“Meeting yield and conservation objectives by harvesting of both juveniles and adults”
Niklas L. P. Lundström, Nicolas Loeuille, Xinzhu Meng, Mats Bodin, and Åke Brännström (Mar 2019)
Fisheries are both economically important and an important source of food. With almost one-third of the world’s fished marine stocks currently being overexploited, there is a need to improve current fisheries practices to secure this valuable resource for future generations.
Contemporary fisheries primarily target adult fish as they are financially more valuable than juvenile fish, and fish stocks are often placed under intense fishing pressure. We show that a small reduction in harvesting intensity and the inclusion of juvenile fish in the harvest can greatly improve the sustainability of fisheries. Specifically, we consider harvesting intensities that still give “Pretty Good Yield,” defined as yields within 80% of the maximum sustainable yields.
In our study, we use simple mathematical and economical tools to investigate when it is possible to increase the fish stocks’ resilience, against e.g. overexploitation and environmental shocks, while at the same time keeping the fished biomass high, by adjusting the harvesting strategy from mainly harvesting large adult fish to also harvesting small juvenile fish at the same rate. The methodology is general and can be tailored to specific fish stocks and extended to interacting population, thus also contributing to the theoretical foundation of an ecosystems approach to fisheries management (EAFM).
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.