Tag shedding by tropical tunas in the Indian Ocean and other factors affecting the shedding rate
Introduction
Mark-recapture techniques can facilitate the collection of useful information for stock assessments, such as stock structure, growth and mortality rates, gear selectivity, and migration patterns. Consequently, tagging studies have become one the key tools used by tuna Regional Fisheries Management Organisations (RFMOs) to improve understanding of how populations are spatially structured and the effects of fishing on these populations. Integral to the use of tagging data are standardization models, such as tag-attrition models for single release events (Kleiber et al., 1987, Hampton, 1997) or Brownie models (derived from bird-banding studies) for multiyear studies (Brownie et al., 1985, Hoenig et al., 1998, Polacheck et al., 2010). The results of tagging studies can, however, be compromised if tags or data are lost (i.e., through tag shedding and non-reporting). Both occurrences can lead to underestimations in tag-return rates, which create a negative bias in fishing mortality estimates, rates of fishery interactions, and tuna movements. Ultimately, this leads to biased estimates of stock status. Thus, the objective of this paper is to update preliminary estimates of tag-shedding rates by tropical tuna in the Indian Ocean (Gaertner and Hallier, 2008, Gaertner and Hallier, 2009).
There are two types of tag losses (Wetherall, 1982, Hampton and Kirkwood, 1990): Type-I losses, which reduce the number of tags initially put out (immediate tag shedding, immediate tagging mortality, and non-reporting), and Type-II losses which occur steadily over time (natural mortality, fishing mortality, permanent emigration, and long-term tag shedding). The current paper is only estimating the Type I and II tag shedding components of total losses. Immediate tag shedding and immediate tagging-induced mortality rates can be estimated by observing tagged fish under controlled laboratory conditions or in field cages (Pollock and Pine, 2007). However, post-release mortality estimates derived under these circumstances may be biased: in general, unlike wild fish, captive fish are not affected by post-release predation. On the other hand, the act of restraining fish in confined conditions can have lethal or sublethal effects. An alternative approach to estimating mortality that is commonly used is double-tagging experiments in which a fish is tagged with two tags simultaneously. Double tagging can also be used to estimate tag-shedding rates by identifying fish that have lost a tag.
In general, shedding rates cannot be estimated from tag-return data directly. Consequently, different methods have been proposed to estimate shedding rates from double-tagging experiments. To maximize the accuracy of these estimates, it is crucial to have a firm understanding of the functioning of the range of other variables known to impact tag shedding. The Regional Tuna Tagging Project—Indian Ocean (RTTP—IO), which focuses on tropical tuna in the Indian Ocean, has already examined some of these variables. For example, there is no evidence, or it remains unclear, whether factors such as tag length (11 cm and 14 cm length tags) or tag position (right side versus left side of the fish) influence the rate of tag returns in RTTP-IO double-tagging experiments (Gaertner and Hallier, 2008). However, multiple taggers have been used over the duration of the program, and tag-return rates are known to vary substantially between taggers. Therefore, in this context, it is desirable to estimate how shedding rates vary among taggers.
Consequently, this study focused on (1) an analysis of the tagger effect and other explanatory variables that were hypothesized a priori to influence tag loss, (2) comparing the constant-rate and time-varying approaches to modeling tag-shedding rates, (3) an analysis of how the insertion position of the tag affects shedding rates, and (4) an investigation into the consequences of ignoring tag shedding and non-reporting on the estimates of exploitation rate and total instantaneous mortality.
Section snippets
Data
Over the duration of the RTTP-IO, a number of different tag types (e.g., conventional, archival) and tag colors have been used. Tag colors are used to indicate the presence of other tag types. Conventional tags are traditionally yellow, but a white version is used if oxytetracycline is also injected into the fish, and a red version is used when an additional archival tag is inserted. In the RTTP-IO, the single- and double-tagging experiments have been alternately performed and always used the
Results
Although the beta-binomial model already naturally incorporates a certain pattern in the variances of the response, it can be necessary to also explore a variable dispersion model by incorporating an additional set of regressors in the precision sub-model. It then becomes possible to check whether the precision parameter ø is constant for all observations. With this in mind, the set of candidate models in this study were ranked on the basis of their BIC values, with the lowest value indicating
Discussion
A number of factors are known to affect tag-shedding rates. For example, it has been established that shedding rates may be impacted by the type of external tag used, with several studies having shown differences in the retention rates of dart and T-bar tags (Boucek and Adams, 2011). For tropical tunas, a comparison has been made between the conventional yellow ‘spaghetti’ tag and another tag commonly used by sport fishermen to opportunistically tag tunas and billfishes, the Betyp tag. Results
Author contributions
J-P. H. was the general coordinator of the RTTP-IO and checked the data; D.G. analyzed the data and led the writing, with J-P. H. contributing.
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