Ray W. Drenner

Lake trophic state and the limnological effects of omnivorous fish

Ecologists have hypothesized that planktivorous fish have greater effects on the plankton and water quality of oligotrophic lakes than eutrophic lakes. We tested this hypothesis in a tank-mesocosm experiment of factorial design in which five biomass levels of filter-feeding omnivorous gizzard shad (Dorosoma cepedianum) were cross-classified with two levels of lake trophic state achieved by filling tank-mesocosms with water and plankton transported by truck from two lakes with different trophic states. The presence of gizzard shad significantly increased total phosphorus, primary productivity, chlorophyll, and particulate phosphorus (PP) 2–20 and 20–200 µm and significantly decreased Secchi depth, cladocerans, copepods and PP > 200 µm. The effects of gizzard shad on chlorophyll, Secchi depth, cladocerans, copepods and PP 2–20 and > 200 µm were dependent on lake trophic state and most intense in the eutrophic lake system. This experiment suggests that filter-feeding omnivorous fish interact synergistically with trophic state so that the limnological effects of omnivorous fish become more intense with increased eutrophication.

Piscivores, Trophic Cascades, and Lake Management

The concept of cascading trophic interactions predicts that an increase in piscivore biomass in lakes will result in decreased planktivorous fish biomass, increased herbivorous zooplankton biomass, and decreased phytoplankton biomass. Though often accepted as a paradigm in the ecological literature and adopted by lake managers as a basis for lake management strategies, the trophic cascading interactions hypothesis has not received the unequivocal support (in the form of rigorous experimental testing) that might be expected of a paradigm. Here we review field experiments and surveys, testing the hypothesis that effects of increasing piscivore biomass will cascade down through the food web yielding a decline in phytoplankton biomass. We found 39 studies in the scientific literature examining piscivore effects on phytoplankton biomass. Of the studies, 22 were confounded by supplemental manipulations (e.g., simultaneous reduction of nutrients or removal of planktivores) and could not be used to assess piscivore effects. Of the 17 nonconfounded studies, most did not find piscivore effects on phytoplankton biomass and therefore did not support the trophic cascading interactions hypothesis. However, the trophic cascading interactions hypothesis also predicts that lake systems containing piscivores will have lower phytoplankton biomass for any given phosphorus concentration. Based on regression analyses of chlorophyll�total phosphorus relationships in the 17 nonconfounded piscivore studies, this aspect of the trophic cascading interactions hypothesis was supported. The slope of the chlorophyll vs. total phosphorus regression was lower in lakes with planktivores and piscivores compared with lakes containing only planktivores but no piscivores. We hypothesize that this slope can be used as an indicator of “functional piscivory” and that communities with extremes of functional piscivory (zero and very high) represent classical 3- and 4-trophic level food webs.

Effect of fish size on the filtering efficiency and selective particle ingestion of a filter-feeding clupeid

Abstract We developed a model predicting the filtering efficiency of different sizes of filter-feeding gizzard shad Dorosoma cepedianum. The model is based on the cumulative frequencies of interraker distances of the gill rakers and was corroborated by feeding trials in which gizzard shad ingested different sizes of suspended plastic microspheres and plankton. The proportion of particles removed by fish increased as a function of particle size, leveling off when particle diameters exceeded 35 μm for small fish (7.1–11.1 cm standard length, SL) and 55 μm for large fish (13.6–16.3 cm SL). According to selectivity index values computed from the filtering efficiency model, gizzard shad of 5, 15, and 25 cm SL would selectively feed on particles larger than 19, 40, and 63 μm, respectively. This change in selective particle ingestion may explain why phytoplankton becomes less important in the diet of gizzard shad and why this clupeid shifts its feeding niche as it increases in length.

Particle-Grazing and Plankton Community Impact of an Omnivorous Cichlid

Abstract We determined selective ingestion of different sizes of suspended polystyrene particles by filter-feeding blue tilapia Tilapia aurea. Blue tilapias from 4.3 to 18.7 cm standard length selectively fed on particles larger than 25 μm. We compared the plankton communities of pond quadrants with and without fish. Blue tilapias suppressed populations of the large-sized algae Uroglenopsis sp. and Ceratium sp. and the zooplankter Keratella sp. Populations of the small-sized algae Rhodomonas sp., Chrysochromulina sp., Chlamydomonas sp. and Cyclotella sp. and the zooplankter Diaptomus sp. were enhanced by the presence of blue tilapia. As an omnivorous filter feeder, blue tilapias act as size-selective phytoplankton grazers and escape-selective zooplankton predators. Received August 26, 1983 Accepted March 24, 1984

Food Web Interactions in Lakes

Why study food webs in lakes? From a basic research perspective, ecological studies of lake food webs provide distinct advantages over studies in many terrestrial systems (Lampert 1987). Lake food webs are composed of organisms with relatively fast population turnover rates which interact in a relatively closed system. These features allow us to readily observe the often rapid dynamics of these systems or to experimentally manipulate these food webs and quickly assess the system response. Enclosures, ponds, and whole-lake manipulations are extremely useful experimental tools that have allowed aquatic ecologists to test hypotheses on food web structure and function that would have been difficult or impossible to address in many terrestrial systems.

Particle ingestion by Tilapia galilaea is not affected by removal of gill rakers and microbranchiospines

Abstract Particle ingestion by filter-feeding Galilee Saint Peters fish Tilapia galilaea increased as a function of particle size, leveling off when particle diameter exceeded 20 μm. Ingestion rates by this cichlid also increased with particle concentration, asymptotically approaching maxima of 4,785 and 84,746 particles˙fish −1˙min−1 for small (3.9–6.0 cm standard length, SL) and large (12.6–14.3 cm SL) fish, respectively. Surgical removal of gill rakers and microbranchiospines did not affect particle ingestion rates or selectivity.

Filter-Feeding Rates of Gizzard Shad

Abstract In laboratory feeding trials, gizzard shad Dorosoma cepedianum of 5.3–17.5-cm standard length fed on zooplankton as pump filter feeders, collecting prey by a series of rapid suctions not directed at individual prey. Buccal volume, determined from plaster casts, increases as a power function of fish standard length. Pumping rates decrease exponentially with length. A model of filtering rate, the product of buccal volume and pumping rate, was corroborated by feeding trials in which gizzard shad cleared zooplankton from a pool. By this model, absolute filtering rates increase as a power function of fish length while filtering rate per gram of fish decreases as a power function of length. Populations of gizzard shad can filter substantial volumes of lake water each hour.

Feeding behavior of the cichlid, Sarotherodon galilaeum: selective predation on Lake Kinneret zooplankton

In laboratory feeding trials, we analyzed the feeding behavior and selectivity of the cichlid, Sarotherodon galilaeum, for zooplankton prey from Lake Kinneret, Israel. The feeding behavior was dependent on fish size. Fish less than 20 mm SL fed on zooplankton as obligate particulate feeders. Fish from 20 to 42 mm SL fed either as particulate feeders or as filter feeders. Fish larger than 62 mm SL fed as obligate filter feeders. Particulate-feeding fish were size selective and had highest feeding electivities for large-sized zooplankton species. Filter-feeding fish had highest feeding electivities for zooplankton species with poor escape ability. In general, S. galilaeum predation pressure would be greatest on Ceriodaphnia reticulata, a large-bodied and easily captured species which is selected by both particulate-feeding and filter-feeding fish.

Habitat-Specific Differences in Mercury Concentration in a Top Predator from a Shallow Lake

Abstract We conducted a survey of mercury contamination in largemouth bass Micropterus salmoides from Caddo Lake, Texas, and found that fish collected from forested wetland habitat had higher concentrations of mercury than those collected from open-water habitat. Habitat-specific differences in largemouth bass size, age, absolute growth rate, trophic position (based on δ15N), and horizontal food web position (based on δ13C), characteristics known to influence mercury accumulation, did not explain the observed differences in mercury contamination. Rather, habitat-related differences in mercury concentration in a primary consumer, Mississippi grass shrimp Palaemonetes kadiakensis, indicated that food webs in forested wetland habitat were more contaminated with mercury than those in open-water habitat. Spatial variation in mercury contamination within lakes and elevated mercury concentrations in forested wetlands should be of special concern not only to researchers but also to public and environmental health...

Effects of Gizzard Shad and Threadfin Shad on Zooplankton and Young-of-Year White Crappie Production

Abstract We examined the effects of gizzard shad Dorosoma cepedianum and threadfin shad Dorosoma petenense on zooplankton abundance and on reproduction by white crappie Pomoxis annularis in a March–July 1986 Texas pond experiment of factorial design. We used four treatment combinations: no shad, gizzard shad, threadfin shad, and gizzard shad + threadfin shad. Densities of cyclopoid copepodids and Daphnia sp. were suppressed in the presence of threadfin shad but not gizzard shad. The presence of gizzard shad or threadfin shad was associated with decreased total number and biomass of young-of-year white crappies, a shift in white crappie size distributions toward larger individuals, and increased mean weight of white crappies in some size-classes. The biomass of young-of-year gizzard shad decreased in the presence of threadfin shad. Our study suggests that gizzard shad and threadfin shad may decrease the density and biomass of young-of-year white crappies, but increase their mean size.