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The Mississippi Alluvial Valley includes hundreds of floodplain lakes that support unique fish assemblages and high biodiversity. Irrigation practices in the valley have lowered the water table, increasing the cost of pumping water, and necessitating the use of floodplain lakes as a source of water for irrigation. This development has prompted the need to regulate water withdrawals to protect aquatic resources, but it is unknown how much water can be withdrawn from lakes before ecological integrity is compromised. To estimate withdrawal limits, we examined descriptors of lake water quality (i.e., total nitrogen, total phosphorus, turbidity, Secchi visibility, chlorophyll-a) and fish assemblages (species richness, diversity, composition) relative to maximum depth in 59 floodplain lakes. Change-point regression analysis was applied to identify critical depths at which the relationships between depth and lake descriptors exhibited a rapid shift in slope, suggesting possible thresholds. All our water quality and fish assemblage descriptors showed rapid changes relative to depth near 1.2–2.0 m maximum depth. This threshold span may help inform regulatory decisions about water withdrawal limits. Alternatives to explain the triggers of the observed threshold span are considered.

The clupeid gizzard shad Dorosoma cepedianum is often the most abundant fish species in North American reservoirs, and this dominance can have cascading trophic effects on entire fish assemblages. Accordingly, a key aspect of managing reservoir fish assemblages involves controlling gizzard shad densities. We used a 33-year time series to evaluate the relative importance of parental stock density, winter temperature, and water regime on recruitment of age-0 gizzard shad in a large reservoir. Recruitment modeled with a Ricker-type curve increased with the size of the adult stock, peaked, and then decreased at high stock densities. This over-compensatory stock-recruitment relationship was made more dynamic by fluctuations in inflow, with recruitment increasing in years of high inflow, however there was no temperature effect at the latitude of the study site. The influence of stock size on recruitment was roughly twice as high as the influence of inflow. This study is the first to report stock-recruitment relationships for a clupeid species in a reservoir and concurs with analyses of marine fishes that have shown that most clupeids exhibit compensatory or over-compensatory patterns in their stock-recruitment relationships.

Fishery yields in reservoirs of the Upper Parana River basin are small when compared to those in other parts of the world. Several hypotheses have been formulated to explain the small yields, but low primary production has not been seriously considered as a possibility. In a sample of seven reservoirs, yield was directly related to fishing effort and chlorophyll a concentration; thus, enhancement of these factors may boost yield. However, primary production is difficult to augment in large reservoirs, plus current levels of effort seem to be nearing maximum sustainable yields in some reservoirs, and therefore increased fishing effort is not likely to boost yields to levels recorded in other geographical regions. Low primary production in reservoirs of the Upper Parana River basin appears to be a result of interactions between physical characteristics of the reservoirs and climatic conditions of the region.

We studied the selective properties of gill net mesh-sizes used in the fisheries at the Itaipu Reservoir to assess the likelihood of growth and recruitment overfishing, and to explore modifications of current harvesting approaches and their effect on the fish stock and the value of the fishery. We used empirical growth estimates and gill net selectivity to model yield per recruit, size distributions, value per recruit, and spawner biomass per recruit under different natural mortalities, fishing mortalities, and gill net mesh-size restrictions. Our results suggest growth overfishing may be limiting fishery yield and commercial value. Increasing mesh-size (in most cases) and increasing fishing effort (in some cases) would lead to higher yields and commercial values. Nevertheless, such increases would further reduce the biomass of the spawning stock, and we could not predict whether such reductions would lead to recruitment overfishing, nor their effect on the interactions between the fishery species and the associated fish assemblage. However, we identify how fishing effort and mesh-size may be juggled to boost yield and commercial value without reducing current biomass of the spawning stock.

White crappie Pomoxis annularis and black crappie P. nigromaculatus are important freshwater sportfishes in North America. Because of erratic recruitment (the number of fish that survive to age 1 each year) fishery yields are inconsistent. We developed a simple age-structured model to help manage crappie fisheries. The model changes population biomass by age-specific somatic growth, and population abundance through recruitment and age-specific natural and fishing mortalities. Using empirical stock-recruitment data and a stochastic component to simulate the effects of abiotic environmental fluctuations on recruitment, the model simulated empirical recruitment dynamics of a crappie population over 30 years in a large impoundment. Effects of density-dependent growth and natural mortality were modeled, but did not substantially affect the yield given the observed magnitude of recruitment fluctuations. Model predictions of population biomass and yield tended to be high relative to empirical estimates, but not unrealistic. Erratic recruitment of crappie populations has made development of management strategies elusive. This model should help fishery managers examine potential management alternatives.