University of Tasmania

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A daily tag return model for lobster fisheries

posted on 2023-05-26, 00:26 authored by Burch, P
A daily tag return model was developed to estimate shing and natural mortality, tag reporting rate and catchability with application to lobster sheries. Tag return data is usually collected with knowledge of the exact date of recapture. By modelling tags individually, each tag contributes information on shing and natural mortality, catchability and tag reporting rate to the likelihood. Providing sucient tags are maintained in the shery to enable recaptures to occur during the shing season, ner resolution of parameters is possible. Model performance was tested by simulation of dierent times of release and recapture as well as a range of dierent seasonal shing patterns typically found in lobster sheries. Precision and accuracy of estimates were improved when there was a contrast in shing eort throughout the season or a seasonal closure within the year. The timing of tag release was not found to aect model performance. Evaluation of the model was undertaken by comparison of estimates from a previous study using an identical dataset. Total mortality estimates were equivalent between models although separation of shing and natural mortality diered between models. Small improvements in the precision of estimates were obtained for the model that incorporated exact times of tag release and recapture. The daily model enabled ner time scale estimates of parameters and this was explored in the estimate of within season catchability. Penalised spline smoothing was applied to estimate catchability, resulting in a curve that captured the timing of biological events such as moulting and mating. Higher shing mortality estimates were obtained when the shing eet was separated into groups based on their reliability at returning tags. This suggests that shing mortality estimates could be compromised when reporting rate is considered cosmopolitan across the entire eet. However, the small number of reliable shers and their shing patterns, make interpretation of results problematic. A cost-benet analysis of the use of PIT tags compared to T-bar tags, the standard tag used in lobster sheries, was undertaken. For a given cost the higher expenses associated with PIT tags and scanners resulted in fewer lobster being tagged compared to T-bar tags. The improved tag reporting rate from PIT tags resulted in improved precision and accuracy of mortality estimates using this technology unless the tag reporting rate for T-bar tags was substantially increased.


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