Kristen E. Ryding

Abundance and Consumption of Fish by California Gulls and Ring-billed Gulls at Water and Fish Management Structures within the Yakima River, Washington

Abstract During 1999-2002, we studied the abundance of fish-eating birds, primarily Ring-billed Gulls (Larus delawarensis) and California Gulls (L. californicus), and estimated their consumption of fish at Horn Rapids Dam and the Chandler Irrigation Canal return pipe on the Yakima River in eastern Washington. Earlier observations of gulls at these structures suggested a high level of predation of juvenile salmonids. The relationship between river flow, gull use at the sites and fish taken was also examined. Numbers of gulls (instantaneous counts of foraging and non-foraging individuals) at the structures varied daily between their arrival in late March-early April and departure in late June. Daily averages across the four years were 9.8 (SE ± 1.5) and 19.1 (SE ± 2.5) gulls at Horn Rapids and Chandler, respectively. Gull numbers at Horn Rapids peaked dramatically during the last two weeks in May, reaching maxima of 37 (SE ± 2.2) to 133 (SE ± 4.2) gulls/day. This increase appeared to be associated with the hatchery release of one to two million juvenile autumn Chinook (Oncorhynchus tshawytscha) above the dam. A comparable peak in gull abundance was not observed at Chandler. Diurnal patterns of gull abundance differed between sites and among years. Relationships between fish take and water flow also varied within and among years at the two sites. Low seasonal flows were associated with increased predation at Chandler, whereas high seasonal flows were associated with increased predation at Horn Rapids. Assuming all fish taken were salmonids, consumption at both sites combined was estimated to be ≤10.3% of the juvenile salmonids passing the two sites.

Using Time Series to Estimate Rates of Population Change from Abundance Data

Abstract Assessing the dynamics of wild populations often involves an estimate of the finite rate of population increase (λ) or the instantaneous rate of increase (r). However, a pervasive problem in trend estimation is that many analytical techniques assume independent errors among the observations. To be valid, variance estimates around λ (or r) must account for serial correlation that exists in abundance data. Time series analysis provides a method for estimating population trends and associated variances when serial correlation of errors occurs. We offer an approach and present an example for estimating λ and its associated variance when observations are correlated over time. We present a simplified time series method and variance estimator to account for autocorrelation based on a moving average process. We illustrate the procedure using a spectacled eider (Somateria fischeri) data set of estimated annual abundances from aerial transect surveys conducted from 1957 to 1995. The analytic variance estimator provides a way to plan future studies to reduce uncertainty and bias in estimates of population growth rates. Demographic studies with policy implications or those involving species of conservation concern should especially consider the correlated nature of population trend data.

The Impact of Hunter Postseason Questionnaire Design on Big Game Harvest Estimation

Abstract Estimating game harvest is among the most important activities of wildlife management agencies. For many state agencies, postseason hunter surveys are the basis for obtaining estimates of harvest. However, the questionnaire design and the questions asked can have a great effect on the accuracy of the harvest estimates. The purpose of this paper is to present alternative models for analyzing postseason hunter questionnaire data. Our model extends the White (1993) approach by incorporating local information from both successful and unsuccessful hunters in cases where that information is available from postseason surveys. Our approach relaxes 2 assumptions: 1) the probability that a hunter reports the area hunted is the same across areas and 2) all hunters have the same probability of success regardless of area hunted. Unless assumptions regarding common harvest rates across areas can be validated a priori, questionnaires should be designed to provide that geographic information. This information should be obtained from both successful and unsuccessful hunters. Previously, managers were primarily concerned whether nonrespondents had the same harvest success as survey respondents. We argue that this concern should be extended to include consideration of differences in reporting rates by geographic area as well. Agency goals, data requirements, and possible differences in harvest rates by region must, therefore, guide the design of hunter harvest questionnaires.

5 – Estimating Survival

This chapter presents a wide range of methods and data structures for estimating survival probabilities emphasizing on demographic methods that use age- and sex-structure data. It uses age-specific radiotelemetry data to illustrate the basic nature of right- and left-censored data and estimation of survivorship curves. Life-table analysis is used to reconstruct survivorship curves from age-structure information. The purpose of survival curve analysis is to characterize the amplitude and shape of the survival function SX ( x ) over the lifetime of a cohort of animals. Both parametric and nonparametric survival curve analyses are possible. Either approach requires information on age of the animals at time of entry to the study and their subsequent age at death. The advantage of parametric analyses over the Kaplan-Meier product-limit and Nelson-Aalen estimators is that the former uses more of the survivorship information collected. Regardless of what age an animal enters the study, information is provided on the same parameters of the parametric distribution as all other animals. Survival analyses generally fall into one of two categories, either cohort or time-specific methods. The vast majority of the age-structure-based methods are time specific in nature. The initial demographic analyses using time-specific methods will often be replaced by cohort-specific approaches as time permits. Proper age classification is key to both cohort and age-specific survival analyses. Three alternative life-table approaches to age-specific survival estimation are presented: horizontal, vertical, and depositional.

The Design and Analysis of Salmonid Tagging Studies in the Columbia Basin; Volume XII; A Multinomial Model for Estimating Ocean Survival from Salmonid Coded Wire-Tag Data.

The purpose of this report is to illustrate the development of a stochastic model using coded wire-tag (CWT) release and age-at-return data, in order to regress first year ocean survival probabilities against coastal ocean conditions and climate covariates.