[Paper Review] Meeting yield and conservation objectives by balancing harvesting of juveniles and adults

Niklas L. P. Lundström, Nicolas Loeuille+3 more|arXiv (Cornell University)|Dec 29, 2017

Ecosystem dynamics and resilience57 references3 citations

TL;DR

This study investigates stage-structured harvesting strategies that balance maximum sustainable yield (MSY) and conservation by varying juvenile and adult harvest rates. It finds that equal harvesting of juveniles and adults often achieves 'pretty good yield' (PGY) while delivering strong conservation benefits, with resilience measures consistently favoring balanced harvest over adult-only exploitation.

ABSTRACT

Sustainable yields that are at least 80% of the maximum sustainable yield are sometimes referred to as pretty good yield (PGY). The range of PGY harvesting strategies is generally broad and thus leaves room to account for additional objectives besides high yield. Here, we analyze stage-dependent harvesting strategies that realize PGY with conservation as a second objective. We show that (1) PGY harvesting strategies can give large conservation benefits and (2) equal harvesting rates of juveniles and adults is often a good strategy. These conclusions are based on trade-off curves between yield and four measures of conservation that form in two established population models, one age-structured and one stage-structured model, when considering different harvesting rates of juveniles and adults. These conclusions hold for a broad range of parameter settings, though our investigation of robustness also reveals that (3) predictions of the age-structured model are more sensitive to variations in parameter values than those of the stage-structured model. Finally, we find that (4) measures of stability that are often quite difficult to assess in the field (e.g.~basic reproduction ratio and resilience) are systematically negatively correlated with impacts on biomass and impact on size structure, so that these later quantities can provide integrative signals to detect possible collapses.

Motivation & Objective

• To identify harvesting strategies that achieve high yields (at least 80% of MSY, i.e., PGY) while also supporting conservation objectives.
• To evaluate the trade-offs between yield and four conservation metrics: biomass impact, size structure impact, resilience, and basic reproduction ratio.
• To compare results across two distinct population models—age-structured and stage-structured—to assess robustness of findings.
• To determine whether equal harvesting rates for juveniles and adults outperform adult-only harvesting in terms of yield and conservation outcomes.
• To assess the reliability of stability indicators (e.g., resilience, basic reproduction ratio) as proxies for population collapse risk.

Proposed method

• Uses an age-structured population model with age-dependent survival, fecundity, and density-dependent recruitment to simulate fish population dynamics.
• Employs a stage-structured consumer-resource model with food-dependent growth, maturation, and fecundity to represent ontogenetic asymmetry and resource feedbacks.
• Introduces selective harvesting by applying separate fishing mortality rates (F_J for juveniles, F_A for adults) across a wide parameter space.
• Quantifies yield as total harvest over time and evaluates conservation via four metrics: impact on population biomass, size structure, resilience, and basic reproduction ratio (R₀).
• Calculates resilience using eigenvalue analysis of the Jacobian matrix and basin-of-attraction methods, and derives R₀ analytically for the age model.
• Compares results across models to test robustness and consistency of findings under varying ecological assumptions.

Experimental results

Research questions

• RQ1 Can PGY harvesting strategies simultaneously achieve high yield and significant conservation benefits?
• RQ2 How do different harvesting strategies (varying F_J and F_A) affect the trade-off between yield and conservation metrics?
• RQ3 Is equal harvesting of juveniles and adults a robust strategy for balancing yield and conservation across different population models?
• RQ4 How do stability indicators like resilience and basic reproduction ratio correlate with observable impacts such as biomass and size structure?
• RQ5 How sensitive are predictions from the age-structured model compared to the stage-structured model under parameter variation?

Key findings

• PGY harvesting strategies can deliver substantial conservation benefits, particularly when juvenile and adult harvesting rates are balanced.
• Equal harvesting rates for juveniles and adults consistently outperform adult-only harvesting in terms of resilience and conservation metrics across both models.
• The age-structured model's predictions are more sensitive to parameter variations than those from the stage-structured model, indicating lower robustness.
• Measures of resilience such as the real part of the dominant eigenvalue and basin-of-attraction dynamics are systematically negatively correlated with biomass and size structure impacts.
• Impact on biomass and size structure serve as reliable, integrative indicators of population stability and collapse risk, potentially replacing harder-to-measure resilience metrics in field applications.
• The basic reproduction ratio (R₀) derived for the age model shows that equal harvesting maintains higher reproductive potential than adult-only harvesting, especially under high adult mortality.