New insights into the life history of Tenualosa ilisha and fishery implications

Tenualosa ilisha (known as hilsa) is the most widespread of the five species of tropical shads Tenualosa spp. It is found from north Sumatra in the east to Kuwait in the west. It is the basis of important fisheries in Bangladesh, India, Burma, Pakistan, and Kuwait and is probably the basis of the largest estuarine fishery in the world. In Bangladesh, T ilisha comprises 25% of total fish production (total = about 200,000 metric tons per year). To date, there has been no attempt to relate the marine, estuarine, and freshwater components of the populations and describe the full life history of the species. To address this, studies were conducted in Bangladesh by the Commonweath Scientific and Industrial Research Organization, Australia, and the Fisheries Research Institute of Bangladesh between 1996 and 2000. Results show that male and female T. ilisha reach sexual maturity at 200 mm when they are 1 year old and that both are able to spawn at this size. Unlike other Tenualosa species, there is no histological or macroscopic evidence for sex change in T. ilisha, but there is a bias in the sex ratio. The majority of individuals over 300 mm and almost all over 400 mm are females. Males predominate between 100 and 250 mm, but the sex ratio below 100 mm is more even. The biases in the sex ratio suggest that males may not live as long as females. Spawning occurs throughout the year from upstream at the Indian border to the coast at Chittagong and even in the sea off Cox's Bazar. Contrary to the previous belief that T. ilisha is strictly anadromous, the data show clearly that T. ilisha spawn in rivers, in estuaries, and on the coast. Otolith core microchemistry indicates that some fish (e.g., from Cox's Bazar) are born in middle to high salinities, which suggests that salinity per se may not be relevant to the location of spawning areas. The previously documented large-scale movements of T. ilisha show a marked correlation with water temperature. Upstream movements during the monsoon (July-September) take place when water temperatures are highest inland. Conversely, the movement toward the sea corresponds with a marked drop in upstream water temperature from about October, when sea temperatures are 3-4degreesC higher than riverine temperatures. Otolith Sr:Ca ratios and reproductive surveys indicate that the movement patterns of T ilisha are complex and variable. Individuals attain 20 cm standard length by the end of their first year and grow at similar rates to other tropical clupeids. The population is now dominated by 1-year-old fish, whereas in the 1960s it was composed mainly of 3-year-old fish. Hence, the probability of recruitment failure has greatly increased because 1-year-old fish now contribute most to egg production and have a relatively low fecundity. This, together with a decline in female fecundity; has implications for the future of the fishery which recorded large declines in catches in 2000 and 2001.