Competition for Open-Access Resources: A Class Exercise that Demonstrates the Tragedy of the Commons

Open-access resources, such as fisheries, often suffer from overexploitation due to competition among individuals. To help students appreciate this phenomenon, we developed a laboratory exercise that uses a computer model of a commercial fishery to illustrate the inevitability of overexploiting open-access resources. Our model contained two identical populations that were fished simultaneously: one on an individual, competitive basis and the other on a collective, cooperative basis. Both populations were governed by identical relationships for recruitment and catch-per-effort, and had constant growth and natural mortality rates. Competition within the individually-run fishery invariably led to population collapse whereas the cooperative group usually maintained sustained yields. This exercise demonstrated that optimal resource use was impossible in a purely competitive fishery and helped students understand fish population biology and commercial fishing strategies. The exercise can show how changes in regulations or population parameters influence yields from a fishery. It can be adapted for use in workshops and other professional settings, even for participants with little background in fisheries.This work was supported in part by the Ohio State University, Department of Zoology, and by the University of Wisconsin Sea Grant Institute under grants from the National Sea Grant College Program, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, and from the State of Wisconsin. Federal grant NA84AA-D-00065, project R/GB-24

Direct and Indirect Effects of Fish Predation on the Replacement of a Native Crayfish by an Invading Congener

In Ohio streams, the crayfish Orconectes rusticus is replacing O. sanborni, and herein we test how predators influence this replacement. In a field survey, crayfish were scarce when fish were abundant, suggesting that predators can adversely affect these prey. In laboratory experiments, we examined underlying mechanisms for this inverse relationship; specifically, we tested how crayfish species, adult aggression, and habitat heterogeneity influenced the predator-prey interaction. In a laboratory stream, smallmouth bass (Micropterus dolomieu) ate similar numbers of equal-sized O. rusticus and O. sanborni, but when sizes mimicked those in the field (i.e., O. rusticus 4 mm > O. sanborni), fewer O. rusticus were eaten. Fish also reduced juvenileactivity and behaviors whereas adult aggression increased the frequency of these risky responses. More affected by adult crayfish, O. sanborni should suffer disproportional predation where adults and juveniles interact. Thus, fish predators should increase replacement rates and adult aggression should further accelerate this process. Manifested through crayfish size, both indirect and direct predator effects contribute to the replacement of O. sanborni by O. rusticusA National Science Foundation Dissertation Improvement Grant, a Sigma Xi Grant-in-Aid, and the Ohio Cooperative Fish and Wildlife Research Unit provided funding

Competition between Larval Fishes in Reservoirs: The Role of Relative Timing of Appearance

Funding for this project was provided by National Science Foundation grants DEB 9407859 and DEB 9107173 to R.A.S and Federal Aid in Sport Fish Restoration, project F-69-P, administered jointly by the U. S. Fish and Wildlife Service and the Ohio Division of Wildlife. A Presidential Fellowship from The Ohio State University supported J.E.G. during part of this research.In small, hypereutrophic reservoirs (100 mg total phosphorus/L), larval gizzard shad Dorosoma cepedianum and threadfin shad D. petenense (henceforth, shad) reach high densities in the limnetic zone, virtually eliminate zooplankton, and perhaps compromise success of other planktivorous larvae, such as bluegill Lepomis macrochirus. Because relative timing of appearance of shad and bluegills probably influences their relative success, we quantified densities of fish larvae and zooplankton during spring through summer in three reservoirs across 8 years (1987–1994), and we conducted three hatchery experiments with varying larval appearance times and gizzard shad densities in plastic bags (1 m3). When shad were abundant in reservoirs, bluegill abundance often peaked either at the same time (36% of reservoirs and years combined) or after (40% of reservoirs and years combined) shad peaks. When gizzard shad were placed in bags 2 weeks before bluegills (N = 1 experiment), they depleted zooplankton, reducing growth (~0.075 g · g-1· d-1) but not survival of bluegills. In experiments (N = 2) in which both species were added simultaneously, zooplankton declined only slightly with gizzard shad, and there was little effect on bluegill growth (~0.21 g · g-1· d-1) and survival; in general, gizzard shad growth declined with time and increasing gizzard shad density. Based on experiments, bluegill success should vary among reservoirs and years as a function of their appearance relative to gizzard shad. In reservoirs, zooplankton availability and bluegill abundances were consistently low during years when gizzard shad dominated reservoir fish assemblages. Because gizzard shad probably reduce bluegill success in hypereutrophic Ohio reservoirs, management strategies that reduce gizzard shad should improve bluegill success

Influence of Larval Gizzard Shad (Dorosoma cepedianum) Density on Piscivory and Growth of Young-of-Year Saugeye (Stizostedion vitreum x S. canadense)

Abstract in English and French.Growth and survival of young-of-year saugeye (Stizostedion vitreum x S. canadense) (stocked into Ohio reservoirs to create sport fisheries) are probably influenced by prey availability, variations in which may account for historically documented variability in stocking success. Because saugeye switch from a diet of zooplankton to fish once stocked, we sought to determine experimentally if saugeye size and available ichthyoplankton, i.e., larval gizzard shad (Borosoma cepediaraum), affected this switch and whether piscivory improved saugeye growth. In an enclosure experiment, saugeye (33.9 mm TL) immediately switched to piscivory when exposed to ichthyoplankton densities of 20 and l00m^-3, growing faster when more gizzard shad were available. In another enclosure experiment, saugeye 30-49 mm TL consumed 14-mm gizzard shad. In ponds (N = 4 ponds-treatment-1) containing zooplankton and chironornids, we compared saugeye growth with and without larval gizzard shad and found, as in the first enclosure experiment, that piscivory improved saugeye growth. Neither saugeye size nor ichthyoplankton density influenced how quickly saugeye switched to piscivory. We conclude that managers should stock saugeye >=30 mm 1-2 wk before peak ichthyoplankton densities to improve saugeye growth and survival by enhancing opportunities for exploitation of young-of-yeas gizzard shad.This work was funded by the Federal Aid in Sport Fish Restoration Program (F-57-R, Evaluation of Fish Management Techniques), administered jointly by the U.S. Fish and Wildlife Service and the Ohio Division of Wildlife