Detecting Alternate States in Ecological Systems: Threshold Effects of Riparian Development and Angling on Fish Population Stability

EPA Grant Number: U916019
Title: Detecting Alternate States in Ecological Systems: Threshold Effects of Riparian Development and Angling on Fish Population Stability
Investigators: Havlicek, Tanya D.
Institution: University of Wisconsin - Madison
EPA Project Officer: Graham, Karen
Project Period: January 1, 2001 through January 1, 2004
Project Amount: $87,619
RFA: STAR Graduate Fellowships (2001) RFA Text |  Recipients Lists
Research Category: Academic Fellowships , Aquatic Ecosystems , Fellowship - Aquatic Ecology and Ecosystems


The objective of this research project is to investigate (through models) whether critical depensation can emerge in fish populations in small, recreationally exploited northern Wisconsin lakes from a few simple assumptions. Collapses in fish populations are a ubiquitous phenomena of recent decades. Aquatic resources such as the groundfish stocks of the Grand Banks and Georges Banks regions that have sustained exploitation for centuries are now closed to harvest with little indication of recovery. This pattern is not limited to marine systems. Lake trout and walleye populations in the Great Lakes and salmon populations in the western United States also have experienced rapid and unexpected declines. In the Northern Highland Lake district of Wisconsin, a popular recreation destination, dominant piscivores sought by recreational anglers no longer recruit and populations are sustained only by hatchery stocking. Explanations for fish population crashes include over harvesting, habitat degradation, environmental fluctuations, gene pool alterations via stocking, and predation. As important as causality, though, is whether or not the decline is reversible. Critical depensation, decreased recruitment at low population levels, can create a threshold that prohibits populations from rebounding to a stable, large equilibrium. It is on this upper equilibrium value that policies like maximum sustained yield are based. The importance of fish stocks for local economies, food, water quality, and recreational opportunities implies the detection of critical depensation is crucial for effective management.


I used age-structured models to explore the effects of habitat-mediated interactions and harvest on fish population stability. Littoral zone structure, coarse woody debris (CWD), provides substrate for nests, refuge from predation for larval and small fish, and surface for benthic invertebrate production. Littoral structure thus affects food availability for small and large fish, controls predator-prey interactions, and can provide hiding refuge from anglers. CWD enters the lake when trees and branches in the riparian forest break and fall via senescence and disturbance. As people develop lake shoreline, however, they remove physical structure from the littoral zone to improve boating and swimming. I explored different development scenarios by simulating a suite of littoral CWD densities. Preliminary results indicate thresholds of critical depensation can emerge as a result of habitat-mediated food web interactions and recreational harvest.

Supplemental Keywords:

fellowship, ecological systems, riparian development, fish, lake trout, fish population crashes, over harvesting, coarse woody debris, CWD, littoral zone, recreational fishing, critical depensation, food we interactions.

Progress and Final Reports:

  • 2001
  • 2002
  • Final