Fraser River sockeye salmon: data synthesis and cumulative impacts

c. I anticipate that a collection of factors whose impacts were felt through acombination of time lags on non-additive and nonlinear stress impactswould be very hard, if not impossible, to detect with this sort of multipleregression analysis – especially when so little is known about some keyfactors like pathogens.Response: These are all excellent points. We had only limited time to conduct statisticalanalyses, and so we chose the simplest approaches. In Appendix 3 and a new section in themain report (3.3.6), we acknowledge the weaknesses and limitations of our approaches. Webelieve that the data limitations (i.e., appropriate covariates reflecting the impact pathwaysof concern; sufficient levels of contrast) are at least as serious a problem as the analyticalproblems.To address specific components of the reviewer’s comments:a) In Section 4.7, we discuss potential cumulative effects over the entire life cycle,though we also note the difficulty of determining the form (e.g. additive or nonadditive),magnitude, location and timing of such effects. We have also addedfurther discussion on the potential for other functional forms for the candidatemodels (i.e. non-linear covariates, interactions) in our descriptions of the methods.The description and rationale for the approach we used with the change-pointanalyses has also been expanded.b) We have discussed functional regression analysis in Appendix 3 as a potentialtechnique to be used in the future.c) We agree.3. These inherent weaknesses notwithstanding, I believe that the authors usedgood judgment in applying these techniques. In particular, I support their use ofscientific knowledge and common sense in limiting the candidate factors for themultiple regression models.4. I would encourage the authors to provide examples of instances in which the sortof approach taken here would not have brought fundamental, underlying causesto light. For example, it seems unlikely that a multiple regression analysis withoutappropriate time lags could have drawn anyone’s attention to the keyphenomenon of bioaccumulation in the early days of research on ecologicalimpacts of DDT.Response: This is a good example, which we will incorporate into our discussion of thelimitations of our analyses (in Appendix 3). Other mechanisms which could cause linearregressions to miss important ecosystem linkages include non-linear relationships betweenecosystem productivity, sockeye abundance and predation (Christensen and Trites 2011,pgs. 13, 76), as well as non-linear threshold effects from contaminants (MacDonald et al.2011).152