Review of the Comparative Survival Study's Draft 2013 Annual Report

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The Northwest Power and Conservation Council’s 2009 amendments to the Columbia River Basin Fish and Wildlife Program call for a regular system of independent and timely science reviews of the Fish Passage Center’s (FPC) analytical products. These reviews include evaluations of the Comparative Survival Study’s (CSS) draft annual reports. This ISAB review of the draft 2013 CSS Annual Report is the ISAB’s fourth review of CSS annual reports in response to the Council’s 2009 Program. The ISAB review begins by suggesting topics for further CSS review, then provides general and specific comments on each chapter of the report, and ends with specific editorial suggestions.

The ISAB suggests five topics for further CSS review:

  1. hypotheses on mechanisms regulating smolt-to-adult survivals (SARs)
  2. life-cycle modeling questions and Fish and Wildlife Program SAR objectives
  3. data gaps
  4. rationalization of CSS's Passive Integrated Transponder (PIT)-tagging, and
  5. publication of a synthesis and critical review of CSS results

The CSS is a large-system study that has collected a substantial amount of PIT-tag data from multiple species and stocks over a 17-year period, but to date identification of hypotheses on the causal mechanisms regulating SARs has been limited. The ISAB suggests a comparative approach to identifying hypotheses that may lead to a greater understanding of causal mechanisms. The CSS posed important questions related to stream productivity and hydrosystem survival that were not addressed by the life-cycle model in this report and need to be addressed by the next version of the model.

A detailed reevaluation of SAR objectives (2-6%) is warranted. These objectives should be reevaluated for each species and Evolutionarily Significant Unit (ESU) of salmon and steelhead based on realistic values needed to support robust viable populations. Discrepancies in SARs between PIT-tagged and non-PIT-tagged fish reported in other publications raise two important issues that could be addressed now: (1) what are the implications of correcting biased SAR estimates from PIT tags with respect to performance against recovery and Fish and Wildlife Program objectives, and (2) what proportion of US Endangered Species Act (ESA)-listed populations are being PIT-tagged and what are the implications for imposing this additional mortality? Further work is needed to analyze the relationship between the ratio of transport/in-river SARs and in-river survival. With many years of experience now, the CSS needs to identify critical data gaps. What crucial pieces of information are not addressed by the CSS, and what improvements can be made to provide them? Some examples provided by the ISAB include the lack of habitat-specific estimates of smolt survival in the estuary, information on how age at maturation affects SARs, the contribution of mini-jacks to SARs, and the relationship between SARs and biomass of adult returns of hatchery and wild salmon.

The ISAB recommends a new focus on rationalization of the PIT-tagging program given the very large detection infrastructure already in place and the overlapping objectives of the different tagging studies (see IEAB document 2013-1 and ISAB 2013-3). It may be possible to reduce the numbers of populations and fish that are PIT tagged without significant loss of information, leading to greater program efficiencies at lower cost. The ISAB also recommends that the CSS prepare and submit a manuscript for peer-reviewed publication that synthesizes and critically reviews the results of the CSS study.

Most of the information in the CSS's 2013 report is an annual update of information in previous year's reports. Our summary, therefore, focuses on new information presented in Chapter 2, which develops and describes a simple life-cycle model. In this model, information from multiple populations is used to estimate parameters common to the different populations (ocean survival) while allowing each population to have a different spawner/recruit relationship. The key advantage of this approach is the reduction in the total number of parameters used to describe dynamics of the populations because of the assumption of common ocean survival. Additionally, estimates can be obtained for certain life states for populations that lack direct data. The conceptual basis of the model appears to be sound, although evidence supporting the primary assumption of common ocean survival was not provided in the report. Three versions of the model with different levels of complexity were evaluated. While the ISAB understands that model development is at the initial stage, there are numerous difficulties in the model’s description that make it uncertain if the three versions of the model have been implemented correctly. Model equations do not match the flow diagram. No estimates of precision from the models were presented. Only a small amount of model assessment was done. Comparison of models using the Akaike information criterion (AIC) should be included.

An alternate but similar approach would be through the use of state-space models in a Bayesian context. This would allow the incorporation of natural variation in the transition between life stages that is currently ignored in this approach and the ability to incorporate prior information on some parameters in a more natural way rather than, for example, assuming the second year of ocean survival is exactly 0.6. Applying a hierarchical approach to the spawning parameters across the populations would also allow some sharing of information across population when data are sparse. Finally, it would also provide probabilistic forecasts of future population trajectories.

See the full report for details.

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