Biology subjects

Howell, D.

Publications and source records attributed to Howell, D..

2 recordsLinked to original sources

bioRxiv2020

A precautionary solution to estimation bias in shaping safe harvest boundaries

Biased estimates of population status are a pervasive conservation problem. This problem has plagued assessments of commercial exploitation of marine species and can threaten the sustainability of both populations and fisheries. We develop a computer-intensive approach to minimize adverse effects of persistent estimation bias in assessments by optimizing operational harvest measures (harvest control rules) with closed-loop simulation of resource-management feedback systems: management strategy evaluation. Using saithe (Pollachius virens), a bottom-water, apex predator in the North Sea, as a real-world case study, we illustrate the approach by first diagnosing robustness of the existing harvest control rule and then optimizing it through propagation of biases (overestimated stock abundance and underestimated fishing pressure) along with select process and observation uncertainties. Analyses showed that severe biases lead to overly optimistic catch limits and then progressively magnify the amplitude of catch fluctuation, thereby posing unacceptably high overharvest risks. Consistent performance of management strategies to conserve the resource can be achieved by developing more robust control rules. These rules explicitly account for estimation bias through a computational grid search for a set of control parameters (threshold abundance that triggers management action, Btrigger, and target exploitation rate, Ftarget) that maximize yield while keeping stock abundance above a precautionary level. When the biases become too severe, optimized control parameters- for saithe, raising Btrigger and lowering Ftarget-would safeguard against overharvest risk (<3.5% probability of stock depletion) and provide short-term stability in catch limit (<20% year-to-year variation), thereby minimizing disruption to fishing communities. The precautionary approach to fine-tuning adaptive risk management through management strategy evaluation offers a powerful tool to better shape sustainable harvest boundaries for exploited resource populations when estimation bias persists. By explicitly accounting for emergent sources of uncertainty our proposed approach ensures effective conservation and sustainable exploitation of living marine resources even under profound uncertainty. Open Research StatementData sets and code utilized for this research are available on Figshare. DOI: https://doi.org/10.6084/m9.figshare.13281266

ecology↗

bioRxiv2020

Managing Arctic predator harvest instability amplified by variable prey productivity and ocean temperature

Sustainable human exploitation of marine living resources stems from a delicate balance between short-term yield stability and long-term population persistence to achieve socioeconomic and conservation goals. However, imperfect knowledge of how oscillations in ecosystem processes regulate fluctuations in exploited populations can obscure the risk of missing management targets. We illustrate how the harvest policy to suppress short-term yield fluctuation inadvertently disrupts population cycles and yield stability of exploited, long-lived predators under stochastically fluctuating environmental forces (food availability and regional climate) using Northeast Arctic (NEA) cod (Gadus morhua, an apex predatory fish) as a case study. We use a stochastic, empirically parameterized multispecies model to simulate NEA cod population dynamics through life-history processes; Barents Sea capelin (Mallotus villosus, a pelagic forage fish) modulates cod productivity through density-dependent cannibalism-predation dynamics, whereas sea temperature regulates cod consumption, growth, and recruitment. We first test how capelin and sea temperature fluctuations regulate patterns in cod yield fluctuation and then quantitatively assess how fishing pattern designed to limit yield between-year variance (within 50-5%) perturbs cod population-catch dynamics. Simulations suggest that capelin and temperature interactively contribute to shifting cyclic patterns in cod yield fluctuation primarily through cod cannibalism-predation dynamics. Wavelet analyses further show that muffling yield variance (30 % or less) reshapes the cyclicity (shorter period and greater amplitude) of cod population size and demography, thereby becoming progressively unsynchronized with fishing pressure. Our work reveals unintended consequences of managing transient dynamics of fished populations: the interworking of population cycle destabilized by inadvertently intensifying fishing pressure, amplifying yield fluctuation and, in turn, elevating overharvest risk when not accounting for compounded effects of stochasticity in ecologically connected processes. These policy implications underscore the need for an ecosystem approach to designing ecologically sound management measures to safely harvest shared living resources while achieving socioeconomic security in increasingly more dynamic oceans in the Arctic and elsewhere.

ecology↗