Population dynamics, production, and prey consumption of fathead minnows (Pimephales promelas) in prairie wetlands: a bioenergetics approach

Abstract

I assessed the population dynamics of fathead minnows (Pimephales promelas) in prairie wetlands and developed a bioenergetics model to estimate their production and prey consumption. I sampled populations in four wetlands weekly from late May through June and biweekly during July and August using a Kushlan 1-m2 throw trap. I imposed commercial harvest on two populations; the other two populations served as controls. Weekly population density estimates ranged from 52 000 to 356 000 ·ha-1 during early June and from 5400 to 19 700 ·ha-1 in late August. Simulated commercial harvest did not influence population density, mortality rates, or size of fathead minnows. Standing stock biomass differed among wetlands sampled, ranging from 144 to 482 kg ·ha-1 in early June and from 1 to 33 kg ·ha-1 during late August. However, differences were attributed to differential predation pressure rather than harvest pressure. Net production during the period ranged from 71.5 to 202.7 kg ·ha-1. Daily net production was greatest in early June (2.6-13.5 kg ·ha-1 ·day-1) and then declined during July and August (0.1-1.2 kg ·ha-1 ·day-1). Total mass of prey consumed by fathead minnows ranged from 332.7-1104.8 kg ·ha-1 among wetlands.

Nutrient excretion by fish in wetland ecosystems and its potential to support algal production

“…Fathead minnows spawn fractionally and are capable of producing multiple cohorts each year; therefore, standard aging methods cannot be used to identify cohorts in populations (Duffy 1998). Thus, we used modal-length distributions to track the growth of individual cohorts during the sampling season (Jearld 1983).…”

“…We used P and N assimilation efficiencies of 0.72 and 0.80, respectively (Nakashima and Leggett 1980) and a value of 0.10 for F (Duffy 1998). Nutrient modeling also requires data on fish body nutrient content, and we estimated N and P content of 50 fathead minnows of variable size collected over the course of the study.…”

“…These factors, coupled with an absence of piscivorous fish and absence or low numbers of potential competitors in many PPR wetlands, facilitate extremely high minnow densities and production rates. Duffy (1998) observed minnow biomass as high as 48.2 g m Ϫ2 (43 fish m Ϫ2 ) and daily net production rates of 1.35 g m Ϫ2 d Ϫ1 in four South Dakota wetlands. Finally, young-of-the-year (YOY) fish appear to have strong potential to affect nutrient dynamics as a result of metabolic allometry, with mass-specific metabolic rates inversely related to fish size (Post 1990;Kraft 1992), and fathead minnow populations are typically dominated by YOY fish.…”

“…Values for total phosphorus, total nitrogen, N : P ratios, Chl a, and turbidity are averages across the 2 yr of the study. sumed (Duffy 1998). Detritus is often a considerable proportion of the diet and can be an important source of energy for these fish (Lemke and Bowen 1998).…”

Land use and wetland drainage affect water levels and dynamics of remaining wetlands

“…The dynamic nature of these wetlands, along with the rare surface water connections to other waters, led to many wetlands in the PPR being devoid of fish (Peterka 1989). The lack of fish in wetlands leads to increased abundance, biomass, and size of aquatic invertebrates (Scheffer et al 1993, Wellborn 1994, Bouffard and Hanson 1997, Duffy 1998, Zimmer et al 2001, Hanson et al 2005 providing better habitat for amphibians and breeding waterbirds (van der Valk and Pederson 2003, Zedler 2003, Brooks 2004, Machtinger 2007. However, our results indicate that wetlands that receive consolidation drainage no longer dry down as far as they did prior to drainage and rarely dry out completely.…”