Rivers and Streams Investigations

Transcription

1 Rivers and Streams Investigations Brian J. Flatter, Regional Fisheries Biologist Lance Hebdon, Regional Fisheries Biologist Jeff Dillon, Regional Fishery Manager ABSTRACT Three sections of the lower Boise River between Barber Park and the Parkcenter Bridge were electrofished in November to compare species composition, abundance, and size structure. Sections were chosen to allow for upstream and downstream comparisons of fish populations with the Quality Management regulation section. Wild rainbow trout abundance estimates were very similar between sections. The estimates for wild rainbow trout 1 mm ranged from 735 to 1,387 fish/km. Wild rainbow trout QSD estimates ranged from 3% to 5%. Our density estimate of wild rainbow trout 1 mm in one section was 2.5 times higher than the highest historical survey. Abundance estimates for brown trout 1 mm ranged from 96/km to 199/km. Very few brown trout between 17 and 24 mm were caught. Brown trout QSD estimates ranged from 19% to 42%. Brown trout were observed spawning in the river and side channels. Mountain whitefish density was 5.5 fish 1 mm/1 m 2, which was about one-third of the observed density in Our results indicate that there was little difference between the size structure or density of wild rainbow trout and brown trout in the special regulation section when compared to adjacent areas of the river with similar habitat. Lower Boise River Electrofishing STUDY AREAS Three sections of the lower Boise River between Barber Park and the Parkcenter Bridge were electrofished to compare species composition, abundance, and size structure. Our results were compared to previous estimates and we also evaluated the special regulation section. The sections were defined as the upper, the middle, and the Municipal Park section (hereafter referred to as the lower section). The upper and lower sections are managed using general regulations, while only two trout 355 mm may be kept each day in the middle section (Quality Management regulation). The upper, middle, and lower sections were.97, 1.1, and 1.3 km in total length, respectively. The lower section had been d previously in 1988 (Mabbot and Holubetz 199), 1992 (Allen et al. 1995), 1994 (Allen et al. 2b), and 1995 (Allen et al. 1998). Sections were chosen to allow for upstream and downstream comparisons of fish populations with the Quality Management regulation section. Other than a slight boundary change in 22 to help clarify the special regulation area, the Quality Management regulation has been in effect since 1996.

2 METHODS Previous Boise River fishery surveys used drift boats (Holubetz and Mabbott 199, Allen et al. 1995), gang probes (Allen et al. 2b and 1998), and rafts (Allen et al. 1999). Sampling efforts with these gears were typically inefficient and labor intensive. For sampling efforts in 24 we used an aluminum canoe fitted with two throwable probes. The canoe served as the cathode and carried our generator, VVP-15, a plastic garbage can for holding fish, and all of our other equipment. Crews consisted of four netters, two probe operators, and one canoe/dead man switch operator. The area for each section was calculated by using maps to determine section length, and mean widths (six measurements per section) were calculated using measurements taken with a hand-held laser range finder. GPS coordinates (NAD27 Conus, in UTM s) were collected at the upper and lower boundaries of each section using a Garmin Rino model 12 handheld GPS receiver. and recapture runs were completed by making a single downstream pass through each section. The first mark run was completed on November 8 in the upper and middle sections, and November 9 in the lower section. runs were completed on the upper and middle sections on November 15 and the lower section on November 16. Boise River flows (below Lucky Peak Dam) were 275 cfs during the mark and recapture runs. Fish were marked using a paper punch in the upper, middle, and lower area of their caudal fin to correspond with the section in which they were collected. We used the different marks to detect general movements between sections. All mountain whitefish, rainbow trout, and brown trout Salmo trutta were marked. The total length of all rainbow and brown trout were measured to the nearest millimeter, as were a sub of mountain whitefish from all sections. Fish recaptured in a section differing from where they were originally marked were removed from the pool of fish used to calculate population estimates, and deleted from total marked where they came from. Population estimates for rainbow and brown trout were calculated using 5. for Windows (MDFWP 1994). The program calculated estimates based on log likelihood and Modified Peterson methods. Chi-square analysis was used to determine if log likelihood estimates were valid. If not, the Modified Peterson method was used. Population estimates and 95% confidence intervals were calculated by 1 mm length s (recaptures permitting) and summed for estimates of fish 1 mm and larger. Population estimates were used to calculate fish/ha, fish/1 m 2, and standardized by to fish/km. Quality stock density (QSD) estimates were calculated for wild rainbow and brown trout by section using 46 mm for the quality length, and 23 mm for the stock length. Because only a sub of the mountain whitefish collected was measured, population estimates were generated by hand using the Modified Peterson method.

3 RESULTS AND DISCUSSION Lower Boise River Electrofishing recapture data for mountain whitefish, brown trout, and rainbow trout by centimeter size are provided in Appendix C. Physical descriptions of the electrofishing sites can be found in Appendix D. Wild rainbow trout abundance estimates were very similar between sections. The estimates for fish 1 mm were 1,387, 735, and 1,149 fish/km in the upper, middle, and lower sections, respectively (Table 5). QSD estimates were 3%, 4%, and 5% in the upper, middle, and lower sections, respectively. The lower section held an estimated 19 wild rainbow trout 46 mm/km, 27% higher than the middle (special regulation) section. There was an average of 11 wild rainbow trout 46 mm/km when the catch from all sections was combined (Table 6). Very few wild rainbow trout were caught between 14 and 22 mm (Figure 15). Our density estimates for wild rainbow trout 1 mm were 4, 2, and 3 fish/1 m 2 in the upper, middle, and lower sections, respectively (Table 5). Abundance estimates for brown trout 1 mm in the middle and lower sections were 199/km and 96/km, respectively. There were insufficient recaptures in the upper section to calculate a brown trout estimate. There were 41 and 33 brown trout 46 mm/km in the middle and lower sections, respectively (Table 6). Very few brown trout between 17 and 24 mm were caught (Figure 16). QSD estimates ranged from 19% in the upper site, to 42% in the lower section (Table 6). The brown trout density in the lower section was.3 fish 1 mm/1 m 2, which was slightly lower than in 1995 but higher than other surveys (Table 7). Brown trout redds were observed in the river adjacent to Municipal Park and the Warm Springs Golf Course, and in the Goodwin Ditch near Warm Springs Mesa. A total of eight redds were observed. The GPS locations of the spawning areas can be found in Appendix E. Abundance estimates for mountain whitefish were 1,739, 2,7, and 1821 fish/km in the upper, middle, and lower sections, respectively (Table 5). All fish collected were used in the estimate and none less than 12 mm were caught (Figure 17). Mountain whitefish density in the lower section was 5.5 fish 1 mm/1 m 2, which was about one-third of the observed density in 1995 and slightly higher than the upper and middle sections (Tables 5 and 7). In addition to the above mentioned species, we observed one 187 mm kokanee O. nerka kennerlyi, many largescale sucker Catostomus macrocheilus and bridgelip sucker C. columbianus, several chiselmouth Acrocheilus alutaceus, and many sculpin Cottus spp. Previous surveys of the Boise River have used a variety of gear with little similarity or comparable results to our work. Past sampling efforts either focused on very short reaches with limited sampling in deeper habitats, or floated through long sections covering a very small portion of the available fish habitat. Our equipment allowed us to most of the river and obtain a very good snapshot of the fish populations, and should be easy to duplicate in the future. However, it made comparisons with past work very difficult. At best, the only long term comparison we were able to make was the density and size structure of fish in the lower section. Our density estimate of wild rainbow trout 1 mm in the lower section was 2.5 times higher than the highest previous survey (Table 7). The low densities observed in surveys conducted between 1988 and 1995 were likely the result of the extended drought southwestern

4 Idaho had been experiencing since the late 198 s. Although we are still experiencing a drought, Boise River winter flows were considerably higher between 1995 and 2 than in the previous twelve years and have likely resulted in better over winter survival of all fish (Figure 18). What we saw in 24 was probably not influenced by the winter flows. The winter flows of were considerably higher ( cfs; 255 cfs average) than occurred in the three years preceeding the 1995 estimate ( cfs; 186 cfs average). The length frequencies of wild rainbow trout, brown trout, and whitefish all indicate multiple missing year classes. In the case of wild rainbow and brown trout, fish between mm and mm, respectively, are essentially nonexistent (Figures 15 and 16). This may be a result of unsuccessful spawning in the spring of 22, limited spawning due to poor adult survival during the fall/winter of 21-22, or simply the size gap between age-1 and age-2 fish. Growth data needs to be collected before concluding we are missing an age class. During this time frame, winter flows were lower than they had been in the previous six years (Figure 18). In very similar winter conditions between 1988 and 1996, a total of five wild rainbow trout and 12 brown trout >21 mm (6% and 29% of the total from all five surveys, respectively) were collected in five surveys of the lower section (Figures 19 and 2). Although few mature wild rainbow and brown trout were collected between 1988 and 1996 (assuming maturity at 35 mm for females and 25 mm for males), the length frequencies from those surveys indicate that there was recruitment in those exceptionally dry years. In contrast, 73% of the wild rainbow and 7% of the brown trout in our survey of the same section were >21 mm following seven out of eight years with winter flows 29 cfs. The length frequency for mountain whitefish revealed a similar trend. There were more mountain whitefish 33 mm in 24 than any previous survey (Figure 21). Another notable feature of the observed size structure for wild rainbow and brown trout was the rapid decline in abundance after approximately 27 mm. This is likely due to a combination of angler harvest and one or more limiting environmental factor, such as low winter flows. Higher than anticipated natural mortality may have contributed to the decline, however we collected no age and growth data to evaluate. Although we are unsure what the optimal winter flow should be, we know that the river conditions between October 1999 and March 23 resulted in improved populations of rainbow and brown trout. Given the size structure and densities of wild rainbow trout currently in the river, the population has and will persist with a few years of lost recruitment. As the missing year classes move through the populations, the likely noticeable difference will be fewer large fish in the size structure (lower QSD) for a few years until the stronger year classes move in to fill the void. Because previous sampling was so different than ours, it is very difficult to make conclusions based on a comparison of our work and historical efforts. However, we suspect that persistent low winter flows will further change the size structure of the population we observed in 24. This conclusion should be validated with standardized sampling in the future. Overall, the wild rainbow trout QSD s were very low but increased slightly as we worked downstream. The QSD was highest in the lower section, and for comparison, was slightly higher than measured in the Big Wood River (near Hailey) in 23. Much like the lower Boise River, the Big Wood River is very productive, contains brown trout, and has had a Quality Trout regulation in effect for more than eight years (Table 8). The gradient of QSD values observed in wild rainbow trout was also apparent with brown trout (Table 6). Although a QSD of 19 is considered good, the upper section held considerably fewer large brown trout than the other sections, resulting in a 45% lower QSD. The gradient of QSD values may be explained by the most noticeable difference in fish habitat between the three sections, water depth. The upper section consisted of sporadic deep pools and woody debris, and predominantly gradually

5 sloping shorelines. As a result, the upper section was more suited for juvenile rearing than for adults. As we moved downstream, the water depth, in general, gradually increased to the point of exceeding the limits of our equipment and crew. Most pools in the upper section were relatively easy to thoroughly, as were many in the middle section. The lower section proved to be the most problematic, with pools frequently exceeding 2.4 meters in depth. Many of our larger rainbow and brown trout were collected from, or in close proximity to, these deep areas. Our sampling and observations suggests that while the upper river is well suited for juvenile rearing, fish likely move downstream to occupy deeper habitats as they grow larger. Wild rainbow trout abundance (fish/km 1 mm) in the middle section was very similar to the South Fork of the Boise River (SFBR) near Cow Creek. The lower section was 4% higher than the SFBR (Table 8). For fish 3 mm, the SFBR had abundance 48% and 69% higher per km than the middle and lower sections, respectively. When comparing the middle and lower sections to the Big Wood River, abundance was 82% and 73% higher in the Big Wood River, respectively. For wild rainbow trout 3 mm, the Big Wood River had 52% and 71% higher abundance than the middle and lower sections, respectively. There has been considerable debate in the local angling community about the effectiveness of the special regulation area (middle section) in the lower Boise River. Our results indicate that there is little difference between the size structure and density of wild rainbow trout in the special regulation section when compared to adjacent areas of the river with similar habitat (the lower section). frequencies indicate that recruitment is occurring and there are very similar densities of wild rainbows and brown trout 356 mm in the middle and lower sections (Table 6). The main difference in size structure between the sections was the density of fish 35 mm. The densities of wild brown and rainbow trout 35 mm in the lower section were 38% and 45% lower than the middle section, respectively (Table 6).

6 Upper n = mm Middle n = mm mm Lower n = 356 Figure 15. Lower Boise River wild rainbow trout length frequency by section, November 24.

7 Upper n = mm mm Middle n = Lower n = mm Figure 16. Lower Boise River wild brown trout length frequency by section, November 24.

8 Upper n = mm mm Middle n = mm Lower n = 323 Figure 17. Lower Boise River mountain whitefish length frequency by section, November 24.

9 4 Cubic feet/ second Mean flow Oct-Mar Year Figure 18. Lower Boise River mean winter stream flow at Glenwood Bridge,

10 n= n= n= n= n= n= Figure 19. Boise River wild rainbow trout length frequencies for Municipal Park area,

11 n= n= n= n= n= n= Figure 2. Lower Boise River wild brown trout length frequencies for Municipal Park area,

12 n= n= n= n= n= n= Figure 21. Lower Boise River mountain whitefish length frequencies for Municipal Park area,

13 Table 5. Population and density estimates for rainbow trout, brown trout, and mountain whitefish 1 mm in the Boise River, 24. Species Section Regulations N^ ± 95% CI /1m 2 /km /ha /mile Wild rainbow trout Upper General 1,34 ± , ,233 Middle 2 Trout None < 14" 87 ± ,187 Lower General 1,477 ± , ,846 All rainbow trout* Upper General 2,56 ± , ,427 Middle 2 Trout None < 14" 89 ± ,19 Lower General 1,549 ± , ,936 Brown trout Upper General - Insufficient s - Middle 2 Trout None < 14" 218 ± Lower General 124 ± Mountain whitefish Upper General 1,68 ± , ,8 Middle 2 Trout None < 14" 2,273 ± , ,343 Lower General 2,342 ± , ,928 * Includes wild and hatchery rainbow trout

14 Table 6. Boise River rainbow trout and brown trout density estimates by section and size, November 24. Species Site Regulations Abundance 35mm /km Abundance 356mm/km Abundance 46mm /km QSD 46mm/ 23mm Wild rainbow trout Upper General 2 16 No recaptures 3% Middle 2 Trout None < 14" % Lower General % Total All rainbow trout* Upper General No recaptures 4% Middle 2 Trout None < 14" % Lower General % Total Brown trout Upper General ** ** ** 19% Middle 2 Trout None < 14" % Lower General % Total * Includes wild and hatchery rainbow trout ** No estimate due to low recaptures

15 Table 7. Comparison of fish densities ( 1 mm/1 m 2 ) in the Municipal Park area of the Boise River, Pass Removal Electrofishing a - b Species Jan 1988 Mar 1992 Mar 1994 Mar 1995 Nov 24 Wild rainbow trout All rainbow Brown trout Mountain whitefish a Population estimate calculated using Microfish 3. b Population estimate calculated using 5. (log-likelihood and Modified- Peterson) Table 8. Comparison of wild rainbow trout densities in three southern Idaho rivers. Site Sample date Regulation Size class fish/km fish/1m 2 fish/ha QSD South Fork Boise River (near Cow Creek Bridge) 1/3 2 Trout none < 2" 1mm mm % Big Wood River (Near Hailey) 1/3 2 Trout none 12"- 1mm 4, ,786 16" 3mm % Boise River Middle section 11/4 2 Trout none < 14" 1mm mm % Lower section 11/4 General 1mm 1, mm %

16 Lower Boise River Electrofishing 1. Take every opportunity to pass on the information learned from the 24 survey to angling s. 2. Repeat the survey in 27. Collect scales/otoliths to determine age structure of brown trout, wild rainbow trout, and mountain whitefish. Duplicate population surveys every 3-4 years to monitor trends. 3. Populations of wild trout appear to have improved since 1995, possibly due to increases in average winter flows in recent years. The Department should continue to advocate the highest possible winter flows to protect or further enhance populations.

17 Appendix C. -recapture data for rainbow trout, brown trout, and mountain whitefish in the Boise River, November 24. All rainbow trout - Upper Section s s <

18 Appendix C. Continued. All rainbow trout - Middle Section s s <

19 Appendix C. Continued. All rainbow trout - Lower Section s s <

20 Appendix C. Continued Wild rainbow trout - Upper Section s s <

21 Appendix C. Continued. Wild rainbow trout - Middle Section s s <

22 Appendix C. Continued. Wild rainbow trout - Lower Section s s <

23 Appendix C. Continued. Brown trout - Upper Section s s <

24

25 Appendix C. Continued. Brown trout - Middle Section s s <

26 Appendix C. Continued. Brown trout - Lower Section s s <

27 Appendix C. Continued. Mountain whitefish Section s Upper Middle Lower

28 Appendix D. Physical descriptions of electrofishing sites in the lower Boise River, 24. Boundaries (UTM's) Section name Upstream Downstream (km) Mean Width (m) Comments Upper E E N N Starts at first diversion below Park and ends where Park C Blvd meets the Boise River ( Crossing). Middle E N E N Starts at Warm Springs Dive (near Warm Springs Mesa) a at Cottonwood Condominium Lower E E N N Starts at first riffle upstream o East Boise River Footbridge at first riffle upstream from th Parkcenter Bridge (near the M Nature Center).