Doctoral Dissertations

Date of Award

5-1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Ecology and Evolutionary Biology

Major Professor

Donald L. DeAngelis

Committee Members

Louis J. Gross, Kenneth A. Rose, J. Larry Wilson

Abstract

An individual-based model of the population dynamics of sympatric rainbow trout (Oncorhynchus mykiss) and brook char (Salvelinus fontinalis) is described and analyzed. The model simulates daily growth, mortality, movement, and spawning over the full life cycle of each species for 100 years in a compartmentalized, hypothetical stream configured for the southern Appalachian mountains of the United States. Egg and alevin development is temperature-dependent with mortality having constant, spatial, and temperature-dependent components. Daily growth of fry, juveniles, and adults is based on bioenergetics and consumption of drift prey. Mortality rate of fry through adults decreases with length. Model predictions of densities, growth, age, and size structure were similar to those observed in southern Appalachian streams. Five different conditions were simulated to explore the population dynamics and competition between the two species: (1) sympatric populations (baseline); (2) allopatric brook char; (3) allopatric rainbow trout; (4) and (5) sympatric populations with reduced or increased spawning season durations. Results indicated that density-dependence mainly operated during the fry and juvenile stages, brook char were more affected by interspecific competition than rainbow trout, and crowding of fry negatively affected brook char (with little effect on rainbow trout), whereas low fry density favored brook char. The individual-based model was used to examine possible explanations for rainbow trout dominance over brook char in southern Appalachian streams. Model simulations were used to quantify the effects of: (1) greater aggressiveness in rainbow trout, (2) latitudinal differences in stream temperatures, flows, and daylight, (3) year-class failures, (4) lower fecundity of brook char, and (5) reductions in spawning habitat on the competitive dynamics between rainbow trout and brook char. Average densities of each species based on 100 year simulations were compared for several levels of each of the five factors and for sympatric and allopatric conditions. Based on model results and empirical information, it was concluded that more frequent year-class failures and the lower fecundity of brook char were both possible and likely explanations for rainbow trout dominance, warmer temperatures due to latitude and limited spawning habitat were possible but unlikely explanations, and rainbow trout aggressiveness was a highly unlikely explanation. Additional field work should focus on comparative studies of the reproductive success and the early life stage mortalities of brook char and rainbow trout among Appalachian streams with varying rainbow trout dominance. The brook char - rainbow trout model was applied to evaluate management strategies for enhancing brook char densities in southern Appalachian streams. Management strategies were compared for 100 year simulations under: (1) electrofishing removal of rainbow trout, (2) stocking of brook char, (3) habitat alteration (changing mean pool size), and (4) angler harvest of rainbow trout. Results indicated that realistic levels of electrofishing removal of rainbow trout and stocking of brook char juveniles could produce allopatric brook char densities, but that habitat alteration and rainbow trout fishing would not significantly increase brook char densities. Electrofishing and stocking results were robust as similar predictions were obtained under conditions that favored rainbow trout (invasion of rainbow trout adults, frequent year-class failures, reductions in spawning habitat). Approximately 10 to 20 years are required for allopatric densities to be achieved. Management strategies that reduce competition in the young-of-the-year life stages offer the most promise for restoring brook char stocks in southern Appalachian streams.

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