-0 survival according to the equation: Age-0 survival = (CMS / D est ), where is an initial estimate of age-0 survival (Table 1), CMS is the Canadian River mean daily discharge in summer in m 3 s -1 (May through August, which corresponds to the peppered chub spawning season in the Canadian River), and D est is a river discharge factor that is fitted using maximum likelihood methods (Hilborn and Mangel 1997). In our initial assessment of the model, we tested several alternative mathematical relationships between discharge and age-0 survival. We also considered alternatives to mean daily discharge in summer. These alternatives failed to match the multiplicative relationship, used herein, and summer mean daily discharge in predictive ability. River discharge information was obtained from USGS gage , located on the Canadian

10 Wilde and Durham 10 River, north of Amarillo, Texas at our most downstream sampling site. This is the only gage located within our study area. In addition to the discharge-related model, we constructed two alternative (null) population models for peppered chub. These models were constructed to allow an assessment of the predictive capabilities of the model versus those of reasonable, null alternatives (e.g., Hilborn and Mangel 1997). The first alternative model assumed no change in population size over the six-year sampling period. This model was parameterized by calculating the mean catch-per-effort of peppered chub (x = 40.2), across all years, and assuming that value in each year. The second (constant-λ) model assumed a constant rate of population change throughout the study period. We regressed ln(catch) in the first and last years of our study against year and used the slope (m = ) of this regression as an estimate of r and then estimated population size using the exponential population growth model, N 2 = N 1 e rt, as described above. Model Assessment. Because the number of years that we sampled (N = 6) is small compared with the number of estimated parameters in each model (K = 3 estimated parameters, including the variance, in each model), we used the second-order Akaike information criterion AIC c to evaluate the three alternative models for each species (Hilborn and Mangel 1997; Burnham and Anderson 2002). The AIC c is calculated as: AIC c = n log(s 2 ) + 2K + 2K (K + 1) / (n K 1),

11 Wilde and Durham 11 where n is the number of samples (years), and s 2 is the least-squares estimate of the population variance (i.e., the sum of squared deviations between the observed population and that predicted by each model, divided by n 1). Elasticity and Loop Analyses. We used elasticity analysis (de Kroon et al. 1986; van Groenendael et al. 1994; Benton and Grant 1999; Caswell 2001) to identify the life history stages, or parameters, that most influenced peppered chub population growth. Elasticity analysis estimates the proportional change in the population growth rate for a given change in a population parameter (e.g., fecundity, survival). These analyses can be used to identify which life-history stages most influence population growth rate and, consequently, which might be most effectively targeted by management (Benton and Grant 1999). Loop analysis decomposes the life history into discrete life-history pathways, or loops (van Groenendael et al. 1994). To better understand the potential effects of uncertainty in our parameter estimates we conducted a number of simulation studies as recommended by Mills et al. (1999). We assumed that age-1 and age-2 fecundities were normally distributed with mean x equal to the observed mean fecundity (Table 1), and with variance s 2 equal to that observed for age-1 (SD = ) and age-2 (SD = 933.8) fecundity. We assumed that age-specific survival rates were binomially distributed with mean x equal to the observed survival rate (Table 1), and with variance s 2 equal to x (1-x ). One thousand replicate simulations were performed and the elasticities of each parameter (age-1 and age-2 fecundity, and age-0 and age-1 survival) were recorded.

12 Wilde and Durham 12 Results We estimated age-specific survival rates to be for age-0 and for age-1 peppered chub (Table 1). Age-specific fecundities were 763 for age-1 and 654 for age-2 fish. Although individual age-2 peppered chub were larger and typically possessed a greater number of oocytes than age-1 individuals, they were common only during the first half of the spawning season after which they decreased in abundance. Consequently, our estimates of age-specific fecundity for age-2 peppered chub were less than those for age-1 fish when extrapolated across the entire spawning season We used age-specific survival and fecundity estimates (Table 1) to parameterize a Leslie matrix model for peppered chub. Age-0 survival and age-1 fecundity had the greatest elasticities, and respectively (Table 2), indicating that incremental changes in these two rates had the greatest effect on peppered chub population dynamics. Similarly, loop analysis showed that peppered chub population dynamics are dominated by survival and reproduction in the first year of life. Summed elasticities for age-0 survival and age-1 fecundity were 0.91, which indicates that peppered chub is essentially an annual species. Elasticity simulations (N = 1000) showed that age-0 survival (mean elasticity + SD = ) and age-1 fecundity ( ) consistently had the greatest effect on peppered chub growth rate. Elasticities of age-0 survival and age-1 fecundity ranged from less than 0.1 to 0.5 (Figure 1). However, 76% of age-0 survival elasticities exceeded 0.45 and 87% exceeded Similarly, 60% of age-1 fecundity elasticities exceeded 0.45 and 69% exceeded Elasticities for age-2 fecundity and age-1 survival ranged from < 0.01 to In 1000 simulations, elasticities for the second life-

13 Wilde and Durham 13 history loop, which consists of age-1 survival and age-2 fecundity, ranged from to 0.63 (mean + SD = ). Our simulation results provide evidence that the elasticities obtained from our initial matrix parameterization are robust with respect to uncertainty in parameter estimates. There was a general decrease in catch-per-effort of peppered chub during our study, which is indicative of a decreasing population (Figure 2). Discharge in the Canadian River generally decreased during this period as well. However, although there was no relationship between Canadian River discharge and peppered chub catch per unit effort (r 2 = 0.32, n = 6, P = 0.25), the discharge-related model, which relates age-0 survival to mean summer discharge, received the best support from the catch data. For the discharge-related, constant-λ, and static models, AIC C s were 36.8, 38.7, and 46.2, respectively. The discharge-factor (D est ) fitted to the peppered chub data was 11.9 m 3 s -1, which implies that a river discharge of that magnitude is necessary for the population to maintain a growth rate λ = 1. Discharge in the Canadian River exceeded 11.9 m 3 s -1, often substantially, in 15 of the 25 years before impoundment for which discharge data are available. In contrast, during the 40-year period, from impoundment of Ute Reservoir in 1963 until the end of our field sampling in 2002, mean summer discharge exceeded 11.9 m 3 s -1 in only 12 years. Thus, our model suggests the Canadian River peppered chub population generally has decreased since Ute Reservoir was impounded.

14 Wilde and Durham 14 Discussion Our estimates of demographic parameters for peppered chub are among the first for Great Plains fishes that broadcast spawn semi-buoyant pelagic ova. This reproductive mode appears to be common among fishes that inhabit the highly variable streams and rivers of the Great Plains region (Moore 1944; Bottrell et al. 1964; Platania and Altenbach 1998; Bonner 2000). Our estimates of fecundity were 382 for age-1 peppered chub and 323 for age-2 fish, which appears paradoxical because age-2 peppered chub are larger than age-1 fish (Bonner 2000). However, age-2 peppered chub was absent from our midsummer samples, presumably as a result of high post-spawning mortality (Bonner 2000). Senescence and high post-spawning mortality appears to be common among short-lived (2- to 3-yr) Great Plains cyprinids and older individuals often are absent in the latter half of the spawning season (Summerfelt and Minckley 1969; Bestgen et al. 1989; Taylor and Miller 1990). Compared with other Great Plains cyprinids, fecundity of peppered chub is comparable to that of sand shiner Notropis stramineus (mean = 281; Summerfelt and Minckley 1969), but is lower than that reported by Durham (2007) for sharpnose shiner N. oxyrhynchus (age-1 fecundity = 379, age-2 fecundity = 1398) and smalleye shiner N. buccula, (age-1 fecundity = 443, age-2 fecundity = 2175), which also are broadcast spawners. Our estimates of fecundity, are substantially less than those for flathead chub Platygobio gracilis (fecundity ranges from 4452 for age-2 fish to 9233 for age-7 fish) and sturgeon chub M. gelida (fecundity ranges from 2750 for age-1 fish to 4000 for age-3 fish) (Rahel and Thel 2004a, b). With the exception of flathead chub, which can reach a

15 Wilde and Durham 15 total length exceeding 300 mm (Rahel and Thel 2004a), these species are comparable in length (maximum total lengths ranging from 65 to 76 mm). Our estimate of age-0 survival for peppered chub, , is near the mid-range of similar estimates ( to 0.004) reported by Durham (2007) and Rahel and Thel (2004a, b), all of which were estimated assuming λ =1. Our estimate is greater than those compiled by Dahlberg (1979) for several highly fecund broadcast-spawning fishes. Thus, the relatively low fecundity of peppered chub appears offset by an increased early survival rate. We were able to accurately model peppered chub population dynamics as a function of average river discharge during the spawning season. This is a rather surprising result given the absence of any statistically significant relationship between peppered chub catch-per-effort and river discharge. Our life-history model explicitly assumes that survival of spawned oocytes and age-0 peppered chub is related to discharge. This assumption is consistent with the spawning behavior of peppered chub (Bottrell et al. 1964; Platania and Altenbach 1998) and observations that young are produced continuously during summer except during periods of zero discharge (Durham and Wilde 2006). Wilde and Durham (in press) examined daily survival of larval and juvenile Canadian River fishes, including peppered chub, in 2000 and In both years, discharge in the Canadian River was lower than average and there were occasional periods of no flow during the reproductive season. Wilde and Durham observed that daily survival was less during 2001, which was characterized by a longer period of noflow conditions.

16 Wilde and Durham 16 We used our life-history model to assess potential affects of past and proposed modifications of Canadian River discharge on peppered chub abundance (Figure 3). In the first simulation, we increased discharge by a multiple of 2.633, which was obtained by dividing mean Canadian River discharge prior to impoundment of Ute Reservoir (1943 to 1962, x = 23.2 m 3 s -1 ) by post-impoundment discharge (1963 to 2002, x = 8.8 m 3 s -1 ). Under this scenario, peppered chub abundance was variable and high. Despite two years of reduced discharge at the end of the simulation, the population was 28% greater than at the beginning of the simulation. In the second simulation, discharge was reduced 6.5% from observed levels. This multiple was obtained by taking the mean of upper (-8%) and lower (-5%) estimates of the potential impacts of salinity control projects on the Canadian River downstream from Ute Reservoir. Abundance of peppered chub generally declined throughout this simulation and, in the end, the projected population was 23% less than that predicted under the observed discharge regime. Our model assumes the relationship between discharge and survival of age-0 peppered chub is multiplicative. This specific relationship has important implications for management of peppered chub. Our model (with D est = 11.9 m 3 s -1 ) predicts that following a year in which discharge was, for example, 10% less than the long-term average, discharge in the following year would have to exceed that average by 11% for the peppered chub population to recover. As discharge is decreased to 30 or 50% below the long-term mean, progressively greater increases in discharge, 41% and 91%, respectively, are required for population recovery in the following year. Thus, persistence of peppered chub in the Canadian River is not guaranteed even if discharge, over a number of years, is at or near the long-term mean.

17 Wilde and Durham 17 In the lower Colorado River of southwest US, Minckley et al. (2003) observed that changes in abundance of fishes did not change dramatically from year to year. Instead, the abundance of most species changed slowly, depending on longevity and other life-history characteristics. Many of the species considered by Minckley et al. (2003), including cyprinids, were long lived. In contrast, peppered chub is short lived and is capable of dramatic population changes over short periods. During the six-year period (1996 to 2001) that we sampled fishes in the Canadian River, a period characterized by below average discharge, the peppered chub population decreased approximately 80%. Based on our estimate of 11% annual survival for age-1 fish, peppered chub abundance could decrease by at least 89% in a single year if reproduction failed completely. Our elasticity analysis shows that peppered chub population growth rate is most sensitive to changes in age-0 survival and age-1 fecundity. The sum of the elasticities of these two parameters is This is consistent with the findings of Vélez-Espino et al. (2006), who constructed projection matrices for 88 species of North American freshwater fishes and conducted elasticity analyses on those matrices. Among short-lived, smallbodied cyprinds, they found that population dynamics were most sensitive to changes in juvenile (usually age-0) survival and reproduction in the first year of maturity. Further, they observed, as did we, that elasticities of these two population-dynamics parameters were approximately equal and summed to > Based on our observed elasticities and those of Vélez-Espino et al. (2006), we speculate that the population growth rates of most short-lived (2- to 3-yr) Great Plains fishes, especially those that broadcast spawn semibuoyant pelagic ova, will be most sensitive to changes in age-0 survival and age-1 fecundity.

18 Wilde and Durham 18 A common use of elasticity analysis is to identify the life-history stages that have the greatest effect on population dynamics because, it is believed, these stages are those that are most sensitive to management intervention (Benton and Grant 1999; Vélez- Espino et al. 2006). Although Benton and Grant (1999) and Mills et al. (1999) caution against an uncritical use of elasticities to direct management actions, our results and those of Durham and Wilde (2006) suggest that peppered chub population dynamics can be readily manipulated by altering the discharge regime, which could result in an increase in early survival. Our results and observations suggest a major paradigm shift for studying and managing Great Plains fishes when contrasted with the historic emphasis on habitat requirements of these fish, which, with the exception of Matthews and Hill (1979) and Polivka (1999), is based almost exclusively on studies of adult fish (Matthews and Hill 1980; Braaten and Guy 1999; Luttrell et al. 1999; Scheurer et al. 2003; Dieterman and Galat 2004; Everett et al. 2004; Welker and Scarnecchia 2004). Human modifications of Great Plains streams and rivers are believed to affect distribution and abundance of fishes directly by affecting extinction-colonization events (Winston et al. 1991; Wilde and Ostrand 1999; Luttrell et al. 1999; Dieterman and Galat 2004) and indirectly by altering discharge, which induces habitat changes (Cross et al. 1985; Cross and Moss 1987; Pigg 1991; Luttrell et al. 1999; Pigg et al. 1999; Dieterman and Galat 2004; Welker and Scarnecchia 2004, 2006) or changes in the competitive and predatory environment (e.g., Pflieger and Grace 1987; Bonner and Wilde 2002). However, there have been no direct tests of these mechanisms. Our model posits a direct and testable mechanism by which altered discharge can affect population dynamics. The validity and generality of this hypothesized mechanism can be assessed as additional

19 Wilde and Durham 19 studies (e.g., Durham 2007) of the population dynamics of Great Plains fishes become available. Acknowledgments. We thank T. H. Bonner, M. J. Brown, K. G. Ostrand, B. Redell, C. D. Smith, and R. Young for assistance in the field and laboratory. We also thank K. Collins, M. Irlbeck, and D. Moomaw for logistical and other support, and C. L. Higgins and C. J. Chizinski for commenting on the manuscript. Funding for this study was provided by the US Fish and Wildlife Service, Tulsa, OK, and the US Bureau of Reclamation, Austin, TX.

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21 Wilde and Durham 21 Bonner, T. H., and G. R. Wilde Effects of turbidity on prey consumption by prairie stream fishes. Transactions of the American Fisheries Society 131: Bottrell, C. E., R. H. Ingersol, and R. W. Jones Notes on the embryology, early development and behavior of Hybopsis aestivalis tetranemus (Gilbert). Transactions of the American Microscopical Society 83: Braaten, P. J., and C. S. Guy Relations between physicochemical factors and abundance of fishes in tributary confluences of the lower channelized Missouri River. Transactions of the American Fisheries Society 128: Brewster-Geisz, K. K., and T. J. Miller Management of the sandbar shark Carcharhinus plumbeus: implications of a stage-based model. Fisheries Bulletin 98: Brown, K. L Population demographic and genetic structure of plains killifish from the Kansas and Arkansas river basins in Kansas. Transactions of the American Fisheries Society 115: Burnham, K. P., and D. R. Anderson Model selection and multi-model inference: a practical information-theoretic approach, second edition. Springer-Verlag, New York. Capra H., C. Sabaton, V. Gouraud, Y. Souchon, and P. Lim A population dynamics model and habitat simulation as a tool to predict brown trout demography in natural and bypassed stream reaches. River Research and Applications 19:

22 Wilde and Durham 22 Caswell, H Matrix population models: construction, analysis and interpretation, second edition. Sinauer Associates, Inc. Sunderland, Massachusetts. Cross, F. B., R. E. Moss, and J. T. Collins Assessment of dewatering impacts on stream fisheries in the Arkansas and Cimarron rivers. Kansas Fish and Game Commission Nongame Wildlife Contract 46: Cross, F. B., and R. E. Moss Historic changes in fish communities and aquatic habitats in plains streams of Kansas. Pages in W. J. Matthews and D. C. Heins, editors. Community and evolutionary ecology of North American stream fishes. University of Oklahoma Press, Norman. Dahlberg, M. D A review of survival rates of fish eggs and larvae in relation to impact assessments. U. S. National Marine Fisheries Service Fisheries Review 41(3):1-12. de Kroon, H., A. Plaisier, J. van Groenendael, and H. Caswell Elasticity: the relative contribution of demographic parameters to population growth rate. Ecology 67: Dieterman, D. J., and D. L. Galat Large-scale factors associated with sicklefin chub distribution in the Missouri and lower Yellowstone rivers. Transactions of the American Fisheries Society 133: Dolliver, P. N Cenozoic evolution of the Canadian River Basin. Baylor Geological Studies Bulletin No. 42. Waco, Texas. Durham, B. W Reproductive ecology, habitat associations, and population dynamics of two imperiled cyprinids in a Great Plains river. Doctoral dissertation. Texas Tech University, Lubbock.

23 Wilde and Durham 23 Durham, B. W., and G. R. Wilde Influence of stream discharge on reproductive success of a prairie stream fish assemblage. Transactions of the American Fisheries Society 135: Durham, B. W., and G. R. Wilde. In press. Elasticity analysis of an age-structured population dynamics model for the Pecos bluntnose shiner. Proceedings of the Sixth Symposium on the Natural Resources of the Chihuahuan Desert Region Emlen, J. M., T. A. Strekal, and C. C. Buchanan Probabalistic projections for recovery of the endangered cui-ui. North American Journal of Fisheries Management 13: Everett, S. R., D. L. Scarnecchia, and L. F. Ryckman Distribution and habitat use of sturgeon chubs (Macrhybopsis gelida) and sicklefin chubs (M. meeki) in the Missouri and Yellowstone rivers, North Dakota. Hydrobiologia 527: Falke, J.A., and K. B. Gido Effects of reservoir connectivity on stream fish assemblages in the Great Plains. Canadian Journal of Fisheries and Aquatic Sciences 63: Fernandez-Delgado, C., and M. Herrera Population structure, growth and reproduction of Tropidophoxinellus alburnoides (Steindachner 1866) in an intermittent stream of the Guadalquivir River basin (southern Spain). Archiv für Hydrobiologie 130: Fitzhugh, G. R., and W. F. Hettler Temperature influence on postovulatory follicle degeneration in Atlantic menhaden Brevoortia tyrannus. Fishery Bulletin 93:

24 Wilde and Durham 24 Gross, M. R., J. Repka, C. T. Robertson, D. H. Secor, and W. Van Winkle Sturgeon conservation: insights from elasticity analysis. Pages in W. Van Winkle, P. J. Anders, D.H. Secor, and D.A. Dixon, editors. Biology, Management, and Protection of North American Sturgeon. American Fisheries Society Symposium 28. Heins, D. C., and J. A. Baker Analysis of factors associated with intraspecific variation in propagule size of a stream-dwelling fish. Pages in W. J. Matthews and D. C. Heins, editors. Community and evolutionary ecology of North American stream fishes. University of Oklahoma Press, Norman. Heins, D. C., and F. G. Rabito, Jr Spawning performance in North American minnows: direct evidence of the occurrence of multiple clutches in the genus Notropis. Journal of Fish Biology 28: Heppell, S. L., L. B. Crowder, and T. R. Menzel Life table analysis of long-lived marine species, with implications for conservation and management. Pages in J. A. Musick, editor. Life in the slow lane: ecology and conservation of long-lived marine animals. American Fisheries Society, Symposium 23, Bethesda, Maryland. Hilborn, R., and M. Mangel The ecological detective: confronting models with data. Princeton University Press. Princeton, New Jersey. Hinton, D. E Histological techniques. Pages in C. B. Schreck and P. B. Moyle, editors. Methods for fish biology. American Fisheries Society, Bethesda, Maryland.

25 Wilde and Durham 25 Jensen, A. L The effect of increased mortality on the young in a population of brook trout, a theoretical analysis. Transactions of the American Fisheries Society 100: Limbird, R. L The Arkansas River- a changing river. Pages In L. W. Hesse, C. B. Stalnaker, N. G. Benson, and J. R. Zuboy, editors. Restoration planning for the rivers of the Mississippi River ecosystem. National Biological Survey, Washington, DC. Lobón-Cervia, J Discharge-dependent covariation patterns in the population dynamics of brown trout (Salmo trutta) within a Cantabrian river drainage. Canadian Journal of Fisheries and Aquatic Sciences 61: Lowerre-Barbieri, S. K., M. E. Chittenden, Jr., and L. R. Barbieri The multiple spawning pattern of weakfish in the Chesapeake Bay and Middle Atlantic Bight. Journal of Fish Biology 48: Luttrell, G. R., A. A. Echelle, W. L. Fisher, and D. J. Eisenhour Declining status of two species of the Macrohybopsis aestivalis complex (Teleostei: Cyprinidae) in the Arkansas River basin and related effects of reservoirs as barriers to dispersal. Copeia 1999: Matthews, W. J., and L. G. Hill Age-specific differences in the distribution of red shiners, Notropis lutrensis, over physiochemical ranges. American Midland Naturalist 101: Matthews, W. J., and L. G. Hill Habitat partitioning in the fish community of a southwestern river. Southwestern Naturalist 25: Mills, L. S., D. F. Doak, and M. J. Wisdom Reliability of conservation actions

26 Wilde and Durham 26 based on elasticity analysis of matrix models. Conservation Biology 13: Minckley, W. L., P. C. Marsh, J. E. Deacon, T. E. Dowling, P. W. Hedrick, W. J. Matthews, and G. Mueller A conservation plan for native fishes of the Lower Colorado River. BioScience 53: Mollet, H. F. and G. M. Cailliet Comparative population demography of elasmobranchs using life history tables, Leslie matrices and stage-based matrix models. Marine and Freshwater Research 53: Moore, G. A Notes on the early life history of Notropis girardi. Copeia 1944: Oakes, R. M., K. B. Gido, J. A. Falke, J. D. Olden, and B. L. Brock Modelling of stream fishes in the Great Plains, USA. Ecology of Freshwater Fish 14: Parrish, R. H., D. L. Mallicoate, and R. A. Klingbeil Age dependent fecundity, number of spawnings per year, sex ratio, and maturation stages in northern anchovy, Engraulis mordax. Fishery Bulletin 84: Pflieger, W. L., and T. B. Grace Changes in the fish fauna of the lower Missouri River, Pages in W. J. Matthews and D. C. Heins, editors. Community and evolutionary ecology of North American stream fishes. University of Oklahoma Press, Norman. Phillip, D. A Reproduction and feeding of the mountain mullet, Agonostomus monticola, in Trinidad, West Indies. Environmental Biology of Fishes 37: Pigg, J Decreasing distribution and current status of the Arkansas River shiner, Notropis girardi, in the rivers of Oklahoma and Kansas. Proceedings of the Oklahoma Academy of Science 71:5-15.

27 Wilde and Durham 27 Pigg, J., R. Gibbs, and K. K. Cunningham Decreasing abundance of the Arkansas River shiner in the South Canadian River, Oklahoma. Oklahoma Academy of Science 79:7-12. Platania, S. P., and C. S. Altenbach Reproductive strategies and egg types of seven Rio Grande Basin cyprinids. Copeia 1998: Polivka, K. M The microhabitat distribution of the Arkansas River shiner, Notropis girardi: a habitat-mosaic approach. Environmental Biology of Fishes 55: Rahel, F. J., and L. A. Thel. 2004a. Flathead chub (Platygobio gracilis): a technical conservation assessment. USDA Forest Service, Rocky Mountain Region. Available: Rahel, F. J., and L. A. Thel. 2004b. Sturgeon chub (Macrhybopsis gelida): a technical conservation assessment. USDA Forest Service, Rocky Mountain Region. Available: Rahel, F. J., and L. A. Thel. 2004c. Plains killifish (Fundulus zebrinus): a technical conservation assessment. USDA Forest Service, Rocky Mountain Region. Available: Rinchard, J., and P. Kestemont Comparative study of reproductive biology in single- and multiple-spawner cyprinid fish. I. Morphological and histological features. Journal of Fish Biology 49: Scheurer, J. A., K. D. Fausch, and K. R. Bestgen Multiscale processes regulate brassy minnow persistence in a Great Plains River. Transactions of the American Fisheries Society 132:

28 Wilde and Durham 28 Starrett, W. C Some factors affecting the abundance of minnows in the Des Moines River, Iowa. Ecology 32: Sublette, J. E., M. D. Hatch, and M. Sublette The fishes of New Mexico. University of New Mexico, Albuquerque. Summerfelt, R. C., and C. O. Minckley Aspects of the life history of the sand shiner, Notropis stramineus (Cope), in the Smoky Hill River, Kansas. Transactions of the American Fisheries Society 98: Taylor, C. M., and R. J. Miller Reproductive ecology and population structure of the plains minnow, Hybognathus placitus (Pisces: Cyprinidae), in Central Oklahoma. American Midland Naturalist 123: van Groenendael, J., H. de Kroon, S. Kalisz, and S. Tuljapurkar Loop analysis: evaluating life history pathways in population projection matrices. Ecology 75: Vaughan, D. S., and S. B. Saila A method for determining mortality rates using the Leslie matrix. Transactions of the American Fisheries Society 105: Vélez-Espino, L. A., M. G. Fox, and R. L. McLaughlin Characterization of elasticity patterns of North American freshwater fishes. Canadian Journal of Fisheries and Aquatic Sciences 63: Welker, T. L., and D. L. Scarnecchia Habitat use and population structure of four native minnows (family Cyprinidae) in the upper Missouri and lower Yellowstone river, North Dakota (USA). Ecology of Freshwater Fish 13:8-22.

29 Wilde and Durham 29 Welker, T. L., and D. L. Scarnecchia River alternation and niche overlap among three native minnows (Cyprinidae) in the Missouri River hydrosystem. Journal of Fish Biology 68: Wilde, G. R., and B. W. Durham. In press. Daily survival rates for juveniles of six species of Great Plains cyprinid fishes. Tranactions of the American Fisheries Society 000: Wilde, G. R., and K. G. Ostrand Changes in the fish assemblage of an intermittent prairie stream upstream from a Texas impoundment. Texas Journal of Science 51: Williams, G. P., and M. G. Wolman Downstream effects of dams on alluvial rivers. U. S. Geological Survey Professional Paper Winston, M. R., C. M. Taylor, and J. Pigg Upstream extirpation of four minnow species due to damming of a prairie stream. Transactions of the American Fisheries Society 120:

30 Wilde and Durham 30 Table 1. Age-specific fecundity and survival rates for peppered chub in the Canadian River, New Mexico and Texas. Estimates are derived from data of Bonner (2000) except for age-0 survival, which was estimated using the method of Vaughan and Saila (1976). Age 0 Age 1 Age 2 Fecundity Survival

31 Wilde and Durham 31 Table 2. Results of an elasticity analysis of peppered chub projection matrix. The elasticities, which sum to 1, show the sensitivity of peppered chub population dynamics to unit changes in each vital rate. Age 0 Age 1 Age 2 Fecundity Age-1 survival Age-2 survival

32 Wilde and Durham 32 Figure Captions Figure 1. Frequency distributions of the elasticities population vital rates of N = 1000 simulated peppered chub projection matrices. Figure 2. Mean daily discharge in the Canadian River, New Mexico and Texas during 1996 to 2001, observed peppered chub abundance, and estimates from a discharge-related population dynamics model and two null population models. Figure 3. Simulated peppered chub population dynamics under unregulated, observed ( ), and reduced river discharge. Unregulated discharge was obtained by multiplying the observed discharge by 2.63, which was obtained by dividing mean Canadian River discharge prior to impoundment of Ute Reservoir (1943 to 1962, x = 23.2 m 3 s -1 ) by post-impoundment discharge (1963 to 2002, x = 8.8 m 3 s -1 ). Reduced discharge was obtained by taking the mean of upper (-8%) and lower (-5%) estimates of the potential impacts of salinity control projects on Canadian River discharge downstream from Ute Reservoir.

33 Wilde and Durham 33 Age-1 fecundity Age-2 fecundity Frequency 200 Frequency Elasticity Elasticity Age-0 survival Age-1 survival Frequency 200 Frequency Elasticity Elasticity

34 Wilde and Durham 34 River discharge (cfs) Catch per effort (N) Catch per effort Uniform model Catch per effort (N) Catch per effort Constant-λ model Catch per effort (N) Catch per effort Discharge-related model

35 Wilde and Durham 35 Population (catch per unit effort) Unregulated Observed Reduced (6.5%)