Growth rates of young-of-year shovelnose sturgeon in the Upper Missouri River By P. J. Braaten1 and D. B. Fuller2 1U.S. Geological Survey, Columbia Environmental Research Center, Fort Peck Project O���ce, Fort Peck, MT 2Montana Department of Fish, Wildlife and Parks, Fort Peck, MT, USA Summary Information on growth during the larval and young-of-year life stages in natural river environments is generally lacking for most sturgeon species. In this study, methods for estimating ages and quantifying growth were developed for field-sampled larval and young-of-year shovelnose sturgeon Scaphirhynchus platorynchus in the upper Missouri River. First, growth was assessed by partitioning samples of young-of-year shovelnose sturgeon into cohorts, and regressing weekly increases in cohort mean length on sampling date. This method quantified relative growth because ages of the cohorts were unknown. Cohort increases in mean length among sampling dates were positively related (P 0.05, r2 0.59 for all cohorts) to sampling date, and yielded growth rate estimates of 0.80��� 2.95 mm day)1 (2003) and 0.44���2.28 mm day)1 (2004). High- est growth rates occurred in the largest (and earliest spawned) cohorts. Second, a method was developed to estimate cohort hatch dates, thus age on date of sampling could be determined. This method included quantification of post-hatch length increases as a function of water temperature (growth capacity mm per thermal unit, mm TU)1), and summation of mean daily water temperatures to achieve the required number of thermal units that corresponded to post-hatch lengths of shovelnose sturgeon on sampling dates. For six of seven cohorts of shovelnose sturgeon analyzed, linear growth models (r2 ��� 0.65, P 0.0001) or Gompertz growth models (r2 ��� 0.83, P 0.0001) quantified length-at-age from hatch through 55 days post-hatch (98���100 mm). Comparisons of length-at-age derived from the growth models indicated that length-at-age was greater for the earlier-hatched cohorts than later-hatched cohorts. Estimated hatch dates for different cohorts were corroborated based on the dates that newly- hatched larval shovelnose sturgeon were sampled in the drift. These results provide the first quantification of growth dynamics for field-sampled age-0 shovelnose sturgeon in a natural river environment, and provide an accurate method for estimating age of wild-caught individuals. Methods of age determination used in this study have applications to sturgeons in other regions, but require additional testing and validation. Introduction Research focusing on sturgeon has become a high priority in many aquatic systems due to the declining population status of sturgeons and the need for basic life history information relevant to management and recovery efforts (Secor et al., 2002). Studies on the adult life stage of sturgeons have provided knowledge of spawning locations and movements, and the influence of natural and anthropogenic factors on these life history attributes (Hurley et al., 1987 Auer, 1996 Bramblett and White, 2001 Paragamian and Kruse, 2001 Paragamian and Wakkinen, 2002). Juvenile sturgeons have also been the focus of research efforts because this life stage is frequently used to augment declining populations, and infor- mation on demographic parameters of juveniles (e.g. growth, survival) is necessary for estimating recruitment to the adult and spawning life stages (Ireland et al., 2002a,b Smith et al., 2002). Similarly, recognizing that growth and mortality dynamics during the larval life stage significantly influence recruitment in fishes (Houde, 1987), focused laboratory research has been implemented to examine factors influencing these early life processes in sturgeon (Gisbert and Williot, 1997 Van Eenennaam et al., 2001 Deng et al., 2002, 2003 Gawlicka et al., 2002 Nathanailides et al., 2002 Hardy and Litvak, 2004). Studies that quantify growth of naturally produced larvae and young-of-year sturgeon in natural habitats are rare. As a consequence, little information is known about sturgeon growth dynamics in natural settings where biotic (e.g. compe- tition) and abiotic (e.g flow regime, water temperature) processes cumulatively interact to influence growth. Lack of growth information for sturgeons during the early life history stages can be attributed to two primary factors. First, larval and young-of-year life stages are di���cult to sample in natural habitats (Secor et al., 2002) unless rearing areas are specifically known. In the Missouri River and Mississippi River, research- ers have only recently identified habitats where large concen- trations of young sturgeon are found these researchers have developed effective techniques to sample young life stages of sturgeon (e.g. Braaten and Fuller, 2003, 2004 Herzog et al., 2005). Second, whereas otolith microstructure techniques have enabled researchers to quantify daily ages and growth of larval and young-of-year individuals in many fish species (Jones, 1992), similar technology does not exist for sturgeons. The objective of this study was to quantify growth of naturally produced larval and young-of-year shovelnose stur- geon Scaphirhynchus platorynchus in the upper Missouri River during 2003 and 2004. As part of this objective, a technique was developed to estimate hatch dates and ages of unknown- age young-of-year shovelnose sturgeon based on water tem- perature and known developmental rates of larval sturgeons. Materials and methods Study area The Missouri River study area was located in western North Dakota between the Yellowstone River confluence at river km 2547 (river km is the distance upstream from the confluence of J. Appl. Ichthyol. 23 (2007), 506���515 Journal compilation �� 2007 Blackwell Verlag, Berlin No claim to original US Government works ISSN 0175���8659 Received: October 20, 2005 Accepted: June 16, 2006 doi:10.1111/j.1439-0426.2006.00821.x U.S. Copyright Clearance Centre Code Statement: 0175���8659/2007/2304���0506$15.00/0 www.blackwell-synergy.com

the Missouri River near St Louis, MO) and river km 2499 (Fig. 1). Discharge in this reach of the Missouri River varies temporally, and is dependent on cumulative input from the free-flowing Yellowstone River and regulated flow releases from Fort Peck Dam located on the Missouri River at river km 2850. Discharge inputs from the Yellowstone River are relatively high (2003 peak discharge �� 1370 m3 s)1, 2004 peak discharge �� 705 m3 s)1 USGS stream gage number 06329500 USGS, 2004a, 2005a) during spring and early summer in accordance with mountain snow-melt patterns (Bowen et al., 2003). Discharge in the Yellowstone River typically decreases during July, August, and September, and mean daily discharge for these months during 2003 and 2004 did not exceed 272 m3 s)1 (USGS, 2004a, 2005a). Regulated releases from Fort Peck Dam alter natural flow patterns in the Missouri River, as maximum flows associated with snow-melt have been eliminated (Bowen et al., 2003). Maximum dis- charge in the Missouri River did not exceed 444 m3 s)1 during spring and early summer 2003 and 2004 (USGS stream gage number 06185500, USGS, 2004b, 2005b) mean daily discharge for July, August, and September was 218 m3 s)1. Water temperature in the study area exhibits seasonal patterns, and averages 17.6��C between April and October (mini- mum �� 8.8��C, maximum �� 27.8��C Braaten and Fuller, 2004). The study area of the Missouri River is characterized by heterogeneous and temporally dynamic habitat conditions typical of natural large river environments. Semi-permanent islands that divide the main channel occur throughout the study area, but smaller islands and numerous sand bar complexes are exposed primarily during low-flow conditions. During low flows, multiple side channels may occur in conjunction with sand bar complexes. In addition, backwater areas associated with stream margins or sand bar island complexes are also present. Sand composes 77% of the substrate in the main channel (Galat et al., 2001). Sampling for larval and young-of-year shovelnose sturgeon Sampling for larval and young-of-year shovelnose sturgeon was conducted at four sites in the Missouri River (Fig. 1). Each site was sampled at about weekly intervals during 2003 (7, 12, 18, 26 August 3 September) and 2004 (21, 29 July 4, 11, 17, 24 August 1, 7 September). Sampling was initiated earlier in 2004 based on our findings from 2003 that indicated young- of-year shovelnose sturgeon were present earlier during the year (see Results). Sampling at each site was conducted in the outside bend thalweg (erosional zone), along the inside bend (depositional zone), and in the channel crossover (i.e. trans- ition area where the thalweg crosses over from one bank to the other bank). The sampling scheme was slightly modified at the most downstream site where other main channel habitats were also sampled. Larval and young-of-year shovelnose sturgeon were sam- pled using a benthic trawl that measured 2.0-m wide, 0.5-m high, and 5.5-m in length. The trawl was fitted with an outer chafe net (3.81-cm mesh) and an inner fish-collecting net (0.32- cm mesh). The trawl was attached to the bow of the boat with ropes, lowered to the riverbed, and towed downstream along the riverbed for 4���5 min (trawl distance of 250���350 m) as the boat motored in reverse. If a young-of-year shovelnose sturgeon was sampled in an individual habitat, then a maximum of eight additional trawls were conducted in the same location. Although repeated sampling removed individ- uals from the population and could potentially influence density-dependent mechanisms of growth through time, repea- ted sampling of habitats where shovelnose sturgeon were concentrated was necessary to obtain adequate numbers for the growth analysis. After trawling was completed, total length (TL excluding the caudal filament) was measured to the nearest 1.0 mm. Fork length is usually measured to quantify length of shovelnose sturgeon however, TL was measured because larval and young-of-year shovelnose sturgeon do not have a well-defined fork in the caudal fin. Growth and age determinations Growth rates and ages of young-of-year shovelnose sturgeon were estimated using multiple techniques. Length-frequency histograms were constructed for each sampling date to facilitate identifying groups of fish that had similar lengths or were clustered within length groups (e.g. cohorts) across the total length range. For lengths less than about 40 mm, groups of young-of-year shovelnose sturgeon were partitioned into individual cohorts when the groups were separated by at least 5 mm. A few exceptions to this process occurred when it was di���cult to assign an individual to a cohort. When this occurred, the individual was randomly assigned to either of the adjacent cohorts. After the initial designation of cohorts, breaks in the length frequency distributions among weekly sampling intervals provided an estimate of cohort growth through time. Cohort mean length was calculated for each weekly sampling interval. Regression analysis was used to estimate relative growth rates (mm day)1) for each cohort by regressing cohort mean length on day of year that sampling occurred. For this analysis, growth rates were considered relative because ages of fish within the cohorts were not known. Ages of young-of-year shovelnose sturgeon were estimated for each cohort using a two-step process. Although cohort relative age and length could be estimated from days elapsed between consecutive sampling dates, an estimate of hatch date and initial cohort mean age on the initial date when a cohort was sampled was necessary to derive a growth function between age and length. First, relationships between length and age for two groups of known-age shovelnose sturgeon (0��� 11 days post-hatch dph) reared in hatcheries were developed for specific water temperature regimes (Table 1). Models for both groups were highly significant, and indicated a strong positive relationship between length and age (P �� 0.004, r2 �� 0.99 Table 1). The growth models were used to estimate Montana Missouri River 8 km Yellowstone River To Lake Sakakawea 4 3 2 1 North Dakota Fig. 1. Missouri River study area and young-of-year shovelnose sturgeon sampling sites (boxes and numbers) in 2003 and 2004 Growth of young-of-year shovelnose sturgeon 507