Changes In Population Abundance Research Articles

Role of disease in abundance of a Pacific herring (Clupea pallasi) population

Disease significantly affects population abundance of Pacific herring (Clupea pallasi). Comprehensive epidemiological study of the Pacific herring population of Prince William Sound, Alaska, U.S.A., from 1994 to 2000 included complete necropsy examination of 230–500 fish each spring and 40–160 fish each fall (total n = 2983 fish). Mortality is best estimated, through modifications of an age-structured assessment model, using a disease index that combines the prevalence of viral hemorrhagic septicemia virus (VHSV) with the prevalence of ulcers. Risk factors for an epidemic include poor body condition and abundant recruitment before spawning in the spring. Prevalence of the pathogen Ichthyophonus hoferi increased as fish aged, but changes in I. hoferi prevalence were not related to changes in population abundance. Disease that caused an epidemic in 1998 (VHSV and ulcers) nearly disappeared from the population when changes in abundance were detected by traditional stock assessment methods in 1999. Disease significantly affects recruitment — the two lowest recruitment estimates on record, in 1994 and 1999, followed increased natural mortality of adults in 1993 and 1998.

A Comparison of Gill Netting and Electrofishing as Sampling Techniques for White Bass in Missouri's Large Reservoirs

I compared fall gill netting with spring electrofishing to determine which method best represented the population abundance and size structure of white bass Morone chrysops in two Missouri reservoirs. Gill nets caught all ages and sizes of white bass, and both sexes were equally represented. Median catch rates were variable but were usually less than 10 fish/gill-net day. Mean annual electrofishing catch rates peaked after mid-April and ranged from about 50–180 fish/h. Electrofishing did not reflect the size structure because it caught primarily male white bass (about 90% of the sample), which are smaller than females of the same age, and the catch rates of immature (age-1) fish were low (usually < 10 fish/h). The catch rates of the two methods were not related to each other, but both appeared to reflect large changes in population abundance. The age frequencies of the sexes and the relative differences in year-class strength were similar with both methods. I recommend fall gill netting for sampling white bass in Missouri reservoirs because all ages and both sexes were represented in the samples, which resulted in earlier assessments of recruitment and better assessments of size structure.

Anaerobic codigestion of municipal solid waste and biosolids under various mixing conditions—II: microbial population dynamics

Microbial population dynamics were evaluated in anaerobic codigesters treating municipal solid waste and sewage sludge. Ribosomal RNA based oligonucleotide probes were used to characterize changes in population abundance of syntrophic volatile fatty acid degrading bacteria and methanogens. Changes in community structure were linked to traditional performance parameters during the recovery of previously unstable codigesters induced by a reduction in mixing levels. Methanosarcina spp. were the most abundant aceticlastic methanogens in unstable codigesters with high acetate concentrations, while Methanosaeta concilii was dominant in stable systems with low levels of acetate. Growth of Syntrophobacter wolinii was enhanced during stabilization of a codigester with a well-developed population of Methanobacteriaceae, possibly because the presence of adequate numbers of these hydrogenotrophic methanogens encouraged the syntrophic oxidation of propionate. Mesophilic saturated fatty acid beta-oxidizing syntrophs were most abundant in previously unstable codigesters. One minimally mixed reactor became unstable after switching to continuously mixed conditions. After the switch, total archaeal abundance decreased sharply, though Methanobacteriaceae and Methanosarcina spp. levels increased as the fermentation became unbalanced. Based on the results presented here, mixing appears to inhibit the syntrophic oxidation of volatile fatty acids, possibly by disrupting the spatial juxtaposition of syntrophic bacteria and their methanogenic partners.

Synergetic Approach and modeling of fish population dynamics

In the last few years it has become increasingly obvious that one of the obstacles in the way of constructing good simulation models of the global ocean ecosystem is a poor understanding of the general principles of marine ecosystems processes. A great number of factors and relationships acting in the marine environment, in combination with the random character of change in many of them, call for the development of new approaches in modeling. In this paper a synergetic approach is proposed. A new paradigm for this approach is discussed. As an example a population with logistic natural growth under different conditions of exploitation is considered. It is shown that the simplest mechanism, that principally changes the behavior of a population in a fluctuating environment, includes fishing and migration. This mechanism explains catastrophic changes in population abundance in cases when no one factor may be seen as exclusive. It is shown that the characteristic level of population number does not correspond to the average balance between input (migration), output (fishing) and the growth of the population. The environment variability leads to stabilization far from equilibrium. This totally conforms to one of the fundamental results in Synergetics which assert that non‐equilibrium in the presence of fluctuations may serve as a source of new order.

Dynamics of North American breeding bird populations

Population biologists have long been interested in the variability of natural populations1,2,3,4,5,6. One approach to dealing with ecological complexity is to reduce the system to one or a few species, for which meaningful equations can be solved. Here we explore an alternative approach7,8 by studying the statistical properties of a data set containing over 600 species, namely the North American breeding bird survey9. The survey has recorded annual species abundances over a 31-year period along more than 3,000 observation routes10. We now analyse the dynamics of population variability using this data set, and find scaling features in common with inanimate systems composed of strongly interacting subunits11. Specifically, we find that the distribution of changes in population abundance over a one-year interval is remarkably symmetrical, with long tails extending over six orders of magnitude. The variance of the population over a time series increases as a power-law with increasing time lag, indicating long-range correlation in population size fluctuations12. We also find that the distribution of species lifetimes (the time between colonization and local extinction) within local patches is a power-law with an exponential cutoff imposed by the finite length of the time series. Our results provide a quantitative basis for modelling the dynamics of large species assemblages.

Large scale variation in abundance of five common species of decapod sampled from seagrass in New South Wales

Abstract Decapods associated with two species of seagrass (Zostera capricorni and Posidonia australis) were sampled at several sites within estuaries on the New South Wales coast for 3 years. Two species of decapod (Macrobrachium intermedium and Penaeus plebejus) were abundant in Z. capricorni. Four species (M. intermedium, P. plebejus, Chlorotocella leptorhynchus and Nectocarcinus tuberculosus) were abundant in P. australis. Abundance of all these species fluctuated greatly among sites and through time. Some seasonal patterns in abundance were evident, and appeared to be caused by recruitment and subsequent mortality or migration of individuals. Similarly, there were some consistent spatial patterns in abundance of most species. In particular, some sites supported consistently more juveniles during the same period of separate years. Within Jervis Bay, P. australis was also sampled in shallow (1–2m) and deep (7–8 m) water. Three species were abundant (C. leptorhynchus, N. tuberculosus and Processa australiense), but showed no consistent differences between the two depths. Abundances of these species did fluctuate greatly among the sites within Jervis Bay and through time. Such observations of the natural rates of change in abundance of populations are essential in order to be able to detect the effects of human disturbances.

Coupling between physical and biological scales

Temporal changes in population abundance display a great diversity of patterns. This diversity is acknowledged by reviewers of marine and terrestrial data, but their explanations tend to fall into two categories: internal factors related to population or com m unity structure; and external aspects determined by the environment. T he former we associate with terrestrial studies and the latter with marine. In this paper we shall stress the importance of the characteristic timescales in the dynamics of the populations and their relation to the scales of relevant environmental forces. We shall suggest that the coupling between these different scales, rather than the nature of the internal or external processes themselves, can provide a partial description, if not an explanation, of the diversity of patterns.

Population Studies of Blue Crabs in Soft-bottom, Unvegetated Habitats of a Subestuary in the Northern Gulf of Mexico

ABSTRACT Studies were conducted to assess seasonal and interannual changes in population abundance, size frequency, recruitment, sex ratio, molt condition, and the reproductive cycle of the blue crab Callinectes sapidus in Weeks Bay, Alabama, a subestuary of Mobile Bay. Populations sampled from soft-bottom, unvegetated sites from May 1989 to August 1991 varied significantly in density, but with little predictable annual pattern. Brief periods of peak abundance occurred sporadically in summer and spring months, followed by relatively long periods of relative stable abundance. Interannual variations in abundance occurred, with numbers of crabs much higher during 1989-1990 than 1990-1991. A cohort of subjuvenile crabs ( 80-mm CW) occurred mostly at the mouth of the bay. Sex ratios were skewed towards males when examined over the entire course of the study period. This was due to an excess of males in 1989. Crabs which had heavily fouled or soft carapaces (near or recent molt) were collected more frequently in the warm season (April through October) than the cool season (November through March). An annual reproductive cycle was evident, with gonadal maturation occurring in the summer and fall. No berried female crabs were collected, indicating that sexually mature females migrate out of the subestuary to hatch their eggs at higher salinities.

Lack of coupling between heterotrophic nanoflagellates and bacteria: A general phenomenon across aquatic systems?

The literature proposes that bacterial abundance (BA) and heterotrophic nanoflagellates abundance (HNFA) are positively related across different aquatic systems, suggesting a strong resource control of HNFA and predatory control of BA. To test this proposed pattern, we collected published and unpublished data and analyzed trends across lakes, rivers, marine, and benthic samples. BA varied between 105 and 1011 cells ml−1, while HNFA ranged from 102 to 106 cells ml−1. The relationship was stronger in systems with low bacterial abundances and weaker in the most bacteria‐rich systems. When data were averaged per study or per system we detected a trend with trophy, with highest HNFA in benthic systems followed by lakes and coastal areas of the sea, and lowest abundances in rivers and the open ocean. This relationship was improved in lakes, where cladoceran abundance was negatively related to HNFA. The effect of cladocerans was stronger in lakes where Daphnia spp. dominated. The weak pattern observed between BA and HNFA across systems suggests that several factors (resources, predators, environment, etc.) are acting differently on BA and HNFA. Changes in population abundances of bacteria and heterotrophic nanoflagellates are mainly determined by nutrient supply and grazing, and also by complex trophic interactions with numerous feedbacks. Empirical relationships between BA and HNFA across systems can only partially explain these changes in terms of predator‐prey dependence.

A Catch-at-Length Analysis that Incorporates a Stochastic Model of Growth

A length-structured population model, which incorporates von Bertalanffy growth, is used to describe changes in population abundance over time. The model is incorporated into a catch-at-length algorithm that uses a nonlinear least squares approach to estimate relative abundance, fishing mortality, selectivity, and the von Bertalanffy growth parameters L∞ and k. The algorithm is applied to a simulated data set for Pacific cod (Gadus macrocephalus) and to catch data on Pseudotolithus typus and Decapterus russellii. The parameter estimates of Pacific cod obtained from this algorithm were comparable with the values that were originally used to simulate the data. Although the catch data of P. typus did not exhibit the full range of sizes present in the population due to differential vulnerability of the population to the fishery, the estimates of L∞ and k reflect the growth over the entire size range of the population. Other population estimates for P. typus were in agreement with observed biological information. The estimated growth parameters L∞ and k of D. russellii showed some discrepancy with the information available on mature individuals present in this fishery but appeared to adequately represent year 1 growth. The estimated population and exploitation parameters fit the observed catch-at-length. Estimates made with the catch-at-length approach can be improved by using auxiliary information that may be available on abundance, fishing effort, recruitment, and growth.

Population Dynamics of Newfoundland Moose Using Cohort Analysis

Changes in abundance, recruitment rates, and age-specific harvest rates of 3 populations of moose (Alces alces) in Newfoundland were estimated with cohort analysis. Populations declined during 1966-73 due to overharvesting, but recovered in all areas following reduction of license quotas in the mid-1970's. Age-specific vulnerability to hunting was highest for young adults and lowest for calves and mid-aged adults. Differences in vulnerability were possibly related to hunter preferences, previous experience of moose with hunters, and frequency of movements by moose. Data on kills/day and moose seen/day obtained from hunter questionaires were positively correlated with moose abundance but negatively correlated with the number of hunters. The latter suggested that interference had a strong influence on hunter success. J. WILDL. MANAGE. 52(1):14-21 A challenging problem facing wildlife managers is how to measure changes in population abundance with sufficient accuracy to make reasonable management decisions. In general, 2 approaches have been employed. Where sufficient funding and manpower are available, many agencies attempt direct estimation of population abundance using aerial surveys, drive counts, or mark-recapture techniques. These methods are costly, time consuming, and often imprecise. Where resources are more limited, wildlife managers are often forced to rely on indirect indices of abundance such as pellet counts, track counts, or hunter success statistics. Such indices can show conflicting trends, and often no other information is available to evaluate which indices are meaningful. Fisheries scientists are faced with similar problems. Their predicament is perhaps even more acute because direct enumeration is impossible for most fish populations. In response, fisheries scientists have developed a wide variety of methods for assessing stock abundance based largely on catch returns from survey vessels or commercial fleets. A powerful technique to reconstruct historical changes in stock abundance from catch-at-age data is the cohort or virtual population analysis (Ricker 1948, Fry 1949, Pope 1972). We used cohort analysis to reconstruct historical changes in moose abundance in 3 areas of Newfoundland from kill-at-age data gathered during 1966-85. Population estimates for 1966-73 (corresponding to cohorts that have completely passed through the population) were used to estimate rates of recruitment and agespec fic vulnerability, and to calibrate indirect indices that have been used in the past to manage moose populations in Newfoundland. Both cohort analysis and indirect indices were then used to evaluate more recent trends in abundance. Funding for this work was provided by the Wildlife Division, Department of Culture, Recreation and Youth, Newfoundland. The Biological Data Centre, University of British Columbia, kindly provided computer facilities. Data were gathered with the assistance of W. A. Collins, S. M. Oosenbrug, and M. J. Strapp. We thank J. S. Collie, P. R. Krausman, M. F. Lapointe, C. J. Walters, G. H. Winters, and an anonymous reviewer for helpful discussions and comments on the manuscript.

Some limitations of biological monitoring

Biological monitoring data are obtained to document changes in population abundance for resource management, to verify compliance with regulatory directives, and to assess cause and effect for research purposes. Particular statistical analyses are often not appropriate because suitable field designs are either not available or not matched a priori to the monitoring objectives. When monitoring to detect normal changes in population numbers, quantitative problems may not be as difficult as they are in detecting and assigning causation induced by a natural or man-made pollutant. Changes in biotic abundance can be caused or influenced by compensation, indirect effects, direct mortality, and interactions among environmental variables. Even though data from baseline monitoring programs should usually only be used to detect change in biotic abundance, they sometimes can be used to draw inferences about cause by correlation or to test laboratory results against a long-term historical record. Available quantitative methodologies useful in evaluating monitoring data are limited and interpretations about cause and effect are difficult.