Choosing a codend mesh when trash fish still have a market
A 14 m mechanised trawler operating off Ratnagiri makes four-day trips. Its codend has 40 mm diamond mesh. In a typical haul of 100 kg, 62 kg are undersized croaker and shrimp that sell only as trash fish at ₹35/kg; the remaining 38 kg sell at ₹180/kg. Fuel, ice and crew cost ₹28,000 per trip, and the vessel makes eight hauls per trip. A net maker offers a 70 mm codend. A trial with 70 mm is expected to retain 30 kg of marketable fish per haul and release 70 kg of small fish, with an estimated release mortality of 30%. The owner says, 'A smaller mesh always earns more because you land more kilos.' He wants a decision before the next trip and is worried about losing his crew's share of the catch.
1. Compare the gross catch value per haul and per trip under the two codends using the given figures. What additional factors must be considered before choosing?
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With the 40 mm codend, each haul gives 62 kg × ₹35 = ₹2,170 from trash fish, plus 38 kg × ₹180 = ₹6,840 from marketable fish. Gross value per haul = ₹9,010. Over eight hauls, gross value per trip = ₹72,080. After the ₹28,000 trip cost, the surplus is ₹44,080.
With the 70 mm codend, each haul gives 30 kg × ₹180 = ₹5,400. Over eight hauls, gross value per trip = ₹43,200. After the same ₹28,000 cost, the surplus is ₹15,200. The short-term difference is about ₹28,880 per trip in favour of the 40 mm codend.
The owner is therefore right about the immediate gross value in this numerical example, but wrong to say that a smaller mesh always earns more. The comparison assumes the trash market holds at ₹35/kg, that the 70 mm codend catches no additional large fish, and that the released fish are the only biological cost. Before deciding, consider: whether 40 mm is legally permitted; the 30% release mortality, which wastes about 21 kg of small fish per haul; whether the trash price will fall or the buyer will stop accepting juveniles; fuel and ice cost per kilogram of saleable fish; the effect on future stocks; and crew share, which is based on gross catch and may make crew resist a larger mesh.
2. The owner wants to keep the 40 mm codend and sell the small catch as trash. How would you assess the biological and economic risk, and what would you recommend?
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Biologically, the 40 mm codend is catching large numbers of undersized croaker and shrimp before they can grow or spawn. This is growth overfishing: the same fish would be worth much more if caught later, and the breeding stock is reduced. The 70 mm trial still has an estimated 30% release mortality, so even the larger mesh is not harmless; about 21 kg per haul of released fish may die. The 40 mm gear also removes small fish that other species feed on, which can affect the wider ecosystem.
Economically, the 40 mm codend looks better only because trash fish fetch ₹35/kg. That price is fragile. If the buyer reduces the price or a discard rule prevents landing juveniles, the 40 mm advantage shrinks or disappears. The vessel is also spending fuel and ice to land a low-value fraction, so the cost per kilogram of marketable fish is high.
I would not simply tell the owner to keep the 40 mm gear. I would recommend a controlled trial of an intermediate mesh, such as 55-60 mm, and possibly a square-mesh codend or separator panel, compared with the 40 mm gear over at least 10 paired hauls. Record total catch, size composition, marketable value, trash value, fuel and time. The decision should be based on value per trip and biological impact, not on kilos landed alone.
3. Design a simple onboard trial to test whether a 60 mm square-mesh codend can improve value without losing too much marketable catch.
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Use a paired-haul design on the same vessel. Fish alternate hauls with the existing 40 mm codend and the 60 mm square-mesh codend in the same area, depth and time of day, so that the two gears experience similar conditions. Aim for at least 10 pairs of hauls, preferably more, and do not change other gear or towing speed between the paired hauls.
For each haul, record total catch weight, weight of marketable fish, weight of undersized fish, species composition, and a length-frequency sample of the main croaker and shrimp species. Record the price obtained for each grade, the time taken to sort, and any damage to fish. For the released catch, estimate survival using a simple holding trial or use the 30% mortality figure as a sensitivity case, not as a fixed truth.
Analyse the results as value per haul and value per trip, not just weight. Calculate the proportion of undersized fish and the catch value lost or gained. Also check whether the 60 mm gear catches fewer small shrimp but more large fish, and whether crew sorting time falls. The trial should be repeated across seasons before a permanent change, because fish size and market prices vary.