Applying catastrophe theory and population dynamics models to understanding fisheries collapses in the Humboldt marine ecosystem
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2025
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Abstract
Abrupt stock and population collapses can occur due to intense fishing pressure, overfishing, orwhen intense exploitation interacts with oceanic climatic stressors; however, the latter of theseinteractions remains poorly understood in many highly productive ecosystems. This thesisexamines 18 data-rich stocks from the Humboldt Current Ecosystem (HCE), utilizingcatastrophe theory modeling in conjunction with theoretical population dynamics models byRoyama (2021) to demonstrate how changes in fishing intensity, combined with climaticvariables, can drive fisheries across ecological tipping points and ultimately lead to collapse.Paper 1 of this thesis utilizes the stochastic cusp model framework, demonstrating that two-thirds of the analyzed stocks, particularly demersal fish and benthic crustaceans, transition fromnon-linear dynamics to a low-biomass equilibrium when warm sea surface Temperatures (SST°C) are considered elevated, leading to destabilizing fishing mortality and exhibitingpronounced hysteresis. The Second paper of the thesis integrates theoretical populationdynamics models developed by Royama (2021), including a predator-prey one revealing thatthe South Pacific hake (Merluccius gayi gayi) collapse was a consequence of the (El NiñoSouthern Oscillation), first providing favorable conditions for the stock to growth underrelatively low fishing pressure, for later transitioning to more La Niña and neutral regimes thatcombine with increased fishing pressure to doom the stock to a pronounced collapse.Additionally, we demonstrated that the leading cause of the collapse was fishing pressure, ratherthan intense predation by the jumbo squid, as was previously evident. This finding is supportedby the fact that the predator-prey model used to test the hypothesis of the jumbo squid did notaccount for the effect of fishing pressure. The last Paper 3 of the thesis extends the Royamaframework to three austral gadids fish stocks: Southern Blue Whiting (Micromesistiusaustralis), Patagonian Grenadier (Macruronus magellanicus), and Southern Hake (Merlucciusaustralis), showing a combination of influences in their dynamics such as SST, the SouthernAnnular Mode (SAM), and a more than a decade of megadrought (expressed as localprecipitation regimes), reforge the recruit dynamics and density dependence of the stocks. Thisthesis concludes that fisheries policies should be climate-ready and must develop adaptiveharvest control rules linked to early warning metrics provided by climatic indices andoceanographic conditions, including precautionary closures that can be triggered by bifurcationthresholds for catastrophic stocks. Finally, multi-national efforts should be carried out acrossthe HCE to buffer the effect of future regimen shifts for coastal communities and fishermenwhose survival relies on the fishing industry.
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Tesis (Doctor en Ciencias Biológicas con Mención en Ecología)--Pontificia Universidad Católica de Chile, 2025.