Robin L. Hofpar and Edward J. Peters, Department of Forestry, Fisheries and Wildlife, University of Nebraska-Lincoln, Lincoln, Nebraska 68583-0814, (402) 472-0825.

Three species of sturgeon have been documented in Nebraska (Nebraska Game and Parks Commission, 1993), including shovelnose sturgeon (Scaphirhynchus platorynchus), pallid sturgeon (Scaphirhynchus albus), and lake sturgeon (Acipenser fulvescens). Declines of all North American river sturgeon populations (Snyder, 1994) and the listing of pallid sturgeon as a Federally Endangered Species (Dryer and Sandvol, 1992) has promoted interest in their habitat selection, food habits and overall biology. The objectives of our research in the lower Platte River were to 1) Document the population structure and distribution of sturgeon using the lower Platte River, 2) Document sturgeon habitat use and food habits, and 3) Determine if the Platte River is acting as a spawning area for sturgeon. Because we still have a lot of data analysis to do before all results are in, a couple aspects of this research were omitted from this report, including results from larval drift net samples and the bulk of our radio telemetry work.

METHODS

To study sturgeon populations in the lower Platte River, six sampling sites were established between the confluence of the Platte and Missouri rivers and the mouth of the Loup River at Columbus, Nebraska approximately 160 km upstream. Other sites include North Bend, Fremont, the mouth of the Elkhorn River, and Louisville, Nebraska (figure 1). Each site was actually a sampling reach encompassing about 8 km of river. Within each reach tributary mouth habitats and river channel habitats were sampled from July through October, 1995 and April through September, 1996.

Fish were collected with gill nets, seines, and drift nets. Gill nets were 2 meters deep and either 15.2 or 30.5 meters long. In 1995 they were made up of four panels of alternating 2.5 and 5.1 cm mesh. In 1996 we used two additional mesh sizes; one larger at 7.6 cm and one smaller at 1.2 cm. Gill nets were drifted with the current and the distance for each drift was measured using a range finder. Catch per unit area (CPUA) was calculated as the number of sturgeon captured per hectare of river sampled at each site each month. One hectare is equivalent to a 30.5 meter net being drifted for 330 meters. The area of river sampled was calculated for each drift by adjusting net lengths to account for bunching up or tearing of nets as they drifted through channels. Seining was conducted at each site in 1995 and 1996 to collect small sturgeon and other species not captured in gill nets. Seines were 6.1 m long by 1.5 m deep with 0.64 cm mesh. Drift nets were used to sample fish eggs and larvae at each site from May through August, 1996. Drift nets were 0.50 m in diameter at the mouth of the net with 500 micron mesh. Both seines and larval nets were used in and around channels that were sampled with gill nets.

Habitat was measured at each sample area regardless of whether or not fish were captured. These measurements included water depth, mean column velocity, bottom velocity, substrate, cover, dissolved oxygen, water temperature, and turbidity. Habitats sampled with gill nets were measured at the beginning, middle, and end of each drift at the point in the channel where the middle of the net was sampling. Dissolved oxygen, water temperature, and turbidity were measured only at the mid-point of each drift.

All sturgeon captured were identified, measured (fork length in mm), weighed (in grams), examined for parasites and disease, and injected with a PIT tag. Pectoral fin rays were removed for age and growth determination and stomach contents were sampled using pulsed gastric lavage (PGL). All other fish species were identified, measured, and released. Fish captured in seines and larval nets were preserved in 10% formalin, labeled, and returned to the laboratory for identification.

RESULTS

A total of 494 gill net drifts resulted in a catch of 607 fish, which included 132 shovelnose sturgeon. One fish captured at Plattsmouth in June, 1996 was recaptured in August, 1996 approximately 1.6 km downstream from the original capture site. No pallid sturgeon or lake sturgeon were found, and no sturgeon were captured in beach seines. However, one larval sturgeon has been identified from a June 10, 1996 drift net sample; captured at the Fremont site. Channel catfish were the most abundant species captured in gill nets, comprising 23.4% of the total catch. Shovelnose sturgeon and river carpsucker were the next most abundant species at 21.1% and 16.1% respectively. All other species each comprised less than 10% of the total catch (table 1).

Length analysis of the shovelnose sturgeon population in the lower Platte River (figure 2) reveals that most fish were between 500 and 625 mm with the most abundant size class being 551 to 575 mm. Figure 3 shows the mean length at each age for shovelnose sturgeon based on examination of pectoral fin rays from 120 fish. Ages ranged from 3 to 9 years spanning the range of lengths collected, with almost 90% of these fish being between age 4 and 6. Only one fish was age 9 at 750 mm.

The relative abundance of sturgeon at each study site appears to change through time. Figures 4 and 5 show CPUA at each site from April through June and from July through September, 1996 respectively. In April CPUA was highest at Louisville while Plattsmouth and Elkhorn were lower but quite similar. No sturgeon were captured at Fremont, North Bend, or Columbus in April. In May, CPUA was highest at Plattsmouth and sturgeon were captured at all sites except Columbus. In June and July we captured sturgeon at all sites, and the Elkhorn site had the highest monthly CPUAs. In August no sturgeon were captured at North Bend or Louisville. Only the lower four sites produced sturgeon in September and the shape of this curve is nearly identical to what it was in April.

Figure 6 shows the mean daily discharge in the lower Platte River from April through September, 1996. Means were averaged over four gaging stations located in the lower Platte, and these data were provided by the U.S. Geological Survey (currently unpublished data). There was an increase in discharge between April and June which corresponds to an increase in CPUA at upstream sites during this time. Despite a significant decrease in flows in July, sturgeon were still captured at all sites in July. It wasn't until September that sturgeon were captured only at the lower four sites. This apparent downstream movement is not as clearly associated with decreasing flows as upstream movement was with increasing flows in the spring.

There appears to be no difference in habitat use from site to site or month to month. Water temperature was different from month to month. Sturgeon were documented throughout the range of water temperatures and turbidities that were recorded in the lower Platte each month. Temperatures ranged from 5.9o in October, 1995 to 30.9o Celsius in July, 1996, and turbidities ranged from 15 to greater than 500 NTU. Figure 7 shows the frequency of depths observed in habitats used by shovelnose sturgeon. Most observations were between 31 and 120 cm, with the highest number of observations between 61 and 90 cm. Figure 8 shows sturgeon occurred most frequently at mean column velocities of 46 to 75 cm/sec, and figure 9 shows they occurred most frequently at bottom velocities of 16 and 60 cm/sec, with the highest number of observations between 31 and 45 cm/sec.

Figure 10 shows the percent of all habitats sampled having silt, sand, or gravel present; and the percent of sturgeon captured over each of these substrate types. In 16% of all habitats sampled, gravel was present and about 18% of all sturgeon captured were found in channels containing gravel. Also in 16% of all habitats sampled, silt was present, which accounted for about 25% of all sturgeon captured. 100% of habitats sampled contained sand to some degree, therefore 100% of all sturgeon were captured in habitats containing sand; but it should be noted that only about 56% of all sturgeon were captured in channels with exclusively sand.

Figure 11 shows the percent of total drifts and percent of total sturgeon captured in cover vs. open channels. Cover, refers to a channel in which our gill net was hung up on debris during a drift. About 51% of our drifts were conducted in channels having cover (although not intentionally). This accounted for about 49% of all sturgeon captured. Approximately 49% of our drifts were conducted in open channels which accounted for about 51% of all sturgeon captured. There doesn't appear to be a significant difference between open channels and channels with cover regarding the number of sturgeon captured.

Figure 12 shows the percent of total drifts and percent of total sturgeon captured in tributary vs. river channel habitats. Only about 17% of our drifts were conducted in tributary habitats which accounted for about 16% of all sturgeon captured. About 83% of our drifts were conducted in river channel habitats which accounted for about 84% of all sturgeon captured. It appears as though the number of sturgeon captured is a result of the amount of effort placed into sampling each habitat type.

Aquatic invertebrates made up the entire diet of shovelnose sturgeon sampled. Table 2 is a summary of the frequency of occurrence, numbers, and numerical percentages of aquatic insect orders found. Diptera were highest in both frequency of occurrence and total organisms, accounting for over 95% of all organisms found, and nearly all of those were in the family Chironomidae. Ephemeroptera and Trichoptera were not nearly as abundant in total organisms, but both were high in frequency of occurrence. Hemiptera and Coleoptera were low in both frequency of occurrence and total organisms.

DISCUSSION

Shovelnose sturgeon are certainly an important species in the lower Platte River. Based on CPUA data, it appears that they migrate upstream in the spring and back down to some degree later in the year. Upstream movement was correlated with an increase in discharge between April and June of 1996. We also saw a similar trend regarding upstream movement using radio telemetry. In late September and early October, when we expected to see fish moving downstream, all four fish with functioning transmitters moved upstream. This again appears to be correlated with an increase in discharge during this time, and the fact that day lengths and water temperatures were decreasing didn't seem to matter. Overall, increased flows appears to be the most important factor triggering upstream movement.

REFERENCES

Dryer, M. P. and A. J. Sandvol. 1993. Recovery plan for the pallid sturgeon (Scaphirhynchus albus). U.S. Fish and Wildlife Service. Bismarck, ND.

Nebraska Game and Parks Commission. 1993. Nebraska's threatened and endangered species: Pallid and lake sturgeons. Nebraska Game and Parks Commission. Lincoln, NE. Snyder, D. E. 1994. Morphological development and identification of pallid, shovelnose, and hybrid sturgeon larvae. Department of Fishery and Wildlife Biology. Colorado State University. Fort Collins, CO.

Return to 1997 Platte River Basin Ecosystem Symposium

Last updated by Darren A. Jack on 4/24/97