Weakened portfolio effects constrain management effectiveness for population aggregates

Area of Science:

• Ecology
• Fisheries Science
• Population Dynamics

Background:

• Population diversity can buffer aggregate population stability via the portfolio effect.
• Weakened portfolio effects, due to increased population synchrony, have been observed but their conservation consequences are understudied.
• Harvested population aggregates, like fisheries stocks, face risks from reduced stability.

Purpose of the Study:

• To quantify the impact of changing variability, synchrony, and productivity on management objectives for Fraser River sockeye salmon.
• To assess how a weakening portfolio effect influences the probability of achieving conservation and management targets.
• To inform adaptive management strategies for harvested stocks facing environmental change.

Main Methods:

• Analysis of historical data on component variability and synchrony in Fraser River sockeye salmon populations.
• Development and parameterization of a stochastic, closed-loop model simulating stock dynamics, fishery, and management.
• Evaluation of management performance metrics under varying levels of variability, synchrony, and productivity.

Main Results:

• Component variability and synchrony have increased in Fraser River sockeye salmon, indicating a weakening portfolio effect.
• While median aggregate abundance and catch were stable, catch stability and achievement of management targets declined with increased variability and synchrony.
• Harvest control methods scaling with aggregate abundance showed increased effectiveness with higher synchrony.
• Reduced productivity broadly decreased performance but moderated the negative impacts of variability and synchrony on stock targets and catch stability.

Conclusions:

• Precautionary management strategies may underestimate risk to socioeconomic objectives if they ignore changes in aggregate variability and portfolio effect strength.
• Climate change is predicted to increase synchrony and variability, necessitating robust recovery strategies that incorporate these dynamics.
• Understanding and incorporating the portfolio effect is crucial for effective fisheries management and conservation, especially for valuable stock aggregates.