Conclusions—Standardization

Estimates of abundance are unchanged, on average, from 1988 to 1993 based on reanalyses of the three indices (two rod and reel indices and one captains' logbook index). The two indices on giant bluefin tuna (one rod and reel index and the captains' logbook index) are concordant. Furthermore, the selection criteria on classes of boats did not alter these conclusions, based on progressively more restrictive data exclusion to remove some inexperienced fishers.

Transformations of catch rates according to logarithm of catch rate plus a large constant are inappropriate because they lack a biological foundation and contradict assumptions about independence of factors on catch rates.

Using nonparametric tests, the Mann-Kendall S-test and Sen's slope estimator for the published SCRS indices, 12 indices were found to have no significant (slope) trends and four indices (see Table 4-6) had significant trends (all negative slopes) at (P ≤ 0.5). These tests also show that there are no significant trends in any of the CPUE data in Tables 4-1 to 4-5.

Recommendations—Standardization

A data management system should be established for catch indices that includes better documentation, quality control, central archiving, and error checking.

A thorough, in-depth review of all indices from all areas of the Atlantic Ocean, Mediterranean Sea, and Gulf of Mexico is needed.

Alternative methods for estimating stock abundance should be evaluated, including aerial survey, spawner surveys in the Florida Straits, purse seining for young bluefin tuna, and expanded larval fish surveys in the Gulf of Mexico.

Selection criteria should be developed to create a rod and reel index for giant bluefin tuna, which excludes fishermen present in the captains' logbook index. That process would create two independent catch rate indices (the captains' logbook index and the modified rod and reel index on giant fish) that could both be used as tuning indices.

Conclusions—Population Assessment

The estimated spawning abundance of western Atlantic bluefin tuna has declined substantially (to about 20% of that in 1975) since the 1970's. 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. Reasons for the committee's view of current abundance result principally from two changes made to the SCRS assessments: (1) reanalysis and correction of some accidental errors



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement