Catch Rate Estimation for Roving and Access Point Surveys

Abstract

Optimal designs of recreational angler surveys may require complemented surveys, in which different contact methods are used to estimate effort and catch. All common methods of estimating catch involve on-site interviews that may either be based on access (complete trip) or roving (incomplete trip) interviews. In roving surveys, anglers to be interviewed on a given day arc intercepted with a probability proportional to the length of their completed fishing trip, whereas in access surveys, anglers arc intercepted as they leave the fishery and are intercepted with the same probability regardless of the length of their completed fishing trip. There are four complemented survey designs which use interviews at access points (mail-access, telephone-access, aerial-access, and roving-access); there are four corresponding designs which use roving interviews (mail-roving, telephone-roving, aerial-roving, and roving-roving). For all of these surveys, total catch is estimated as the product of total effort and catch rate. We show that, when access interviews are used, the appropriate catch rate estimator is the ratio of means estimator (i.e., mean catch from interviews divided by mean effort); whereas when roving interviews are used, the appropriate estimator is the mean of the individual ratios of catch divided by effort for each angler. In the roving method, it is necessary to ignore short trips of less than about 0.5 h duration when calculating the mean of the ratios. This stabili/es the variance of the estimates and does not appear to cause any appreciable bias. A bias occurs in the estimates of catch rate and total catch from roving interviews when anglers are subject to a bag limit. This bias can be substantial if the bag limit is effective in limiting the catch. Recently, Jones et al. (1995) discussed a special

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