An Assessment of Atlantic Bluefin Tuna (1994) / Chapter Skim
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4 Fish Stock Assessment
4 Fish Stock Assessment
Pages 79-108
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From page 79... ...
Environmental and/or genetic factors may affect the shape parameter of the growth models for Atlantic bluefin tuna (in particular, in the Mediterranean Sea versus the Gulf of Mexico)
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From page 80... ...
Nonlinear relationships between CPUE and abundance were not considered in this report owing to data limitations, but it is due Hat the presence of nonlinear relationships would induce additional uncertainty about population status and should be considered further. One popular technique for standardizing catch rates is the use of general linear models.
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From page 81... ...
These two goals may not be compatible. To resolve some of these problems, other methods have been investigated to standardize Atlantic bluefin tuna CPUEs (Porch and Scott, 1993)
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From page 82... ...
For the constant-added log transformation, see Porch and Scott (1993~. Another complication that arises in standardizing CPUEs with general linear models is weighting.
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From page 83... ...
, and captains' logbook data for giant fish. The committee reanalyzed these data as explained below.2 Rod and Reel Indices for Small Fish Catch rates of small bluefin tuna were standardized by the general linear model method of Brown and Browder (1993)
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From page 84... ...
Rod and Reel Indices for Giant Bluefin Tuna These data were analyzed in a similar fashion as the data for small fish. Bow the back-~ansformed, corrected, least-squares means from He general linear model of In(CPUE+O.OS)
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From page 85... ...
FISH STOCK ASS~ Cal Cal Cal Cal o ~ · · - d 3 1 ~ o o Cal ~ 4 - ~ _ ~ ad ~ 3 o o ~ .~ ~ .,= · Ct o o .
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From page 86... ...
1992; however, after data processing errors were corrected by Inter-American Tropical Tuna Comunission scientists under the comunittee's direction, no such decline could be detected. Captains' Logbook Data for Giant Bluefin Tuna Captains' logbook data were summed for each permit number in each year over all days of fishing to reduce the number of zero values.
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From page 87... ...
FISH STOCK ASSESSMENT .s Ct o o no , _ .
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From page 88... ...
88 ANASSESSMENT OF ATLANTIC BLUEFIN TUNA Cal ~ .O ~ ' o o ~ In d O ~ ' &, 3 c, no ~ .s ·= o ·s o^ ~ o a, 11 Cal 4 - ~ Cal ~ o ·C~ Cal ~ Cal m ~ ~ Cal o a' ~ O O ~ ~ ~ · O ~ ~ ~ 0 V)
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From page 89... ...
. _ o 83 8S 87 ~91 93 Ye" FIGURE 4-2 Western Atlantic Ocean giant bluefin tuna indices.
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From page 91... ...
The four significant Mends had negative slopes; no significant positive slopes were found (Table 4-6~. Second, the sets of CPUE and abundance estimates in Tables 4-1 to 4-5 (rod TABLE 4-6 Trend analysis of abundance indices for western Atlantic bluefin tuna.
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From page 92... ...
Errors introduced during data processing can be serious. For example, an error in preprocessing the data for Atlantic bluefin tuna resulted in indices that showed a dramatic decline in all sizes of bluefin tuna in the 1992 ICCAT SCRS assessments.
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From page 93... ...
Methods Because most of the estimation methods gave similar results, Be choice of methods may not be as critical as quality control. However, the committee recommends the effort-weighted, nonlinear, least-squares method (our
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From page 94... ...
The methodology used in Be SCRS assessments of Atlantic bluefin tuna, ADAPT,4 falls into this general category. It is based essentially on a backcalculation procedure that allows reconstruction of the abundance of each cohort in Be population on the basis of the catches and estimates of natural mortality.
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From page 95... ...
The committee followed the method of equal weighting of normalized indices as was employed in the 1993 SCRS assessments of Atlantic bluefin tuna. The normalization procedure consists simply of construction of a so-called normalized index by dividing each value of Be original index by its overall average.
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From page 96... ...
rod and reel index for small fish, because it is the only one Rat includes ages 1 and 2. At the other extreme, several indices offer conflicting views of the trend in older age groups: while the Japanese longline index for the northwestern Atlantic Ocean suggests that the population has been stable over the past 18 years, other indices suggest declines.
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From page 97... ...
For example, the most obvious alternative model not considered by the SCRS assessments is the one that assumes that exchange occurs between the eastern and western components of the fisheries. The Two-Area Mixed-Stock Case Assessments by the SCRS working group have focused on one of the possible stock hypotheses for the Atlantic bluefin tuna, namely, that Were are western and eastern stocks and that no exchange occurs between them.
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From page 98... ...
of the abundance in the eastern Atlantic Ocean represents an important proportion of the western component. In years when either the recruitment in He east is high (e.g., 1983)
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From page 99... ...
SSB Case East West 93/88 93/75 93188 93/75 1 2%/yr 1%/yr 92.30% 18.10% 99.95% 24.54% 2 3%1yr 1%/yr 126.70% 36.10% 126.29% 45.70% 3 2%/yr 1 %/yr 100.30% 21.30% 102.76% 27.35% 4 2%/yr 1 %/yr 88. 10% 20.60% 98.31% 25.28% 5 2%/yr 1%/yr 92.40% 18.20% 100.32% 24.65% 6 2%1yr 1%/yr 98.90% 20.70% 102.89% 26.98% 7 0%/yr 0%/yr 75.80% 13.80% 87.91% 19.09% 8 2%/yr 1 %/yr 129.80% 43.00% 125.87% 50.85% Note: Case 1: Base case, Case 2: Increase eastern emigration rate, Case 3: Omit GOM index, Case 4: Vary natural mortality at age, Case 5: Delete Canadian CPUE index, Case 6: Increase catch of age 1 in Eastern Atlantic Ocean, Case 7: No migration (SCRS base case with data processing errors corrected)
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From page 100... ...
The effect of underreporting catches of young fish in the eastern Atlantic Ocean was assessed by increasing the catch of one-year-old fish in the eastern SIn this section of the report, spawning stock is defined as fish on a given fishing ground (either the east Atlantic Ocean or the west Atlantic Ocean) that are capable of reproduction, but whose spawn localities are unknown.
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From page 101... ...
rod and reel indices for small and large fish have been revised, and the missing points in the Japanese longline index for the northwestern Atlantic Ocean have been added. In terms of the trends in spawning stock abundance, Me assessment in case 7 offers about the same view of the situation of the stock (spawning abundance at 76% relative to 1988 and at 14% relative to 1975)
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From page 103... ...
FISH STOCK ASSESSMENT West- Larval oo~ N O C C ~_ ~3 ~5 : ~O t5 U)
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From page 105... ...
Trends in spawning stock abundance offer a more optimistic view (spawning stock abundance is 130% relative to 1988 and 43% relative to 1975~. Overall, based on the anaylsis, estimates of fishing mortality for 1992 are much higher in the east than in the west (Table 4-10~.
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From page 106... ...
Under this hypothesis, control of levels of spawning abundance in the Gulf of Mexico may not be easily accomplished by controls on fisheries in the western Atlantic Ocean: spawning site fidelity may occur with western Atlantic progeny, which may cause migrants in the eastern Atlantic Ocean to represent a portion of the Gulf of Mexico spawners. A second hypothesis to account for the divergence is that bluefin tuna are opportunistic spawners that will spawn whenever environmental conditions are favorable.
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From page 107... ...
The reasons for this decline are unknown, because the spawner-recruit relationship is uncertain. The current abundance of Atlantic bluefin tuna in the western Atlantic Ocean has been stable since 1988, in contrast to the roughly 50% decline in the age 8+ abundance reported in the 1993 ICCAT report.
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From page 108... ...
The mixing model points to the importance of bluefin tuna in the eastern Atlantic Ocean and Mediterranean Sea when evaluating the status and future of bluefin tuna in the western Atlantic Ocean. Even with relatively low mixing rates, the extremely large abundance of young tunas estimated recently in the eastern Atlantic Ocean causes the abundance of fishable age classes in the west to be strongly influenced by westward migrants.
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