Salmon Escapement Research Articles

Estimating spawning escapements from periodic counts: a comparison of methods

The escapement of Pacific salmon is often estimated by periodic counts of spawners, calculating the number of fish-days present and dividing by the average number of days a fish spends in the survey area. We present a maximum likelihood method to calculate the number of spawning fish and compare this approach with the most commonly used method, which relies on linear interpolation between observations. The maximum likelihood method is computationally more demanding; however, it does provide a statistical basis for describing uncertainty and can also be used to deal with data sets where the first or last counts are nonzero or where there are few observations. We compared escapement estimation methods using data from 18 experimental streams where the number of fish in the stream was evaluated by weir and carcass counts. In this comparison, the method of linear interpolation deviated from the weir count by an average of 19%, whereas the maximum likelihood method deviated by 23, 24, 30, or 40% depending upon which likelihood and arrival time model was used. We conclude that for most data sets where measures of uncertainty are not required, the linear interpolation method is adequate but recommend an examination of maximum likelihood methods when an estimate of uncertainty is required.

Estimators of stream residence time of Pacific salmon (Oncorhynchus spp.) based on release-recapture data

The area-under-the-curve method is a widely used method for estimating salmon escapement. The method depends on obtaining an accurate estimate of stream residence time, or stream life. This paper develops two estimators of stream residence time based on release-recapture data: a nonparametric estimator and a parametric estimator. Monte Carlo simulations showed that with an adequate release size and number of sampling occasions, both estimators provide precise estimates of stream residence time. If there is significant right censoring, however, the parametric estimator is significantly less biased. If the data are too sparse, the parametric estimator performs poorly and often fails. The stream residence time of spawning sockeye salmon (Oncorhynchus nerka) in Iliamna Lake, Alaska, was estimated using the estimators developed here. Because the estimators also provide the variance of the estimates, the precision of the stream residence time estimate could be assessed, and we were able to test and reject the hypothesis that the stream residence time for females is equal to that of males. Both estimators are applicable to estimating the life expectancy of any fish or wildlife population with release-recapture techniques.

Observer Accuracy and Precision in Aerial and Foot Survey Counts of Pink Salmon in a Southeast Alaska Stream

Within- and between-observer variability and bias in aerial and foot survey estimates of spawning abundance of pink salmon Oncorhynchus gorbuscha were determined for survey counters from Chaik Bay Creek on southern Admiralty Island, Southeast Alaska, in 1992–1994. Multiple counts of pink salmon escapement in predetermined sections of the creek were made. At the same time, a mark–recapture experiment was performed to estimate the actual number of pink salmon and chum salmon O. keta. Observer counting abilities were also examined by using computer images that simulated aerially observed fish; four object types aggregated at predetermined numbers were tested. Observers tended to undercount the actual number of salmon present, and accuracy decreased nonlinearly with an increase in actual salmon escapements. On average, observers counted between 25% and 68% of the salmon present. Correction factors determined for each observer can be used to reduce bias in estimates of total abundance and make counts comparable between observers. Comparison of relative biases of observers making aerial surveys in both 1993 and 1994, years of nearly equal pink salmon abundance, suggested that learning occurred during the study.

Estimating the value of in-season estimates of abundance of sockeye salmon (Oncorhynchus nerka)

Fishwheels have recently replaced a gillnet test fishery for in-season estimation of escapement of sockeye salmon (Oncorhynchus nerka) to the Nass River, British Columbia. We used an empirically based Monte Carlo simulation model to estimate the potential benefits (in terms of future harvests) of using this new method. We found that using the information from the fishwheels in place of the gillnet test fishery may increase the average annual catch from current ocean fisheries by 2.7% ($229 000) due to the greater accuracy of in-season estimates. When the additional costs of operating the fishwheels instead of the gillnet test fishery were taken into account, the fishwheel program still resulted in an annual net benefit of about $180 000. Eliminating the deviations from the target escapement for Nass River sockeye salmon by having perfect monitoring of escapement and perfect control over harvesting has a potential annual value of more than $1 million (CAN). However, much greater benefits from fishwheels are unlikely to be realized because of the highly variable response of the commercial fishing fleet to additional fishing opportunities and the 5-day lag of the escapement estimates provided by fishwheels.

Review on monitoring adult salmonid ( Oncorhynchus and Salmo spp.) escapement using fixed-location split-beam hydroacoustics

Since 1992, split-beam hydroacoustic techniques have been used to monitor adult salmonid escapement ( Oncorhynchus and Salmo spp.) in 14 rivers in North America and Europe. Monitoring in rivers is one of the more challenging applications for fisheries acoustics. Rivers typically have a high reverberation level, uneven bottom bathymetry, and nonlaminar hydraulics, requiring sophisticated equipment and careful deployment, calibration, and testing. The major issues that were addressed in order to obtain estimates of adult salmon escapement included hydroacoustic equipment and techniques, site selection, transducer deployment, and fish behavior. Fixed-location hydroacoustic techniques were employed, utilizing narrow-beam transducers aimed horizontally, monitoring migrating fish in side-aspect. Fish were tracked in three dimensions as they passed through the acoustic beam. A bottom substrate of low acoustic reflectivity enabled the acoustic beam to be aimed close to the bottom. Sites were selected where fish were actively migrating, not holding or milling. In most cases, migrating salmonids were strongly shore- and bottom-oriented, where water velocities were slowest. Diel distributions of fish passage were weighted toward nighttime. Other results included fish size and velocity. Potential improvements in riverine monitoring capabilities include quadrature demodulation and FM Slide/Chirp signals.

Use of the stratified-Petersen estimator in fisheries management: estimating the number of pink salmon (Oncorhynchus gorbuscha) spawners in the Fraser River

The simple-Petersen estimator is a well-known mark-recapture method to estimate animal abundance. Two key assumptions are equal catchability in both samples and complete mixing of tagged and untagged animals. If these are violated, severe bias can occur. The stratified-Petersen estimator can be used to account for some of the heterogeneity in catchability or mixing. In this paper, we first review recent developments in the stratified-Petersen experiment for fisheries audiences and demonstrate some of the practical problems that can occur that have not been discussed in the theoretical literature. Second, we present a case study to estimate the gross escapement of Fraser River pink salmon (Oncorhynchus gorbuscha) in 1991. The motivation for this study is a discrepancy of over 5 million fish between the estimates as derived by the Pacific Salmon Commission (PSC) (7.5 million fish based on a hydroacoustic method) and the Department of Fisheries and Oceans (DFO), Canada (13.0 million fish based on a mark-recapture method). One hypothesis put forward was that the discrepancy may be due to the use of a pooled-Petersen estimator when there is differential migration over time. The stratified-Petersen model suggests that little of this discrepancy can be explained by differential migration.

Estimating the value of in-season estimates of abundance of sockeye salmon (<i>Oncorhynchus nerka</i>)

Fishwheels have recently replaced a gillnet test fishery for in-season estimation of escapement of sockeye salmon (Oncorhynchus nerka) to the Nass River, British Columbia. We used an empirically based Monte Carlo simulation model to estimate the potential benefits (in terms of future harvests) of using this new method. We found that using the information from the fishwheels in place of the gillnet test fishery may increase the average annual catch from current ocean fisheries by 2.7% ($229 000) due to the greater accuracy of in-season estimates. When the additional costs of operating the fishwheels instead of the gillnet test fishery were taken into account, the fishwheel program still resulted in an annual net benefit of about $180 000. Eliminating the deviations from the target escapement for Nass River sockeye salmon by having perfect monitoring of escapement and perfect control over harvesting has a potential annual value of more than $1 million (CAN). However, much greater benefits from fishwheels are unlikely to be realized because of the highly variable response of the commercial fishing fleet to additional fishing opportunities and the 5-day lag of the escapement estimates provided by fishwheels.

Recent events concerning Atlantic salmon escapees in the Pacific

Atlantic salmon (Salmo salar L.) escapees from commercial salmon farms in Washington State, USA, and British Columbia, Canada, arecaught in fisheries from Washington State to Alaska. A large escape from a farm near Cypress Island, Washington, was likely to have been the cause of increased abundance of Atlantic salmon in Juan de Fuca Strait and higher than average catches on the west coast of Vancouver Island during the summer of 1996. The catch of Atlantic salmon in Alaska in 1996 exceeded the sum of catches in all previous years. The number of escapees recorded in fresh water has been increasing, with more than 210 adult fish observed in 29 rivers in British Columbia during 1996. During the winter of 1995/1996, 39 Atlantic salmon were reported caught in the Salmon River on Vancouver Island. Analysis of the stomach contents of these fish indicated that some were feeding in fresh water.

Implications of Trends in Marine-derived Nutrient Influx to South Coastal British Columbia Salmonid Production

Marine nutrients and carbon transported by adult salmon are important to the productivity of the oligotrophic lakes and streams in which salmon spawn. Reduced carcass availability results in a decline in nutrient and carbon sources for stream-rearing salmonids. We examined 42 years of escapement records for five species of Pacific salmon for Georgia Strait, the west coast of Vancouver Island, and the mainland coast of British Columbia to estimate the status of this nutrient source. Salmon stocks from enhanced streams frequently dominated the total escapement of entire regions. As a result, most of the influx of marine nutrients is focused toward a few large stream systems already undergoing significant salmon enhancement, while nutrient influx to the more-numerous unenhanced streams is declining. In the large number of streams with smaller salmon escapements, stream-rearing species already in decline may decrease further from oligotrophication. Risk-averse escapement targets for wild salmon stocks need to include sufficient spawners to provide the nutrient influx linked to the maintenance of stream productivity. Declining trends in nutrient influx to wild salmon streams in most regions are a cause for concern and more intensive examination.

ESTIMATION OF SALMON ESCAPEMENT: MODELS WITH ENTRY, MORTALITY AND STOCHASTICITY

Understanding the dynamics of Prince William Sound pink salmon requires knowledge of the size of the spawning population in a stream over time. Periodic aerial surveys provide observations on the number of spawners, but the lack of daily observations requires a model to fill in the gaps. We develop a differential equation framework to represent the dynamics of escapement during the season. An exponential population growth model with a time‐varying rate of growth is used for the number of spawners. The rate of growth consists of two primary components: the entry of salmon to the stream (escapement) and the mortality of spawners in the stream. The models for entry and mortality are also functions of time. The stochastic element of the model is based on a nonhomogeneous birth‐and‐death process which leads to a least squares estimation approach with either additive measurement or process errors. We illustrate the approach for a stream in Prince William Sound by fitting various models to observed spawner abundance, mortality counts from ground surveys and weir counts of the entry to the stream. We believe this approach could improve salmon escapement estimation, because the processes governing entry and mortality are explicitly considered.

Status of Fall Chinook Salmon Populations in the Mid-Columbia River, 1948–1992

Abstract Historically, fall Chinook salmon Oncorhynchus tshawytscha spawned in the main-stem Columbia River over a distance of approximately 900 km. However, because of hydroelectric development and other human activities, most natural production of upriver stocks of fall Chinook salmon (upriver bright, URB) is now restricted to a 90-km-long section of river known as the Hanford Reach. The adjacent Hanford Nuclear Reservation was managed for production of nuclear materials from 1943 to 1971, which restricted many development activities, including additional hydroelectric projects. Aerial counts of fall Chinook salmon redds since 1948 at the Hanford Reach provide an index of relative abundance among spawning areas and years. Redd counts during peak spawning were less than 1,000 annually from 1948 to 1961, but they increased to as high as 8,800 in 1989 following construction of several main-stem dams on both the Columbia and Snake rivers. Escapement of adult fall Chinook salmon to the Hanford Reach averaged...

Optimizing Aerial Count Frequency for the Area-under-the-Curve Method of Estimating Escapement

Abstract The area-under-the-curve estimate of salmon escapement is computed from aerial counts of fish and estimates of the residence time of spawners. Imprecision in both of these factors leads to error in the final escapement estimate and, thus, affects its reliability as an index of escapement over time. The population of Chinook salmon Oncorhynchus tshawytscha in the Nechako River, British Columbia, was chosen for a case study, and this paper illustrates how simple simulation models can be used to explore the increases in precision about escapement estimates associated with increasing the frequency of aerial flights. Results show that confidence in escapement estimates for the Nechako River Chinook salmon population decreases as the frequency of survey flights decreases; results also show that this pattern is robust to the typically small annual differences in mean residence time in the Nechako River. Such models can help managers optimize the frequency of aerial flights by comparing the benefits of f...

Adjusting for missed tags in salmon escapement surveys

One of the assumptions in mark-recapture studies is that all tags recovered are correctly identified. In some cases, tags are overlooked, and a subsample of fish previously classified as untagged is examined to look for missed tags. In this paper, a method for estimating the salmon escapement using a mark-recapture methodology after correcting for missed tags is developed. The variance of the revised estimator is derived and the loss of efficiency compared with a Petersen estimate, where no tags are overlooked, is found. Optimal allocation of effort between the initial and second examination is considered. Finally, a numerical example using actual data from the 1994 Fraser River sockeye salmon (Oncorhynchus nerka) season is used to illustrate the results.

Atlantic salmon in the North Pacific

The first catches of Atlantic salmon, Salmo salar L., in British Columbia (BC) waters occurred in 1987. The first reported escape of Atlantic salmon (2000 individuals) occurred in 1988. From 1988 to 1995, 97 799 Atlantic salmon were reported escaped from net pens in BC but the true number was higher as not all escapes are reported. Since 1987 a total of 9096 Atlantic salmon was caught in the coastal marine waters of BC, Washington and Alaska, and 188 were caught in fresh water. Most catches occurred in the Johnstone Strait area, where the abundance of salmon farms is highest. The most distant recovery occurred in 1994 when an Atlantic salmon was caught near the western end of the Alaska Peninsula. There have been no reports of successful reproduction of Atlantic salmon in the wild and no feral juveniles have been found. Atlantic salmon caught in the ocean in BC have substantial amounts of adipose tissue and they are heavier at length than fish caught in Alaska. The proportion of fish with prey items in their stomachs is generally low but higher in Alaska (13.1%) than in BC (5.8%). Most fish caught in fresh water are either maturing or mature.

The incidence of reared Atlantic salmon (Salmo salarL.) of fish farm origin at West Greenland

The proportion of escaped farmed Atlantic salmon in the commercial fishery at West Greenland was estimated using samples of fish caught with drift-nets in the commercial fishery in 1991 and 1992. Identification of reared fish was carried out using scale analysis. Despite the fact that large numbers of salmon escape from fish farms around the north Atlantic, the proportion of farmed fish in this fishery was low, 1.1 % in 1991 and 1.4 % in 1992, and much lower than in the area of the Faroes fishery in the east Atlantic. The low number of fish farm escapees at Greenland is surprising since large numbers of farmed salmon are produced in countries from which most of the wild slamon that occur at West Greenland originate.

Adjusting for missed tags in salmon escapement surveys

Adjusting for missed tags in salmon escapement surveys

Results of Bayesian methods depend on details of implementation: an example of estimating salmon escapement goals

Bayesian methods have been proposed to estimate optimal escapement goals, using both knowledge about physical determinants of salmon productivity and stock-recruitment data. The Bayesian approach has several advantages over many traditional methods for estimating stock productivity: it allows integration of information from diverse sources and provides a framework for decision-making that takes into account uncertainty reflected in the data. However, results can be critically dependent on details of implementation of this approach. For instance, unintended and unwarranted confidence about stock-recruitment relationships can arise if the range of relationships examined is too narrow, if too few discrete alternatives are considered, or if data are contradictory. This unfounded confidence can result in a suboptimal choice of a spawning escapement goal.

Estimation of Pacific Salmon Escapement with a Time-Lapse Video Recording Technique

Abstract A time-lapse video system was designed and used to record the migration of adult Pacific salmon at Tumwater Dam on the Wenatchee River, Washington. From 1989 through 1991, the system was evaluated to determine its suitability and usefulness for escapement estimation of three Pacific salmon species, chinook salmon Oncorhynchus tshawytscha, sockeye salmon O. nerka, and steelhead O. mykiss. The accuracy and precision of estimates based on these videotape records were tested. Video-based sockeye salmon escapement estimates were found to be within 4% of an independent estimate based on the on-site visual counts made by experienced observers. An among-observer variance test was performed comparing the counts of five different individuals who examined the same videotape records. Analysis of variance showed no significant differences among the five observers. A test that compared the effect on counts of different time-lapse recording speeds indicated no significant differences between the two speeds test...

A Likelihood Method for Estimating Pacific Salmon Escapement based on Fence Counts and Mark–Recapture Data

Pacific salmon escapements in natural streams are often determined by conducting fence counts in conjunction with mark–recapture operations. Typical field conditions are characterized by protracted floods, undetected immigration, variation in sampling rates, mortality during the census periods, small sample sizes, and few successive recaptures. Closed population models tend to overestimate escapement under such conditions, and traditional open population models cannot always be relied on due to the lack of sufficient data. A likelihood-based estimation method was specifically designed for such conditions. A distinguishing feature of this model is that individual recapture histories are not required for parameter estimation. The model is also structured to incorporate ancillary data in the estimation process to improve the precision and accuracy of the estimates. The model is described in detail, and suggestions are made to facilitate its application. Constructed data sets resembling those obtained under field conditions were used to evaluate the performance of the model. The estimates generated with simulated data and actual field data were compared with those based on visual foot surveys and other mark–recapture models.

Estimating Salmon Spawning Escapement Using Capture–Recapture Methods

We describe a method of estimating the spawning escapement of coho salmon (Oncorhynchus kisutch) from capture–recapture data. Traditional capture–recapture analyses do not directly provide estimates of escapements; however, we show how simple modifications to the Jolly–Seber method can estimate the total number of fish returning to a river including those that enter and die between sampling occasions. Spawning runs of Pacific salmon were simulated and their escapements estimated using capture–recapture. The performance of the maximum likelihood estimators (MLEs), the censored MLEs, the constrained MLEs, and less-biased estimators in estimating the run sizes and providing estimates of precision were evaluated. Simulation results indicated that constrained MLEs provided the most appropriate estimates of escapement and that standard errors be computed using the large-sample variance formulae evaluated at these estimates. These methods were used to estimate the escapements of coho salmon to a small river on Vancouver Island in 1989 and 1990.