Estimates Of Vital Rates Research Articles

Is mean squared error a consistent indicator of accuracy for spatially structured demographic models?

Animal demographic models are used in many disciplines and it is well-understood that their results and interpretation depend greatly on their structure and complexity. However, when models are constructed using data compiled from multiple sources it is difficult to objectively assess optimal structure and complexity. We explore the use of mean squared error (MSE) to identify optimal model structure. To illustrate, we compare the performance of a single-patch model versus a two-patch model for a migratory songbird in a spatially structured environment. In the single-patch model, mean fecundity across patches is estimated, and movement between patches is ignored. In the two-patch model, patch-specific reproductive rates are estimated, which requires the estimation and use of rates of movement between patches. MSE performed well as an indicator of optimal model structure, consistently conforming to our intuition about which model should be preferred under different combinations of model parameters. We show that (1) when dispersal offsets patch-specific demography, the single-patch model will always be favored, (2) with good estimates of vital rates, only modest sample sizes for dispersal are required to make the two-patch model more accurate, and (3) the same factors that result in large bias in the simple model also result in large sampling error for the two-patch model. Finally, our analyses also suggest that erroneous conclusions about optimal model complexity can be reached when additional structure is added to an already poorly parameterized model and we recommend a stepwise approach to the study of model complexity.

Application of photographic capture–recapture modelling to estimate demographic parameters for male Asian elephants

AbstractIn addition to the threats of habitat loss and degradation, adult males of the Asian elephant Elephas maximus also face greater threats from ivory poaching and conflict with humans. To understand the impact of these threats, conservationists need robust estimates of abundance and vital rates specifically for the adult male segment of elephant populations. By integrating the identification of individual male elephants in a population from distinct morphology and natural markings, with modern capture–recapture (CR) sampling designs, it is possible to estimate various demographic parameters that are otherwise difficult to obtain from this long‐lived and wide‐ranging megaherbivore. In this study, we developed systematic individual identification protocols and integrated them into CR sampling designs to obtain capture histories and thereby estimate the abundance of adult bull elephants in a globally important population in southern India. We validated these estimates against those obtained from an independent method combining line‐transect density estimates with age–sex composition data for elephants. The sampled population was open to gains and losses between sampling occasions. The abundance of adult males in the 176 km2 study area was N̂(SÊN̂) = 134(14.20) and they comprised 14% (±1%) of the total elephant population. Time‐specific abundance estimates for each sampling occasion showed a distinct increase in adult male numbers over the sampling period, explained by seasonal patterns of local migration. CR‐based estimates for adult male abundance closely matched estimates from distance‐based methods. Thus, while providing abundance data of comparable rigour and precision, photographic CR methods permit estimation of demographic parameters for the Asian elephant that are both urgently needed and difficult to obtain.

Dynamics of Newly Established Elk Populations

Abstract: The dynamics of newly established elk (Cervus elaphus) populations can provide insights about maximum sustainable rates of reproduction, survival, and increase. However, data used to estimate rates of increase typically have been limited to counts and rarely have included complementary estimates of vital rates. Complexities of population dynamics cannot be understood without considering population processes as well as population states. We estimated pregnancy rates, survival rates, age ratios, and sex ratios for reintroduced elk at Theodore Roosevelt National Park, North Dakota, USA; combined vital rates in a population projection model; and compared model projections with observed elk numbers and population ratios. Pregnancy rates in January (early in the second trimester of pregnancy) averaged 54.1% (SE = 5.4%) for subadults and 91.0% (SE = 1.7%) for adults, and 91.6% of pregnancies resulted in recruitment at 8 months. Annual survival rates of adult females averaged 0.96 (95% CI = 0.94‐0.98) with hunting included and 0.99 (95% CI = 0.97‐0.99) with hunting excluded from calculations. Our fitted model explained 99.8% of past variation in population estimates and represents a useful new tool for short‐term management planning. Although we found no evidence of temporal variation in vital rates, variation in population composition caused substantial variation in projected rates of increase (Λ = 1.20‐1.36). Restoring documented hunter harvests and removals of elk by the National Park Service led to a potential rate of Λ = 1.26. Greater rates of increase substantiated elsewhere were within the expected range of chance variation, given our model and estimates of vital rates. Rates of increase realized by small elk populations are too variable to support inferences about habitat quality or density dependence.

Survival and Recovery Rate of Canada Geese Staging in Interior Alaska

Abstract: Lesser Canada geese (Branta canadensis parvipes) are indistinguishable from other subspecies of small Canada geese on the wintering grounds using current survey methods. Consequently, managers are unable to adequately measure their abundance. Without direct estimates of abundance, researchers often use estimates of vital rates that influence abundance (e.g., annual survival) to monitor potential impact of harvest on the population. Based on capture and re‐sighting data records of 567 geese marked from 1994 through 1998, we calculated annual survival and recovery rates for different age and sex classes of white‐cheeked geese staging in interior Alaska. We compared those survival and recovery rates with those of other neck‐collared white‐cheeked geese. The best approximating model allowed survival to vary by age class while holding Seber's recovery probability (r̂) constant over sex, age class, and time. We estimated annual survival to be 0.49 (SE = 0.05) for hatch‐year geese and 0.68 (SE = 0.03) for after‐hatch‐year geese based on the weighted average of all models with a change in Akaike's Information Criterion adjusted for small sample size and lack of fit < 4. Estimates of annual survival of white‐cheeked geese in this study are among the lowest and recovery estimates are among the highest for migratory populations of neck‐collared geese. Low survival estimates of Canada geese in our study suggest that harvest rates may be higher than in many other populations. Surveys to estimate abundance or other population parameters such as reproductive success and recruitment are necessary to determine whether this population is self‐sustaining. Furthermore, we recommend monitoring abundance and harvest of small white‐cheeked geese east and west of the Cascade Mountain Range separately to better determine harvest pressure on white‐cheeked geese wintering east of the Cascades.

Estimator of upgrowth transition rates for size-classified matrix from small samples

Matrix models can incorporate demographic, environmental or sampling stochasticity. The first two have intensively been studied, and we shall focus on the latter. It comes from the uncertainty on the estimation of vital rates, that generates an uncertainty on the model predictions. When dealing with size-classified models, the continuous information brought by the size may or may not be taken into consideration to estimate vital rates. The proportion estimator is obtained in the former case, whereas the increment estimator is obtained in the latter case. We compared these two estimators on the basis of their bias, variance and probability of being null, and applied the results to a tropical rain forest in French Guiana. The proportion estimator is unbiased, whereas the increment estimator is generally biased. We specified some conditions under which the increment estimator is also unbiased. However, the increment estimator generally has a lower asymptotic variance than the proportion estimator. As a consequence, the increment estimator is generally more efficient than the proportion estimator for small samples. Moreover, the increment estimator cannot bring null estimates, contrary to the proportion estimator, which reinforces its suitability for small samples.

The persistence niche: what makes it and what breaks it for two fire-prone plant species

Persistence niches are expected to favour qualitatively different plant life histories compared with regeneration niches. In fire-prone habitats, for example, resprouting plants may be expected to exploit persistence niches, whereas obligate-seeders by definition exploit regeneration niches. Resprouter life histories should be typified by high rates of survival, which may be offset by relatively low rates of growth and reproduction. This combination of characters is expected to result from trade-offs in resource allocation and because the longevity of individual plants should buffer their populations against the effects of recruitment failure. We asked whether two resprouting perennial shrubs, Epacris barbata Melville and Xanthorrhoea resinifera (Sol. Ex Kite) E.C.Nelson & D.J.Bedford, exhibited the life-history character combinations that are expected for species exploiting a persistence niche. We also investigated how a change in habitat suitability caused by the invasion of a root pathogen may limit the ability of these species to occupy persistence niches. Demographic censuses of several years’ duration in two populations of each species yielded estimates of vital rates that were consistent with the life-history profile expected for a persistence niche. Rates of background survival were high and rates of fire-related mortality were low in both species. As expected, these were associated with low rates of growth and seedling establishment, although rates of seed production and viability were relatively high in both species. The importance of survival was confirmed by stochastic population models, which showed that population viability was more sensitive to decreases in survival of mature plants and increases in fire mortality of established plants than to changes in other vital rates. Seedling growth rates were also relatively important in E. barbata. Populations of both species that had been infected by root rot disease, Phytophthora cinnamomi, had substantially reduced survival rates and, consequently, reduced population viability. These effects were more extreme in E. barbata than in X. resinifera. We conclude that processes that reduce survival, such as disease infection and habitat loss, rather than processes that impede seed production and recruitment mediate the persistence niche. However, we discuss the possibility that this dependency might be mitigated by high fecundity if infrequent conditions that permit large recruitment events have so far eluded detection.

REDUCED POPULATION GROWTH OF GRAY SEALS AT SABLE ISLAND: EVIDENCE FROM PUP PRODUCTION AND AGE OF PRIMIPARITY

AbstractPup production on Sable Island, Nova Scotia, has been increasing exponentially since the early 1960s and by 1997 Sable Island was the largest gray seal colony worldwide. Using an aerial photographic survey, as in previous years, we estimated pup production in January 2004 to determine if this exponential rate of increase had continued. A total of 33,268 pups was counted on the color positives. When corrected for the proportion pups missed on the imagery (1.106 for the 12th; 1.527 on the 13th), the proportion of pups that died prior to the survey (0.031), and the proportion of pups born before the survey (east colony 0.966, west colony 0.962), estimated total pup production was 41,500 with SE = 4,381. The 2004 estimate indicates that pup production on Sable Island has continued to increase, but suggests that the rate of increase (r) may have declined (0.070 compared to previous 0.128). Females from the 1998–2000 cohorts were about 16 times less likely to give birth for the first time at age 4 yr and more than twice as likely at age 6 yr compared to those in the mid‐late 1980s. The new estimate of pup production and observed changes in age of primiparity provide the first indication of changes in vital rates of this population. However, additional estimates of pup production and vital rates are needed to confirm this conclusion and to investigate the underlying mechanisms.

Effects of Falconry Harvest on Wild Raptor Populations in the United States: Theoretical Considerations and Management Recommendations

We used recent population data and a deterministic matrix model that accounted for important aspects of raptor population biology to evaluate the likely impact of falconry harvest (including take of different age classes) on wild raptor populations in the United States. The harvest rate at maximum sustainable yield (MSY) ranged from 0.03 to 0.41 for the species examined. At least for peregrine falcons (Falco peregrinus), harvest rate at MSY was greatest for nestlings and lowest for adults. The quality of demographic data for the species influenced MSY. For most species the state of current knowledge probably underestimates the capacity for allowed harvest because estimates of vital rates, particularly survival, are biased low, because emigration is not distinguished from survival. This is offset somewhat by biases that might overestimate sustainability inherent in MSY-based analyses and deterministic models. Taking these factors into consideration and recognizing the impracticality of monitoring raptor populations to determine actual effects of harvest, we recommend that falconry harvest rates for juvenile raptors in the United States not exceed one-half of the estimated MSY up to a maximum of 5%, depending on species-specific estimates of capacity to sustain harvest. Under this guideline, harvest rates of up to 5% of annual production are supported for northern goshawks (Accipter gentilis), Harris's hawks (Parabuteo unicinctus), peregrine falcons, and golden eagles (Aquila chrysaetos); lower harvest rates are recommended for other species until better estimates of vital rates confirm greater harvest potential.

INCORPORATING DIVERSE DATA AND REALISTIC COMPLEXITY INTO DEMOGRAPHIC ESTIMATION PROCEDURES FOR SEA OTTERS

Reliable information on historical and current population dynamics is central to understanding patterns of growth and decline in animal populations. We developed a maximum likelihood-based analysis to estimate spatial and temporal trends in age/sex-specific survival rates for the threatened southern sea otter (Enhydra lutris nereis), using annual population censuses and the age structure of salvaged carcass collections. We evaluated a wide range of possible spatial and temporal effects and used model averaging to incorporate model uncertainty into the resulting estimates of key vital rates and their variances. We compared these results to current demographic parameters estimated in a telemetry-based study conducted between 2001 and 2004. These results show that survival has decreased substantially from the early 1990s to the present and is generally lowest in the north-central portion of the population's range. The greatest temporal decrease in survival was for adult females, and variation in the survival of this age/sex class is primarily responsible for regulating population growth and driving population trends. Our results can be used to focus future research on southern sea otters by highlighting the life history stages and mortality factors most relevant to conservation. More broadly, we have illustrated how the powerful and relatively straightforward tools of information-theoretic-based model fitting can be used to sort through and parameterize quite complex demographic modeling frameworks.

Environmental stochasticity: empirical estimates of prairie vole survival with implications for demographic models

A rich theory exists regarding the potential impact of correlations among vital rates on population projections derived from demographic models. However, relatively little is known about the magnitude of correlations among vital rates in natural populations, particularly in mammals. We used 30 years of mark–recapture data from a population of prairie voles ( Microtus ochrogaster (Wagner, 1842)) to test for differences in survival among mass classes and sexes, in correlations among vital rates, in correlations between vital rates and environmental factors, and in autocorrelation in vital rates. Estimated monthly survival rates did not differ significantly among mass classes and there were no significant cross-correlations among mass classes. Survival of large prairie voles increased in mild winters (i.e., warm temperatures and low snowfall). Survival rates of medium and large voles were negatively autocorrelated at time lags of 9–12 months, and survivals of large voles were positively autocorrelated for time lags of <3 months. These autocorrelations were not explained by patterns of temperature or precipitation. The observed degree of autocorrelation in vital rates is sufficient to affect projections from demographic models, particularly in short-lived taxa that require seasonal or monthly estimation of vital rates.

Can we use the young : female ratio to infer ungulate population dynamics? An empirical test using red deerCervus elaphus as a model

SummaryManagement decisions for exploited species often rely on count ratios, such as the young to adult female ratio as a proxy of birth rate, and the decrease in the ratio between yearling and females in yeart+ 1 and the young : female ratio in yeartto obtain juvenile survival. However, the reliability of such estimates has yet to be demonstrated.We investigated the consistency of the young to female ratio method by using observations on individually marked female red deer. These females were known to have raised their calves to weaning successfully. We compared temporal variation in the proportion of females with a calf at heel from count ratios with the probability of observing a calf with its mother from marked animals.The proportion of females observed with a calf at heel decreased over time (from 0·43 early May to 0·15 early March). This measure was influenced by individual variation and by interactive effects of year and habitat type, although there was no direct relationship with the density of vegetation cover.The proportion of females seen with a calf at heel assessed from counts did not correspond with the probability of observing a female with a calf at heel, given that she successfully weaned a calf in autumn (estimated from direct observation). The probability of observing a female with a calf was highest in September (0·4) prior to the rut and lowest just after the birth season (0·08) and during the rut (0·18).The temporal pattern in the probability of observing a calf with its mother, given that she successfully weaned a calf, reflected changes in the mother–calf bond from birth to weaning, rather than changes in offspring mortality or changes in pregnancy rates. These variations in the probability of observing the mother–calf pair in the field may lead to strong biases in count ratio‐based methodologies.Synthesis and applications. This study has important implications for the management of ungulate populations when decisions are based on count ratios. Our results indicate that count‐based estimates of vital rates are misleading, especially in closed habitats. Pregnancy rate and juvenile survival rate are both key parameters in detecting density‐dependence responses. Reliable estimates of survival and pregnancy rates obtained from marked animals and foetus sampling are more accurate. Alternatively, yearly monitoring of young body mass or bone length coupled with an index of grazing pressure would improve the assessment of population dynamics.

Demographic meta‐analysis: synthesizing vital rates for spotted owls

Summary Effective resource management ultimately influences vital rates of fecundity and survival for target species. Meta‐analysis can be used to combine results from multiple demographic studies replicated in time and space to obtain estimates of vital rates as well as metrics of population growth. Workshop formats were used to conduct meta‐analyses of mark–recapture experiments on spotted owls Strix occidentalis in the western USA. The implied motivation for demographic studies of spotted owls has been that changes in vital rates and population growth, λ, reflect the success of conservation strategies, but how to interpret results may not be obvious. Demographic analysis is of little practical utility until vital rates can be linked to management. In the case of spotted owls, future meta‐analyses must focus on co‐variation between vital rates and habitat variables, and experiments will be necessary. Sensitivity of population growth to variation in vital rates is central to demographic analysis, but results must be interpreted cautiously because these sensitivities are not likely to identify the vital rates most responsible for variation in population size, and cannot reveal which vital rates will be most responsive to conservation investments. Difficulties in documenting dispersal seriously compromised estimates of juvenile survival and thereby biased estimates of λpm from a projection matrix, a problem that was resolved in later workshops by estimating λRJS directly using a reparameterized Jolly–Seber mark–recapture method. Several sources of bias for estimates of vital rates and λ were reviewed. Bias exists in meta‐analysis estimates of λ combined over spatial replicates because λ is a non‐linear function of vital rates. Bias also exists in estimates of average population growth where λt varies over time. This problem can be reduced by calculating the geometric mean of λ. Research to measure biases associated with the estimation of vital rates and the selection of study areas will be necessary to validate meta‐analyses of demography for spotted owls. Synthesis and applications. Meta‐analysis is ideally suited to studies of the demography of long‐lived species because of the large areas involved, high costs for each individual study, and multiple jurisdictions within which the organisms occur. Mixed models selected using information–theoretic approaches provide a powerful way to combine research results from several studies in a meta‐analysis.

Multiple cancer sites incidence rates estimation using a multivariate Bayesian model.

In Brazil cancer incidence rates have to be estimated from occasional surveys, due to lack of continuous cancer registries. Many estimated rates have very large variances, because only few years of data were collected. When dealing with a single cancer site, it is possible to adopt a Bayesian method which borrows information about the cancer rates from other geographical areas to estimate the cancer rate in a given area. We suggest an additional improvement to this method which explores the correlation between multiple cancer sites rates in a same area and in different areas. Our method works with a multivariate vector of different cancer sites rates in several areas and it borrows information from both, across geographical areas and across different cancer sites. We applied our method to data from a survey carried out in 18 Brazilian cities in São Paulo State in 1991. We estimated age and sex indirect standardized incidence rates for the six most common cancers in men and women, and calculated the 95% interval estimation for the incidence rates. The usual indirect standardized incidence rates had very large confidence intervals for many cancers and cities due to small expected number of cases. The use of the multivariate Bayesian method led to more precise estimates. More precise age-standardized cancer incidence rates can be calculated using data from other cancers. The method is conceptually simple, easy to perform, has low cost, and can improve substantially the estimation of cancer incidence and other vital rates.

ASSOCIATIONS BETWEEN PATCH AREA AND VITAL RATES: CONSEQUENCES FOR LOCAL AND REGIONAL POPULATIONS

The negative association between patch area and extinction risk is one of the most ubiquitous observations in ecology. This association is often attributed to the disproportionate effects of stochastic fluctuations on small populations, but it also may be explained by deterministic relationships between patch area and vital rates (i.e., fecundity and survival). We explored the consequences of including deterministic relationships between patch area and vital rates in metapopulation models. Field studies have revealed that fecundity and monthly survivorship were positively associated with patch size for the Florida scrub lizard (Sceloporus woodi). Using simulation techniques, we compared metapopulation models of artificial landscapes that included vital rates for this species (scaled for patch area) to models that used averaged estimates of vital rates for all patches. These two types of metapopulation models produced dramatically different results with respect to the proportion of occupied patches and the extinction probability of the regional population. Including deterministic relationships between patch area and vital rates often resulted in either all patches being occupied or all patches going extinct, depending on the size of the local patches. Few artificial landscapes resulted in a dynamic colonization–extinction equilibrium characteristic of a “classic metapopulation.” In contrast, model simulations with demographic parameters equal for all patch sizes produced a more linear decline in extinction probability with patch size. Our results highlight considerations for application of metapopulation theory in conservation.

Potential causes of population declines in forest fragments in an Amazonian frog

Forest fragmentation results in population declines and extinctions for many forest vertebrates, but little is known about the mechanisms causing declines in fragments. We investigated potential causes of declines in forest fragments for an Amazonian forest frog (Colostethus stepheni) at an experimental fragmentation study site in central Amazonian Brazil using field estimates of abundance and vital rates coupled with population simulations. Although adult male survival was not reduced by fragmentation, mean clutch size was reduced by 17%. Population simulations demonstrate that a reduction in clutch size of this magnitude is sufficient to cause the observed magnitude of population declines in fragments. Female snout-vent length was also reduced in fragments and may be related to the observed reduction in clutch size.

Modeling Spatial Variation in Waterfowl Band-Recovery Data

Historically, the statistical complexity of modeling spatial relationships in band-recovery data has limited the use of spatial models in the management of waterfowl populations. Consequently, managers have assumed simplified spatial relationships (e.g., by stratification and pooling data over large geographic areas) to obtain spatially explicit estimates of vital rates. As an alternative, we used a binomial random effects approach to modeling spatial variation in band-recovery data. The model accommodates spatial correlation and heterogeneity in recovery rates and facilitates spatially explicit estimation of recovery rates with sparse data and at arbitrary levels of spatial resolution. Although the model is structurally simple, estimation using conventional likelihood techniques is complex. Instead, we rely on a technique known as Markov chain Monte Carlo (MCMC) simulation. We used this model to construct a map of mallard (Anas platyrhynchos) recovery rates on a relatively fine-grained grid and for estimation of recovery rates within predefined geographic strata. The results show a strong gradient in recovery rate, with lower values in the western United States and higher values in the eastern United States. The spatial correlation in the model allows useful stratum-level estimates to be produced for strata with small sample sizes.

Relative Cohort Size: Source of a Unifying Theory of Global Fertility Transition?

Using United Nations estimates of age structure and vital rates for 184 countries at five‐year intervals from 1950 through 1995, this article demonstrates how changes in relative cohort size appear to have affected patterns of fertility across countries since 1950—not just in developed countries, but perhaps even more importantly in developing countries as they pass through the demographic transition. The increase in relative cohort size (defined as the proportion of males aged 15–24 relative to males aged 25–59), which occurs as a result of declining mortality rates among infants, children, and young adults during the demographic transition, appears to act as the mechanism that determines when the fertility portion of the transition begins. As hypothesized by Richard Easterlin, the increasing proportion of young adults generates a downward pressure on young men's relative wages (or on the size of landhold‐ings passed on from parent to child), which in turn causes young adults to accept a tradeoff between family size and material wellbeing, setting in motion a “cascade” or “snowball” effect in which total fertility rates tumble as social norms regarding acceptable family sizes begin to change.

Relative Cohort Size: Source of a Unifying Theory of Global Fertility Transition

Using United Nations estimates of age structure and vital rates for nearly 200 nations at five year intervals from 1950 through 1995, this paper demonstrates how changes in relative cohort size appear to have affected patterns of fertility across nations since 1950-not just in developed countries, but perhaps even more importantly in countries as they pass through the demographic transition. The increase in relative cohort size (defined as the proportion of the population aged 15 to 24 relative to that aged 25 to 59) which occurs as a result of declining mortality rates among children and young adults during the demographic transition, appears to act as the mechanism of transmission which determines when the fertility portion of the transition begins. As hypothesized by Richard Easterlin, the increasing proportion of young adults would generate a downward pressure on young men's relative wages, which in turn causes young adults to accept a trade-off between family size and material well-being, setting in motion a cascade or snowball effect in which total fertility rates tumble as social norms regarding acceptable family sizes begin to change. Thus relative cohort size can be thought of as the mechanism which prevents excessive rates of population change- reducing fertility when previous high rates, in combination with low mortality rates, have caused relative cohort size to increase, and increasing fertility when previous low rates have caused relative cohort size to decline.

Relationship of Larval-Stage Growth and Mortality to Recruitment of Striped Bass, Morone saxatilis, in Chesapeake Bay

Variable recruitments of striped bass were hypothesized to be caused by factors influencing growth and survival of larvae. Eggs and larvae were collected in the Potomac River from 1987 to 1989 and in the Upper Chesapeake Bay in 1988 and 1989 to estimate abundances, larval growth and survival rates, and environmental variability. Larval batch dates, ages, and growth and mortality rates were estimated from analysis of otolith daily increments. A retrospective analysis of Potomae River ichthyoplankton data from 1974–1977 and 1980–1982 provided additional estimates of larval abundances and vital rates for comparative purposes. Significant correlations betweens vital rates (growth and mortality) and abundances of striped bass larvae, and the Maryland juvenile recruitment index indicated that recruitment level may be fixed during the larval stage. The ratio of mean daily growth and mortality rates (G:Z) of larvae in the Potomac River for 1987–1989 was highest in 1987 when the juvenile index was relatively high, and was lower in 1988 and 1989 when juvenile indices were low. In the Upper Bay, mean larval growth rate, survival rate, and the G:Z ratio were highest in 1989 when the juvenile index also was high. In both tributaries, abundances of late-stage larvae (8 mm SL) were correlated with juvenile-stage recruitment indices. The retrospective analysis provided additional evidence that Potomac River larval abundances and G:Z ratios were positively correlated with juvenile recruitment indices in the 1974–1977 and 1980–1982 periods. Conditions favoring striped bass larval abundance and potential recruitment differed between the Potomac River and the Upper Bay. In the Potomac, late-stage larval abundances coincided with late-season water temperatures that were relatively warm, low river discharges and high, late-season densities of zooplankton prey, which favored larval growth. In the Upper Bay, the high abundance of late-stage larvae in 1989 relative to 1988 was attributed to a higher egg production that was coincident with high zooplankton abundances.

Demographic data collection in less developed countries 1946-1996.

This evolution over the last 50 years of data collection systems in less developed countries is assessed. The progress made by civil registration systems has been extremely disappointing. Except in Central and South America, their role in providing vital rate estimates is still very limited. In contrast, the promulgation of regular population censuses has been a success, particularly in Africa. The relative merits and demerits of different types of demographic surveys are described. To some extent multi-round designs have given way to single-round surveys, such as the Demographic and Health Surveys (DHS). DHS-style enquiries are particularly suitable for evaluation of interventions but are less appropriate if the main aim is to measure vital rates.