Weakened portfolio effects constrain management effectiveness for population aggregates

Abstract

Population diversity can reduce temporal variability in aggregate population abundances in a process known as the portfolio effect. Portfolio effects may weaken, however, due to greater synchrony among component populations. While weakened portfolio effects have been previously documented, the consequences of reduced stability on meeting conservation goals for population aggregates that are harvested (e.g., stock aggregates in fisheries) are rarely quantified. Here, we demonstrate how changes in variability within components, synchrony among components, and population productivity interact to influence the probability of achieving an array of management objectives for Fraser River sockeye salmon: a stock aggregate of high economic, ecological, and cultural value. We first present evidence that component variability and synchrony have increased over the last two decades, consistent with a weakening portfolio effect. We then parameterize a stochastic, closed‐loop model that simulates the population dynamics of each stock, the fishery that harvests the stock aggregate, and the management framework used to establish mixed‐stock exploitation rates. We find that while median aggregate abundance and catch through time were relatively insensitive to greater aggregate variability, catch stability and performance metrics associated with achieving management targets generally declined as component variability and synchrony increased. A notable exception we observed is that harvest control means that scale exploitation rates based on aggregate abundance may be more effective as synchrony increases. Reductions in productivity led to broad declines in performance, but also moderated the impacts of component variability and synchrony on the proportion of component stocks above management targets and catch stability. Our results suggest that even precautionary management strategies that account for declines in productivity may underestimate risk, particularly to socioeconomic objectives, if they fail to consider changes in aggregate variability. Adequately incorporating changes in portfolio effect strength may be particularly relevant when developing recovery strategies that are robust to climate change, which is likely to increase synchrony and component variability.

Conservation risks and portfolio effects in mixed‐stock fisheries

“…Exploring three major sockeye salmon fisheries (Bristol Bay, Alaska; Fraser River, BC; Skeena River, BC; Table 1) revealed both trade‐offs and opportunities in mixed‐stock fisheries. As previously shown (Freshwater et al., 2019; Price et al, 2021; Schindler et al., 2010), all three systems benefit from portfolio effects, where aggregate catches were more stable than would be expected based on observed variation in the individual stock abundances (Table 1). Further, as predicted by simulations (Figure 1) and theory (Doak et al., 1998), this portfolio effect was smaller in the Skeena where there was substantially lower evenness (Table 1).…”

“…First, building on examinations of risks in mixed‐stock fisheries (Cheung & Sumaila, 2008; Connors, Staton, et al. 2020; Hilborn et al., 2012; Ricker, 1958; Freshwater et al., 2019; Okamoto, Hessing‐Lewis, et al 2020), we illustrate how different aspects of biodiversity (asynchrony, evenness, richness, productivity) exacerbate trade‐offs among conservation and fishery objectives. Second, building on research on biodiversity and fisheries performance (Schindler et al., 2010; Worm et al., 2006) and more general research on biodiversity and ecosystem functioning (Cardinale et al., 2013; Hooper et al., 2005; Loreau et al., 2001; Naeem et al., 2012; Pasari et al, 2013), we explore how these same dimensions of biodiversity influence fishery performance.…”

“…All three fishery complexes target a diversity of stocks (at least 8 stocks) that are relatively asynchronous and exhibit a range of stock productivities (Table 1). Accordingly, portfolio effects have been observed to dampen interannual catch variability in all three systems: Bristol Bay (Schindler et al., 2010), Skeena (Price et al, 2021), and Fraser (Freshwater et al, 2019). From a qualitative perspective, management control across these systems, at the scale at which it occurs, varies from high (Bristol Bay), intermediate (Fraser), to low (Skeena) due to factors such as spatial configuration of the fishery complex (multiple rivers vs. single river), run‐timing diversity, and the extent to which in‐season management occurs (Table 1).…”

Influence of warming temperatures on coregonine embryogenesis within and among species

“…Intersexual selection and mate pairing has been proposed as an important component affecting coregonine offspring fitness (Wedekind et al, 2008;Huuskonen et al, 2011;Karjalainen & Marjomäki, 2018), and may play a role in conserving natural biodiversity within populations (Anneville et al, 2015). The long-term stability of commercially exploited stocks, which can experience fisheriesinduced evolution, has been linked to population diversity (Schindler et al, 2010;Freshwater et al, 2019). Spawning stocks that comprise individuals of variable sizes and ages (e.g., portfolio effect; Schindler et al, 2010) may contribute differently to spawning, offspring performance, and recruitment (Luck et al, 2003;Figge, 2004), and are likely an important consideration as the rapid rate of climate change adds additional stressors on populations.…”

Influence of warming temperatures on coregonine embryogenesis within and among species

“…Intersexual selection and mate pairing has been proposed as an important component affecting coregonine offspring fitness (Wedekind et al, 2008; Huuskonen et al, 2011; Karjalainen & Marjomäki, 2018), and may play a role in conserving natural biodiversity within populations (Anneville et al, 2015). The long-term stability of commercially exploited stocks, which can experience fisheries-induced evolution, has been linked to population diversity (Schindler et al, 2010; Freshwater et al, 2019). Spawning stocks that comprise individuals of variable sizes and ages ( e.g., portfolio effect; D. E. Schindler et al, 2010) may contribute differently to spawning, offspring performance, and recruitment (Luck et al, 2003; Figge, 2004), and are likely an important consideration as the rapid rate of climate change adds additional stressors on populations.…”