Estimation of Tag-Reporting Rates for Sablefish in the Northeastern Pacific Ocean

January 11, 2001

An essential component of any mark and recapture study that seeks to estimate fish population abun-dance, exploitation rates, or migration rates from tagging data is the tag-reporting rate. We obtained tag-reporting rates for the sablefish Anoplopoma fimbria fishery during 1980–1998 by comparing tag returns in the fishery to tag returns from a scientific survey where all tag recoveries were assumed to be reported. Analytical formulae were derived for the measurement error associated with the estimates. When pooled over geographic areas or years, estimates of reporting rates were reasonably precise with coefficients of variation (CVs) usually less than 25%. Reporting rates were highest in the central (0.385) and eastern (0.315) Gulf of Alaska, intermediate in the western Gulf of Alaska (0.269), and lowest in the Aleutians (0.174) and Bering Sea (0.169). Rates pooled over all areas increased from lows of 0.102–0.248 in 1980–1982 to a peak of 0.465 in 1985 before declining to 0.199 in 1986 and 0.157 in 1987. The reporting rate increased gradually and fluctuated between 0.376 and 0.450 since 1995. The increase in reporting in 1995 was coincidental with the implementation of the Individual Fishing Quota (IFQ) system. The linear increase in reporting rates during 1986–1998 was significant. Factors that may have influenced the reporting rate were the number of tags available for recovery, the length of the commercial fishing season, the presence of scientific observers on commercial vessels, and the tag reward program. Pooled over all years and areas the tag-reporting rate has been 0.276 with a CV of 4.2%.

The proportion of recovered tags that are returned, the tag-reporting rate, is an important component of any mark and recapture study used to estimate fish abun-dance, exploitation rates, or migration rates from tag-ging data (Seber 1982; Hearn et al. 1999). Methods to estimate reporting rates include tag-seeding experiments (Campbell et al. 1992; Hampton 1996), sequential ob-servations of tags at different stages of catch handling (Hilborn 1988), and comparison of tag returns to those in a control sample with a reporting rate near or equal to unity (Paulik 1961).

Sablefish Anoplopoma fimbria is a long-lived and migratory species (Bracken 1982; Sasaki 1985; Fujioka et al. 1988; Maloney and Heifetz 1997). Results from tagging studies of sablefish have been used to esti-mate movement rates, evaluate harvest strategies, and determine ageing accuracy (Heifetz and Fujioka 1991; Heifetz et al. 1997; Heifetz et al. 1999). The age-structured model used to estimate absolute abundance and recommend catch quotas for sablefish in Alaska relies primarily on relative abundance indices and age composition from annual longline surveys (Sigler 1999, 2000). There is a wealth of tagging data available for Alaskan sablefish that can provide auxiliary information for age-structured analyses. Within the framework of an age-structured model, such data can be included by tracking cohorts of tagged fish (e.g., Haist 1998). Although there has been a desire to integrate tagging data into the age-structured model, the tagging data have not been used, in part because the magnitude and accuracy of the tag-reporting rate has not been formally evaluated. Estimates of tag-reporting rates independent of the age-structured model are desired because this parameter is confounded with tag loss and fishing and total mortality estimates (Heifetz and Fujioka 1991; Haist 1998).

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