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1992-1993 SEASON




Bernard Aguilar and Mark Zuspan



Staff of the California Department of Fish and Game's Trinity Fisheries Investigations Project conducted a mark-and-recovery, salmon spawner survey of the mid-Trinity River basin from 15 September through 17 December 1992. We surveyed the mainstem Trinity River from the upstream limit of anadromous migration at Lewiston Dam to the confluence of the North Fork Trinity River. Selected portions of major tributaries that were accessible to anadromous fish were also surveyed. We examined 982 chinook salmon (Oncorhynchus tshawytscha) and 52 coho salmon (O. kisutch) carcasses during the mainstem Trinity River survey.

Chinook and coho salmon spawned throughout the entire mainstem survey area. Spawner density was highest in the uppermost 3.2 km of the river, with decreased densities in downstream survey zones. Spawner density was more uniform between survey zones than in past years. We found 44 chinook and 11 coho salmon carcasses during the tributary surveys. All chinook which spawned in the tributaries surveyed this season were fall-run.

We recovered both spring-run and fall-run chinook salmon carcasses in the survey. Spring-run chinook salmon dominated recoveries in the mainstem until early November, thereafter fall-run fish became the predominant race. Coho salmon were first noted in the mainstem survey during the first week in November, reaching peak numbers in late November, and were gone by mid-December.

Mainstem female prespawning mortality was 0.7% for spring-run chinook salmon, and 5.9% for fall-run chinook salmon. These were the lowest prespawning mortality rates for chinook salmon on record. The probable causes for the decreased pre-spawning mortality were the low spawner escapement, and increased holding and spawning habitat in downstream survey areas provided by higher river flows this year.

Based on the recovery of adipose-fin-clipped chinook salmon carcasses, we estimated that 16.1% of the spring-run and 14.0% of the fall-run chinook salmon spawners observed in the mainstem survey were of hatchery origin.

Fork lengths of spring- and fall-run chinook salmon from the mainstem Trinity River averaged 70.7 cm and 72.4 cm, respectively. Adult spring-run chinook salmon composed 81.3%, and fall-run fish composed 92.2%, of each respective run. Fork lengths of coho carcasses examined in the mainstem Trinity River averaged 65.3 cm. Adult coho composed 95.1% of the total number of coho carcasses examined in the mainstem. In the tributaries, fork lengths of fall-run chinook carcasses averaged 57.3 cm. Adult chinook composed 61.1% of the carcasses examined in the tributaries.



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-fifth 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- one (La Faunce 1965; Rogers 1970, 1973, 1982; Smith 1975; Zuspan 1991, 1992a, 1992b, 1994; and works 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 (U.S. 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.



Mainstem Trinity River Spawner Survey

Our study area included the mainstem Trinity River from the upstream limit of anadromous fish migration at Lewiston Dam (river km 180.1) to the confluence of North Fork Trinity River, 63.4 km downstream (Figure 1). We surveyed this area once a week throughout the salmon spawning season. 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, 1990, and 1991 by TFIP (Zuspan 1991, 1992a, 1992b, 1994). 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 1992-1993 data were collected based on both zone systems. We 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 the tributaries surveyed in the 1992-93 spawner survey.

TABLE 1. Description and lengths of river zones used in the 1992-93 mainstem Trinity River spawner survey.

River zone

Length (km)

Zone description



Lewiston Dam (RKMa/ 180.1) - Old Lewiston Bridge (RKM 176.9)



Old Lewiston Bridge (RKM 176.9) - Browns Mtn. Bridge (RKM 169.0)



Browns Mtn. Bridge (RKM 169.0) - Steel Bridge (RKM 158.8)



Steel Bridge (RKM 158.8) - Douglas City Camp (RKM 148.4)



Douglas City Camp (RKM 148.4) - Junction City Weir (RKM 136.4)



Junction City Weir (RKM 137.1) - McCartney Pond (RKM 123.9)



McCartney Pond (RKM 123.9) - mouth of North Fork Trinity (RKM 116.7)

a/ RKM = distance from the mouth of the river in km.

River kilometers (RKM) for location references were taken from a series of 7.5-minute United States Geological Survey topographic maps, and refer to distances upstream from the mouth of the Trinity River (Appendix 2).

TFIP staff conducted the survey using 12-ft Avon (Note: brand or trade names used for identification only, no endorsement by CDFG is implied or expressed) 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 zones, 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.

All carcasses we observed were identified by species and examined for an adipose fin-clip (Ad-clip) indicating the possible presence of a coded-wire tag (CWT) in their snout. To minimize the number of Ad-clipped fish missed during the spawner survey, all carcasses recovered were passed through a CWT detector. Fish which produced a positive reading with the detector, regardless of the condition of their adipose fin, were considered Ad-clipped.

Carcasses 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) were placed on returning adult fish at two trapping and tagging stations for estimating escapement and harvest of adults. Spaghetti-tagged salmon also received an identifying operculum punch in order to estimate tag shedding rates. The downstream-most trapping site was Willow Creek Weir (WCW), located at RKM 32.2 on the mainstem Trinity River. The other trapping site, Junction City Weir (JCW), was located in the spawner survey area at RKM 137.1. Spring-run and fall-run chinook salmon, coho salmon, and steelhead were trapped and tagged at both WCW and JCW.

We determined spawning condition in female salmon by direct observation of their 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.

Chinook Salmon

All recovered chinook salmon carcasses were further classified into four categories for data collection purposes: 1) Ad-clipped fish; 2) Program-marked fish; 3) unmarked (no Ad-clip or Program-mark), condition-one fish; and 4) unmarked, condition-two fish. The category assigned determined the subsequent processing of each carcass.

We designated chinook salmon carcasses as either condition-one or -two, based on the extent of body deterioration. Condition-one carcasses were the freshest, having at least one clear eye and a relatively firm body. Condition-one carcasses 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 Program-marked or condition-two fish which had already been counted and chopped in half during a previous week's survey.

Heads of Ad-clipped carcasses were removed and retained for later CWT recovery and decoding.

Program-marked carcasses were sexed and the females' spawning condition assessed. We removed any spaghetti tags, then cut the carcass in half to prevent recounting in future weeks. Spaghetti tags had a unique number which allowed determination of the date and location of tagging.

Unmarked condition-one carcasses were flagged and returned to moving water for subsequent recovery. We flagged and measured the first 30 chinook carcasses from each zone and tallied the remainder. 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, upon recovery, the week of flagging could be determined. The hog rings used to attach the flagging were also color-coded to indicate in which zone they were affixed, so that we could determine the incidence of carcasses drifting into another recovery zone. A systematically collected sample of carcasses was measured to the nearest cm of fork length (FL). 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 was based on the post-season size determinations.

Unmarked condition-two carcasses were checked for the presence of a flag and, if possible, the sex and females' spawning condition were assessed. If a flag was present, the color of the flagging tape and the underlying ring were recorded. All carcasses were then cut in half to prevent future recounting.

Coho Salmon

All coho salmon (coho) carcasses recovered were measured (cm FL) and checked for the presence of Ad-clips or Program-marks only. When possible, sex and females' spawning condition were determined and then they were cut in half to prevent future recounting. Coho carcasses were not flagged because they would have required a separate series of flag colors to differentiate them from flagged chinook salmon. Condition-one or -two was recorded only for Program-marked and Ad-clipped coho.

Tributary Spawner Surveys

Tributaries to the mainstem Trinity River, specifically Rush Creek, Grass Valley Creek, Indian Creek, Reading Creek, Browns Creek, Weaver Creek, Canyon Creek, the 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 (Figure 1). 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. The surveys began with the onset of chinook salmon spawning in each tributary and continued until spawning ended. During the first week of our surveys, Grass Valley, Indian, and Reading creeks were the only tributaries surveyed because the others contained little water and were inaccessible to salmon.

We designated all identifiable chinook salmon carcasses into the four categories used in the mainstem spawner survey and handled them accordingly. However, spawning condition was not assessed for tributary carcasses. In past surveys, too few fish were observed in the tributaries to compose a representative sample, and most of those observed were condition-one fish which we needed to flag for spawner estimates. Coho were measured, 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 that was represented by the length of stream we surveyed. 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 proportion of redds present in a survey zone was assumed to represent the percentage of a tributary's total spawning taking place within the zone.

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