Separation of the Taiwanese regular and deep tuna longliners in the Indian Ocean using bigeye tuna catch ratios
Resource
Fisheries Science 71 (6): 1256-1263
Journal
Fisheries Science
Journal Volume
71
Journal Issue
6
Pages
1256-1263
Date Issued
2005
Date
2005
Author(s)
Abstract
Taiwanese longline (LL) fisheries operating in the Indian Ocean usually target albacore tuna (ALB), swordfish (SWO) and yellowfin tuna (YFT) using regular LL. Bigeye tuna (BET), however, is targeted using deep LL. Thus, these two types of LL are considered to be different gears as they target different tuna species. Regular or deep LL fishing is defined by number of hooks per basket (NHB): regular LL if 6 ≤ NHB ≤ 10 and deep LL if 11 ≤ NHB ≤ 20. However, NHB information was available in only some of the recent LL data (1995-1999). This situation had caused problems of biased results in stock analysis in the past. Thus, the objective of our study was to explore an effective method to separate the two types of LL fishing by considering species composition. Some intervals of BET catch ratios were found to be effective in separating the regular and deep LL catches, i.e. 0.0 ≤ BET/(BET + ALB + SWO) ≤ 0.4 and 0.8 ≤ BET/(BET + ALB) ≤ 1.0, respectively. Using these two separators, the LL known data set (1995-1999) (learning data set) was classified. Correct classification occurred in 67.7% of the data, while 23.1% of the data were unclassified (11.9% due to zero catches and 11.2% due to classification into both LL types), and 9.2% were misclassifications. Then, using the methods developed, the LL unknown data set in the historical data (1979-1999) was classified and nominal CPUE values were calculated for four species. The CPUE trends based on this study were likely to be more reliable than those of previous studies.
Subjects
Bigeye tuna catch ratios; Indian Ocean; Regular and deep tuna longline; Separators
SDGs
Other Subjects
Scombridae gen. sp.; Thunnus alalunga; Thunnus albacares; Thunnus obesus; Xiphias gladius
Type
journal article
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