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|Title: ||Evaluating data and model structure uncertainty for the stock assessment of swordfish ( Xiphias gladius ) in the Indian Ocean.|
|Authors: ||Sheng-Ping Wang;Mark N. Maunder;Tom Nishida;Ying-Ru Chen|
|Issue Date: ||2017-03-09T01:56:52Z
|Publisher: ||The Twelfth Session of the IOTC Working Party on Billfish, Indian Ocean Tuna Commission (IOTC)|
|Abstract: ||Abstract:Swordfish in the Indian Ocean (Xiphias gladius) have historically been exploited
by Japan and Taiwan. Since the early 1990s, the catch of swordfish in the Indian
Ocean increased substantially owing to the seasonal targeting of the Taiwanese
fishery, the targeting of EU longline fisheries, and exploitation of semi-industrial
longline and artisanal fisheries. Although the recent stock assessments suggested that
the MSY-based reference points were not exceeded for the Indian Ocean population,
these assessment results may be misleading because they lacked the consideration of
uncertainty about changes in fishing operations and model structure assumptions. In
this study, we conducted a stock assessment using an integrated age-structured model
and evaluated estimates of management quantities under alternative assumptions for
changes in catchability for CPUE-based indices of abundance and for gear selectivity.
The results of this study indicated that assuming time-blocks for catchabilities may be
appropriate to reflect the changes in fishing operations of Japanese and Taiwanese
longline fleets. This assumption also provided better model performance and more
optimistic assessment results because it implied that the decline in indices of
abundance may have resulted from changes in catchabilities rather than depletion of
biomass. However, assuming time-blocks for selectivities, misspecifying shapes of
selectivity curves and changing weights for CPUE data may not be appropriate for
this stock assessment because model performance was deteriorated under these
assumptions. More generally, substantial changes in catchability (e.g. due to changes
in targeting) may not be fully addressed in CPUE standardization and may require
modelling catchability within the stock assessment model. Time varying should be
applied judiciously because unmodelled changes in catchability may cause distortion of the selectivity curve to compensate for the changes in selectivity.
|Appears in Collections:||[環境生物與漁業科學學系] 演講及研討會|
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