Estimating delayed density-dependent mortality in sockeye salmon (Oncorhynchus nerka): a meta-analytic approach

Abstract

Delayed density-dependent mortality can be a cause of the cyclic patterns in abundance observed in many populations of sockeye salmon (Oncorhynchus nerka). We used a meta-analytical approach to test for delayed density dependence using 34 time series of sockeye data. We found no consistent evidence for delayed density-dependent mortality using spawner - spring fry or spawner-recruit data. We did find evidence for delayed density-dependent mortality at a 1 year lag for the spawner - fall fry and the spawner-smolt data; however, effects at lags greater than 1 year were weak and not statistically significant. Between-cohort interactions at a 1-year lag are not sufficient to cause the observed cycles in adult abundance, although they will tend to reinforce patterns established by other mechanisms. Our results imply that reductions in fishing mortality in off-peak years should result in an increase in abundance.

Meta-Analysis of Cod–shrimp Interactions Reveals Top-Down Control in Oceanic Food Webs

“…To further test this and related hypotheses for other species and ecosystems, we propose the following formal procedure: (1) Use diet composition or behavioral data to establish possible food web linkages (e.g., predation on particular species or functional groups); (2) assemble biomass time series for species that are believed to interact; (3) correct for measurement error and autocorrelation; (4) correlate time series and use random-effects meta-analysis to combine estimates of effect size (z-transformed correlation coefficients); (5) examine data sets for spatial correlation (Myers et al 1997c) and adjust weightings accordingly (Myers et al 1997a), or, alternatively, perform a sensitivity analysis in which potentially correlated data sets are eliminated; (6) test alternative hypotheses using the same framework; and (7) fit alternative models. We admit that loglog correlations represent a very simplistic mathematical model of population interactions, equivalent to a power model of the form s ϭ ␣/c ␤ , where s and c represent shrimp and cod biomass and ␣ and ␤ are parameters.…”

“…This suggests that data sets in regions that are Ͻ500 km away may not be entirely independent. Unfortunately, our data set was too small to accurately estimate the covariation among stocks and use this as a measure of spatial independence (Myers et al 1997a). Therefore, we report both the results of the complete analysis and those from an analysis in which regions that were Ͻ500 km from neighboring ones were excluded.…”

Stock and recruitment: generalizations about maximum reproductive rate, density dependence, and variability using meta-analytic approaches

“…A natural scaling would be = / and = / . Using this approach, Myers et al (1997a) have found evidence of moderate intercohort density-dependent mortality at lag one for sockeye salmon populations. For sockeye, intercohort density-dependent mortality at greater lags was at most weak and not significant.…”

Migration links ocean‐scale competition and local ocean conditions with exposure to farmed salmon to shape wild salmon dynamics

“…Delayed density‐dependent processes, which result from high spawner abundance in one brood year reducing the productivity of subsequent brood years in a given population, have long been hypothesized to maintain the cyclic dominance characteristic (i.e., large changes in abundance during each 4‐year period) of many Fraser sockeye populations (e.g., Larkin 1971; Myers et al 1997; Martell et al 2008). Management actions that successfully increased the abundance of spawners of Fraser River sockeye (Figure 2A) have therefore led to concerns that delayed density‐dependent processes in fresh water, including competition for food and buildup of predator populations (e.g., rainbow trout), may have contributed to depressed productivity of subsequent sockeye generations.…”