Potential importance of spatial and temporal heterogeneity in pH, Al, and Ca in allowing survival of a fish population: A model demonstration

Abstract

Discrepancies usually exist between the results of bioassay studies and the status of natural fish populations. One cause of such discrepancies is the spatial and temporal heterogeneity in the stressing factors or contaminant concentrations in natural systems. To provide a means of relating laboratory and field bioassay results to natural populations, we have modified an existing Monte Carlo mathematical model to track the movement and survival of fish in a body of water, represented by a two-dimensional grid, subject to acidification stress. We assumed that the fish (adults or the less mobile and generally more sensitive early life stages) are able to sense realistic gradients of pH, aluminium (Al), and calcium (Ca) (or alkalinity) and to move to reduce the chemical stress. We have also considered the effects of food availability and habitat preference on fish movement. Each fish takes one spatial ‘step’ at a time, the direction being randomly selected but also biased by the above factors. A function for accumulation and repair of damage is implemented within the model to relate mortality to the variable exposure history the fish accumulate by moving in the chemically heterogeneous environment. Using the model, we evaluated the influence of the strength and sensitivity of fish avoidance behavior, the presence or absence of the refuge, and the timing of multiple pulses on fish survival. The results highlight the potential importance of a chemically heterogeneous natural environment in allowing a population to survive under average water-quality conditions that laboratory bioassays suggest should prohibit survival. Laboratory and field behavior experiments, including characterization of water chemistry on a microscale in the field, are suggested to explore this phenomenon further.