Fish/Shellfish Research and Management - Fish/Shellfish Research
Date Published: July 2003
Number of Pages: 61
Author(s): Dave Seiler, Steve Neuhauser and Lori Kishimoto
Skagit River chinook returns (spring and summer/fall combined) have steadily declined over the last fifty years (PSSSRG 1992, 1997). In 1994, the Joint Chinook Technical Committee of the Pacific Salmon Commission designated the status of these stocks as â€œNot Rebuilding.â€ To address this poor stock status, resource managers formed the Skagit River Chinook work group in 1995. Composed of state, tribal, and federal fish biologists, this group recommends and coordinates restoration and monitoring programs. A major goal of this work group is to determine the limiting factors for chinook. Necessary data for this purpose include an indicatorstock tagging program, habitat inventory, annual adult escapement estimation, and wild juvenile chinook assessment. The juvenile production evaluation is a vital link in this process because it provides a direct measure of freshwater survival.
Seattle City Light (operators of several dams on the Skagit River), through a 1991 fisheries settlement agreement with WDFW, the Skagit tribes (Skagit System Cooperative or SSC) and federal agencies â€“ National Marine Fisheries Service (NMFS), US Fish & Wildlife Service (USFWS), US Forest Service (USFS) and National Park Service (NPS) â€“ created the Skagit Non-Flow Plan Coordinating Committee (NCC). The NCC is responsible for funding several non- flow fisheries programs including the â€œChinook Research Program.â€ Beginning in 1997, this program provided funding to conduct chinook studies. This report documents our 2002 downstream migrant trapping project in the Skagit River which, with funding from the NCC, we expanded to continue estimating wild 0+ chinook production.
Understanding the major sources of inter-annual variation in run size is critical to improving harvest and habitat management. Quantifying anadromous salmonid populations as seaward migrants near saltwater entry is the most direct assessment of stock performance in freshwater because the variation resulting from marine survival and harvest are precluded. Relating smolt production to adult spawners over a number of broods empirically determines the watershedâ€™s natural production potential (provided escapement and environmental conditions are sufficient), its stock/recruit function if escapements are less than that required to achieve maximum production, and enables identification of the major density- independent source(s) of inter-annual variation in freshwater survival. To accomplish these and other fish management objectives, the WDFW implemented a long-term research program directed at measuring wild salmon production in terms of smolts and adults in selected watersheds, beginning in 1976 (Seiler et al.1981). In 1981, this program, which was directed primarily at coho salmon, was expanded to include additional large watersheds (Seiler et al.1984).
In 1990, we initiated downstream migrant trapping in the Skagit River system to quantify wild coho smolt production to, among other objectives, resolve a discrepancy in escapement estimates (Conrad et al. 1997). This program, which in 2002 was in its thirteenth year, involves trapping and marking wild coho smolts emigrating from a lower river tributary, Mannser Creek (R.M. 35), and sampling a portion of the entire population via floating traps in the lower mainstem (R.M. 17, Burlington Northern railroad bridge).
In past years we evaluated returns of coho adults coded-wire tagged as smolts at the gulper in Baker Lake. The upstream migrant trap below the dam provided a reliable accounting of all salmon returning to this system. Applying the marine survival estimated from the tag-based estimates of harvest and escapement to respective estimates of total system wild coho smolt production yielded estimates of adult recruits, escapement, and harvest for the entire Skagit River system (Seiler et al.1995). Technical support for this program was eliminated in 2000 and 2001, suspending this portion of the Skagit coho production and survival evaluation. This work resumed in 2002.
Although our trapping in the mainstem was originally directed at coho smolts, we identify and enumerate all fish captured. For the first seven years of this study (1990-1996), season total 0+ chinook catches in the one scoop trap varied six- fold, from 1,700 to 10,500 chinook. (As of 1993, we have simultaneously operated both a scoop and a screw trap.) In addition to abundance, these catch totals are influenced by fishing effort (the time fished on each date and for the season), migration timing relative to the interval we trapped, and instantaneous trap efficiency. Many such variables as discharge, water velocity, turbidity, debris, channel configuration, trap placement, and fish size combine to affect both instantaneous and season average trap efficiency.
Preliminary expansion of these 0+ chinook catches, based on the season average recapture rates of wild coho and several other assumptions held consistent between years, has yielded annual chinook production estimates that range from 0.5 to 6.5 million. The accuracy and precision of these estimates is presently incalculable because the assumptions remain unverified. We believe, however, that these estimates reflect the abundance of wild 0+ chinook production from these broods, at least in a relative sense. We base this contention upon the significant negative correlation between the freshwater survival estimates and the severity of flow during the period that the eggs were incubating in the gravel. The survival rates in this relationship are the ratio of total 0+ chinook emigrants estimated past the traps to the potential egg deposition. System total egg deposition is simply the product of the estimated total adult chinook escapement, an assumed sex ratio, and a fecundity of 5,500 eggs/female (Pete Castle pers. comm.). This relationship indicates that overall egg-to- migrant survival for Skagit River chinook has varied over ten-fold within just the first seven broods, almost entirely as a function of flow during egg incubation.
In 1997, we began trapping in mid-February and continued into September. This first season of extended trapping produced our first insight into the migration timing of wild chinook. Over the season, we estimated a total of 2.4 million 0+ chinook, of which about one third emigrated before April.
Measuring the biological attributes of outmigration timing and size contributes to our understanding of juvenile chinook freshwater life history. This information is useful for flow management (dams and other flow controls), habitat protection, and designing hatchery programs to minimize hatchery/wild interactions.
We estimate coho smolt production from the Skagit River with the mark and recapture strategy that we developed and have used successfully in a number of large watersheds throughout the state over many years. This method involves the following components:
- Trapping all the wild coho smolt s emigrating from a selected tributary;
- Identifying each of these smolts with an external mark; and
- Capturing a portion of the smolt population migrating through the lower mainstem and examining each fish for the mark.
This design produces relatively precise and (we believe) unbiased production estimates, because a temporally- representative portion of the coho population is marked via 100% trapping at an upstream tributary. Therefore, trapping in the mainstem does not have to be continuous or even representative with respect to timing (Seber 1982). We explicitly developed this design to avoid the requirement of estimating gear efficiency.
Because of the early life history characteristics of chinook in freshwater, estimating their smolt production with the same statistical precision we achieve for coho smolts is not possible. Chinook originate in discrete portions of the mainstem, and subsequently rear for variable intervals in various reaches. Therefore, the methodology we use with coho, capturing and identifying a representative portion of the entire population, is not feasible for chinook. Each component likely has different survival patterns that result from the complex interactions of a number of factors: their parent's spawning timing and distribution; genetically-programmed juvenile rearing strategies; and the flow and habitat conditions each brood and sub-population within it encounters. In a system as wide as the lower Skagit River, the migration pathways selected may also vary between sub-populations, which would affect capture rates. The susceptibility of migrants to capture also va ries as a function of flow and environmental conditions in effect at the trap and upstream of it.
Sources of Variation Affecting Wild 0+ Chinook Estimates
Given the foregoing problems, estimating wild juvenile 0+ chinook production from the trapping data we have collected in the lower Skagit River involves a number of assumptions. Accuracy of the resultant estimates is a direct function of the veracity of these assumptions. Each assumption deals with the uncertainty resulting from the following five major sources of variation we have identified.
- Trap efficiency. Expanding catches to estimate wild 0+ chinook production requires estimates of instantaneous gear efficiency, ideally as a function of some measurable variable such as flow.
- Day vs. night trap efficiency. Trap efficiency may be influenced by light. For example, it may be lower during the daylight than at night.
We have operated the traps primarily at night because catch rates, especially for coho and to a lesser extent chinook, are higher at night than during the daylight. Estimating instantaneous trap efficiency during the daylight hours, however, is probably not possible because it would require that a sufficient and known number of marked wild chinook pass the traps within a single daylight period. The traps fish only the top 4 ft of the water column, and the depth at our site is 20-30 ft, depending on discharge. If, as a function of increasing light intensity, juvenile chinook migrate at greater depth and/or their ability to avoid the trap increases, then trap efficiency during daylight hours would be lower. The behavior of juvenile chinook and the biases imposed by releasing marked fish immediately upstream of the traps precludes estimating instantaneous efficiency within such a limited time interval as a single daylight period. Catches during daylight hours appear to be positively affected by increasing turbidity. If true, this positive correlation between daytime catch and turbidity results from either increased migration rate and/or an increase in trap efficiency because avoidance is reduced.
- Day vs. night migration. Efficiency-based estimates rely on trapping either continuously or randomly throughout the time strata that migration is estimated. We developed our experimental design for estimating coho production to avoid the requirement of continuous trapping in the mainstem. Therefore, trapping in previous years was conducted almost entirely at night.
- Migration interval. Skagit River 0+ chinook emigrate over a longer season than coho smolts. Chinook begin their downstream migration in January or earlier, and continue through the summer. In the first four years, we operated the traps only over the coho smolt migration period, early-April through mid-June. Beginning in 1994, and continuing through 1996, we extended trapping as late as mid-July. In 1997, we began trapping in mid-February and continued into September. To better define the early portion of the migration period, in 1998 and 1999, we began trapping in mid-January and extended trapping into September. In 1999 and 2000 we assessed late migration by operating the traps intermittently during October.
- Incidence of hatchery-produced fish. Prior to 1994, releases of hatchery-produced 0+ chinook in the Skagit River were unmarked. Consequently, our estimates of wild chinook production for the first four years rely on an assumption for the number of hatchery-produced fingerlings we caught. Estimating wild and hatchery components of the migration relies on assumptions of how many hatchery fish survived to pass the trap during the interval trapped. Beginning with the 1993 brood, (released in 1994) all hatchery-produced zero-age chinook released into the Skagit River have been marked with an adipose fin-clip (ad-mark) and coded-wire tagged.
Study Plan for 2002
The study plan for the 2002 trapping season was directed at continuing to improve the estimates of Skagit River chinook production through achieving a better understanding of the sources of variation. In addition to continuing our analysis of the chinook and coho trapping data collected over the previous eleven years, the 2002 work plan included the following six operational elements.
- Trapping season. A critical uncertainty in estimating Skagit River wild 0+ chinook production is their emigration timing. In 2002 we began trapping in mid-January and continued through July. Migration was in progress at a low level when trapping began and was essentially over in mid-July.
- Nightly trap operation. We fished the scoop and screw traps nightly throughout the season, unless high flows, debris or damaged gear prevented trap operation.
- Daytime trap operation. Daytime trapping occurred every third day. We enumerated catches shortly after dawn and around dusk to enable us to separate day and night catches.
- Wild coho marking. In 1999 and 2000, we assessed differences in recapture rates of wild coho trapped and marked in the upper river with those marked in the lower watershed by using different marks. Coho smolts marked and released by the NPS and the WDFW Habitat Program were identified with a left ventral fin-clip (LV- mark), as in past years. Smolts captured at Mannser Creek in the lower river were right ventral finclipped (RV-marked) by our trapping personnel. During the two- year evaluation we discovered significant differences in recapture rates between the two mark groups. Smolts released high in the river were recovered at lower rates than those released from Mannser Creek in the lower watershed. Inclusion of the upper-river marked smolts in the coho production calculations biased the estimate high. Therefore, we discontinued marking fish in the upper watershed in Spring 2001. Smolts that were RV-marked at Mannser Creek provided the basis for the coho smolt production estimate.
- Trap efficiency. In addition to the marked wild coho released from the Mannser Creek tributary trap and the groups of ad-marked/coded-wire tagged hatchery chinook fingerlings released from the three production facilities (Countyline Ponds, Baker River and Skagit Hatchery), we marked and released six groups of hatchery chinook above the trap to serve as calibration groups.
- Measuring visibility. To better understand the influence of water clarity on migration behavior, we measured visibility each day over the 1999, 2000, 2001, and 2002 spring seasons. Visibility data will be correlated with flow, turbidity measured at the Mount Vernon water intake, and fish catch data.
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