Cascading food-web effects of piscivore introductions in shallow lakes

摘要

1. Anthropogenic landscape modifications have resulted in increased water depths and greater connectivity among the remaining shallow lakes throughout eastern portions of the Prairie Pothole Region of North America. This has created conditions favourable for the establishment of planktivorous fish populations, notably fathead minnows Pimephales promelas. Such fish populations may reach high densities and reduce zooplankton and macroinvertebrate abundance and diversity. Resulting reductions in algal grazing by zooplankton favour transition to a turbid-water state, characterized by increased phytoplankton, decreased water clarity, and reduced macrophytes. Ultimately, such lakes are characterized by low habitat complexity and biodiversity, and conditions less favourable for waterbirds and other wetland-dependent species. 2. We conducted a 3-year study to assess the efficacy of walleye Sander vitreus stocking as a means of initiating trophic cascades, thus testing the ecological theory that piscivore addition can promote shifts from the turbid to the clear-water state by suppressing planktivores (fathead minnows), increasing zooplankton and macroinvertebrates, and reducing phytoplankton. Treatments consisted of six sites stocked with walleye fry, six lakes stocked with walleye age-1 and older, and six control lakes containing antecedent fathead minnows only. 3. Stocking walleye fry resulted in much lower densities of fathead minnows and higher densities of cladocerans and some macroinvertebrates. Chlorophyll a also decreased in the fry treatment sites. Stocking age-1 and older walleye induced few changes at the stocking density we tested. 4. The success of walleye fry appears to be partly attributable to their rapid diet shift to fish prey and their ability to consume and suppress all life stages of planktivores (fry, juveniles, and adults). However, most responses to walleye fry additions were short-lived, indicating that achieving long-term improvements may require repeated restocking. 5. Synthesis and applications. The available evidence indicates that catchment-level restoration alone is often of limited utility in restoring clear-water conditions in shallow lakes. Our results indicated that stocking piscivorous fish has the potential to improve ecological characteristics of shallow lakes, overcoming tendencies towards phytoplankton dominance. We suggest that managers consider using piscivorous fish stocks, ideally in conjunction with catchment-level restoration measures, to stimulate improvement of ecological characteristics in shallow lakes.