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Hagen J.,J. Hagen and Associates | Decker S.,Decker and Associates | Korman J.,Ecometric Research | Bison R.G.,Environment Canada
North American Journal of Fisheries Management | Year: 2010

The refinement of methodologies for estimating the abundance of juvenile salmonids in small streams has been an important area of fisheries research, but the development of methods suitable for larger streams has received insufficient attention. Using a novel approach for obtaining marked populations of fish, we evaluated the effectiveness of night snorkeling counts for estimating the abundance of steelhead Oncorhynchus mykiss parr in streams within a large river basin. Sampled streams ranged from headwater tributaries with wetted widths of less than 10 m to a large, main-stem river with a wetted width approaching 100 m. Estimates of snorkeling detection probability for steelhead parr were consistently high (overall mean= 0.65) and exhibited moderately low variability among sites (coefficient of variation = 0.24). Detection probability exhibited a dome-shaped relationship to fork length and declined with increasing cross-sectional site area. The high and relatively consistent detection probabilities we estimated indicate that night snorkeling can be an effective technique for estimating the basinwide abundance of steelhead parr. © The American Fisheries Society 2010.


Melnychuk M.C.,University of British Columbia | Korman J.,Ecometric Research | Hausch S.,University of Calgary | Welch D.W.,Kintama Research Services Ltd | And 2 more authors.
Canadian Journal of Fisheries and Aquatic Sciences | Year: 2014

We observed large survival differences between wild and hatchery-reared steelhead trout (Oncorhynchus mykiss) during the juvenile downstream migration immediately after release, which persisted through adult life. Following a railway spill of sodium hydroxide into the Cheakamus River, British Columbia, a short-term conservation hatchery rearing program was implemented for steelhead. We used acoustic telemetry and mark-recapture models to estimate survival of wild and (or) hatchery-reared steelhead during 4 years of the smolt migration, with both groups released in 2008. After adjusting for estimated freshwater residualization, 7%-13% of wild smolts and 30%-40% of hatchery smolts died in the first 3 km of the migration. Estimated survival from release to ocean entry was 71%-84% for wild fish and 26%-40% for hatchery fish and to exit from the Strait of Georgia system was 22%-33% for wild fish and 3.5%-6.7% for hatchery fish. A calculated 2.3-fold survival difference established during the downstream migration was similar to that after the return of adult spawners, as return rates were 8.0% for wild fish and 4.1% for hatchery fish. Contrary to current understanding, a large proportion of salmon mortality in the smolt-to-adult period, commonly termed "marine mortality", may actually occur prior to ocean entry.


Korman J.,Ecometric Research | Schick J.,Box 504 | Mossop B.,BC Hydro
North American Journal of Fisheries Management | Year: 2016

Abstract: Estimating riverwide abundance of juvenile fish populations is challenging because detection probability is typically low and juveniles can be patchily distributed over large areas. We used a hierarchical Bayesian model to estimate the abundance of juvenile steelhead Oncorhynchus mykiss in two rivers in British Columbia over 3 years based on a multigear, two-phase sampling design. These estimates were used to drive a simulation model to evaluate how the precision of abundance estimates varied with the number of single-pass index and mark–recapture sites that were sampled, the proportion of shoreline sampled, and the mean and variation of detection probability and fish density across sites. The extent of variation in fish densities across index sites was the most important factor influencing the precision of river-wide abundance estimates, and increasing the number of index sites was the best approach to reduce variability in abundance estimates. River size, which controls the proportion of habitat sampled for a given level of sampling effort, had a moderate effect on precision, but only when the extent of site-to-site variation in fish density was high. Factors affecting detection probability, such as the number of mark–recapture sites, the mean detection probability, or the extent of variation in detection probability across sites, had much less influence on precision of abundance estimates unless the proportion of river sampled was high. Hierarchical Bayesian models are no substitute for collecting informative data, but they improve our understanding of variance structure, which is critical for providing realistic estimates of uncertainty and designing informative and efficient sampling programs. Received July 16, 2015; accepted October 7, 2015 Published online March 8, 2016 © American Fisheries Society 2016.


Korman J.,Ecometric Research | Scott Decker A.,Decker and Associates | Mossop B.,BC Hydro | Hagen J.,J. Hagen and Associates
North American Journal of Fisheries Management | Year: 2010

We compared nighttime electrofishing- and snorkeling-based mark-recapture methods for estimating the detection probability and abundance of juvenile steelhead Oncorhynchus mykiss in the Cheakamus River, British Columbia. The reliability of abundance estimates largely depends on the precision and accuracy of detection probability (the fraction of marked individuals detected) as well as a few key assumptions of closed population models that we evaluated in this study. There was minimal bias (-2.5%) in diver estimates of the fork lengths of juvenile steelhead, and the relationship between measured and estimated fork lengths was very precise (r2 = 95%). With a hierarchical Bayesian model, estimates of the detection probability for smaller juveniles (40-60 mm) ranged from 0.4 to 0.6 with electrofishing and were near zero with snorkeling. In contrast, snorkeling-based detection probability was 0.6 and independent of size for larger juvenile steelhead (>60 mm) and much greater than that with electrofishing. These results provide strong evidence that there is considerable individual heterogeneity in detection probability driven by fish size for both methods. Owing to these differences, the abundance of age-0 steelhead based on snorkeling was underestimated by 50%, but that of larger, age-1 fish was unbiased and more precise (10-fold) than that based on electrofishing. The use of electrofishing during marking resulted in a substantive reduction in snorkeling-based detection probability during recapture, but the converse was not true. Thus, there is strong evidence of behavioral heterogeneity in detection probability induced by electrofishing, but only when snorkeling is used to detect fish during recapture. The differences in detection probabilities among size-classes and sampling methods were probably driven by differences in concealment behavior, spatial distribution, and fright responses to sampling. Our results indicate that snorkeling is the better way to estimate abundance for larger juvenile steelhead, whereas electrofishing is preferred for smaller fish. © The American Fisheries Society 2010.


Bradford M.J.,Simon Fraser University | Higgins P.S.,BC Hydro | Korman J.,Ecometric Research
Freshwater Biology | Year: 2011

1.Water managers must make difficult decisions about the allocation of streamflows between out-of-channel human uses and environmental flows for aquatic resources. However, the effects environmental flows on stream ecosystems are infrequently evaluated. 2.We used a 13-year experiment in the regulated Bridge River, British Columbia, Canada, to determine whether an environmental flow release designed to increase salmonid productivity was successful. A hierarchical Bayesian model was used to compare juvenile Pacific salmon (Oncorhynchus spp.) abundance before and after the flow release. 3.We found that the total number of salmonids did increase after the release, but most of the gains could be attributed to the rewatering of a previously dry channel located immediately below the dam. In reaches that had flowing water during the baseline period, the response of individual salmon species to the increase in flow was variable, and there was little change in total abundance after the flow release. Our results were inconsistent with both habitat modelling, which predicted a decrease in habitat quality with increasing flow, and holistic instream flow approaches, which imply greater benefits with larger flows. 4.We question whether biotic responses to flow changes can be predicted reliably with currently available methods and suggest that adaptive management or the use of decision tools that account for the uncertainty in the biotic response is required for instream flow decisions when the competing demands for water are great. © 2011 Blackwell Publishing Ltd and Crown in the right of Canada.

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