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Special situations, which are difficult to predict and to model by traditional methods, develop rapidly in population processes. A sudden collapse of commercial fish stocks, in addition to outbreaks of forest pests, are among the most economically significant nonlinear phenomena in ecosystems. According to systems analysis of statistical data on the dynamics of catches, the transitional stages of rapid degradation in completely different species of fish and aquatic invertebrates occur in a similar way. The most representative stages of the development of fish resources collapse have been distinguished. Restoration of the already critically depleted aquatic bioresources occurs at different rates. Based on the method of dynamically redefinable hybrid computational structure, we considered situations of collapse that occurred with crab off the coast of Alaska and northern cod off the coast of Canadian provinces of Newfoundland and Labrador. The resulting computational scenarios for collapse manifestation consist of three stages, all the way up to the degradation of bioresources. Bifurcations are implemented purposefully and follow the logic of the process. The modeling method is generalized for cases with stationary food resources and with oscillatory dynamics of food organisms. [ABSTRACT FROM AUTHOR]