Evolutionary Impact of Size-Selective Harvesting on Shoaling Behavior: Individual-Level Mechanisms and Possible Consequences for Natural and Fishing Mortality

16 pages, 5 figures, 1 table, supplementary data https://doi.org/10.5061/dryad.x95x69pkf; supplementary material https://www.journals.uchicago.edu/doi/suppl/10.1086/718591.-- Data and Code Availability: Experimental data and the R code have been deposited in the Dryad Digital Repository (https://doi.org/10.5061/dryad.x95x69pkf; Sbragaglia 2021). The code to run the burst-and-coast model with the fishing/predator scenarios is available at GitHub (https://doi.org/10.5281/zenodo.5651937; https://github.com/PaPeK/sde_burst_coast). The optimization was done with a Python implementation of the CMA-ES (https://github.com/CMA-ES/pycma)

Intensive and size-selective harvesting is an evolutionary driver of life history as well as individual behavioral traits. Yet whether and to what degree harvesting modifies the collective behavior of exploited species are largely unknown. We present a multigeneration harvest selection experiment with zebrafish, Danio rerio, as a model species to understand the effects of size-selective harvesting on shoaling behavior. The experimental system is based on a large-harvested (typical of most wild-capture fisheries targeting larger size classes) and small-harvested (typical of specialized fisheries and gape-limited predators targeting smaller size classes) selection lines. By combining high-resolution tracking of fish behavior with computational agent-based modeling, we show that shoal cohesion changed in the direction expected by a trade-off between individual vigilance and the use of social cues. In particular, we document a decrease of individual vigilance in the small-harvested line, which was linked to an increase in the attention to social cues, favoring more cohesive shoals. Opposing outcomes were found for the large-harvested line, which formed less cohesive shoals. Using the agent-based model, we outline possible consequences of changes in shoaling behavior for both fishing and natural mortality. The changes in shoaling induced by large size-selective harvesting may decrease fishing mortality but increase mortality by natural predators. Our work suggests an insofar overlooked evolutionary mechanism by which size-selective harvesting can affect fishing and natural mortality of exploited fish

V.S. was supported by a Leibniz-DAAD (German Academic Exchange Service) postdoctoral research fellowship (91632699), and he is now supported by the Spanish Ministry of Science and Innovation (Juan de la Cierva Incorporación Research Fellowship, IJC2018-035389-I). V.S. also acknowledges the Spanish government through the Severo Ochoa Centre of Excellence accreditation to the Department of Marine Renewable Resources, Institute of Marine Sciences (ICM-CSIC; CEX2019-000928-S). P.P.K. and P.R. received financial support from the German Research Foundation (DFG) under RO 4766/2-1 and under Germany’s Excellence Strategy (EXC 2002/1 Science of Intelligence, project 390523135)

Peer reviewed