Separation of the Taiwanese regular and deep tuna longliners in the Indian Ocean using bigeye tuna catch ratios

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)(1996)(1997)(1998)(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)(1996)(1997)(1998)(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 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.KEY WORDS: bigeye tuna catch ratios, Indian Ocean, regular and deep tuna longline, separators.

Association between the interannual variation in the oceanic environment and catch rates of bigeye tuna (Thunnus obesus) in the Atlantic Ocean

“…Monthly nominal catch rates were calculated as the number of fish captured per 1,000 hooks (fish/10 3 hooks). The number of hooks between floats was classified as regular (6–12 hooks) or deep (13–20 hooks; Lan et al., ; Lee et al., ). However, some logbook data did not contain this information, including all data from 1990 to 1994 and nearly 50% of the data from 1995 to 2002.…”

“…Monthly nominal catch rates were calculated as the number of fish captured per 1,000 hooks (fish/10 3 hooks). The number of hooks between floats was classified as regular (6-12 hooks) or deep (13-20 hooks; Lan et al, 2012;Lee et al, 2005).…”

Changes in Fishing Pattern From Surface to Deep Longline Fishing by the Indonesian Vessels Operating in the Indian Ocean

“…Longline gear was arbitrarily classified based on the number of branch lines or hooks between floats (Suzuki et al, 1977;Marcille et al, 1984;Lee & Nishida, 2002;Lee et al, 2005), as the latter is considered an index of the maximum fishing depth (Bach et al, 2000;Ward & Hindmarsh, 2007). Note that the number of branch lines and hooks between floats are used interchangeably.…”

Standardization of a geo-referenced fishing data set for the Indian Ocean bigeye tuna, <i>Thunnus obesus</i> (1952–2014)

“…The introduction of monofilament for the mainline, allowing deeper longline sets (Okamoto and Shono, 2006;Okamoto et al, 2001); the installation of super-cold freezers for fish storage (Haward and Bergin, 2001;Matsumoto et al, 2013;Okamoto and Shono, 2006;Ward and Hindmarsh, 2007); and increasing market demand for sashimi (Miyake et al, 2004;Sakagawa et al, 1987) have led to stronger targeting of bigeye tuna. These changes particularly affected the Japanese longline fleet, which has the longest periods of exploitation and the largest market demand (Haward and Bergin, 2000;Lee et al, 2005;Yeh and Chang, 2013). Consequently, the catchability of the fishing gears and thus the CPUE were modified over time (Fonteneau et al, 2000;Maunder et al, 2006).…”