Capacity for increase, compensatory reserves, and catastrophes as determinants of minimum viable population in freshwater fishes

Population recovery targets are a pressing need for the conservation and management of species at risk, and demographic sustainability, expressed as the size of a minimum viable population (MVP), is frequently used to guide conservation efforts. Here, we conduct population viability analyses of stage-structured populations to explore the roles of maximum capacity for increase (rmax), density-dependent compensatory reserves, and catastrophes as determinants of MVP in freshwater fishes. Computer simulations of random-walk population trajectories for 31 freshwater fish species listed as threatened and endangered in Canada were repeated at various probabilities of random catastrophes within the range observed in vertebrate populations. MVP increased with rmax and decreased with the strength of compensatory reserves; low compensatory reserves are associated with greater variability in population size. Although large-bodied species exhibit lower rmax values, their greater compensatory reserves allow them to buffer density-dependent variability in early mortality. Catastrophes have a small influence on MVP sizes of large-bodied freshwater fishes (e.g., sturgeons). Conversely, MVP sizes of small-bodied freshwater fishes are strongly sensitive to catastrophes, in most cases requiring larger MVP sizes than determined for other vertebrate taxa even at moderate catastrophe probabilities. Thus, knowledge of primary catastrophic events and their probabilities are particularly essential for setting population recovery targets of small-bodied species.