Gary K. Meffe

Effects of Abiotic Disturbance on Coexistence of Predator-Prey Fish Species

Behavioral responses of animals to large—scale abiotic disturbance are poorly known. Organisms living in habitats with frequent perturbations may evolve behaviors that minimize the impact of, or exploit, the disturbance. Organisms from environments that have fewer or different perturbations should not evolve these behaviors. The Sonoran topminnow (Poeciliopsis occidentalis) a poeciliid fish native to the arid American southwest, and the mosquitofish (Gambusia affinis), a morphologically similar poeciliid introduced from the mesic central and eastern United States, occur sympatrically in a few Arizona localities. Since flash floods of the mountainous Southwest are quantitatively and qualitatively different from the typical spring floods of mesic lowland drainages, I predicted that introduced G. affinis populations would incur greater losses during Southwestern floods than would native P. occidentalis. This was supported by preflood and postflood populations surveys in a natural habitat and by subsequent laboratory simulations of floods. The native species apparently possesses innate behaviors, such as quick response and proper orientation to high discharge, that allow its persistence under flash flood conditions. Mosquitofish show little such behavior and are more easily displaced by floods. Mosquitofish typically extirpate topminnows from native habitats via predation within 1—3 yr after introduction. Replacement is most rapid in localities that rarely or never flood, while long—term coexistence may occur in frequently flooded habitats. Abiotic disturbances may promote coexistence by periodically reducing population sizes of the exotic predator before the native prey species is eliminated. This hypothesis is consistent with data from a variety of other Arizona aquatic systems containing communities of native prey and introduced predatory fishes.

INFLUENCE OF BEAVERS ON STREAM FISH ASSEMBLAGES: EFFECTS OF POND AGE AND WATERSHED POSITION

We used a spatial survey of fish assemblage structure in streams and beaver ponds to: (1) determine the effects of beavers on fish assemblage structure at the reach and drainage basin scales, and (2) assess the influences of pond age, watershed position, and pond environment on fish assemblage structure within beaver ponds. Stream impoundment by beavers affected species richness at the reach scale, but this effect was highly dependent on pond age and drainage area above the pond. In headwater streams, species richness per pond increased to a high in ponds 9-17 yr old and then decreased to a low in ponds >17 yr old. Farther downstream, species richness showed little change with pond age. Assem- blage structure varied significantly among unimpounded, impounded, and previously im- pounded stream reaches, and variation in assemblage structure among ponds was related to pond age and physical habitat. In large ponds there was a shift from lotic to lentic species, and as ponds aged, small-bodied minnows were replaced by larger predators. Because there was high species turnover among impounded and previously impounded reaches, beavers affected patterns of species richness at the drainage basin scale; we collected more species in first- and second-order streams (32 and 38, respectively) than in third-order streams (26). Taken together these results indicate: (1) that beavers have a positive effect on fish species richness in low-order, blackwater streams, but maintenance of this effect requires preser- vation of both spatial and temporal dynamics of beaver pond creation and abandonment, and (2) the positive relationship between stream fish species richness and drainage area described for many streams may be a recent phenomenon resulting from the extirpation of beavers from much of their historical range.

Conservation Genetics and the Management of Endangered Fishes

Abstract The emerging field of endangered fishes management has yet to fully incorporate conservation genetics into recovery programs. Genetic aspects of small populations must be considered at the outset of management programs in order to maximize probability of their long‐term survival and continued adaptability. Total genetic variance of a species consists of within population genetic diversity, and the differences found among populations; both types of variance should be maintained to maximize adaptive flexibility of endangered fishes. Forces that erode genetic variation include small population size, population bottlenecks, genetic drift, inbreeding depression, artificial selection in captivity, and mixing of distinct genetic stocks. These can lead to increased homozygosity, loss of quantitative variation, and exposure of deleterious recessive alleles, all of which may reduce fitness. Suggestions for genetically sound management of endangered fishes include genetic monitoring of natural and captive populations, use of large numbers for captive breeding where feasible, selective mating to avoid inbreeding where necessary, minimization of time in captivity, and separate maintenance of distinct stocks.

POST-DEFAUNATION RECOVERY OF FISH ASSEMBLAGES IN SOUTHEASTERN BLACKWATER STREAMS'

The authors analyzed fish assemblage structure at 37 sites in South Carolina streams before and nearly one year after experimental defaunation to test assemblage resiliency. Decreases in stream depth and width in the second year reflected an intervening drought, but habitat structure remained highly correlated between years. Fish assemblages recovered well over four scales of analysis. Total fishes sampled, collective assemblage properties (species richness, density, biomass, and mean mass of fish), local assemblage structure, and single-species attributes generally did not significantly differ after defaunation, as determined by species- and individual-abundance correlations, detrended correspondence analysis, and a proportional similarity index. These assemblages were not randomly structured units, but were largely deterministic systems highly predictable from local habitat structure.

Short-Term Recolonization by Fishes of Experimentally Defaunated Pools of a Coastal Plain Stream

Fishes were removed from pools in a South Carolina stream to observe subsequent recolonization patterns and development of community structure. Recolonization, measured between two and 32 days, was rapid; and downstream sites attained preexperimental density, richness, and proportional composition by day 32. However, recovery of upstream sites was incomplete in that time. Species colonized in proportion to abundance, and rare species returned later; both of these results are interpreted as simple proportional sampling of a mobile fauna. In a longer experiment, defaunated pools did not differ from controls; and density, richness, and composition showed no temporal trend over 31-63 days. Movement, measured by directional trapping, was dominated by spawning yellowfin shiners (Notropis lutipinnis), but 16 of 26 species present were trapped, demonstrating active movement by much of the fauna. Rapid recolonization of defaunated areas is plausibly explained by a combination of high mobility and equilibrium with source populations.

Plasticity of life-history characters in eastern mosquitofish (Gambusia holbrooki: Poeciliidae) in response to thermal stress

Eastern mosquitofish (Gambusia holbrooki) from a population exposed to abnormally high temperatures for 28 years were raised from birth in environmental chambers at 25 C (a normal temperature) or 32 C (a stressful temperature). Fish grew faster and larger at 25 C and were in better somatic condition than were fish grown at 32 C. There were no differences in ovum production at the two temperatures. At the higher temperature, fish matured at a younger age and a smaller size than did those at the normal temperature. This particular age-size at maturity relationship is not included as one of the predicted norms of reaction of a model by Stearns and Koella (1986) that deals with age and size at maturity under various growth rates. Gambusia holbrooki is a highly plastic species, which makes it adaptable to a variety of conditions, but also complicates interpretations of its interpopulation life-history differences; much of its life-history variation may be a consequence of different developmental environments rather than adaptive responses to demographic conditions.

Conserving fish genomes: philosophies and practices

The earth’s biota is facing an unprecedented crisis. Massive habitat change, introduction of exotic species, careless disposal of toxins,and countless other anthropogenic ills have placed many species in potential or immediate danger of extinction. Global biotic diversity will continue to decline with man’s expanding domain over the planet (Wilson 1985). This loss of species diversity is accompanied by a decline in genetic diversity. The former problem is generally obvious either a species exists or it does not. This is observable and quantifiable. Loss of genetic diversity, however, is inconspicuous and thus easily overlooked. How important is global erosion of genetic diversity? What is lost when populations or alleles vanish? Can lost genetic diversity be recovered? These are exceedingly difficult questions to answer, but nonetheless worthy of our consideration. I recently outlined basic principles to consider when dealing with genetic problems in conservation of endangered fishes (Meffe 1986) and will not repeat those details here. However, in reflecting and expanding upon some of the issues raised in that paper, one word repeatedly comes to mind: ignorance. We simply do not yet know much about conserving genetic diversity in fishes or other taxa. Many of the approaches to, and basic tenets of, conservation genetics are educated guesses based on theoretical models of simple genetic systems. They are good starting points, but much needs to

Lipid Dynamics and Growth Relative to Resource Level in Juvenile Eastern Mosquitofish (Gambusia holbrooki: Poeciliidae)

We examined early energy allocation patterns of eastern mosquitofish (Gambusia holbrooki) reared on three dietary regimes by measuring growth and lipid storage at several points during the first 40 days of life. A high food treatment produced growth increments similar to those previously found in natural mosquitofish populations, whereas intermediate (one-half the quantity of high) and low (one-quarter the quantity of high) food treatments produced significantly lower growth increments. Lipid content was about 20% of dry mass at birth and declined for the first five days of life in all three treatments. After this initial period of lipid use, lipid levels increased asymptotically in all treatments. Lipid content was positively correlated with feeding level and plateaued at significantly different levels for each treatment. These three laboratory-reared groups had significantly higher percent somatic lipids than two populations of wild fish of similar size, probably because of differences in food and activity levels between laboratory and field environments. Size at birth was positively correlated with size at two weeks of age but was not significantly correlated with size beyond 15 days of age. Birth size was uncorrelated with subsequent growth or lipid content. No trade-offs between growth and lipid storage were found at any level examined in this study. We suggest that differences in energy acquisition or metabolic efficiency may swamp differences in resource allocation between these two energy compartments.

Genetic and Ecological Guidelines for Species Reintroduction Programs: Application to Great Lakes Fishes

Complete restoration of the Great Lakes is unlikely, due to naturalization of exotic species, habitat degradation and destruction, heavy fishing mortality, lack of native gene pools, and complicated political jurisdictions that rarely work toward a common vision. A more realistic goal is rehabilitation, a movement along the trajectory toward complete restoration. Proper rehabilitation employs an evolutionary-genetic perspective, which protects and works with the remaining genetic variation available in lake trout or other species of concern. A difficult question is how to define the units of genetic conservation; one answer is to determine Evolutionary Significant Units. To do this, population structure must be defined, and various conceptual models are here presented that determine genetic population structure as a function of geographic structure of the habitat. Several concepts from the developing field of conservation biology should also be incorporated into lake trout rehabilitation. These include metapopulations—groups of populations that experience some degree of regular or intermittent gene flow, and that serve as recolonization sources after local extinctions. Related to this is “source-sink dynamics,” a recognition that habitats typically are unequal in quality. Habitat fragmentation is also relevant because it can stop inter-population movement and disrupt metapopulation structure and source-sink dynamics. Finally, hatcheries should be used in rehabilitation only with great caution and skepticism. Hatcheries tend to address the symptoms, rather than the causes, of fish declines, and may mask the underlying problems by continually replacing declining fish. Improved habitat quality, rather than artificial production, is the key to rehabilitation of the Great Lakes for lake trout and other native species, and hatcheries should only be used as a stopgap measure to prevent further losses of genetic diversity.

Multivariate analysis of feeding relationships of fishes in blackwater streams

SynopsisThe autumnal diets of 23 fish species in 9 families from South Carolina blackwater streams were characterized by rank order statistics and analyzed by detrended correspondence analysis (DCA). DCA ordinations of fishes and their food items indicate two primary gradients. The first axis contrasts fine particle feeders and omnivores (Catostomidae, some cyprinids) eating algae, detritus, microcrustacea and bivalves, with predators(Anguilla, Esox, largeAmeiurus) on decapods and vertebrates. The second is a gradient from benthic feeders (percid darters,Noturus catfishes) with varied invertebrate diets, to taxa (Gambusia, some cyprinids and centrarchids) feeding at the surface. With increasing size, fishes shift to larger prey and increased surface feeding. Much of the trophic differentiation within the assemblage reflects diversification at generic and family levels. DCA is an effective method for summarizing trophic relations in diverse assemblages.