W. John O'Brien

APPARENT SIZE AS THE DETERMINANT OF PREY SELECTION BY BLUEGILL SUNFISH (LEPOMIS MACROCHIRUS)

Copyright by the Ecological Society of America. This is the publishers version, also available electronically from http://www.jstor.org/stable/1935055.

Piscivorous Feeding Behavior of Largemouth Bass: An Experimental Analysis

Abstract Largemouth bass Micropterus salmoides are the dominant top carnivores of many North American lakes and reservoirs and are very popular sport fish, but the actual behavioral mechanisms of their feeding are still poorly known. In laboratory experiments we broke predation into its component parts: Location, pursuit, attack, and handling of prey. The distance at which largemouth bass can locate forage fish increases with prey size, with prey motion (when prey are small), and with light intensity. In the pursuit phase of the predation cycle, largemouth bass are more likely to choose prey with large apparent size, closer proximity, or greater motion. When bluegills Lepomis macrochirus were the experimental prey, the number of attempted attacks by largemouth bass before the prey was ingested increased with bluegill size, within broad limits. At high light intensities bluegills can locate modest-size largemouth bass (29 cm total length) long before the predators locate them, but at low light intensities,...

Flexible search tactics and efficient foraging in saltatory searching animals

SummaryForaging is one of the most important endeavors undertaken by animals, and it has been studied intensively from both mechanistic-empirical and optimal foraging perspectives. Planktivorous fish make excellent study organisms for foraging studies because they feed frequently and in a relatively simple environment. Most optimal foraging studies of planktivorous fish have focused, either on diet choice or habitat selection and have assumed that these animals used a cruise search foraging strategy. We have recently recognized that white crappie do not use a cruise search strategy (swimming continuously and searching constantly) while foraging on zooplankton but move in a stop and go pattern, searching only while paused. We have termed thissaltatory search. Many other animals move in a stop and go pattern while foraging, but none have been shown to search only while paused. Not only do white crappie search in a saltatory manner but the components of the search cycle change when feeding on prey of different size. When feeding on large prey these fish move further and faster after an unsuccessful search than when feeding on small prey. The fish also pause for a shorter period to search when feeding on large prey. To evaluate the efficiency of these alterations in the search cycle, a net energy gain simulation model was developed. The model computes the likelihood of locating 1 or 2 different size classes of zooplankton prey as a function of the volume of water scanned. The volume of new water searched is dependent upon the dimensions of the search volume and the length of the run. Energy costs for each component of the search cycle, and energy gained from the different sized prey, were assessed. The model predicts that short runs produce maximum net energy gains when crappie feed on small prey but predicts net energy gains will be maximized with longer runs when crappie feed on large prey or a mixed assemblage of large and small prey. There is an optimal run length due to high energy costs of unsuccessful search when runs are short and reveal little new water, and high energy costs of long runs when runs are lengthy. The model predicts that if the greater search times observed when crappie feed on small prey are assessed when they feed on a mixed diet of small and large prey, net energy gained is less than if small prey are deleted from the diet. We believe the model has considerable generality. Many animals are observed to move in a saltatory manner while foraging and some are thought to search only while stationary. Some birds and lizards are, known to modify the search cycle in a manner similar to white crappie.

The development and field test of a tactical model of the planktivorous feeding of white crappie (Pomoxis annularis)

A computer model was developed to predict the feeding selectivity of planktivorous white crappie (Pomoxis annularis) from a known distribution of zooplankton. The model was based on the assumption that each predation event could be subdivided into a series of independent steps: prey location, pursuit, attack, and retention. The probability that white crappie successfully completed each step was determined for potential zooplankton prey species in a series of laboratory experiments. The four steps were then incorporated into a stochastic model where the probability of a particular prey type being consumed is equal to the product of the probabilities of the individual steps. The model was field tested by sampling fish, zooplankton, and physical parameters from discrete depth strata in a small reservoir on nine dates from October 1978 through November 1979. The model proved to be very accurate at predicting the species and size distribution of the ingested prey across the range of light intensities, turbidities, temperatures, and zooplankton densities encountered. Prey consumption could not be characterized as simply size selective; rather, it reflected the selectivity expressed at each step in the feeding cycle.

The Dynamics of Nutrient Limitation of Phytoplankton Algae: A Model Reconsidered

A model of phytoplankton nutrient dynamics similar to that proposed by Dugdale (1967) simulated the dynamics of phytoplankton—nutrient interactions in lakes. Phyoplankton population crashes similar to those commonly observed in spring phytoplankton blooms occur during simulation whenever large amounts of the limiting nutrient are initially provided. The model shows the phytoplankton death rate as an important influence on the steady state nutrient concentration and population density. Zooplankton grazing appears to be potentially important in triggering blue—green algal blooms and in maintaining high phytoplankton species diversity; phytoplankton mortality caused by settling may be a factor in the seasonal succession of plankton algae. Two potentially confounding factors–nutrient regeneration from dead cells and multiple limiting nutrients–are shown to have no significant effect on the properties of the model. See full-text article at JSTOR

Helmets and Invisible Armor: Structures Reducing Predation from Tactile and Visual Planktivores

Several arctic Alaskan lakes were found to have populations of both visual—feeding planktivorous fish and the tactile—feeding predaceous copepod Heterocope septentrionalis. Feeding experiments and reactive distance measurements were carried out to determine the relationship between zooplankton size and susceptibility to both types of predation. Heterocope was found to feed selectively on small prey forms and fish on large forms, thereby confirming that the zooplankton in these lakes are subject to conflicting selective pressures. Two of the species of zooplankton appear to have adapted to this situation by developing structures that increase their actual size, thus helping to protect them from invertebrate predation while not altering the visual body size they present to fish. See full-text article at JSTOR

The biogeochemistry and zoogeography of lakes and rivers in arctic Alaska

Water samples from 45 lakes and 8 rivers in arctic Alaska were analyzed for major anions, cations, nutrients, chlorophyll, zooplankton, and benthos. The waters were dilute (conductivities of 30 to 843 µS cm−1), and their composition varied from Na-Ca-Cl waters near the Arctic Ocean to Ca-Mg-HCO3 waters further inland. Sea salt input in precipitation was important in determining the chemistry of coastal lakes, partly because of low groundwater flow and less time for water to react with shallow unfrozen soils. Further inland, variations in water chemistry among sites were related mainly to differences in bedrock, the age of associated glacial drift, and the input of wind blown sediment. Variations in zooplankton species composition among the lakes were related more to latitude, lake morphometery, and biotic interactions than to water chemistry. The presence of fish as predators mostly determined the overall size structure of the zooplankton community. The chironomid taxa identified have been previously reported from the Neararctic, except for Corynocera oliveri which is a new record. The abundance of the widely distributed chironomid Procladius appears to be controlled by sculpin predation.

An Analysis of the Components of Chaoborus Predation on Zooplankton and the Calculation of Relative Prey Vulnerabilities

We formulated a model for Chaoborus predation on zooplankton and analyzed its various components for fourth—instar Chaoborus americanus and three prey species (Daphnia pulex, Bosmina longirostris, and Diaptomus pallidus). This model is based upon the encounter rates between predator and prey, the probability of a first strike by Chaoborus, and the probability of ingestion given a first strike. This latter probability is a function of the strike efficiency, contact efficiency, and second—strike probability of the predator with each prey. The strike volume of Chaoborus was variable in both size and shape for the different prey species. In each case, however, it could be closely approximated by a cylinder. This strike volume was largest for Daphnia and smallest for Bosmina. The strike efficiency of Chaoborus (probability that a strike will result in a contact) was not significantly different for the three prey types, indicating that the different evasion behaviors of the prey do not influence this parameter. The contact efficiency (probability that a contact will result in a capture) on Daphnia was significantly lower than for the other ability that a contact will result in a capture) on Daphnia was significantly lower than for the other prey; this is a function of the differing sizes and shapes of the prey. Second—strike probability (probability that if a prey is initially missed by a Chaoborus strike, it will remain in the strike volume and be attacked again) was significantly different for each of the three prey species tested, with Bosmina > Daphnia > Diaptomus. This is mainly the result of the different evasion behaviors of the prey. Our model predicts that Chaoborus is equally efficient at capturing each prey once a first strike occurs. Differential vulnerabilities of these prey to Chaoborus predation are therefore the result of differing encounter rates, which are influenced by both the swimming speeds of the prey and the strike volume of the predator.

Photosynthetically Elevated pH as a Factor in Zooplankton Mortality in Nutrient Enriched Ponds

During a nutrient enrichment study of eight small ponds, the mortality of crustacean zooplankton species was investigated using a life table technique. There was close agreement between high pH values and the disappearance of crustacean zooplankton in four of the highly fertilized ponds. Pond water containing phytoplankton was toxic to the dominant Ceriodaphnia reticulata, yet pond phytoplankton isolated from pond water and resuspended in chlorinated tap water supported excellent growth of C. reticulata. Millipore—filtered pond water was not toxic. Analysis of chemical changes occurring in pond water with filtration showed a slight decrease in pH as the only significant alteration that is directly related to zooplankton mortality. The existence of such a pH mortality factor was demonstrated by testing the survivorship of newborn. C. reticulata in artificially adjusted pH treatments. A change from pH 10.8 to 11.2 radically altered the survivorship. Other hypothesis to explain the disappearance of C. reticu...

A New Estimate of Zooplankton Retention by Gill Rakers and Its Ecological Significance

Abstract The probability that prey will be retained by gill rakers of white crappies Pomoxis annularis (9–15 cm total length) was determined by comparisons of size-frequency distributions of prey in a laboratory pool and in stomach contents. White crappies trained to feed on large-bodied Daphnia magna were released individually into a pool containing that species and an assemblage of small-bodied species. Most attacks were directed at Daphnia magna, but several prey were ingested for every attack, and nonselective capture was assumed for the small-bodied among them. Estimated retention probabilities for species of Ceriodaphnia, Bosmina, and Cyclops were less than 10% for mean prey sizes smaller than 0.35 mm and increased linearly with mean prey size to 100% for prey larger than 0.55 mm. In contrast, retention probabilities calculated by traditional means from distances between gill rakers predicted that all prey larger than 0.23 mm would be retained. The new retention estimates, based on actual results of...