Indices of reduced fish biomass, changes in fish biodiversity and seafloor macrolitter – implications for fisheries management in Sri Lanka

The Norwegian research vessel “Dr. Fridtjof Nansen” operating under United Nations flag, conducted four surveys in the waters of Sri Lanka from 1978 – 2018. During the three first surveys carried out annually in 1978 - 1980, a total fish abundance of about 280 000 – 395 000 tons were measured by fisheries acoustics. From the first two surveys signals of potential increases in catches by a factor of two, and use of longlining for catch of deepwater rock fishes were given. In the 2018 survey with the larger and better equipped third “Dr. Fridtjof Nansen”, the total fish abundance recorded by fisheries acoustics was just 121–000 tons, and an additional 52 000 tons estimated by the bottom trawl swept area method. These numbers indicated a drastic reduction in the standing biomass of fish species off Sri Lanka during this forty-year period. Total catch per unit effort declined from the 1978–1980 surveys to the 2018 survey, especially for six common fish families. However, for ten other fish families the catch per unit of effort increased. For eight commercially important fish species average length had decreased by about 50% from the 1978–1980 surveys to the 2018 survey. For seven other fish species the average length seems generally to have increased during this forty-year period. Macro-litter as plastic waste and remains of fishing gears were common in the 2018 bottom trawls. Our results support further development of fisheries management plans with harvest control rules as tools to support the sustainability of Sri Lankan fisheries and environmental action plans to reduce macro litter accumulation on the seabed.