Staff of the California Department of Fish and Game's Trinity Fisheries Investigations Project conducted a mark-and-recovery, salmon spawner survey of a portion of the mid-Trinity River basin from 16 September through 19 December 1991. We surveyed the mainstem Trinity River from the upstream limit of anadromous migration at Lewiston Dam to a point 63.4 km downstream at the confluence of the North Fork Trinity River. Selected portions of its major tributaries that were accessible to anadromous fish were also surveyed. We examined 690 chinook salmon (Oncorhynchus tshawytscha) and 127 coho salmon (O. kisutch) carcasses during the mainstem Trinity River survey.
Chinook and coho salmon spawned throughout the entire mainstem survey section, but spawner density was highest in the uppermost 3.2 km of river, generally decreasing in a downstream direction. Few salmon spawned in the tributaries this year. We found only 29 chinook and 12 coho salmon during the tributary surveys.
Only 1.2% of the fall-run chinook salmon and none of the spring-run chinook and coho salmon females died prior to spawning. These are the lowest prespawning mortality rates for chinook salmon on record. The probable cause for the high spawning success was the low spawner escapement and resulting low spawning density in comparison to previous years.
We recovered both spring-run and fall-run chinook salmon in the survey. Spring-run chinook salmon dominated recovery until late October, thereafter fall-run fish became the predominant race. Coho salmon were first noted in the mainstem Trinity River survey during mid-October, their numbers peaked in mid-November, and they were essentially gone by mid-December.
Based on the recovery of adipose fin-clipped chinook salmon, we estimate that none of the spring-run and 66.9% of the fall-run chinook salmon spawners observed in the survey were of hatchery origin. We could not determine the proportion of spawning coho salmon which were of hatchery origin because fish from the 1988 brood year released from the hatchery were not adipose fin-clipped.
Fork lengths of adult spring- and fall-run chinook salmon from the mainstem Trinity River averaged 74.9 cm (range: 57-94 cm) and 68.8 cm (range: 52-91 cm), respectively. Adult chinook salmon composed 97.5% of the spring-run chinook salmon and 95.3% of the fall-run chinook salmon with grilse composing the remainder. Fork lengths of adult coho in the mainstem Trinity River averaged 68.5 cm (range: 58-85). Adult coho salmon composed 99.1% of the fish measured with grilse composing the remainder. Adult fall-run chinook salmon in the tributaries averaged 63.5 cm FL (range: 54-74 cm) and composed 92.9% of the fish measured, with grilse composing the remainder.
1. To determine, through a system of spawning ground surveys, the distribution of naturally spawning chinook and coho salmon in the mainstem Trinity River and its tributaries upstream of, and including the North Fork Trinity River.
2. To determine the incidence of pre-spawning mortality among naturally spawning salmon in the mainstem Trinity River and its tributaries upstream of, and including the North Fork Trinity River.
3. To determine the size, sex composition, and incidence of marked and tagged individuals among the naturally spawning populations in the mainstem Trinity River and its tributaries upstream of, and including the North Fork Trinity River.
4. To determine spawner distributions within the mainstem Trinity River upstream of the North Fork Trinity River.
This year the California Department of Fish and Game's (CDFG) Trinity Fisheries Investigations Project (TFIP) completed the twenty-fourth salmon spawner survey conducted in the mainstem Trinity River since 1942. The first three surveys (Moffett and Smith 1950, Gibbs 1956, and Weber 1965) were fishery evaluations prior to the construction of Lewiston Dam. The remaining twenty (La Faunce 1965; Rogers 1970, 1973, 1982; Smith 1975, Zuspan 1991, 1992a, 1992b; and work by Miller and Stempel [Appendix 1]) were designed to evaluate the effects of the existing dam on the salmon resource.
In 1984, The Trinity River Basin Fish and Wildlife Management Program was enacted by Congress (Public Law 98-541). This law appropriated approximately $57 million to be spent for fishery and wildlife restoration, and monitoring within the Trinity River basin.
This survey, and those scheduled for following years by CDFG's TFIP, will help to evaluate the effectiveness of increasing spawning and holding habitat within the basin through habitat improvement efforts that are part of the restoration program.
Our study area included the mainstem Trinity River from its upstream limit to anadromous fish migration at Lewiston Dam (river km 180.1) to the confluence of North Fork Trinity River, 63.4 km downstream (Figure 1). Previous studies have divided the river into either a four- or seven-zone system. The seven-zone system (Table 1) was used in 1987 by the United States Fish and Wildlife Service (USFWS) (Stempel Appendix 1) and again in 1988, 1989 and 1990 by TFIP (Zuspan 1991, 1992a, 1992b). Prior to this, with the exception of Moffett and Smith 1950, all surveys were based on a system using four zones in the river reach below Lewiston Dam (Gibbs 1956; La Faunce 1965; Rogers 1970, 1973, 1982; Smith 1975; Weber 1965; and work by Miller [Appendix 1]). Our 1991-1992 data were collected based on both zone systems. We have summarized data in this report based only on the seven-zone system as it allows comparisons of different river sections in finer detail. By also recording data using the four-zone system, we will be able to compare historic and current trends in other reports.
FIGURE 1. Map of the Trinity River basin showing the mainstem spawner survey zones and areas of tributaries surveyed in the 1991-92 spawner survey.
TABLE 1. Description and lengths of river zones used in the 1991-92 mainstem Trinity River spawner survey.
River kilometers (RKM) for location references were taken from a series of 7.5 minute United States Geological Survey topographic maps (Appendix 2).
TFIP staff conducted the survey using 12-ft Avon inflatable rafts equipped with rowing frames. Raft crews consisted of a rower, and one or two personnel to recover carcasses. To increase coverage of the highly productive upper two sections, two rafts were used simultaneously, with one covering each side of the river. Carcasses were recovered on foot along the shore or, in deep water, from the rafts with long handled gigs. We surveyed the entire mainstem Trinity River study section once a week throughout the salmon spawning season.
We determined spawning condition in female salmon by direct observation of the ovaries. Fish were classified as either spawned or unspawned based on egg retention. Females which retained over 50% of their eggs were classified as unspawned. Male spawning condition was not assessed, as its determination was considered to be too subjective.
All carcasses we observed were identified by species and examined for an adipose fin-clip (Ad-clip) indicating the presence of a coded-wire tag (CWT) in their snout. To increase our likelihood of recovering all Ad-clipped fish, we passed all recovered salmon through a coded-wire tag detector. In this manner, fish that carried a coded-wire tag but had an unidentifiable adipose fin-clip were identified as an Ad-clip fish. Fish were further examined for the presence of an external tag (spaghetti tag) and an operculum punch, applied as part of an ongoing study by the Trinity River Project of the CDFG's Klamath-Trinity Program. Spaghetti tags and operculum punches (Program marks) are placed on returning adult fish by CDFG staff at two trapping and tagging stations downstream of the spawner survey area, to monitor escapement and harvest of returning adult salmonids. The spaghetti-tagged salmon also receive an identifying operculum punch in order to estimate tag shedding rates of fish tagged at the two sites. The most downstream trapping site is Willow Creek Weir, located at RKM 32.2 on the mainstem Trinity River. The other trapping site, Junction City Weir, is located in the spawner survey area at RKM 136.4. Spring-run and fall-run chinook salmon, coho salmon, and steelhead are trapped and tagged at both Willow Creek and Junction City weirs.
We classified all chinook salmon carcasses as either condition one or two, based on the extent of body deterioration. Condition-one fish were the freshest, having at least one clear eye and a relatively firm body. Condition-one fish were assumed to have died within one week prior to recovery. Condition-two fish were in various advanced stages of decomposition and assumed to have died more than one week prior to recovery. We did not count partially intact fish skeletons, because they could have represented Project-marked or condition-two fish which had already been counted and chopped in half during a previous week's survey.
All chinook salmon we recovered were further classified into four categories: 1) Ad-clipped fish; 2) Program-marked fish; 3) condition-one, unmarked fish; 4) condition-two, unmarked fish. The category assigned determined what data we collected from each fish.
We determined the species and condition (i.e. one or two) of Ad-clipped fish. Heads of Ad-clipped fish were removed and retained for later CWT recovery and decoding.
Program-marked fish were sexed and their spawning condition assessed. We removed any spaghetti tags and then cut the fish in half with a machete to prevent recounting in future weeks. Spaghetti tags had a unique number which allowed determination of the date and location of tagging.
Condition-one fish which were neither Ad-clipped nor Program-marked were flagged and returned to moving water for subsequent recovery, and a systematically collected sample of them were measured to the nearest cm fork length (FL). Flags consisted of plastic surveyor's tape wrapped tightly around a colored hog ring and affixed to the left mandible of the carcass. The surveyor's tape was wrapped so tightly around the hog ring, that it amounted to no more than a colored coating, with less than 2.5 cm of tape extending from the hog ring at any time. Flag colors were changed weekly so that, on recovery, the week of flagging could be determined. The hog rings used to attach the flagging were color-coded to indicate in which zone they were affixed, so that we could determine the incidence of carcasses drifting into another recovery zone. Chinook < 55 cm were preliminarily classified as grilse during the carcass surveys. Actual grilse to adult ratios for the whole population of chinook salmon in this year's run were determined from post-season evaluations of length frequency and CWT data. Adult and grilse salmon analysis in this report is based on the post-season size determinations.
Condition-two fish which were neither Ad-clipped or Program-marked were checked for the presence of a flag and, if possible, the sex and spawning condition were assessed. If a flag was present, the color of the flagging tape and the underlying ring were recorded, and all fish were then cut in half to prevent later recovery and re-counting of the same fish.
All coho salmon collected were measured (cm FL) and checked for the presence of Ad-clips or Program-marks. When possible, sex and spawning condition were determined and then all coho salmon were cut in half to prevent future re-counting. Coho carcasses were not used in the flagging experiment, since they would have required a separate series of flag colors to segregate them from flagged chinook salmon.
Tributaries to the mainstem Trinity River, specifically Rush Creek, Grass Valley Creek, Indian Creek, Reading Creek, Browns Creek, Weaver Creek, Canyon Creek, East Fork of the North Fork Trinity River, and the mainstem North Fork Trinity River, were surveyed on foot once a week throughout the chinook salmon spawning season. Sections surveyed for each tributary ranged in length from 0.5 to 2.5 km, and were chosen based on accessibility and their historic use by chinook salmon spawners (Figure 1). The survey began with the onset of chinook salmon spawning in each tributary and continued until spawning ended (Table 2) The lower reach of Weaver Creek was dry and inaccessible to salmon until 21 November, so the survey of that tributary was delayed until that date.
TABLE 2. Trinity River tributaries surveyed in the 1991-92 spawner survey.
We classified all identifiable chinook salmon recovered into the four categories used in the mainstem spawner survey and handled them accordingly (see page 6). However, sex and prespawning condition were assessed only for fish collected from the mainstem Trinity River. Too few fish were observed in the tributaries to compose an adequate sample and most of those observed were condition-one fish which we needed to flag for spawning escapement estimates. Coho salmon were counted and cut in half upon recovery. Chinook salmon redds, when observed for the first time, were counted and recorded.
Aerial flights and ground-truthing surveys were made of each tributary to determine the percentage of the total available spawning area within each tributary represented by each of our ongoing spawner survey zones. Flights were made during the peak of spawning activity to observe redds and locate the upstream limit of spawning. Follow-up ground-truthing surveys were made, when necessary, to make total redd counts for both the whole tributary and its spawner survey zone. The percentage of the total redds occurring in a survey zone during the aforementioned count was assumed to represent the percentage of the total spawning in each tributary that took place within the survey zone.
Chinook Salmon. We examined 690 chinook salmon carcasses during the spawner survey. These included 30 Ad-clipped fish, 73 Program-marked fish (five also ad-clipped), 251 unmarked condition-one fish which we flagged, and 270 unmarked condition-two fish. We also recaptured and re-examined 87 fish which we had flagged in previous weeks. No whole skeletons were observed (Appendix 3).
Coho Salmon. We recovered 127 coho salmon carcasses in the spawner survey, including one Ad-clipped and 17 Program-marked fish (Appendix 4), and did not see any whole skeletons.
Chinook Salmon. We found only 29 chinook salmon carcasses in the nine tributaries surveyed this season. These included 14 condition-one fish which we flagged and 15 skeletons. Included in the fish we flagged were four Program-marked fish. We re-examined four chinook which we had flagged in prior weeks (Appendix 5).
Coho Salmon. We examined 12 coho salmon in the tributaries this season, and no skeletons were observed (Appendix 5).
We only considered chinook salmon recovered in the mainstem Trinity River in determining a date to separate the two chinook salmon runs. Both spring- and fall-runs of chinook salmon were observed in the mainstem survey. A date separating the two races was determined from CWTed and Program-marked chinook salmon. Spring-run chinook salmon dominated our recoveries through the fifth week of the survey ending 20 October 1991. Some overlap of spring- and fall-run chinook salmon occurred during the sixth week ending 27 October 1990. Fall-run chinook salmon became predominate by the seventh week of the survey which began 28 October 1991. For the purposes of this report, all chinook recovered prior to 28 October are considered spring race while those recovered from that date onward are considered fall race (Figure 2). For comparison, the date separating spring and fall-run chinook in previous years was 11 October in 1988, 23 October in 1989, and 29 October in 1990 (Zuspan 1991, 1992a, 1992b).
FIGURE 2. Weekly proportion of spring- and fall-run chinook salmon observed in the 1991-92 Trinity River spawner survey. .
Mainstem Trinity River. We measured 81 spring-run chinook salmon to the nearest cm FL during the survey. Adults are fish >53 cm FL (Bill Heubach Calif. Dept. Fish and Game, pers. comm.) and composed 97.5% (79/81) of the spring-run chinook salmon observed in the survey, while grilse (fish < 53 cm FL) composed the remaining 2.5% (2/81) (Table 3, Figure 3). For comparison, the percentage of grilse in the spring-run chinook sampled at Junction City Weir, and Trinity River Hatchery ranged between 8% and 10% (Table 3). Data from Willow Creek Weir are not included in this analysis as only a small portion of the late spring-run chinook salmon population was sampled there. There was a significant difference in the percentage of grilse sampled in the survey and at the two fixed sites (X2=6.03, df=2, P=0.049).
TABLE 3. Numbers and percentages of spring-run chinook salmon grilse observed in the spawner survey and at two fixed locations in the mainstem Trinity River during the 1991-92 season.
FIGURE 3. Fork length distribution, in 2-cm increments, of spring-run chinook salmon measured in the mainstem Trinity River during the 1991-92 spawner survey.
Tributaries. Based on the date at which we first observed spawning activity, we assume that only fall-run chinook were recovered in the tributaries.
Mainstem Trinity River. We measured (cm FL) 170 fall-run chinook salmon this season. Based on a minimum of 52 cm FL2/ for adults (Bill Heubach Calif. Dept. Fish and Game, pers. comm.), 95.3% of the fall-run chinook salmon measured were adults and 4.7% were grilse (Table 4, Figure 4). The percentage of fall-run chinook salmon grilse at the different sampling sites, including the tributary survey, ranged from 12.1% to 4% (Table 4) and when tested for independence, the difference was highly significant (X2=34.38, df=4, P=.00001). The reason for the difference in rates between the sample sites is unknown.
TABLE 4. Numbers and percentages of fall-run chinook salmon grilse observed in the spawner surveys and at three fixed locations in the Trinity River basin during the 1991-92 season.
FIGURE 4. Fork length distribution, in 2-cm increments, of fall-run chinook salmon measured in the mainstem Trinity River during the1991-92 spawner survey.
Tributaries. We measured (cm FL) 14 chinook salmon in the tributaries this year. Of these, 92.9% were adults (>52 cm FL) and 7.1% were grilse (Table 4).
We measured (FL cm) 113 coho salmon in the mainstem Trinity River. Adults are fish >49 cm FL2/ (Bill Heubach Calif. Dept. Fish and Game, pers. comm.) and composed 99.1% of the coho measured, with grilse composing the remaining 0.9% (Table 5, Figure 5). The percentage of coho salmon grilse at the different sampling sites ranged from 4.1% to 0.9% (Table 5), but the differences were not significant (X2=3.275, df=3, P=.351).
TABLE 5. Numbers and percentages of coho salmon grilse observed in the spawner surveys and at three fixed locations in the Trinity River basin during the 1991-92 season.
FIGURE 5. Fork length distribution, in 2-cm increments, of coho salmon measured in the mainstem Trinity River during the 1991-92 spawner survey.