3
Transatlantic Movement of Atlantic Bluefin Tuna

INTRODUCTION

The degree of fish movement between fishing grounds in the eastern and western Atlantic Ocean and the degree of genetic mixing in the spawning areas in the eastern and western Atlantic Ocean are both important topics relevant to the management of Atlantic bluefin tuna. The review and reanalysis of tagging data constitute the committee's evaluation of the scientific basis for physical movement or mixing of fish between fishing grounds. These tagging data, however, do not provide information on the degree of genetic mixing.

TAG-RECAPTURE DATA

Tag-recapture data provide the strongest evidence available for transatlantic movement of fish. Atlantic bluefin tuna were marked and released in the western Atlantic Ocean from 1954 in varying numbers until 1987 (Parrack, 1990). The total number tagged by the National Marine Fisheries Service (NMFS) since the inception of the program has been over 15,000. Early releases were primarily of large (giant) fish, although more recently releases have included numerous small fish.

West to East

An overview of movement from the west to the east by size class (i.e., from age 0 fish to giant fish), as indicated by tag returns, appears in Suzuki (1990).



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An Assessment of Atlantic Bluefin Tuna 3 Transatlantic Movement of Atlantic Bluefin Tuna INTRODUCTION The degree of fish movement between fishing grounds in the eastern and western Atlantic Ocean and the degree of genetic mixing in the spawning areas in the eastern and western Atlantic Ocean are both important topics relevant to the management of Atlantic bluefin tuna. The review and reanalysis of tagging data constitute the committee's evaluation of the scientific basis for physical movement or mixing of fish between fishing grounds. These tagging data, however, do not provide information on the degree of genetic mixing. TAG-RECAPTURE DATA Tag-recapture data provide the strongest evidence available for transatlantic movement of fish. Atlantic bluefin tuna were marked and released in the western Atlantic Ocean from 1954 in varying numbers until 1987 (Parrack, 1990). The total number tagged by the National Marine Fisheries Service (NMFS) since the inception of the program has been over 15,000. Early releases were primarily of large (giant) fish, although more recently releases have included numerous small fish. West to East An overview of movement from the west to the east by size class (i.e., from age 0 fish to giant fish), as indicated by tag returns, appears in Suzuki (1990).

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An Assessment of Atlantic Bluefin Tuna TABLE 3-1  Synopsis of release and recapture (tagging experiments) of western Atlantic bluefin tuna. Data are from Mather (1980) and Brunenmeister (1980). Tag locality # Released Size # Recaptured   Bahamas 1,709 Giant fish 17   NW Atlantic Ocean 1,881 Giant fish 81   US Coast* 468 Medium fish 10   NW Atlantic Ocean >17,700 Small fish >2,180   Bahamas to eastern Atlantic Ocean 9 [giant fish]     US Coast* to eastern Atlantic Ocean 1 [medium fish]     NW Atlantic to eastern Atlantic Ocean 46 [small fish]     Trans-Atlantic giant fish released in Bahamas 9/17 52.9% Trans-Atlantic giant fish released in W Atlantic Ocean 9/98 9.2% Trans-Atlantic medium fish released off US Coast* 1/10 10.0% Trans-Atlantic small fish released in NW Atlantic Ocean 46/2,180 2.1%** * From Mather (1980): "mostly" medium fish were released north of 35°N and west of 60°W. ** More than 2,180 fish released; estimate of 2.1% is maximum. Recaptures through 1978 of large fish released in the Bahamas in the 1960s, and of small fish released along the U.S. Atlantic coast, document unequivocally that transatlantic migration occurs (Table 3-1). Transatlantic migrants include nine giant fish taken off the Norwegian coast, one medium fish (recovered as a giant tuna over 10 years later) taken off the southern coast of Spain, and 46 small fish taken chiefly from the Bay of Biscay. Estimates of the total proportion of fish tagged in the western Atlantic Ocean in eastern Atlantic samples (Table 3-1) range from 9.3% for giant fish and medium fish to 2.1% for small fish. Estimates for giant and medium fish differ slightly from those given in Mather (1980), who found a total of 10 western migrants in 126 eastern Atlantic recaptures (10/126 = 7.9%). Suzuki (1990) estimated an "overall" proportion of western-tagged fish in eastern Atlantic samples of 3.2% as of 1988. Estimates of west to east transatlantic movement are variable and depend on the size of fish tagged and tagging locality. All but one of the giant fish transatlantic recoveries were tagged near the Bahamas, and five of the nine migrants were recovered in Norwegian waters in the same year. These results indicate that giant bluefin tuna can travel long distances; only one of the remaining 91 giant tuna or medium fish tagged in the western Atlantic Ocean was recovered in the eastern Atlantic Ocean. This fish was tagged near Bermuda and recovered 10 years later. Mather (1980) hypothesized that the transatlantic migration of

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An Assessment of Atlantic Bluefin Tuna giant fish was not part of an annual pattern but rather represented irregular migration of varying numbers of fish in different years. This hypothesis deserves consideration because bluefin tuna tend to follow ocean currents (Sella, 1929, cited in Mather, 1980) and because of the proximity of the Bahamian tagging sites to the Gulf Stream. This possible transatlantic "route" for giant fish is strikingly similar to the one for larvae proposed by Murphy (1990). Finally, Mather (1980) also noted that the transatlantic movement of small bluefin tuna was variable and that virtually all transatlantic movements of small fish were from the Mid-Atlantic Bight to the Bay of Biscay. Mather concluded that the west to east transatlantic movement of small fish might be due to unusual stimuli and that the great majority of Atlantic bluefin tuna remain on one side of the Atlantic Ocean or the other. Alternatively, the "nonrandom" movement of small bluefin tuna from the Mid-Atlantic Bight to the Bay of Biscay may reflect the importance of both nursery areas for small bluefin tuna. East to West Eastern Atlantic/Mediterranean bluefin tuna have been tagged and released since 1911, although most of the tag and release data are from 1957 to the present (Cort and Liorzou, 1990c). As in the western Atlantic Ocean, most of the early releases were giant tuna, with the number of small fish released and recovered increasing over the past two decades. Numbers of fish released and recaptured from the eastern Atlantic Ocean and Mediterranean Sea from 1911 through 1992 are listed in Brunenmeister (1980), Cort and Liorzou (1990c), and Cort and de la Serna (1993). A synopsis of these listings is shown in Table 3-2. The proportion of eastern-tagged fish among western Atlantic recaptures varies from 0% for giant fish to 4.5% for small fish. Virtually all of the transatlantic migrants recaptured off the U.S. coast were released from the Bay of Biscay, and most were captured between 10 and 20 months after first released. The "total" proportion of eastern fish among western Atlantic recaptures is 4.0% (19/472) after adjustment for the greater number of small fish among recaptures. This estimate is similar to the estimate of 4.4% of Cort and Loirzou (1990c). Estimates of eastern Atlantic fish among Mediterranean recaptures are 11.3% for giant fish and 2.8% for small fish. The proportion of Mediterranean fish among eastern Atlantic recaptures is 2.8% for small fish. The absence of documented east to west transatlantic movement of large fish is striking and merits further investigation. Considering only small fish, the proportion of transatlantic immigrants in western and eastern Atlantic samples differs: estimates made here suggest an overall proportion of small western fish among eastern Atlantic recaptures of 2.1% and a proportion of 4.5% eastern-tagged fish among western recaptures.

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An Assessment of Atlantic Bluefin Tuna TABLE 3-2  Synopsis of release and recapture (tagging experiments) of eastern Atlantic/Mediterranean bluefin tuna. Data are from Brunenmeister (1980), Cort and Loirzou (1990c), and Cort and de la Serna (1993). Tag Locality # Released Size # Recaptured   Eastern Atlantic Ocean 599-604 Giant fish 53     6,144 Small fish 418-420     107-232 Unknown* 3   Mediterranean Sea 3,993 Small fish 70     20-30 Unknown* 0   Eastern Atlantic to western Atlantic Ocean 19 [small fish] Eastern Atlantic Ocean to Mediterranean Sea 13 [6 giant fish, 7 small fish] Mediterranean Sea to eastern Atlantic Ocean 11 [small fish] Trans-Atlantic giant fish released in eastern Atlantic Ocean 0   Trans-Atlantic small fish released in eastern Atlantic Ocean 19/419 4.5% Mediterranean small fish to eastern Atlantic Ocean 11/70 15.7% Eastern Atlantic giant fish to Mediterranean Sea 6/53 11.3% Eastern Atlantic small fish to Mediterranean Sea 13/472 2.8% * From studies carried out prior to 1940 in Italy and Portugal. Reanalysis of Tagging Data Tagging data suggest that physical movement or mixing of Atlantic bluefin tuna in fishing grounds from opposite sides of the Atlantic Ocean is significant. Thus, the committee reanalyzed the tagging data to estimate rates of transatlantic movement between fishing grounds. This analysis was undertaken to provide rigorous estimates of transfer across the Atlantic Ocean, which has not been attempted previously. Because it is not possible to estimate transfer rates without knowledge of the total mortality rates and the nonreporting rates (tags recovered but not reported), these quantities are also estimated. METHODS The problem of transfer rates is described by the rates of change in the population sizes, Nw for the west and Ne for the east. Equations for the transfer of fish from the west to the east, applicable to fish tagged with a single tag, are: (1)

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An Assessment of Atlantic Bluefin Tuna (2) M = annual rate of instantaneous natural mortality plus shedding. 1 Fw = annual rate of instantaneous fishing mortality in the west. Fe = annual rate of instantaneous fishing mortality in the east. Tw = annual rate of instantaneous transfer from west to east. Integration of this pair of equations provides estimates of the population size at any time t, over the interval for which Fw is constant in the west and Fe is constant in the east. The solutions are given by: where X = M + Fe and Y = Fw-Fe + Tw Similarly, the equations for the caches are: When t is sufficiently large: When Pw and Pe are the proportions of tags recovered that are reported for the west and east, respectively, then: (3) Analogous equations (with the subscripts e and w reversed) are used to estimate the transfer rate of fish from the east to the west. Because the models given by Equations (1) and (2) do not admit roundtrip movement, any such return movement would cause the transfer rates estimated by Equation (3) to be underestimates of movement rates from one side of the ocean to the other. Also, if fishing mortality decreases with age, as it appears to do, the movement rates estimated by assuming a constant fishing mortality will be underestimated. Tag-recovery information from Corte and de la Serna (1993) (Table 2; which is reproduced below as Table 3-3) and from NMFS archives (file name MRFISH) 1   Shedding refers to tags that have detached from the fish.

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An Assessment of Atlantic Bluefin Tuna TABLE 3-3  Spanish tagging data for Atlantic bluefin tuna in the Cantabrian Sea (Bay of Biscay) from 1976 to 1991. Recoveries   Recoveries in the Eastern Atlantic With Known Years At Liberty Year N 0 1 2 3 4 Trans Atlantic Med. Sea East Atlantic* No data Total 1976 3 - - - - - 0 - 0 0 - 1977 10 - - - - - 0 - 0 0 - 1978 170 29 2 4 1 - 1 - 0 5 42 1979 101 1 10 - - - 2 - 0 1 14 1980 302 15 2 2 - - 3 - 2 8 32 1981 293 3 5 - 1 - 0 2 0 2 13 1982 395 5 6 4 2 - 1 3 2 2 25 1983 370 2 1 1 - - 0 1 0 1 6 1984 513 8 7 1 - 1 0 1 2 6 26 1985 407 12 2 - 2 - 1 - 0 4 21 1986 838 37 8 6 1 1 5 - 0 10 68 1987 - - - - - - 0 - 0 0 - 1988 1,151 26 17 5 - 2 1 - 0 7 58 1989 122 2 1 2 2 - 0 - 0 0 7 1990 973 8 22 2 - - 3 - 0 0 39 1991 15 4 1 - - - 0 - 0 0 5 Total 5,663 152 84 29 11 4 17 7 6 46 356 * This column refers to recoveries in the eastern Atlantic without known recovery year.

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An Assessment of Atlantic Bluefin Tuna was used to obtain values of catch (C). Some descriptions of qualitative features of the tagging data with respect to movement are given below in the Results section. Shedding (L = M -0.14) (0.14 was taken from the literature as the natural mortality rate; ICCAT, 1993) was estimated from these catch data using the method of Chapman, Fink, and Bennet (1965, equations 5, 6, and 7) to estimate L. This method was used rather than that of Bayliff and Mobrand (1972), because shedding rates do not appear to be constant and appear to be insensitive to type I (initial) shedding. Once the estimate of M (L + 0.14) was available, VPA analysis was done on the catches over time to obtain estimates of F. Results Tables 3-4, 3-5, and 3-6 show the number of fish tagged in the western Atlantic Ocean (from MRFISH) and the number of recoveries by year after tagging. Year 0 means fish caught in the same calendar year as tagged; year 1 means fish caught in the calendar year following tagging, etc. Table 3-7 (eastern Atlantic Ocean, ICCAT data) and Table 3-8 (western Atlantic Ocean, NMFS data) show the months in which the fish were tagged and the months in which the fisheries captured the tagged fish. Because most tagged fish were small (less than four years old), inferences about fishing times based on these tag recoveries should apply to small fish. From 1971 to 1978, there was intensive tagging in the west with both single-and double tagged fish (a small number of double tagged fish actually had more than two tags). Table 3-9 shows the number of recoveries from single tagged fish (SS), the number of recoveries of fish with two tags from double tagged fish (DD), and the number of recoveries of fish with one tag from double tagged fish (DS), by year of recovery. Tagging time varied, so data from the west were sorted into periods of one-quarter year; that is, if a tag was recovered with the same quarter as the release, it was placed in quarter 1. If the recovery was made after more than 91 days but before 182 days, it was placed in quarter 2, and so on. Table 3-10 shows the recovery data for recoveries SS, DD, and DS by year of release and quarter of recovery. The western data from Table 3-10 were used to estimate shedding rates (Table 3-11). Comparison of SS to DD (Db Db vs. Sing in Table 3-11) shows the most variability, and comparison of DD to DS (Db Db vs. Db Sing) shows the most stability. After examining Table 3-11, it was decided to try L = 0.26 for all ages and also to try L(1) = 0.50, L(2) = 0.40, L(3) = 0.30, and L(4+) = 0.20, corresponding to quarter 1, quarter 2, quarter 3, and all quarters 4 or more. These values are not much different from the value of 0.205 estimated by Baglin et al. (1980). With the shedding rates mentioned above, and the catch vectors for SS and DD in Table 3-11, VPA analysis was performed showing that the two sets of

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An Assessment of Atlantic Bluefin Tuna TABLE 3-4   Atlantic bluefin tuna release and recovery data from the United States tagging program in the western Atlantic Ocean. Tag returns are by year tagged and years out for all tag types. Year Tag 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Total 1954 193 1 - - - - 2 - - - - - - - - - - - - - 3 1955 232 - - - - - - - - - - - - - - - - - - - - 1956 99 - - - - - - - - - - - - - - - - - - - - 1957 39 - - 1 - - - - - - - - - - - - - - - - 1 1958 38 - - - - - - - - - - - - - - - - - - - - 1959 147 1 - - - - - - - - - - - - - - - - - - 1 1960 237 - - 3 1 - - - - 1 - - - - - - - - - - 5 1961 188 2 2 3 2 - - - - - - - - - - - - - - - 9 1962 128 1 4 - - - - - - - - - - - - - - - - - 5 1963 223 11 3 1 - - - - - - - - - - - - - - - - 15 1964 553 100 32 - - - - - - - - - - - - - - - - - 132 1965 1,812 164 63 36 3 - - - - - - - - - - - - - - - 267 1966 4,128 524 577 51 8 6 - 2 - - 1 1 - - 2 - - - - - 1,172 1967 718 98 60 16 13 - - - - - - - - - - - - - - 1 188 1968 521 88 19 9 - - - - - - - - - - - - - - - - 116 1969 567 14 81 12 2 2 - 1 1 - 1 - - - - - - - - - 114 1970 729 52 119 9 7 3 - - - - - - - - - - - - - - 190 1971 432 11 59 12 - - 1 - - - - - - - - - - - - - 83 1972 284 10 56 6 2 - - - - - - - - - - - - - - - 74 1973 393 40 22 7 2 - - - - - - - - - - - - - - - 71 1974 1,752 75 126 51 14 7 - - - - 1 1 1 1 - - 1 - - - 278 1975 349 22 27 6 2 - - 1 - - - - - - - - - - - - 58 1976 2,460 185 63 75 8 - 1 - - 1 - - - - - - - - - - 333 1977 2,115 52 193 81 2 - - 1 2 1 1 1 2 - - - - - - - 341 1978 1,680 24 110 34 7 7 3 1 1 2 - 1 2 - - - - - - - 192 1979 1,124 6 30 11 1 3 1 1 - 1 - - - - - - - - - - 53 1980 3,074 141 100 6 3 4 2 2 4 - 2 - - - - - - - - - 268 1981 1,797 57 2 2 1 1 1 2 - 4 - - - - - - - - - - 68 1982 210 1 1 1 2 - - - - 2 - - - - - - - - - - 5 1983 149 1 2 4 4 - - 1 - - - - - - - - - - - - 12 1984 86 1 2 - - - - - - - - - - - - - - - - - 3 1985 130 - 1 2 - - - - - - - - - - - - - - - - 3 1986 51 - 1 1 - - - - - - - - - - - - - - - - 2 1987 66 1 1 - - - - - - - - - - - - - - - - - 2 1988 96 - 2 - - - - - - - - - - - - - - - - - 2 1989 113 - - - - - - - - - - - - - - - - - - - - 1990 127 - - - - - - - - - - - - - - - - - - - - Note:  Dash indicates zero value.

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An Assessment of Atlantic Bluefin Tuna

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An Assessment of Atlantic Bluefin Tuna TABLE 3-5   Atlantic bluefin tuna release and recovery data from the U.S. tagging program in the western Atlantic Ocean. Tag returns are by year tagged and years out for fish tagged with a single tag. Year Tag 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Total 1954 192 1 - - - - 2 - - - - - - - - - - - - - 3 1955 231 - - - - - - - - - - - - - - - - - - - - 1956 99 - - - - - - - - - - - - - - - - - - - - 1957 39 - - 1 - - - - - - - - - - - - - - -   1 1958 38 - - - - - - - - - - - - - - - - - - - - 1959 147 1 - - - - - - - - - - - - - - - - -   1 1960 237 - - 3 1 - - - - 1 - - - - - - - - - - 5 1961 185 2 2 3 2 - - - - - - - - - - - - - - - 9 1962 128 1 4 - - - - - - - - - - - - - - - - - 5 1963 183 7 3 1 - - - - - - - - - - - - - - - - 10 1964 544 100 32 - - - - - - - - - - - - - - - - - 132 1965 1,751 164 63 36 3 - - - - - - - - - - - - - - - 267 1966 4,114 523 575 51 8 6 - 2 - - 1 1 - - 2 - - - - - 1,169 1967 718 98 60 16 13 - - - - - - - - - - - - - - 1 188 1968 448 60 12 6 - - - - - - - - - - - - - -   - 78 1969 547 14 77 12 1 2 - 1 1 - 1 - - - - - - - - - 109 1970 724 51 119 9 7 2 - - - - - - - - - - - - - - 188 1971 115 2 11 3 - - 1 - - - - - - - - - - - - - 17 1972 130 3 14 6 1 - - - - - - - - - - - - - - - 21 1973 105 4 8 1 1 - - - - - - - - - - - - - - - 14 1974 466 17 43 14 7 4 - - - - 1 - - 1 - - - - - - 87 1975 239 20 16 4 - - - 1 - - - - - - - - - - - - 41 1976 1,916 145 52 66 5 - 1 - - - - - - - - - - - - - 269 1977 1,590 39 129 50 1 - - - 1 1 1 1 1 - - - - - - - 224 1978 848 20 36 13 4 4 1 1 - - - - 1 - - - - - - - 80 1979 1,110 5 29 10 1 3 1 1 - - - - - - - - - - - - 50 1980 3,066 141 100 6 3 4 2 2 2 4 4 2 - - - - - - - - 268 1981 1,787 55 2 2 1 1 1 1 - 2 - - - - - - - - - - 65 1982 200 1 1 1 2 - - - - - - - - - - - - - - - 5 1983 146 1 1 4 4 - - 1 - - - - - - - - - - - - 11 1984 84 - 2 - - - - - - - - - - - - - - - - - 2 1985 128 - 1 2 - - - - - - - - - - - - - - - - 3 1986 51 - 1 1 - - - - - - - - - - - - - - - - 2 1987 66 1 1 - - - - - - - - - - - - - - - - - 2 1988 96 - 2 - - - - - - - - - - - - - - - - 2 1989 112 - - - - - - - - - - - - - - - - - - - - 1990 127 - - - - - - - - - - - - - - - - - - - - Note: Dash indicates zero value.

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An Assessment of Atlantic Bluefin Tuna

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An Assessment of Atlantic Bluefin Tuna TABLE 3-20   Atlantic bluefin tuna tagged in the eastern Atlantic Ocean and recovered in the eastern Atlantic Ocean versus time (years) before recapture and a regression analysis of the natural logarithm of catch number against years out (Cort and LaSerna, 1993).  

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An Assessment of Atlantic Bluefin Tuna TABLE 3-21 Atlantic bluefin tuna tagged in the western Atlantic Ocean from 1960 to 1981 with a single tag and recovered in the western Atlantic Ocean vs. years before recapture and a regression analysis of the natural logarithm of catch number against years out (data in file MRFISH provided by NMFS).  

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An Assessment of Atlantic Bluefin Tuna TABLE 3-22   Atlantic bluefin tuna. VPA analysis on fish tagged in the eastern Atlantic Ocean (near Spain) from 1976 to 1991. Six different solutions. Time Initial pop Catch Fish. Mort. Nat. Mort. Shed Rate Time Units 1 553.8 193.00 0.9608 0.1400 0.2600 0.50 2 280.5 107.00 0.6047 0.1400 0.2600 1.00 3 102.7 37.00 0.5605 0.1400 0.2600 1.00 4 39.3 14.00 0.5522 0.1400 0.2600 1.00 5 15.2 5.00 0.5000 0.1400 0.2600 1.00 TOTALS   356.00 2.6978 0.6300 1.1700 4.50 Time Initial pop Catch Fish. Mort. Nat. Mort. Shed Rate Time Units 1 587.3 193.00 0.8922 0.1400 0.2600 0.50 2 307.8 107.00 0.5351 0.1400 0.2600 1.00 3 120.8 37.00 0.4558 0.1400 0.2600 1.00 4 51.3 14.00 0.3954 0.1400 0.2600 1.00 5 23.2 5.00 0.3001 0.1400 0.2600 1.00 TOTALS   356.00 2.1325 0.6300 1.1700 4.50 Time Initial Pop Catch Fish. Mort. Nat. Mort. Shed Rate Time Units 1 753.3 193.00 0.6599 0.1400 0.2600 0.50 2 443.4 107.00 0.3419 0.1400 0.2600 1.00 3 211.2 37.00 0.2370 0.1400 0.2600 1.00 4 111.7 14.00 0.1640 0.1400 0.2600 1.00 5 63.5 5.00 0.0999 0.1400 0.2600 1.00 TOTALS   356.00 1.1729 0.6300 1.1700 4.50

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An Assessment of Atlantic Bluefin Tuna Time Initial Pop Catch Fish. Mort. Nat. Mort. Shed Rate Time Units 1 1,000.3 193.00 0.4765 0.1400 0.2600 0.50 2 645.4 107.00 0.2226 0.1400 0.2600 1.00 3 346.3 37.00 0.1381 0.1400 0.2600 1.00 4 202.2 14.00 0.0874 0.1400 0.2600 1.00 5 124.2 5.00 0.0499 0.1400 0.2600 1.00 TOTALS   356.00 0.7363 0.6300 1.1700 4.50 Time Initial Pop Catch Fish. Mort. Nat. Mort. Shed Rate Time Units 1 2,047.9 193.00 0.2193 0.1400 0.2600 0.50 2 1,502.6 107.00 0.0902 0.1400 0.2600 1.00 3 920.4 37.00 0.0499 0.1400 0.2600 1.00 4 586.9 14.00 0.0294 0.1400 0.2600 1.00 5 382.0 5.00 0.0160 0.1400 0.2600 1.00 TOTALS   356.00 0.2952 0.6300 1.1700 4.50 Time Initial Pop Catch Fish. Mort. Nat. Mort. Shed Rate Time Units 1 2,969.6 193.00 0.1485 0.1400 0.2600 0.50 2 2,257.3 107.00 0.0591 0.1400 0.2600 1.00 3 1,426.4 37.00 0.0319 0.1400 0.2600 1.00 4 926.1 14.00 0.0185 0.1400 0.2600 1.00 5 609.4 5.00 0.0099 0.1400 0.2600 1.00 TOTALS   356.00 0.1936 0.6300 1.1700 4.50

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An Assessment of Atlantic Bluefin Tuna TABLE 3-23   Atlantic bluefin tuna. VPA analysis on fish tagged in the western Atlantic Ocean (near USA) from 1960 to 1981. Five different solutions. Time Initial Pop Catch Fish. Mort. Nat. Mort. Shed Rate Time Units 1 19,162.9 1,471.00 0.1768 0.1400 0.2600 0.50 2 14,361.5 1,386.00 0.1241 0.1400 0.2600 1.00 3 8,503.7 309.00 0.0450 0.1400 0.2600 1.00 4 5,449.3 59.00 0.0133 0.1400 0.2600 1.00 5 3,604.6 26.00 0.0087 0.1400 0.2600 1.00 6 2,395.3 7.00 0.0035 0.1400 0.2600 1.00 7 1,600.0 44.00 0.0113 0.1400 0.2600 10.00 TOTALS   3,302.00 0.3961 2.1700 4.0300 15.50 Time Initial Pop Catch Fish. Mort. Nat. Mort. Shed Rate Time Units 1 13,739.9 1,471.00 0.2507 0.1400 0.2600 0.50 2 9,924.0 1,386.00 0.1845 0.1400 0.2600 1.00 3 5,531.6 309.00 0.0700 0.1400 0.2600 1.00 4 3,457.1 59.00 0.0209 0.1400 0.2600 1.00 5 2,269.4 26.00 0.0139 0.1400 0.2600 1.00 6 1,500.3 7.00 0.0056 0.1400 0.2600 1.00 7 1,000.0 44.00 0.0184 0.1400 0.2600 10.00 TOTALS   3,302.00 0.6044 2.1700 4.0300 15.50 Time Initial Pop Catch Fish. Mort. Nat. Mort. Shed Rate Time Units 1 9,218.3 1,471.00 0.3859 0.1400 0.2600 0.50 2 6,223.0 1,386.00 0.3114 0.1400 0.2600 1.00 3 3,055.1 309.00 0.1305 0.1400 0.2600 1.00 4 1,797.4 59.00 0.0404 0.1400 0.2600 1.00 5 1,157.1 26.00 0.0276 0.1400 0.2600 1.00 6 754.5 7.00 0.0114 0.1400 0.2600 1.00 7 500.0 44.00 0.0386 0.1400 0.2600 10.00 TOTALS   3,302.00 1.1003 2.1700 4.0300 15.50

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An Assessment of Atlantic Bluefin Tuna Time Initial Pop Catch Fish. Mort. Nat. Mort. Shed Rate Time Units 1 6,945.2 1,471.00 0.5296 0.1400 0.2600 0.50 2 4,363.3 1,386.00 0.4771 0.1400 0.2600 1.00 3 1,815.0 309.00 0.2293 0.1400 0.2600 1.00 4 967.3 59.00 0.0768 0.1400 0.2600 1.00 5 600.5 26.00 0.0539 0.1400 0.2600 1.00 6 381.4 7.00 0.0224 0.1400 0.2600 1.00 7 250.0 44.00 0.0854 0.1400 0.2600 10.00 TOTALS   3,302.00 1.9787 2.1700 4.0300 15.50 Time Initial Pop Catch Fish. Mort. Nat. Mort. Shed Rate Time Units 1 5,570.1 1,471.00 0.6844 0.1400 0.2600 0.50 2 3,238.9 1,386.00 0.7082 0.1400 0.2600 1.00 3 1,069.4 309.00 0.4243 0.1400 0.2600 1.00 4 468.9 59.00 0.1646 0.1400 0.2600 1.00 5 266.6 26.00 0.1253 0.1400 0.2600 1.00 6 157.7 7.00 0.0554 0.1400 0.2600 1.00 7 100.0 44.00 0.3143 0.1400 0.2600 10.00 TOTALS   3,302.00 4.9628 2.1700 4.0300 15.50

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An Assessment of Atlantic Bluefin Tuna TABLE 3-24  Estimates of transfer rates from east to west in the Atlantic Ocean, using tagging data and assuming a natural mortality rate of 0.14, a tag shedding rate of 0.26, and an eastern catch of 339 and a western catch of 17, with a ratio of 0.0501.   Pe = 0.5244 Fe = 0.0430 Pe = 0.3616 Fe = 0.0656 Pe = 0.1330 Fe = 0.2606 Pw = 0.9147 Fw = 0.0256 Te = 0.0206 Te = 0.0216 Te = 0.0316 Pw = 0.6558 Fw = 0.0390 Te = 0.0194 Te = 0.0204 Te = 0.0298 Pw = 0.4400 Fw = 0.0710 Te = 0.0171 Te = 0.0179 Te = 0.0262 Pw = 0.3315 Fw = 0.1277 Te = 0.0141 Te = 0.0148 Te = 0.0217 Pw = 0.2659 Fw = 0.3202 Te = 0.0096 Te = 0.0101 Te = 0.0147 Pw = Fraction of captured tags which are reported in the west. Pe = Fraction of captured tags which are reported in the east Fw = Annual instantaneous fishing mortality rate in the west. Fe = Annual instantaneous fishing mortality rate in the east. Te = Annual instantaneous rate of transfer from the east to the west.

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An Assessment of Atlantic Bluefin Tuna TABLE 3-25 Estimates of transfer rates from west to east in the Atlantic Ocean, using tagging data and assuming a natural mortality rate of 0.14, a tag shedding rate of 0.26, and a western catch of 3037 and an eastern catch of 65, with a ratio of 0.0214.   Pw = 0.9147 Fw = 0.0256 Pw = 0.4400 Fw = 0.0710 Pw = 0.2659 Fw = 0.3202 Pe = 0.5244 Fe = 0.0430 Tw = 0.0099 Tw = 0.0131 Tw = 0.0358 Pe = 0.3616 Fe = 0.0656 Tw = 0.0098 Tw = 0.0130 Tw = 0.0356 Pe = 0.1330 Fe = 0.2606 Tw = 0.0096 Tw = 0.0127 Tw = 0.0347 Pe = 0.1037 Fe = 0.4739 Tw = 0.0089 Tw = 0.0119 Tw = 0.0324 Pe = 0.0978 Fe = 0.5995 Tw = 0.0085 Tw = 0.0114 Tw = 0.0311 Pw = Fraction of captured tags which are reported in the west. Pe = Fraction of captured tags which are reported in the east. Fw = Annual instantaneous fishing mortality rate in the west. Fe = Annual instantaneous fishing mortality rate in the east. Tw = Annual instantaneous rate of transfer from the west to the east.

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An Assessment of Atlantic Bluefin Tuna   Western Atlantic Ocean (1) Pw = 0.9147 Fw = 0.0256 Zw = 0.4256 (2) Pw = 0.6558 Fw = 0.0390 Zw = 0.4390 (3) Pw = 0.4400 Fw = 0.0710 Zw = 0.4710 (4) Pw = 0.3315 Fw = 0.1277 Zw = 0.5277 (5) Pw = 0.2659 Fw = 0.3202 Zw = 0.7202 These estimates were used to compute a variety of transfer rates from east to west (Table 3-24) and from west to east (Table 3-25). Not all combinations of P values are used in Tables 3-24 and 3-25 because other results were inconsistent with VPA estimates of Chapter 4. DISCUSSION Shedding rates appear to be 0.20 or higher (Table 3-11). Rates estimated by comparing double-double returns to double-single returns should be the most robust estimates, because the two groups originated from the same group of tags and should have been exposed to similar histories. Rates appear to be higher in the first several quarters. Two different sets of shedding rates were used in the VPA analysis (Tables 3-12 to 3-15). The results of this reanalysis of tagging data provide quantitative confirmation of the empirical results on transfer rates presented earlier in Chapter 3. Although the committee was unable to obtain variance estimates for these transfer rates due to time constraints, results are robust with respect to reporting rate, the parameter most likely to affect the results. Thus, the results are expected to be statistically significant. However, movement of fish from west to east seems to have varied from year to year (Table 3-16). Movement from west to east also seems to have a cumulative effect, with the percent caught in the east increasing as a function of the amount of time the tags were attached to the fish (Table 3-17). Annual transfer rates estimated with the model (equations 1 and 2) are about 0.01 for west to east, about 0.02 for east to west, and 0.03 overall. These rates depend to a great extent on the nonreporting rate for captured tagged fish. The VPA analysis on the Spanish data is especially indicative of a high nonreporting rate. The nonreporting rate for the west could also be high. Little can be said about the estimate of natural mortality from these data. However, even though there is good reason to be skeptical about the nonreporting rate and natural mortality rate, the transfer rates are almost certainly low, because the number of tags recovered in the other area is small compared to the total tags recovered. Information is not available to determine whether a transfer from one side

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An Assessment of Atlantic Bluefin Tuna FIGURE 3-1  Proportion of tagged bluefin tuna in area 2 (eastern Atlantic Ocean) that were originally released in area 1 (western Atlantic Ocean) for a hypothetical population with Tw = 0.03, F = 0.1, and M = 0.14. to the other is permanent. The consequences of returning would give a different meaning to the data. If fish that transfer each year do not return, even a low transfer rate will cause a significant portion of the spawning-age fish produced as recruits on one side to be on the other side when they are sexually mature. Figure 3-1 shows the increasing proportion of tagged fish in area 2 that were originally released in area 1 for a hypothetical population with T = 0.03, F = 0.1, and M = 0.14. After 300 weeks (about 6 years), the proportion is up to 0.20 even though the annual transfer rate is only 0.03; that is, 20% of the fish on one side of the Atlantic Ocean will have originated from the other side. This example shows that small annual transfer rates can have a large effect on population dynamics over a relatively short time period. Because none of the fish tagged in the eastern Atlantic Ocean were from the Mediterranean Sea, caution should be exercised in interpreting the east to west movement rate. Estimates of Pe and Pw obtained from Tables 3-22 and 3-23 were calculated with the assumption of no type I (initial) tagging mortality. If some, or most, of the differences between the VPA estimates of age 1 fish in the east and west result from type I mortality, the estimates of Pe and Pw would be incorrect. However, it is the ratio Pe/Pw that appears in equation 3, and if the type I mortality was the same for east and west, the ratio would still be the same as the estimates used to estimate Tw.

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