Nonlinear and density-dependent fish habitat selection across physiochemical gradients in an invasive macrophyte habitat

Invasive macrophytes drive substantial changes to freshwater fish habitats. Such changes can influence how fish select for habitats, but fish-habitat relationships in many invasive macrophytes are often poorly understood at micro-scales. Fish habitat use is influenced by dissolved oxygen (DO) and habitat complexity, but this response can be nonlinear and dependent upon fish density. This study tested whether microhabitat use of sunfish (Lepomis spp.) in invasive macrophyte beds was density-dependent. Fish were sampled with underwater video point counts in six 0.405 ha experimental ponds with surface-matted hydrilla Hydrilla verticillata and stocked with varying densities of sunfishes. Regression models were used to evaluate the key drivers for fish habitat selection across DO, complexity and fish densities. Both fish occurrence and fish counts were positively influenced by DO and negatively influenced by habitat complexity, but the fish counts-DO relationship was dome-shaped and both fish occurrence and counts depended upon fish densities. For example, at high fish densities, fish used low and high DO and high macrophyte complexity; at low fish densities, fish avoided such areas. Fish counts peaked at intermediate DO and low macrophyte complexity. Density-dependent fish habitat selection appeared to mitigate detrimental effects of invasive macrophytes on fish habitats, indicating that fish can and do use 'inhospitable' habitats with potential positive population growth. Understanding density-dependent habitat selection is needed when evaluating the quality of habitats, as apparently inhospitable habitats can be utilized when fish density is high.