Mark H. Olson

Ontogenetic Niche Shifts in Largemouth Bass: Variability and Consequences for First‐Year Growth

The ontogeny of many size-structured organisms is marked by a series of niche shifts that divides a population into ecologically distinct stages. Consequently, overall population dynamics are affected both by processes operating within individual stages and by factors that influence transitions between stages. In largemouth bass (Micropterus sal- moides), there is a diet shift from littoral invertebrates to fish that typically occurs sometime in their 1st yr. I examined factors that influenced this shift by tracking diet and growth trajectories of young-of-year (YOY) bass cohorts in several Michigan lakes over 2 yr. Cohorts differed in consumption rates of invertebrate prey, which led to an early divergence in bass growth among lakes. This variation in early growth had dramatic consequences for the timing of the shift to piscivory. In lakes with rapid growth in the invertebrate feeding stage, bass were able to gain a size advantage over YOY bluegill (Lepomis macrochirus), their primary fish prey, which facilitated a shift to piscivory as soon as these bluegill became available as food. In contrast, slow growth during the invertebrate feeding stage reduced the size advantage of bass over bluegill and delayed or even prevented the shift to piscivory in the 1 st yr. Niche shifts in bass were also sensitive to environmental conditions early in the growing season. In 1992, low temperatures caused a reduction in growth rates that prevented the establishment of a size advantage over bluegill in all but one lake. Bass grew much faster after switching to piscivory; thus the shift from invertebrates to fish strongly influenced sizes attained by YOY bass by autumn. Because survivorship of YOY bass is strongly size dependent (particularly through the first winter), the timing of the shift to piscivory may be important in determining survival and recruitment rates. Therefore, processes that operate on bass in the invertebrate feeding stage can influence the shift to piscivory, and this can potentially have important effects on the dynamics of the population as a whole.

Competition between Predator and Prey: Resource-Based Mechanisms and Implications for Stage-Structured Dynamics

In predator-prey interactions between size-structured populations, small (young) predators may compete with species that ultimately become their prey. We tested experimentally whether such competition occurs between young-of-year (YOY) largemouth bass and their eventual prey, bluegill. In a divided experimental pond, target densities of YOY bass and juvenile bluegill were subjected to different densities of bass and bluegill neighbors to examine the dependence of juvenile growth on fish density and species com- position. After 7 wk, clear differences in growth rates of both species existed across den- sities. Bass growth was reduced in the presence of both bass and bluegill neighbors, whereas bluegill growth was primarily affected intraspecifically. Bluegill had strong competitive effects on bass despite substantial resource partitioning between the two species. These effects were mediated through changes in the size-structure of important invertebrate prey in both open water and vegetation habitats. Data collected from a set of seven lakes in Michigan indicate that interspecific competition between juvenile bluegill and YOY bass is an important process in natural populations. However, once bass become piscivorous, bluegill become a significant resource for bass. As a result, bass populations are split into two functionally distinct stages that respond differently across a gradient of bluegill density. Because of the competitive stage between predator and prey, dynamics of the interaction differ from predictions based on classical predator-prey or competition theory.

Managing Macrophytes to Improve Fish Growth: A Multi-lake Experiment

Abstract Macrophyte harvesting often has been suggested as a way to improve fish growth and size structure in lakes with high densities of submergent macrophytes and stunted fish populations. However, previous experimental tests have provided no clear consensus on whether the technique works for management. We conducted a series of whole-lake manipulations to test the effects of macrophyte removal on growth of bluegill and largemouth bass. We selected four lakes in southern and central Wisconsin for experimental manipulation and nine others for controls. In August 1994, we removed macrophytes from approximately 20% of the littoral zone by cutting a series of evenly spaced, deep channels throughout each treatment lake. In the first year after manipulation, we observed substantially increased growth rates of some age classes of both bluegill and largemouth bass in treatment lakes relative to controls. Growth rates of other age classes were less responsive to manipulation. We observed increased bluegill and ...

Predator-prey interactions in size-structured fish communities: implications of prey growth

Predator-prey interactions among size-structured populations may be strongly influenced by factors which affect growth rates of prey. I examined the importance of prey growth in the interaction between large-mouth bass (Micropterus salmoides) and their prey, bluegill (Lepomis macrochirus), by analyzing diets and growth rates of bass in a set of seven lakes in south-central Wisconsin. Sizes of bluegill consumed by bass changed dramatically across a gradient of bluegill growth, which resulted in differing patterns of bass growth. In lakes with slow bluegill growth, small bass fed on the youngest bluegill cohort, but large bass were capable of feeding on several age classes. Consequently, bass growth rates were strongly size-dependent; small bass ate small prey and had low growth, but growth rates increased substantially with size as bass ate progressively larger prey. When bluegill had high growth rates, they quickly reached a size refuge from predation and bass of all sizes were restricted to feeding on the youngest/smallest prey. In these lakes, bass growth rates were more uniform across bass sizes. Because growth rates influence population size-distributions, variation in bluegill growth can have strong effects on the structure of bass populations. These effects could potentially feed back to further influence the interaction between predator and prey.

Size-Dependent Alewife Predation on Larval Walleyes in Laboratory Experiments

Abstract Alewives Alosa pseudoharengus can be significant predators on larval fish, and fishery managers must consider potential predation when developing stocking strategies. We conducted laboratory experiments to determine sizes of larval walleyes Stizostedion vitreum that alewives can capture. Adult alewives (122 ± 1.52 mm total length, TL) preyed intensely on larval walleyes from the time the larvae hatched until they reached about 16 mm TL. All larval walleyes smaller than this size were captured on the first strike by an alewife. Avoidance behavior by larval walleyes was first observed when they reached 16–19 mm TL, and although some survived the first strike, all were consumed by the end of 1 h. When larvae averaged about 25 and 30 mm TL, few were captured on the first strike, and 75% and 33%, respectively, were consumed within 1 h. When larvae were about 34 mm TL, only 5% were eaten by the end of 1 h. Alewife gape height and width were both about twice as large as the largest larval walleye body h...

Patterns of Diet and Growth in Co-occurring Populations of Largemouth Bass and Smallmouth Bass

Abstract Largemouth bass Micropterus salmoides and smallmouth bass M. dolomieu both undergo ontogenetic diet shifts from insects and zooplankton (as juveniles) to fish and crayfish (as adults). This diet similarity leads to the expectation that the growth rates of co-occurring largemouth bass and smallmouth bass should covary positively. However, the relative abundance of the two species varies widely among lakes, suggesting that there are important ecological differences and making it difficult to predict how the individual growth rates of the two species will covary among systems. We quantified the diets and growth rates of largemouth bass and smallmouth bass in a set of lakes in New York State that varied in the relative abundance of the two species. Despite changing diets as a function of size, largemouth bass and smallmouth bass consumed similar prey across the size range. As expected based on diets, the growth rates of the two species were also positively correlated among lakes for most size-classes...

Interspecific variation in UV defense mechanisms among temperate freshwater fishes

Abstract An important step in predicting the effects of future increases in UV radiation (UVR) is to evaluate the mechanisms that organisms use to prevent and repair DNA damage and determine how those mechanisms influence UVR sensitivity. Damage is prevented to varying degrees through photoprotection and repaired via two main pathways: nucleotide excision repair and photoenzymatic repair. At present, little is known about the generality or similarity of these defenses among temperate freshwater fishes. We used laboratory experiments to compare UVR defense mechanisms among five freshwater fish species representing four families and three orders. Purified DNA, freeze-killed larvae and live larvae were exposed to UVB radiation for 12 h in the presence or absence of photorepair radiation. After exposure, we quantified frequencies of cyclobutane pyrimidine dimers in each exposure treatment. All five species used photoprotection and proportional decreases in dimer frequency were similar for all species. Evidence of excision repair was also found for all species but proportional decreases in photoproduct frequencies varied among species. Finally, evidence of photoenzymatic repair was found for only two of the five species.

Quantifying Ultraviolet Radiation Mortality Risk In Bluegill Larvae: Effects Of Nest Location

Ultraviolet (UV) radiation (280-400 nm) is an increasing threat to aquatic organisms due to stratospheric ozone depletion and reductions in concentrations of dissolved organic carbon. Because fish are most vulnerable to UV during the egg and larval stages, parental spawning site selection can strongly influence mortality risk. We examined the role of nest location in determining UV-induced mortality risk for bluegill (Lepomis macrochirus) in Lake Giles, Pennsylvania, USA. In a series of five short-term incubation experiments, we found that survival of yolk sac larvae across the range of depths at which bluegill spawn was significantly lower in the presence of ambient-UV levels relative to larvae that were shielded from UV radiation. In addition, survival decreased as a function of cumulative UV exposure, as measured by the number of cyclobutane pyrimidine dimers per megabase DNA in DNA dosimeters. Although UV had the potential to significantly reduce larval survival, DNA dosimeters placed in bluegill nests concurrently with incubation experiments indicated that most nests were exposed to relatively low levels of UV. Only 19% of nests had predicted UV-induced mortality greater than 25%. Consequently, current levels of UV may be an important mortality source at the level of individual nests, but not at the population level. One reason for the weak predicted effect of UV on bluegill survival is that many nests were located at depths by which much of the incident UV had been attenuated. In addition, many of the shallower nests were protected by overhanging trees or other submerged structures. It is important to note that Lake Giles is highly transparent and therefore not representative of all lakes in which bluegill are found. Nevertheless, Lake Giles is a natural system and may be representative of north temperate lakes in the future.

Implications of climate change for Daphnia in alpine lakes: predictions from long-term dynamics, spatial distribution, and a short-term experiment

Alpine lakes may be particularly useful as sentinels of climate change because they are highly sensitive to environmental conditions. To explore the potential biotic consequences of climate change in these systems, we conducted paleo- and neoecological observational studies, as well as a short-term experiment to examine Daphnia responses to changing environmental conditions in Rocky Mountain alpine lakes. Our analysis of a sediment core from Emerald Lake representing two periods from the Holocene revealed a significant positive relationship between the abundance of Daphnia remains and fossil Aulacoseira lirata, a diatom associated with deeper mixing depths. In addition, we detected a significant increase in mean Daphnia density in the long-term record (1991–2005) from Pipit Lake, a trend that correlated well with increases in mean surface temperature. In our survey of Daphnia in 10 lakes in the Canadian Rocky Mountains, Daphnia abundance was positively correlated with both dissolved organic carbon concentration and temperature. Finally, our short-term incubation experiment demonstrated significant effects of physical conditions (i.e., temperature and/or UV radiation) and water chemistry on the juvenile growth rate of Daphnia. Overall, our findings highlight the sensitivity of Daphnia to changes in mixing depth, water temperature, and dissolved organic matter, three limnological variables that are highly sensitive to changes not only in air temperature, but also to precipitation and location of the treeline in alpine catchments. Thus, we conclude that Daphnia abundance could serve as a powerful sentinel response to climate change in alpine lakes of the Rocky Mountains.

Mechanisms underlying habitat use of juvenile largemouth bass and smallmouth bass

Abstract Differences in habitat use between adult largemouth bass Micropterus salmoides and adult smallmouth bass M. dolomieu have been well documented, with largemouth bass occupying vegetated habitats and smallmouth bass occupying areas with cobble substrate. However, less is known about the habitat use of juveniles. Direct observations of three co-occurring populations of largemouth bass and smallmouth bass in New York State indicated that habitat use by age-0 fish was very similar to that of adults. To examine potential mechanisms underlying this pattern of habitat use, we conducted a pair of laboratory experiments. The first experiment, concerned with habitat-specific feeding rates, found that juvenile largemouth bass consumed aquatic insects at equal rates in vegetated and cobble habitats. In contrast, smallmouth bass fed at higher rates in cobble than in vegetation. The second experiment, which examined habitat-specific predation risk, found qualitative differences between the two species. Largemou...