For every 1kg of usable fish, 4kg of other marine life are killed: StudyUpdated: Feb 17, 2020, 00:10 IST
The rise in fish consumption has led to indiscriminate trawling of juveniles of commercially important fish and a significant increase in the removal of non-target fish, says a study, adding that the practice could have long-term consequences such as depletion of targeted fish species and, in turn, marine biodiversity.
Researchers, led by ICAR-Central Marine Fisheries Research Institute (CMFRI), Mumbai research centre, analysed 3,561kg of subsamples from 236 hauls operating at a depth of 10 to 60mts found that for every kg of targeted catch, shrimp trawlers operating in the commercial trawl fishing grounds off New Ferry Wharf (NWF) landing centre ended up with 4kg of non-targeted by-catch comprising juveniles, non-commercial fish and benthic debris, which is either dumped into the sea or discarded at the fish landing site.
“The abundance of juveniles throughout the (fishing) seasons shows the behaviour of year-round spawning and recruitment patterns of tropical fish. Continuous removal of juveniles of commercially important species will result in reduction in their mean size that can lead to growth in overfishing,” said Sugumar Ramkumar, principal investigator, CMFRI.
Ramkumar said, “The reasons for discarding by-catch in multiday trawlers are non-availability of space for storage, fear of spoiling of high-valued or large fish and non-edibility of the catch.”
Of the average catch rate of 178.64 kg per haul (kg/h) every year, the target catch was only 39.6 kg/h (21.4%), while the non-target catch comprising 145 species stood at 139.04kg/h (78.59%) of the total trawl catch. Catch rate is the number of fish caught per fishing trip. A 1983 study had reported 68.7% by-catch of the total catch in Mumbai waters.
Although the five-member team’s assessment of non-target by-catch from multiday shrimp trawlers (with a duration of 7-15 days) spanned between January 2013 and December 2014, based on collected samples since then researchers have estimated an average 10% increase in by-catch till 2019.
With NFW landing centre alone accounting for 33% of the total trawl landings in Maharashtra, researchers said the study is important because the trawl net – considered one of the most destructive type of fishing – is dragged across the ocean floor which, in addition to catching targeted fish or shrimps, also traps a wide range of marine life that can have a negative impact on the entire ecosystem.
Of the 35,228 trawlers that go in Indian waters, 71.2% operate in the west coast, while the remaining along the east coast. In 2018, trawl fishing contributed to 57% of the total marine fish production in Maharashtra.
In the present study, the average discarded by-catch every year was 33.25% of the total catch, and 42.19% of the total by-catch. Juveniles of commercially important species comprised 52.44% of the by-catch with an average catch rate of 73.3 kg/h/year for juveniles – highest in December (95.43kg/h) and lowest in March (40kg/h). Reason being, trawlers of the west coast that mostly sweep shallow waters with 10-50mts depth is a fertile ground for food that supports juvenile population.
“Continuous removal of predators, irrespective of seasons, with a wide diet spectrum feeding on squilla, crabs, gastropods and 20 families of finfishes throughout the year pave way for aggregation or increase in the population of non-commercial/non edible species in trawl grounds,” stated the study.
To address the issue of handling non-target discards, Mangalore and Calicut have shown a decrease in discards at sea between 2008 and 2011 owing to the high demand for raw material demand from fish meal and fish oil plants in Mangalore, Karnataka driven by aquaculture and poultry feed factories.
“In our study also, 43.7% of discard consists of crabs, gastropods, bivalves and stomatopods that could similarly be utilised as raw material for low-cost fish meal preparation in order to decrease the discard trends in the sea of Mumbai trawlers,” said Ramkumar. “The quantum of juvenile catch should also be limited within a regulatory framework.”
The study has also recommended restrictions on fleet size and fishing days per trip, strict enforcement of use of mesh size by trawlers, and closure of entry permits of boats in to sea once their catch limit on by-catch is reached. “A suitable market chain for permissible by catch landings needs to be established. Awareness among fishers on destructive nature of trawl fishing, sustainable harvest of fishery resources and participation of fishermen in the decision-making process can sustain the stocks of Maharashtra fishing grounds forever,” said Vinay Deskmukh, co-author and former principal scientist, CMFRI.