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An Assessment of Atlantic Bluefin Tuna
that there is mixing of about 15% in both directions. Brunenmeister's (1980) analysis predates access to all of the Walters' data and so is incomplete. Bossert (1993) does not consider Walters' data. It is important then to restate Walters' view of his own data. He uses the terms "reasonable to postulate," "may have spawned," "a plausible explanation,'' "further study is indicated," "it appears that Nasicola sp. could be used," and "Elytrophora sp. appears to be acquired." Walters offers a possible explanation, but it is not more than that and always should be couched in those terms.
Most giant bluefin tuna in the western Atlantic Ocean are infested with Nasicola sp. (93% [Walters, 1980], 81% [Phipps, 1980]). None have been sampled from the eastern Atlantic Ocean.
The most definitive result in Walter's study is that mean parasite size increases with host size. Data of Phipps (1980) and Wheeler and Beverley-Burton (1987) confirm and extend this relation (Figure E-1). Walters suggested that the parasite may be very long lived because parasite size increases with host size and the host is long lived (20+ years). An alternative explanation, not uncommon in the parasitological literature, is that the parasites grow larger because they have more room to do so and that the giant fish repeatedly acquire the parasite each time they pass through water harboring immatures.
Table E-1 and Figure E-2A provide a summary of the data for Nasicola sp. The data of Phipps (i.e., worm size ) suggest that the giant bluefin tuna used by Walters were probably small giant fish. Neither Walters nor Wheeler and Beverley-Burton (1987) give the host size of giant fish.
Walters used the data to suggest the following. The decrease in prevalence in the western Atlantic tuna (i.e., 0.86 at age 0 to 0.31 at age 3) was taken to indicate that the fish were being diluted by uninfested fish from the east; there is mixing. The presence of infested fish in the Bay of Biscay was taken to indicate that the uninfested fish from the Mediterranean Sea were being diluted by infested fish from the west; there is mixing.
The Nasicola klawei holotype (i.e., the specimens used to define the parasite species) was obtained from tuna other than bluefin (Yamaguti, 1963). Wheeler and Beverley-Burton (1987) proposed a new species (N. hogansi) for specimens obtained from western Atlantic bluefin tuna (Prince Edward Island, Canada). Their description is identical to that for a much larger sample described by Phipps (1980) also from western Atlantic giant bluefin tuna. It is likely the taxonomy is confused because workers have not sampled parasites from hosts of the same size. It is likely that the species from bluefin tuna described by Walters is not N. klawei but rather N. hogansi.
The high prevalence at age 4 was ignored by Walters; presumably it was taken to be anomalous. The slope of the line describing the decrease in prevalence