The California Department of Fish and Game's Trinity River Project conducted tagging and recapture operations from May 1992 through March 1993 to obtain chinook salmon (Oncorhynchus tshawytscha), coho salmon (O. kisutch), and fall-run steelhead (O. mykiss) run-size, angler harvest, and spawner escapement estimates in the Trinity River basin. We placed weirs in the Trinity River near the towns of Junction City and Willow Creek, and trapped 689 spring-run and 1,124 fall-run chinook salmon, 500 coho salmon, and 219 fall-run steelhead.
Based on tagged fish recovered at Trinity River Hatchery and on the return of reward tags by anglers, we estimated 4,030 spring-run chinook salmon migrated into the Trinity River basin upstream of Junction City Weir and that 298 (7.4%) were caught by anglers, leaving 3,732 fish as potential spawners. We estimated 14,164 fall-run chinook salmon migrated past Willow Creek Weir and that 9,584 of these fish continued up the Trinity River past Junction City Weir. Anglers harvested an estimated 472 (3.3%) of the fall-run chinook salmon that passed Willow Creek Weir, leaving 13,692 fish as potential spawners.
The coho salmon run in the Trinity River basin upstream of Willow Creek Weir was 10,339 fish, of which 5,683 continued their migration past Junction City Weir. Anglers harvested an estimated 24 (0.2%) of the coho salmon that migrated past Willow Creek Weir, leaving 10,315 fish as potential spawners.
An estimated 3,046 adult fall-run steelhead entered the Trinity River basin upstream of Willow Creek. Anglers harvested 292 (9.6%) of the adult fall-run steelhead that migrated past Willow Creek Weir, leaving 2,754 fish as potential spawners.
1. To determine the size, composition, distribution and timing of adult chinook and coho salmon, and steelhead runs in the Trinity River basin.
2. To determine the angler harvest and spawner escapements of Trinity River chinook and coho salmon, and steelhead.
The California Department of Fish and Game's (CDFG) Trinity River Project (TRP) conducts annual tagging and recapture operations for adult chinook and coho salmon, and fall-run steelhead in the mainstem Trinity River. This effort determines the composition (race and proportion of hatchery-marked or Project-tagged fish), distribution, and timing of the chinook and coho salmon, and fall-run steelhead runs in the Trinity River basin. Recaptures of hatchery-marked or Project-tagged fish are used to develop run-size, angler harvest, and spawner escapement estimates for each chinook and coho salmon, and steelhead run.
This is a continuation of studies that began in 1977 with the trapping, tagging, and recapture of fall-run chinook salmon (fall chinook), coho salmon (coho), and fall-run steelhead (steelhead) in the Trinity River in order to determine run-size and angler harvest rates. In 1978, similar studies were added to include spring-run chinook salmon (spring chinook). Steelhead were dropped from the program in 1985 through 1989, and reinstated in 1990. Results of these studies are available from California Department of Fish and Game (Heubach 1984a, 1984b; Heubach and Hubbell 1980; Heubach et al. 1992a, 1992b; Lau et al. 1994; Zuspan et al. 1985)
The earlier studies were funded variously by the U.S. Bureau of Reclamation (USBR), and with Anadromous Fish Act funds administered by the U.S. Fish and Wildlife Service and National Marine Fisheries Service. The USBR (PL 98-541) has funded the program from 1 October 1989 through the present.
Prior to the current program, all efforts to measure salmon and steelhead populations in the Trinity River basin had been restricted to portions of the upper mainstem Trinity River and certain of its tributaries, or the South Fork Trinity River and some of its tributaries (Gibbs 1956; La Faunce 1965a, 1965b, 1967; Miller 1975; Moffett and Smith 1950; Rogers 1970, 1972, 1973a, 1973b, 1982; Smith 1975; Weber 1965). These earlier efforts did not include fish which used the mainstem and tributaries of the lower Trinity River, nor attempt to determine the proportion of hatchery fish in the runs and the rates at which various runs contributed to the fisheries. To develop a comprehensive management plan for the Trinity River basin, all salmon stocks utilizing the basin must be considered.
Trapping and tagging operations were conducted by TRP personnel from May through December 1992 at the same temporary weir sites near the towns of Willow Creek and Junction City in the mainstem Trinity River that were used since 1989. The downstream site, Willow Creek Weir (WCW), was located 8.4 km upstream from the town of Willow Creek, 48.4 km upstream from the Trinity River's confluence with the Klamath River, and 131.4 km downstream from Trinity River Hatchery (TRH) (Figure 1). The upstream site, Junction City Weir (JCW), was located 9.8 km upstream from the town of Junction City, 137.1 km upstream from the Klamath River confluence, and 42.7 km downstream from TRH (Figure 1).
The WCW is used to obtain Trinity River run-size and angler harvest estimates for fall chinook, coho, and steelhead as far downstream as possible. The JCW is used to obtain run-size and angler harvest estimates of spring chinook as far downstream as is feasible during periods of high spring flows. We operated the JCW into December to obtain run-size estimates of fall chinook, coho and steelhead in the upper Trinity River basin.
We trapped at WCW from 20 August through 2 December 1992. We trapped at the JCW from 21 May through 8 December 1992.
At both sites, we attempted to trap four to six nights per week, from mid-afternoon on Monday through Friday or Sunday morning. We trapped and tagged fish only at water temperatures <21BC to avoid severely stressing the fish.
Since 1989, we have used the Bertoni (Alaskan) weir design at both sites (Figure 2). The weir was supported by wooden tripods set 2.5 m apart. Weir panels consisted of 3.0-m X 1.9-cm (10-ft X :-in) electrical conduit spaced 5.1 cm apart on center, leaving a gap of 3.2 cm between conduits. Conduits were supported by three pieces of aluminum channel arranged 0.92 m apart, that connected to the supporting tripods.
We anchored the tripods with cable attached to 1.8-m stakes driven into the stream bottom. The weir panels were angled, with the top of the weir standing 1.8 m above the river bottom (Figure 2).
The trap was made of 1.9-cm electrical conduit spaced 2.5 cm apart and welded into panels. The panels were wired together at the corners to produce a 2.4-m square box, which was bolted to a plywood floor and covered with plywood to prevent fish from jumping out. A fyke, also made of conduit panels, was installed in the trap. Its purpose was to guide the fish into the trap and prevent their escape.
The trap was placed on the upstream side of the weir. About 12 weir conduits were raised to allow fish to pass through the weir and into the trap.
A gate, inserted between two weir panels, allowed boat passage at both weirs. The gate was made of welded conduit panels with 2.5-cm spacing between conduits.
At both weirs, we identified all trapped salmonids to species, measured them to the nearest cm fork length (FL), and examined them for hook and gill-net scars, fin clips, and tags. Each untagged salmonid judged in good condition or unspawned was tagged with a serially numbered FT-4 spaghetti tag (Project-tagged). To determine angler harvest rates upstream of the weirs, a portion of these spaghetti tags bore $10 rewards while the remaining tags were non-reward. The proportion of each species receiving reward tags was inversely related to the number of each species we expected to effectively tag during the season. In no case did we reward-tag less than one-third of the fish tagged.
To determine tag shedding rates, we removed one-half of the left ventral fin from all spring chinook tagged at JCW. We gave all fall chinook and coho tagged at WCW a single 6.4-mm diameter puncture on the left operculum, while those tagged at JCW received two punctures. Tagged steelhead did not receive a secondary mark at either weir. We released all fish at the respective capture sites immediately after processing.
Each year there is a temporal overlap in the spring and fall chinook runs in the Trinity River. Since the timing of runs varies between years, each season we assign a new date separating the two runs so that numbers of spring and fall chinook used to estimate the run-size and angler harvest could be determined. To make this separation, we compared the proportions of known spring and fall chinook trapped at each weir each week. The week at which the proportion of fall chinook exceeded spring chinook was designated as the first week of the fall-run at that weir. A recovered chinook was identified as either a spring or fall chinook based on two separate criteria. First, some chinook tagged at the weirs carried coded-wire tags (CWT), placed in their snouts as juveniles at the hatchery. If these fish were recovered at the hatchery or during spawning surveys, the tag's code indicated whether they were spring or fall fish. Secondly, non-CWTed chinook tagged at the weir and recovered at the hatchery were classified as either spring or fall fish based on the date they entered the hatchery. If they entered the hatchery during the period associated with the spring run (based on CWT recoveries at the hatchery) they were considered spring chinook. Those chinook entering the hatchery during the period associated with the fall run (again, based on CWT recoveries) were considered fall chinook.
As at the weirs, there is an overlap in the migration of spring and fall chinook into TRH. To estimate the respective numbers of spring and fall chinook without CWTs entering TRH, we expanded the numbers of tags recovered from each returning CWT group by the ratio of tagged to untagged chinook salmon when they were originally released (same strain, brood year [BY], release site and date). For example, 97,569 fall chinook of CWT group 065632 plus 968,475 unmarked fall chinook were released directly from TRH in September 1987. Since there were 9.9 unmarked chinook salmon released for every CWTed chinook salmon released (968,475 unmarked/97,569 marked = 9.9), we multiplied the total TRH recovery numbers of CWTed chinook salmon of code group 065632 by 9.9 to estimate the number of unmarked chinook of that release group that returned to TRH. In doing so, we assumed that return rates to TRH of both CWTed fish and their unmarked counterparts were the same.
If more chinook salmon entered the hatchery on a particular sorting day than could be accounted for by the expansion of all of the CWT groups, we assumed the additional fish were naturally produced. We designated these fish as spring- or fall-run in the same proportions that were determined by the expansion of the CWT groups on that day.
We designated the size separating an adult fish from a grilse for spring and fall chinook, and coho based on length frequency data obtained at the two trapping sites and at TRH, compared against length data obtained from groups of CWTed fish that entered TRH whose exact age was known. Daily chinook salmon FL data from TRH were assigned to either spring or fall chinook only when the expansion of the number of CWTs indicated >90% of the chinook salmon entering TRH were from either spring or fall runs.
The length data collected at the weirs and TRH were smoothed with a moving average of five, 1-cm increments to determine the nadir separating grilse and adults.
All steelhead >41 cm FL were considered adults, and steelhead <41 cm FL captured at the weirs were assumed to be half-pounders (assumed to have migrated to the ocean). Steelhead <41 cm FL that entered TRH were classified as sub-adults, since we did not know whether they had migrated to the ocean or were resident steelhead.
River surveys were not conducted in the 1992-93 season because very few dead, tagged fish were recovered during river surveys in the previous seasons. We continued to recover dead, tagged fish at the weirs. We examined dead salmonids for tags, fin clips, and spawning condition, and measured them to the nearest cm FL. Heads of adipose fin-clipped (Ad-clipped) (potentially hatchery-marked) fish were removed for the recovery of the CWT. After examination, the carcasses were cut in half to prevent recounting.
We defined all tagged salmonids recovered dead at the weir or reported dead by anglers as tagging mortalities, if there was no evidence they had spawned and they were recovered dead <30 days after tagging. Tagged fish recovered dead more than 30 days after tagging, or those that had spawned, regardless of the number days after tagging, were not considered tagging mortalities.
We used the information from Project-tags returned by anglers to assess sport harvest. All the tags placed on fish at the weirs were inscribed with our address so anglers could return the tags to us. If, when returned, the angler failed to indicate the date and location of their catch, we requested the information in a follow-up thank-you letter. The letter also informed them of the fish's tagging date and location.
The Trinity River Fisheries Investigation Project (TFIP), another element of CDFG's Klamath-Trinity Program, conducted salmon spawner carcass surveys in the mainstem Trinity River and its spawning tributaries from Lewiston to the confluence of, and including the North Fork Trinity River (Figure 1). Staff of the TFIP routinely provided us records of the species, tag number, date, and recovery location of Project-tagged fish seen during surveys from 15 September through 17 December 1992. These recoveries are not reported in this Chapter, but are contained in Chapter I.
The TRH fish ladder was open from 1 September 1992 through 28 March 1993. Hatchery personnel conducted fish sorting and spawning operations two-days-per-week through December, and up to seven-days-per-week and twice daily from 2 January through 27 March 1992. Increased sorting frequency was an attempt to reduce predation by river otters (Lutra canadensis) on steelhead in the fish ladder and holding raceway. We considered the initial day a fish was observed during sorting as the day it entered the hatchery.
On all sorting days, salmon and steelhead entering TRH were identified to species, sexed, and examined for tags, fin clips, and secondary tagging marks. We measured all salmon to the nearest cm FL, except those that were Project-tagged fish from the weirs. Project-tagged salmon and steelhead recovered at TRH were assigned the FL initially recorded for them at the weir when they were tagged.
We removed Project-tags from unmarked (non-Ad-clipped) salmon on the initial sorting day, while Project-tags were removed from hatchery-marked (Ad-clipped) salmon the day they were spawned. On each sorting day, we gave a distinguishing fin-clip to hatchery-marked salmon before they were placed in ponds to ripen. Thus the day they initially entered the hatchery (i.e., were sorted) could be later determined when they were spawned. Salmon with a secondary tagging mark and no tag were measured to the nearest cm FL and sexed. At the end of the season, we assigned each of these secondary-marked salmon with a shed tag, the tag number from a fish of the same species, FL, sex, and weir location where they were originally tagged and released. Tag numbers of the recovered Project-tagged steelhead were recorded the initial day the steelhead were sorted but the tags were not removed.
On the day they were spawned, we removed the heads of all Ad-clipped salmon and placed each in a plastic bag with a serially numbered tab noting the date and location of recovery, species, sex, and FL. Salmon heads were given to the CDFG's Ocean Salmon Project for CWT recovery and decoding. The Ocean Salmon Project provided us with a computer file of the CWTs recovered for editing and analysis.
We estimated the number of effectively tagged fish by subtracting, from the total tagged, those fish we classified as tagging mortalities, tagged-fish recovered downstream of the tagging site, and angler-caught-and-released fish.
We determined the run-size estimates in 1992-93 by using Chapman's version of the Petersen Single Census Method:
N = (M+1) (C+1)/ (R+1) , where
N = estimated run-size, M = the number of effectively tagged fish, C = the number of fish examined at TRH, and R = the number of Project-marked fish recovered (including fish with a secondary tagging mark and no tag) in the hatchery sample.
We attempted to tag and recover enough fish to obtain 95% confidence limits within +10% of the run-size estimate. We used criteria established by Chapman (1948) to select the type of confidence interval estimator.
We examined the grilse and adult composition of the effectively tagged salmon, the sample of Project-tagged salmon recovered at TRH, and the untagged sample of salmon at TRH to determine if the run-size estimate should be stratified by grilse and adults. Run-size estimates were stratified by grilse and adult salmon when: 1) the proportions of grilse and adult salmon in each of the above samples were significantly different statistically; and 2) there were sufficient grilse and adult salmon recovered in the Project-tagged sample at TRH to obtain 95% confidence limits of +10% of each of the stratified portions of the run-size estimate.
If we were not able to stratify the salmon run-size estimate by grilse and adults, we used the proportions of grilse and adult salmon trapped at each weir to estimate the numbers of grilse and adults comprising the run upstream of that respective weir.
All steelhead run-size estimates were for adults only. This year, we made independent estimates of naturally and hatchery-produced steelhead. Commencing with the 1989 BY, all TRH-produced steelhead have been fin-clipped. This allowed us to distinguish naturally produced (non-fin-clipped) from hatchery-produced (fin-clipped) steelhead at the weirs. We used the proportions of non-fin-clipped and fin-clipped steelhead observed at each weir to estimate the numbers of naturally and hatchery-produced steelhead in the run upstream of that respective weir.
For the run-size estimates, we assumed that: 1) fish trapped and released at a weir were a random sample representative of the population; 2) tagged and untagged fish were equally vulnerable to recapture at TRH; 3) all Project tags and secondary tagging marks were recognized upon recovery; 4) tagged and untagged fish were randomly mixed throughout the population and among the fish recovered at TRH; and 5) we accounted for all tagging mortalities.
Generally, anglers returned reward tags at higher rates than non-reward tags. When this was the case, we used only reward tag returns to determine harvest rates. When non-reward tags were returned at higher rates than reward tags, we combined the two to determine harvest rates.
We computed the harvest rate for each species (and race of chinook) by dividing the respective number of angler-returned tags by the number of fish we effectively tagged.
We made independent harvest rate estimates for grilse and adult salmon.
The assumptions for the numbers of effectively reward- and non-reward-tagged fish released were the same as those for determining the run-size estimate (See "Run-size Estimates", above).
We estimated the numbers of fish harvested upstream of each weir by multiplying the harvest rate (for each species and race) by the respective run-size upstream of each weir.
The mean FLs of samples were compared statistically using a Student's t-test with the assumption of unequal variances (Dixon and Massey 1969). We did not conduct comparisons for sample sizes <20 fish and differences in such cases were not considered statistically different. We analyzed the percentages or ratios of adults and grilse, marked and unmarked fish, and the angler return of non-reward and reward tags in samples by Chi-square. A continuity correction (Yates correction) was used for contingency tables of one degree of freedom (Dixon and Massey 1969).
Weekly sampling data collected by Project personnel at the weirs are presented in Julian week (JW) format. Each JW is defined as one of a consecutive set of 52 weekly periods, beginning 1 January, regardless of the day of the week on which 1 January falls. The extra day in leap years is included in the ninth week (Appendix 1). This procedure allows inter-annual comparisons of identical weekly periods.
