We addressed the problems of recall and nonresponse biases in self-report angling participation surveys. While each source of bias has been recognized as problematic, previous research has not addressed the interaction effects of these biases. Recall bias was assessed by comparing immediate, 3-month, and 6-month recall periods. A diary format was used for the immediate recall period and mail surveys for the 3-month and 6-month recall periods. Nonresponse bias was assessed by conducting telephone interviews with subjects who did not respond to the mail or diary surveys. The dependent variable was total number of days fished over two separate 3-month periods. Analysis showed significant first-order interactions (P ^ 0.05), suggesting that recall bias and nonresponse bias are not independent factors. Respondents are more likely to report higher levels of participation and nonrespondents lower levels of participation as the length of recall period increases. Findings indicate that studies which use long recall periods, or do not control nonresponse bias, overestimate use. Future studies can control recall and nonresponse biases by combining frequent sampling with telephone interviews that request short recall periods.Effective management, planning, and develop-gent validity. Convergent validity addresses the ment of recreational fisheries resources are in-extent to which different methods give the same creasingly dependent upon accurate and reliable estimates of angling participation. When estimates data sources. An important aspect of this data col-of angling participation vary widely among estilection process involves the retrieval, analysis, and mation methods, it calls into question the validity evaluation of information concerning the behav-of the estimates and limits their utility to manior, preferences, catch, and effort of sport anglers, agers. This information can be useful in a variety of ways,The accuracy of estimates can be improved by including the identification of use trends, projec-examining the magnitude of biases that occur in tion of angler demand, evaluation of the impacts existing survey methods and refining methodolof management decisions, allocation of funding, ogies so they explicitly control for method effects, and examination of user conflict.This study examines the effects of method bias in As fisheries planners and managers have be-the Illinois Sport Fishing Survey (ISFS). The ISFS come more involved in the collection and use of is the primary mechanism used to assess angler angler information, they have become increasingly information in Illinois (Illinois Department of aware of measurement problems that can affect Conservation 1988). the validity and reliability of study results. OneThe ISFS uses a mail survey for collecting statearea of concern examined in our study is conver-wide angler information, as do many other agencies (e.g., USFWS 1982;Harris and Bergersen ----1985), because mail surveys are a cost-effective