An Assessment of Atlantic Bluefin Tuna (1994) / Chapter Skim
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2 Historical Evidence for Stock Structure
2 Historical Evidence for Stock Structure
Pages 9-38
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From page 9... ...
Included are sections on stock concepts, history of stock and population designations in Atlantic bluefin tuna, genetics, life history, climate, and movement. Except for the first two sections (on stock concepts and historical stock designations in Atlantic bluefin tuna)
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From page 10... ...
Geographic distribution, parasite markers, microconstituent analysis, tag-recapture data, population parameters, morphological variability, and genetic information can be used to assign populations to these categories. A variety of evidence is available for assessing the population structure of Atlantic bluefin tuna, but much of the data are equivocal.
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From page 11... ...
(1974) suggested two alternative hypo~eses: a single Atlantic population and one or more Mediterranean populations; or two Atlantic populations, one spawning in the western Atlantic Ocean and the other in the eastern Atlantic Ocean or the Mediterranean Sea, or both, and one or more Mediterranean populations.
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From page 12... ...
l l..-~7,............ are 40o go 7oo 60° I, 40° -oo 4oo FIGURE 2-1 General distribution of bluefin tuna in the Atlantic Ocean (darkened areas indicate known spawning areas [adapted from FAO, 19681~.
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From page 13... ...
the Gulf Stream, via its extension in the Norm Atlantic Current, could transport larval and postlarval bluefin tuna into the middle of the Atlantic Ocean. GENETIC STUDIES Molecular techniques that assay genetic variability have an obvious advantage over techniques that measure life history traits where the genetic component is unknown.
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From page 14... ...
First, mitochondria are inherited only from Be female parent; second, there is no recombination among mlDNA molecules, greatly simplifying interpretation of phylogenetic trees based on mtDNA; and, third, Mere is a higher mutation rate in mtDNA than in nuclear genes typically assayed for population discrimination, thus providing the opportunity to examine more recent divergences. These characteristics provide a f~ner-scale resolution of genetic differences than is possible for the analysis of nuclear genes assayed with protein elec~ophoresis.
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From page 15... ...
gene in four species of tuna from the northeast Atlantic Ocean: bluefin (n=33) , yellowf~n (n=33)
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From page 16... ...
This was taken as evidence for the lack of genetic differentiation between eastern and western Atlantic bluefin tuna. The oceanwide fit of SOD genotypes to proportions expected from random mating is consistent with the hypothesis that a single population of bluefin tuna occurs in the Norm Atlantic Ocean.
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From page 17... ...
The genetic analysis of within-ocean basin diversity of Atlantic bluefin tuna (e.g., Atlantic Ocean, Mediterranean Sea) , would benefit from a worldwide study of molecular genetic variation among bluefin tuna.
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From page 18... ...
In theory, natural selection, genetic drift, and migration between or among localities determine the degree that life history variables change from one locality to the next: high levels of migration tend to minimize differences among localities, whereas extremely low levels of migration could allow population differences to appear in only a few generations. Geographic Locality of Spawning Grounds Although bluefin tuna have been found as far north as Newfoundland in the western Atlantic Ocean and as far north as Norway in the eastern Atlantic Ocean, and a fishery existed for a short time as far south as Brazil, extensive searching has detected only two spawning localities: the Gulf of Mexico and the Mediterranean Sea (figure 2-1~.
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From page 19... ...
Differences in spawning times do not necessarily indicate that bluefin tuna produced in the Gulf of Mexico and maturing there or in the western Atlantic Ocean would be precluded from spawning, as adults, in the Mediterranean Sea, or vice versa. Tagging experiments demonstrated that fish can cross the Atlantic Ocean in less Han 60 days.
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From page 20... ...
suggested that western Atlantic bluefin tuna also might become sexually mature before the age of 10 years.
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From page 21... ...
Figure 2-1 shows the distribution of bluefin tuna in the Atlantic TABLE 2-1 Migration speed of Atlantic bluefin tuna (kilometers/day) calculated from tag-recovery data (Clay, 1990~.
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From page 22... ...
Western Atlantic bluefin tuna occur in waters as cold as 6.6°C (Squire, 1962) , whereas eastern Atlantic bluefin tune have not been reported in waters cooler than 12°C (Tiews, 1962)
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From page 23... ...
It seems appropriate to estimate the availability of food such as squid, herring, and mackerel in the various regions of the western Atlantic Ocean. This infor mation should not be used as a bluefin tuna index, per se, but might be useful in choosing or weighting bluefin tuna abundance indices in the virtual population analyses (VPA)
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From page 25... ...
25 to to on ~ ~ to can ~ ~ ~ ~ O cry can ~ ~ oo cat to ~ ~ ~ ~ to ~ By en of 1 1 1 1 1 1 1 1 1 1 1 1~ 1 1 1 1 1 1 1 1 1 1 1 -~ ~ ~ as ~ en of 0 0 cry can ~ 1 - ~ ~ ~ 1 1 en ~ ~ ~ ~ ~ ~1 1 1 <)
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From page 26... ...
· i: O l l _ in F Sum of all values in Table 2 - 2 y In -- i I 'Jot o 19 60 1970 1980 1990 PIGURE 2-2 Landings of giant Atlantic bluefin tuna (by numbers of fish) in Canadian waters showing variation in catch from year to year.
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From page 27... ...
Lawrence arid surrounding Atlantic Ocean regions presented in Figure 2-2. Numbers correspond to the areas listed in Table 2-2 column headings.
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From page 28... ...
Some move as far west as He western Atlantic Ocean; Hat is, they mix with those spawned in the west. Little is known about He movements of "medium" fish, except that Hey move farther eastward offshore than small fish.
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From page 29... ...
There is no evidence that the large geographic separation of spawning locations represents reproductively isolated spawning grounds. Giant bluefin tuna tend to occur in patches where food is abundant and in areas of suitable temperatures.
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From page 30... ...
, which is on the order of millions of years for marine fish. This assumption of equilibrium is probably not completely met for North Atlantic pelagic fish, in large part because the present-day population structure in the North Atlantic Ocean has arisen since the last glaciation (15,000 to 20,000 years ago)
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From page 31... ...
, even though it is unlikely that transocean movement presently occurs in these fish. Thus, the lack of allozyme differentiation in Atlantic bluefin tuna may reflect the recent glacial history of the North Atlantic Ocean rather than ongoing migration across the Atlantic Ocean.
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From page 32... ...
(b) AN ASSESSMENT OF ATLANTIC BLUEFIN TUNA Rno ~ ;no 65°w 45° w 40o W `s w '^1 10°V' 5°V' HIP 10°1 ~ / ME FIGURE 24 Barometric shifts and wind patterns driving the Russell Cycle (from Mann and Lazier [1991]
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From page 33... ...
When a high-pressure system dominates, the prevailing westerly winds are diverted north of the high, resulting in a northeastward flowing body of warm air and water along North America in the western Atlantic Ocean and a southeastward flowing body of cool air and water along Europe and the British Isles (Figure 2-4~. When a low-pressure system dominates in the north Atlantic Ocean, the westerlies are diverted to the south, and this results in warmer weather and sea temperature in the eastern Atlantic Ocean (Figure 2-44.
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From page 34... ...
Smaller individuals of Atlantic bluefin tuna also appear to be restricted to areas in the Gulf of Maine win surface temperatures of at least 16°C, whereas adults are found in waters as cold as 1012°C, which appears to be their lower thermal limit in the Gulf of Maine (Bigelow and Schroeder, 19534. Similarly, 12°C also appears to be the lower level of thermal tolerance for bluefin tuna in the eastern North Atlantic Ocean (Luhmann,1959; Tiews, 19624.
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From page 35... ...
Such strategies should also account for the dynamic nature of geographic distributions in space and time. MOVEMENT BASED ON NONGENETIC MARKERS Nongenetic markers can provide only indirect evidence for gene flow across the Atlantic Ocean because Were is no assurance that fish moving from one area to the next actually breed in the new area; mixing on Be fishing grounds may not necessarily reflect mixing of the same magnitude, or mixing of the same fish, on the spawning grounds.
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From page 36... ...
Conclusions The microconstituent studies support the hypothesis of mixing between the two regions. Some giant fish harvested in the western Atlantic Ocean were spawned in the Mediterranean Sea, and some that were harvested in the eastern Atlantic Ocean were spawned in the Gulf of Mexico.
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From page 37... ...
A random sample from the western Atlantic Ocean must include samples from several localities, and not just from a single school.
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