Relationships of otter populations with fish, macroinvertebrates and water quality across three Korean rivers revealed by inferential modelling based on evolutionary computation

Depending on the life cycles of trophic biota, cascades of biological transition are established between top-down and bottom-up relationship. In South Korea, flows of the large rivers have been regulated with large weirs and estuary barrages construction. The Eurasian otter Lutra lutra is a semi-aquatic carnivore and top predator in freshwater systems. In order to understand the aquatic community variability under the impact of river modifications, we assessed the strengths of otter populations in relation to limnological data monitored between 2014 and 2016 at 250 sites of the Nakdong River, 92 sites of the Yeongsan River and 81 sites of the Seumjin River. The habitat preference and resource use of otter populations have been measured for each of the 423 river sites by the number of spraint (= otters' feces) per 600 m. The limnological data included water quality parameters, abundances of Mollusca, Anthropoda, Annelida, Nematomorpha, and Platyhelminthes as well as abundances of benthivore, herbivore, planktivore and piscivore fish. The hybrid evolutionary algorithm HEA has been applied to model relationships of otter with water quality, the benthic invertebrates and fish to determine habitat and food preferences of otter across the three rivers. Despite the stochastic noise of the data, the modelling results addressed well relationships between spraint densities and densities of fish and macroinvertebrate communities. Threshold conditions of the IF-THEN-ELSE-models discovered by HEA revealed otter responses to changing habitats and aquatic communities across the river sites but also between the three rivers. In addition, sensitivity analysis suggested possible relationships between the otter and aquatic communities. According to the results of sensitivity analysis, otter populations seemed not to control the piscivorous fish densities by showing a negative relationship, but preyed preferably on benthivorous and planktivorous fishes. Water quality parameters and benthic macro-invertebrates correlated well with otter populations suggesting favorable habitat and food-web structures. There is evidence that river flow regulation measures have a negative impact on otter populations, and that growing otter populations will alter future food web structures of the three rivers.