Little is known about the habitats of young pallid sturgeon Scaphirhynchus albus and shovelnose sturgeon S. platorynchus. Therefore, we compared the distribution and abundance of age‐0 fish of these species collected in benthic trawls among most of the mesoscale habitats comprising the channelized lower Missouri River (i.e., rock‐dike channel modifying structures, large‐channel sandbar, bankline, tributary, floodplain, and wooded island) from 2007 through 2009. Most age‐0 sturgeon were less than 110 mm (fork length) by the end of the growing season. Age‐0 fish were most frequently collected in areas with a relatively fast bottom velocity (0.5–0.7 m/s) associated with channel sandbars, rootless‐dikes, and wing‐dikes and were rarely found at L‐dikes, along banklines, or in tributaries where bottom velocity was slower (≤0.2 m/s). Age‐0 sturgeon habitat use in the lower Missouri River supports the hypothesis that main‐channel areas are important habitats throughout the life cycle of some fluvial specialist species. In light of existing literature on habitat use by wild age‐0 sturgeon, our results highlight their complex ecology in contemporary large rivers throughout their range. Therefore, we do not predict the response of pallid sturgeon to contemporary habitat restoration goals but discuss current knowledge gaps important to characterizing the early life history of Scaphirhynchus spp. Habitat restoration design on the Missouri River should consider the impact on ecosystem structure and the response by early life stages of these species.
We examined use of channel modifying structures (CMS; e.g., engineered rock dikes, channel sandbars) and their associated smaller spatial-scale habitats (SSH) on lower Missouri River, USA, by speckled chub (Macrhybopsis aestivalis), sicklefin chub (M. meeki) and sturgeon chub (M. gelida) from June through October, 2006 to identify important habitat characteristics that could aid conservation. We sampled chubs primarily in six CMS types: L-dikes, wing-dikes, kicker dikes, rootless dikes, channel sandbars and bank-lines. Ninety-three per cent of chubs were collected from L-dikes, wing-dikes and channel sandbars. Unidentifiable Macrhybopsis chubs (UHY) were less than 35-mm TL, were collected early in the sampling season and were most associated with L-dikes where mean bottom current velocity was slow (0.07 m/s). Chubs large enough to be identified to species were associated with large channel sandbars where mean bottom current velocity was faster (0.34 m/s). Chubs from wing-dikes were more abundant in shallow water (<1.2 m deep) adjacent to bars than in deeper (>1.2 m deep), open-water areas ( p ¼ 0.007). Month accounted for 54% and CMS with SSH 18% of explained variation in chub species distribution and abundance. Our results demonstrate that CMS in the contemporary lower Missouri River channel provide multiple habitats for multiple life-stages of Macrhybopsis chubs (e.g., nursery and post-nursery habitats). L-and wing-dikes may provide surrogate nursery habitats for chubs where in-channel slow-velocity areas were lost after river channelization. Managing CMS to address life history stages and an ecologically appropriate proportion and distribution of dike to channel sandbar habitats may be critical for conservation of native Macrhybopsis chubs in lower
Summary We synthesized wild and stocked pallid sturgeon Scaphirhynchus albus capture data collected in four recovery areas of the Missouri River during 1998–2007, providing the first basin‐wide analysis of size structure, growth, and condition. Proportional size distribution (PSD) values ranged from 20 to 33 and were indicative of past stockings given the continued lack of natural recruitment. A new weight‐length regression derived from 2268 captured wild (8%), hatchery‐stocked (75%), and unknown origin (16%) pallid sturgeon had a significantly lower slope and intercept from a previously published model that used only 214 wild fish and a truncated size range. Relative condition (Kn) declined after stocking throughout the basin but stabilized at 0.94 within 3 years. Spatially, Kn of juvenile pallid sturgeon (330–629 mm) was generally highest in the reaches of the Missouri River with large tributaries. In accordance with the latitudinal counter gradient growth hypothesis, similar absolute growth increments (both length and weight) of tagged age‐1 pallid sturgeon in the Upper and Lower Missouri River indicated upstream fish grew at faster rates given the 1.3 fold difference in growing season length. From North to South, von Bertalanffy growth coefficients (k) showed a latitudinal increase while L∞ decreased, providing additional support of the latitudinal counter gradient growth hypothesis. However, growth rates of tagged juvenile pallid sturgeon aged 2–6 years were highest at the two most downstream reaches showing an increased influence of growing season length. In the Missouri River, pallid sturgeon (≤ ages 1–9 or 10 years) exhibited linear growth in the two most upstream reaches (Montana and North Dakota), exhibited logistic growth in the inter‐reservoir reach in South Dakota/Nebraska while a power function best described growth downstream of the lowermost dam (Nebraska, Iowa, Kansas, and Missouri). Differing growth models among reaches highlights the confounding affect of habitat fragmentation by dams in the Missouri River on growth as well as potential latitudinal affects on sexual maturation. Relative condition and growth of pallid sturgeon appears adequate throughout the Missouri River indicating success of past stocking efforts to forestall extirpation.
We compared variability in catch per unit effort (CPUE) and size structure of shovelnose sturgeon Scaphirhynchus platorynchus collected in 2003 and 2004 with stationary winter gill nets, drifted trammel nets, hoop nets, and otter trawls in the Lower Missouri River, USA to determine the most precise types of gear to collect all sizes of sturgeon so that refinements of longterm monitoring protocols can be made. A total of 1947 net sets or trawls collected 8743 shovelnose sturgeon, with 67% of the fish collected during winter gill netting (16% of total samples). Mean coefficient of variation (CV) among all months for juvenile (age 3 and younger; <250 mm fork length) sturgeon was highest for gill nets and lowest for otter trawls (P = 0.0008). Mean CV of subadult and adult shovelnose sturgeon ( ‡250 mm) was highest in hoop nets compared to other gear types (P = 0.0002). All gear and mesh sizes collected the most common sizes of shovelnose sturgeon (500-600 mm), but only otter trawls and trammel nets collected fish <150 mm. The higher precision of winter gill nets and summer otter trawls led to fewer samples needed to detect changes in CPUE as compared to hoop and trammel nets. Sampling only with types of gear that do not collect younger shovelnose sturgeon may hinder management decisions that rely on recruitment trends to determine the effects of management actions (e.g. channel modifications).
Occupancy modeling was used to determine (1) if detection probabilities (p) for 7 regionally imperiled Missouri River fishes (Scaphirhynchus albus, Scaphirhynchus platorynchus, Cycleptus elongatus, Sander canadensis, Macrhybopsis aestivalis, Macrhybopsis gelida, and Macrhybopsis meeki) differed among gear types (i.e. stationary gill nets, drifted trammel nets, and otter trawls), and (2) how detection probabilities were affected by habitat (i.e. pool, bar, and open water), longitudinal position (five 189 to 367 rkm long segments), sampling year (2003 to 2006), and season (July 1 to October 30 and October 31 to June 30). Adult, large-bodied fishes were best detected with gill nets (p: 0.02-0.74), but most juvenile large-bodied and all small-bodied species were best detected with otter trawls (p: 0.02-0.58). Trammel nets may be a redundant sampling gear for imperiled fishes in the lower Missouri River because most species had greater detection probabilities with gill nets or otter trawls. Detection probabilities varied with river segment for S. platorynchus, C. elongatus, and all small-bodied fishes, suggesting that changes in habitat influenced gear efficiency or abundance changes among river segments. Detection probabilities varied by habitat for adult S. albus and S. canadensis, year for juvenile S. albus, C. elongatus, and S. canadensis, and season for adult S. albus. Concentrating sampling effort on gears with the greatest detection probabilities may increase species detections to better monitor a population's response to environmental change and the effects of management actions on large-river fishes.
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