Genetic History of Walleyes Spawning in Lake Erie's Cattaraugus Creek: a Comparison of Pre- and Poststocking

Fish stocking (artificial supplementation) has been used to augment populations and angling opportunities. However, genetic composition and adaptations of native fish populations may be affected, raising management concerns. From 1995 to 2000, the New York State Department of Environmental Conservation stocked Walleye Sander vitreus fry and fingerlings from the Maumee River (western Lake Erie) into Cattaraugus Creek (eastern Lake Erie). We analyzed nuclear microsatellite (msat) DNA and mitochondrial DNA (mtDNA) variation in Cattaraugus Creek Walleyes for comparisons between prestocking and poststocking groups, among annual spawning runs (1998–2011), among age cohorts, and between sexes. Results for genetic differentiation (index FST) were not significant between prestocking and poststocking groups (msat: FST D 0.003; mtDNA: FST < 0.001), and the two groups did not resemble stocked Maumee River fingerlings (msat: FST D 0.003–0.012; mtDNA: FST D 0.076–0.090). Tests for differentiation were not significant among annual spawning runs (msat: FST D <0.001–0.007; mtDNA: FST D <0.001–0.049), among age cohorts (msat: FST D <0.001–0.006; mtDNA: FST D <0.001–0.097), or between sexes (msat: FST < 0.001; mtDNA: FST < 0.001). Genetic diversity levels were high and consistent (msat: observed heterozygosity [mean § SE] D 0.71 § 0.04; mtDNA: haplotype diversity D 0.79 § 0.01). Thus, despite stocking, the genetic signature of the native spawning run remained distinctive. However, the genetic composition of the local wild population and the stocking source should be assessed prior to any future supplementation plans. Stocking—the artificial supplementation of an existing population or the founding of a new population, frequently by using individuals raised in hatcheries—has been a common fishery management practice dating to the 1800s (Smith and Needham 1942; Knapp et al. 2001; Pister 2001). Present-day fishery managers and scientists increasingly have become concerned with the effects of stocking on the genetic composition and adaptations of native populations (see Radomski and Goeman 1995; Stepien et al. 2004; and Halverson 2008). Stocking may act to homogenize genetic diversity (Radomski and Goeman 1995) through admixture, founder effects, genetic drift, outbreeding depression, or a combination thereof (see Cena et al. 2006; Kerr 2011). This can lead to the loss of unique adaptations and fitness or to extirpation of the genetic signature of the original population (Reed and Frankham 2003; V€ali et al. 2008). Contemporary management strategies increasingly recognize the importance of matching the genetic composition of stocked individuals to that of the native population to preserve variability and local adaptedness (Eldridge et al. 2002; Kerr 2011). The objective of the present study was to evaluate the possible genetic effects of stocking on the gene pool of an indigenous spawning run of Walleyes Sander vitreus. Walleyes are ecologically and economically important (Locke et al. 2005; Nate et al. 2011). The largest Walleye metapopulation and fishery are located in Lake Erie (Hubbs and Lagler 2004; Stepien et al. 2012) and generate in excess of US$600 million per year (Schmalz et al. 2011). After maturing at ages 2–3, Walleyes migrate annually in spring to early summer to reproduce at specific spawning grounds (summarized by Barton and Barry 2011), exhibiting natal site fidelity (Jennings et al. 1996; Stepien and Faber 1998). Adults provide no parental care or nest guarding; they range widely to feed after spawning, traveling 50–300 km (Bozek et al. 2011). Fine-scale population genetic structure of Walleyes is largely due to differences among spawning groups (Stepien and Faber 1998; Strange and Stepien 2007; *Corresponding author: [email protected] Present address: Delaware State University, 1200 North Dupont Highway, Dover, Delaware 19901, USA. Present address: Department of Biology, East Carolina University, 1001 East Fifth Street, Greenville, North Carolina 27858, USA. Received December 6, 2013; accepted June 12, 2014 1295 Transactions of the American Fisheries Society 143:1295–1307, 2014 American Fisheries Society 2014 ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1080/00028487.2014.935477 D ow nl oa de d by [ U ni ve rs ity o f T ol ed o] a t 0 7: 55 1 7 O ct ob er 2 01 4 Stepien et al. 2009, 2010), which possess considerable genetic consistency from year to year and from generation to generation at given locations (Stepien et al. 2012). Walleyes have been widely stocked to establish new populations and to augment existing ones (USFWS and GLFC 2010); Walleye stocking in the Great Lakes dates to the early 1890s (McDonald 1893; Bowers 1901). Between 2001 and 2012, eight reported stocking events occurred at three separate Lake Erie sites; all were in the eastern basin (Conneaut Creek, Pennsylvania, and two sites were at Presque Isle Bay, Pennsylvania; USFWS and GLFC 2010). Prior to this, Cattaraugus Creek (New York), located in the eastern basin of Lake Erie, was stocked with an estimated 2.2 million unmarked Walleye fingerlings and 44,000 Walleye fry per year from 1995 to 2000 by the New York State Department of Environmental Conservation (NYSDEC; Stepien et al. 2004). Cattaraugus Creek is an eastern Lake Erie tributary that is about 109.4 km long; approximately 22.5 km of its lower reaches flow through the Seneca Nation’s Cattaraugus Reservation (NYSDEC 2009). The creek’s discharge ranges from 0.17 to 979.77 m/s (from 6 to 34,600 ft/s; USGS 2014). Cattaraugus Creek supports a popular Walleye fishery that is under the jurisdiction of the Seneca Nation, which sells its own fishing licenses and enforces its own regulations (NYSDEC 2009; Shane Titus, Seneca Nation Fish and Wildlife Department, personal communication). The Walleye spawning run in Cattaraugus Creek numbers only a few thousand individuals (Donald Einhouse, NYSDEC, personal communication). For years, the NYSDEC routinely sampled Cattaraugus Creek and recovered only one or two fish per effort. Believing there to be no native Walleye run, NYSDEC released unmarked fingerlings and fry into the creek near Irving, New York, downstream of the reservation (Figure 1; Donald Einhouse, personal communication); however, a later study revealed that Cattaraugus Creek houses a historic and genetically diverse native Walleye spawning run (Stepien et al. 2004, 2009, 2010). The broodstock used for stocking was derived from Walleyes spawning in the Maumee River (western Lake Erie); this run differs genetically from the native Walleye population of Cattaraugus Creek (Stepien et al. 2004, 2009, 2010; Strange and Stepien 2007). Thus, it appeared likely that the genetic composition of the stocked Walleye fry and larvae varied from that of the Walleyes spawning in Cattaraugus Creek; we tested this hypothesis in the present study. Additionally, since the stocked individuals were unmarked, we used analyses of nuclear DNA and mitochondrial DNA (mtDNA) to determine the potential genetic effects of stocking on the native run. We investigated the possible impact of the annual (1995– 2000) stocking events on the small spawning population in Cattaraugus Creek by comparing genetic composition (1) between adult Walleyes that were born during the prestocking and poststocking periods and (2) between each of these groups and the stocked Maumee River fingerlings. The genetic structure and temporal stability of Walleyes spawning in Cattaraugus Creek were assessed over a 13-year time span (six separate run years: 1998, 1999, 2001, 2003, 2005, and 2011) using microsatellite (msat) DNA loci and mtDNA control region sequences from available fish samples that were supplied by the NYSDEC. Specific questions addressed were 1. Does the population genetic composition and diversity (nuclear msat and mtDNA allelic variation) of Walleyes spawning in Cattaraugus Creek differ between the prestocking and poststocking groups? 2. Has population genetic variation remained similar across annual spawning runs? 3. Do birth year cohorts possess similar msat and mtDNA allelic compositions? 4. Do spawning males and females exhibit similar genetic patterns?