Albacore (Thunnus alalunga) population structure in the Indo-South Atlantic Oceans and stock assessment of South Atlantic albacore
Date Issued
2006
Date
2006
Author(s)
Lee, Liang-Kang
DOI
en-US
Abstract
Albacore (Thunnus alalunga) is a very abundant and widely distributed temperate tuna species that inhabits mainly inside an oceanic gyre of the World’s Oceans. There are often two gyres in an Ocean, one in the northern hemisphere and the other in the southern hemisphere. Two stocks of albacore have been thus acknowledged in the Pacific and the Atlantic Oceans. In the Indian Ocean, mainly because of lacking a northern gyre, only one albacore stock is postulated.
Land barrier of African continent is not extending southward enough to totally separate two oceanic temperate water masses systems. Instead, shifting frontal boundary of the two oceanic temperate water masses, particularly the Agulhas current off South Africa, is quite often. One of the main purpose of this study is thus to elucidate albacore stock structures in the two oceans by using mitochondria DNA D-loop sequence characters. The results of D-loop sequence character analyses indicated: (1) 866 base pairs are observed in the albacore D-loop segment; (2) a total of 164 albacore specimens in-situ collected from 8 sampling areas in the Indo-Atlantic Oceans, 142 unique haplotypes are identified; (3) Within those 142 unique haplotypes, two specimens (one from the South Atlantic and the other from the Indian Ocean) having the identical haplotype which strongly implies that they possessed the same maternal ancestor; (4) Indicators as haplotype diversity (h), nucleotide diversity (π), pairwise gene flow (Nm), population differentiation (Φct) were adopted and all indicate that albacore from off northern Madagascar is significantly different from those from the Indo-Atlantic Oceans and the slight differences appeared alongside the Indian and the South Atlantic water mass frontal regions imply that there is a certain degree of gene flow existing between the boundary.
Growth equation is essential if a proper assessment of the stock is intended. Lee and Yeh (1993) provided a growth equation for Southern Atlantic albacore, derived from ring-reading results on vertebra and 1st dorsal spine cross sections, based on assumptions that a ring is formed per year and the first opaque band is the mark of the first year of age. It was not until recently when some small albacore (40-44cm) otoliths were obtained with its daily ring counts performed (Lu et al., 2006b), these results indicate that the assumption of first opaque ring mark implied the 1st year of age is not correct. For this reason, the data of Lee and Yeh (1993) were re-analyzed in this study for providing a better growth equation of albacore in the South Atlantic. The parameters of von Bertalaffy’s growth equation thus obtained are as follows: L∞ = 147.5 cm; k = 0.126 per year; and t0 = - 1.89 year.
Catch and effort statistics of Taiwanese longliners, which is the major fishing fleet able to provide extensive and consistent abundance information for this resource, was standardized by adopting generalized linear model (GLM) for elucidating the historic abundance trend of the South Atlantic albacore from 1968 to 2003. Factors of year, quarter, area and target effect of bigeye tuna were chosen, according to the results of covariates selection procedure, for the application of GLM. The standardized CPUE trend thus obtained shows that a significant decline in the beginning of the fishery (from 1968 to 1973) followed by a slightly increase and leveled off until 1977; after that a decline is observed to the lowest level in 1989; from 1989 to present 2003 the index fluctuate between levels from 5 to 8.
For assessing the current stock status of South Atlantic albacore resource, explanatory fittings on Japanese, South African, and Taiwanese CPUE trends were performed by using age structured production model (ASPM) with stochastic recruitment. The prospects of this model indicate that (1) Bmat/ B ratios of the stock from 1959 to 2003 are always greater than unity and the current B /B value of 1.53 is considered rather high and implies that the stock is still in a healthy condition; (2) current exploitable biomass is estimated to be 0.35 that of 1959, which is larger than the threshold (0.20) of biomass to be concerned of; (3) despite of yields in recent years were close to the MSY of this resource, fishing mortality F were always smaller than its corresponding values at MSY , in particular the F2003 is at the level of 80.4 % FMSY. The current status of albacore stock in the South Atlantic can be concluded as in a condition of not being over-exploited.
Subjects
長鰭鮪
系群
資源評估
albacore
population structure
stock assessment
SDGs
Type
thesis
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