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This paper provides information on the length-length (fork-length, FL with pectoral length, PL and first dorsal length, LD1) and length-weight relationships of albacore tuna (Thunnus alalunga) in five regions of the Western Indian Ocean. Data were obtained for a total of 923 female and 867 male albacore, caught by different fishing gears, and sampled from 2013 to 2015. The regression coefficients of the different relationships are presented. Possible causes of variations in length-weight, including tissue weights (gonad, liver and the rest of the viscera), sex and region are assessed using analysis of covariance (ANCOVA) and linear regressions on log-transformed equations of length and weight. There were significant differences in the relationships FL-PL (F(5,1054) = 5553, P<0.001) and FL-LD1 (F(5,921) = 307.2, P<0.001) between regions but no significant differences were found between sex. Significant interactions were also found between log(FL) and region (F(4,1637) = 25.3, P<0.001), and log(FL) and sex (F(1,1512) = 7.62, P<0.01). For the relationship of somatic gutted weight with length, significant interactions were observed between FL and region (F(4,1509) = 71.43, P<0.001) but not with sex (F(1,1515) = 0.062, P>0.05). The study shows that fixed values of a and b for the entire region may be misleading. To minimise fluctuations in length-weight relationships, it is suggested to use somatic-gutted weight instead of whole fish weight and to use separate relationships for the northern part of the Western Indian Ocean (where albacore may be in a fattening stage at their feeding ground) and the southern part, particularly between 10 and 30°S where spawning occurs.
