Square Cod-end Mesh to Reduce By-catch

Introduction

Demersal trawls are used throughout the world and account for almost a quarter of the global landed marine catch

Trawls retain large ratios of incidental (i.e. by catch) to targeted catches

That is because of their active fishing mechanisms, the frequent use of small diamond-shaped mesh

Trawling is a method of fishing that involves pulling a fishing net through the water behind one or more boats….

Trawl fisheries sector account more than 50% of the marine fisheries production of India.

The boats that are used for trawling are called trawlers or draggers

The net that is used for trawling is called a trawl

In Karnataka, bottom trawling was first introduced by the Japanese trawler M.S. Maru in 1961. (Kurup et al., 1987)

Trawling

An Overview

The use of the term “L V F” varies from country to country

One category of LVF are those not used for direct human consumption, which may be either landed and discarded at the sea itself.

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L V F

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TRASH FISH

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Discards are that portion of the total catch which is dumped or thrown overboard at sea.” (FAO).

What is a discard ?

•Discards are generally considered a waste of fish resources and inconsistent with responsible fisheries.

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The estimated landing of low value bycatch (LVB) in trawl fisheries, increased from 14 % in 2008 to 25 % in 2011

At Mangalore: An estimated 25,067 t of LVB valued at Rs. 25 crores was landed by multiday

trawlers (MDF) at Mangalore Fisheries Harbour.

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CMFRI, 2011

Landing of LVB

The high demand for trash fish has been the major reason for increased landing.

A. P. Dineshbabu (30 Jan 2014)

A global problem

The FAO has recently estimated that nearly 7 million tonnes of fish by-catch is discarded globally by commercial fishermen every year.

This is equivalent to about 8% of the global catch from marine capture fisheries.

Industrial shrimp trawling in the capture of by-catch and accounts for about 27% of all global discards.

Estimations of Low value/Trash fish production in Asia-Pacific (tonnes)

Country Low value/trash

fish

% of total catch

Dominant gear Year of estimation

Bangladesh 71 000 17% Gill nets (48%)Non-mechanised set bags (42%)

2001-2002

China 5 316 000 38% Trawl 2001

India 271 000 10-20% Trawl 2003

Viet Nam 933 183 36% Trawl 2001

, 12FAO 2005

Although the overall design of a trawl influences the type and quantity of catch that enters the gear,

Selection is largely determined by (i) the lateral mesh openings in the cod end, and

(ii) the probability that these are encountered

Towards achieving desired selection in otter trawls is to regulate a suitable mesh size in the codends.

Performance of square-mesh codends 

The performance of square-mesh codends was examined as a discard and by-catch reduction device for the Shark, scallop etc. trawl fishery.

Of the three different-sized square-mesh codends (50, 55, 60 mm) tested against the standard 100-mm diamond-mesh codends,

The 50-mm square-mesh codends performed poorly with relatively high retention of small (<85 mm shell height) scallops,

Whereas the 55- and 60-mm square-mesh codends retained 22–33% less smaller scallops than did the diamond-mesh codends.

Overall, a mean of 5% loss in commercial-sized scallops across all three square-mesh codends and significant by-catch reductions of up to 95% were achieved compared with the diamond mesh.

Catch rate of prawns by the square-mesh codend was less than 2% of that of the standard prawn net.

The performance of the 60-mm square-mesh codend presents a good basis for its use in commercial trials in the Shark Bay scallop trawl fishery.

A move to square-mesh codends could result in a significant reduction of small scallops and by-catch and may potentially increase the catches of commercial-sized scallops as a result in improvements in water flow and net efficiency.

Effects of Square and Diamond Mesh

Asia-Pacific Fishery Commission

The demand for targeted resources has paved way for indiscriminate bottom trawling along the coast with an ultimate result of massive wastage of low value, high volume by-catch including a wide spectrum of non-edible benthic biota.

The small cod end mesh of bottom trawlers has also exploited juveniles and sub-adults of commercial species in large quantities.

Global response to reducebycatch

In response to concerns over the capture of turtles and other bycatch in shrimp trawls, fishermen in many countries have taken steps to modify the trawl net and use bycatch reduction devices.

Can square-mesh codendshelp reduce bycatch?

The mesh openings of a traditional diamond-mesh codend collapse as it fills with catch and prevents the escape of small fish. The mesh openings of a square-mesh codend retain their shape as it fills with catch and are available for fish to escape.

What is a square-meshwindow and how might itreduce bycatch?

A square-mesh window is usually a panel of square mesh netting located in the top panel of the codend or trawl body.

As fish pass through the trawl they orientate directionally toward the device and swim through the square escape openings.

Dan Watson devised a system based on a series of escape rings for fish – which can be fitted to a fisherman's trawler net

won a prestigious international award for creating a "humane" net to make fishing more sustainable by preventing small fish

Designed by: Dan Watson Thursday 8 Nov 2 01 2

Source: www.guardian.co.uk/environment/2012/nov/08/humane-fishing-net-dyson-award23

Another way to reduce bycatch

Fishing net with emergency exits

The net is designed to make commercial fishing more sustainable by reducing the number of non-target and juvenile fish picked up by trawlers,

Watson explained to the BBC that about 20 rings are needed per net, costing a total of around $800 .Rs. ~ 48000

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Close Season

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Fishing Holiday

Karnataka 1986

Marine Fisheries Regulation Act

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