| Summary: | The study attempted to understand the difference in growth and mortality rate, between two phenotypic variants of the oceanic squid Sthenoteuthis oualaniensis. Specimens (1015 nos.) of the species collected off the south-west coast of India (lat. 07 to 11oN; long. 74 to 77o E) at a depth range of 180-2601 m were used in this study. Data on dorsal mantle length (DML) of dwarf and medium forms of the species were collected and analysed using the length-based FiSAT routine. L∞ varied from 17.37 to 18.05 cm with an average of 17.89 cm for the dwarf and 29.5 to 30.49 cm with an average of 30.08 cm for medium form. Dwarf forms had a higher average K value (3.65 cm y-1) with 􀂛 of 3.06, whereas, medium form had a lower average K value (2.38 cm y-1) with 􀂛 of 3.33. Growth curve and longevity study indicated longer than a year life span for medium forms (461 days) with growth rate of 13.3 cm DML in three months (91 days), 20.8 cm in six months (182 days) and 25.0 cm in nine months (273 days). Dwarf form attained maximum length in ten months (302 days) with a growth of 10.5 cm in three months (91 days), 14.9 cm in six months (182 days) and 16.7 cm DML in nine months (274 days). The average total instantaneous mortality (Z) was estimated as 4.58 y-1 for dwarf and 6.03 y-1 for medium form. As there was no commercial fishing for this species, fishing mortality was considered nil. Therefore, total mortality rate Z was considered equivalent to natural mortality.
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