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Estimating tag-shedding rates for skipjack tuna, Katsuwonus pelamis, off the Maldives

2001; National Oceanic and Atmospheric Administration; Issue: 1 Linguagem: Inglês

1937-4518

M. Shiham Adam, G.P. Kirkwood,

Marine and fisheries research

One source of uncertainty in fi shery assessments based on tag release and recapture data arises from tag shedding—the loss of tags from fi sh from the time of tagging until tag recovery. Independent estimation of tag-shedding rates from double tagging experiments is an integral part of well-designed tagging experiments. Failure to allow for tag shedding can result in biases in estimates of important parameters derived from tag-recapture data, such as fi shery-induced and natural mortality, and migration rates. A wide variety of methods have been proposed for estimating shedding rates using data from double tagging experiments, the variety in part resulting from the nature of the data available from the experiments. The earlier literature has been well integrated and reviewed by Wetherall (1982). More recently, Xiao (1996) has developed a general model that unifi es the estimation of tag-shedding rates from double tagging experiments with exact and pooled times at liberty. Some recent examples of application of these methods include Xiao et al. (1999) for school gummy shark and Fabrizio et al. (1999) for lake trout. Tag shedding is of two types (Wetherall, 1982). Type-I shedding is a one-time event and occurs immediately after tagging, usually as a result of suboptimal placement of tags in the fi sh. Effectively, it reduces the number of tags initially put out to sea. Type-II shedding is the loss of a tag or both tags over a period of time after the fi sh has been tagged and released back into the sea. For long-lived species, it may not occur at a constant rate because some tags are likely to have been applied more effectively than others, and some may become fi rmly embedded (with growth of muscle tissues), such that they are very unlikely to be detached from the fi sh (Kirkwood, 1981). The Ministry of Fisheries and Agriculture, Maldives, carried out two tagging experiments: one in 1990–91 (Yesaki and Waheed, 1992) and the other between 1993 and 1995 (Anderson et al., 1996). The latter tagging experiment included a double tagging experiment in which 504 skipjack tuna, Katsuwonus pelamis, were double tagged, by using the same type of tags and techniques of tagging as used in the single tagging experiment (Anderson et al., 1996). As of end 1996, a total of 53 of these fi sh had been recovered. These data are considered to be of immediate importance for designing largeEstimating tag-shedding rates for skipjack tuna, Katsuwonus pelamis, off the Maldives