Fish display enormous reproductive potential and highly variable survival rates during their juvenile year. Individual females of many species annually produce thousands or hundreds of thousands of larvae, most of which die during the first few weeks of life. However, if favorable conditions occur at critical times, larval fish survival may increase substantially, resulting in unusually abundant year classes. Consequently, the number of young fish that eventually reach maturity, or "recruit," may depend to a large extent on the conditions during their early life history.
One idea that biologists have devised to explain the dynamics of early life history and variable fish recruitment is the Match-
Mismatch Hypothesis. According to Match- Mismatch, adult animals should spawn when resources are plentiful, thereby enhancing the chance their offspring will recruit. Fish produce more offspring and enjoy greater fitness if they "match" spawning to the peak abundance of the zooplankton diet of their larvae. Individuals that spawn too late or early or during years with low zooplankton abundance "mismatch" and produce slow- growing offspring that either starve or are consumed by predators. Although this idea has been applied primarily to marine ecosystems, freshwater flood pulses also create discrete periods of high productivity similar to those assumed by the Match- Mismatch Hypothesis.
Analyses from Lake Shelbyville, Illinois, a flood-control reservoir built by the Corps of Engineers (COE), have sought to apply the principle of Match- Mismatch to a freshwater system. Each fall, the COE lowers Lake Shelbyville to a level below normal pool. The next year, a significant portion of the spring flood is impounded to inhibit flooding downstream. This flow regulation strategy results in a single, large flood peak within the reservoir that generates a burst of phytoplankton and zooplankton production. Field data from this system show that higher abundance and survival of larval fish occur during the period following floods.
Using seven years of electrofishing data collected in Lake Shelbyville by the INHS Kaskaskia Biological Laboratory, and lake-level data from the COE, we created a simple empirical model based on the principles of the Match- Mismatch Hypothesis to predict juvenile abundance of the omnivorous fish species gizzard shad (Dorosoma cepedianum). Assumptions of the model for Lake Shelbyville include 1) adult gizzard shad always produce larvae in excess of carrying capacity, 2) the height of the flood pulse determines the abundance of resources for larval fish, 3) the total number of surviving larval and therefore juvenile fish increases with the availability of flood- generated resources during the larval stage, and 4) larval survival increases as flood pulses near an optimal date that corresponds with a predictable annual peak of sexual maturity within the adult gizzard shad population. Model parameters include the height and the date of the flood peak. For the past two years, this model has produced successful a priori predictions of juvenile gizzard shad abundance, and now explains 83% of the variability in juvenile gizzard shad abundance over the last nine years.
Gizzard shad from Lake Shelbyville.
Application of the Match- Mismatch Hypothesis in flood- prone waters may benefit researchers, managers, and resource users. For instance, predictions from the flood model could guide water-level manipulations to manage shad populations to the benefit of sportfish. If Match- Mismatch principles are sufficiently general in freshwaters, knowledge of these interactions may facilitate conservation of economically important or endangered species, particularly in systems with managed flows. Current needs include an exploration of flood- driven recruitment in systems and with species that challenge or violate model assumptions to varying degrees.
Timothy B. Smith and David H. Wahl, Center for Aquatic Ecology
Charlie Warwick, editor