Extensively reared juvenile walleyes Stizostedion vitreum subjected to capture and transport stressors associated with stocking procedures in South Dakota and Minnesota displayed acute increases in plasma cortisol and extended declines in concentrations of plasma chloride. In four separate South Dakota transport hauls, mean plasma cortisol in walleyes rose from 12–49 ng/mL at the time of trap‐net capture to 138–174 ng/mL after 1 h of transport; these concentrations remained increased at 3 h. Plasma chloride decreased by about 19% during this period; by 3 h, mean values had dropped about 30%, to 53–61 meq/L. Similarly, in Minnesota mean plasma cortisol in captured and transported walleyes increased from 13–26 ng/mL during trap‐net capture to 154–214 ng/mL by 1 h after transport in three separate trials and remained at the higher values at 3 h posttransport. Mean plasma chloride concentrations dropped about 28%, to 65–72 meq/L, by 3 h after transport and remained significantly decreased at 68–82 meq/L 24 h later, even though plasma cortisol, an acute stress indicator, had returned to near pretransport values. Results from these experimental transport hauls indicate that the juvenile walleyes experienced considerable physiological stress when captured and transported to lakes for stocking. This stress was reflected by an appreciable decrease in plasma chloride that failed to recover to pretransport values after 24 h, which suggests a possible mechanism for poststocking mortality.

Summary 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.

We surveyed anglers on four Minnesotta lakes who fish for bluegill Lepomis macrochirus to determine (1) if they would support regulation changes designed to increase bluegill size structure, (2) if their behavior would allow increases in bluegill size structure to be sustainable, and (3) what they viewed as the causes and remedies for declining bluegill fisheries. The majority of anglers surveyed would not support regulation changes on bluegill fisheries. However, most said they would increase the number of fishing trips they took if increased bluegill size structure could be reestablished. We estimated that a modest increase of two trips annually would result in a 16–34% increase in the rate of exploitation on the four lakes. On certain lakes this could jeopardize the sustainability of a quality fishery. The majority of anglers believed that stunting was the cause of the decline in bluegill populations and that removal by managers and anglers was the most important management tool available.

Fisheries managers throughout the Prairie Pothole Region of Minnesota often use semi-permanent and permanent wetland basins to extensively culture walleye Sander vitreus fry. Waterfowl managers have expressed concern over this practice because of the potential influence that fish have on food resources used by waterfowl during development and migration.