Artificial spawning grounds rescue hundreds of thousands of cuttlefish eggs in South Korea, study finds

The problem the team set out to solve is rooted in the biology of the golden cuttlefish itself. This coastal benthic cephalopod, distributed from the East China Sea and southern Japan to China, Hong Kong, Taiwan Province of China, Viet Nam and possibly Singapore and western Indonesia, inhabits sandy seabeds at depths of 10 to 150 metres in water temperatures of 15 to 25 degrees Celsius. When sea temperatures rise, the animals migrate from deeper winter waters into coastal areas to spawn, and from April to May they move into inner bays, attaching their eggs to available substrates such as seagrasses growing at depths of 2 to 10 metres. Cephalopods are choosy about where they spawn, and the availability of suitable substrate is an important factor in maximising egg production and hatching rates; a shortage of spawning substrate can lead to decreased or delayed spawning.

Golden cuttlefish are an economically important species and a source of high-quality protein, but their populations are reported to be rapidly declining due to overfishing and marine climate change. They face high fishing pressure during the spawning season and are a major target of trap fishing operations. Because embryonic development continues for 20 to 50 days, fertilized eggs are often deposited directly on traps. To restore the functionality of their gear, fishers remove the attached eggs with pressure washers, and most of the deposited eggs are lost in the process.

The intervention itself was simple in design. Artificial spawning grounds were established in the waters of Maando and Eunjeom in Mulgeon-ri, Namhae-gun, with installation beginning on 25 April 2024 and completed by 1 May 2024. Each site received 20 netting structures and 160 bamboo structures. The netting structures were built around a central stone measuring 490 by 520 by 500 millimetres and weighing 280 kilograms, with polyethylene netting of 20 by 10 millimetre mesh fixed around it in the form of a square-based pyramid. Each netting installation covered an area of 25 square metres, with base dimensions of 5 by 5 metres, and the structures were spaced 2 metres apart using guidelines and 1-ton chain blocks placed underwater. The bamboo structures used Phyllostachys bambusoides, cut to size with small branches intact, transported by boat and fixed into underwater support stones drilled on land with a hammer drill. These were placed 1 metre from the netting structures, in the centres of the gaps between them, at distances of less than 1 metre from one another.

The structures were monitored 12 times, every week from 4 May to 19 July 2024, using underwater photography. The structures were checked 12 times, every week from 4 May to 19 July 2024, using underwater photography with a Canon 5D Mark IV in manual mode at ISO 400. Within the reconstructed panoramas, the egg attachment area was defined as the total pixel area within the outlined attachment zone, and the attachment percentage was calculated with ImageJ as the ratio of attachment area to total pixel area. For egg density, four randomly placed 50 by 50 centimetre quadrats were imaged at each site and eggs were counted per unit area. On the bamboo structures, the presence or absence of eggs was simply checked at each visit, and when eggs were found they were counted.

The behavioural observations were as detailed as the engineering. Frequent mating was recorded among the golden cuttlefish, with males protecting females from mating through egg attachment and blocking the approach of other males while the females spawned under their guard. Analysis of underwater images of the attachment process showed that camouflaging each egg with sediment took 2 to 3 minutes, followed by an additional 4 minutes for attachment. Females repeatedly attached eggs one at a time, ultimately depositing 150 to 200 eggs at a single location. The authors note that males have previously been reported to exhibit such protective behaviours while females attach their eggs, and their own observations are consistent with that record.

The results revealed a clear substrate preference. Eggs attached to the netting structures included both golden cuttlefish and bigfin reef squid, whereas only bigfin reef squid eggs were observed on the bamboo structures. Golden cuttlefish eggs appeared on the Maando netting from the very first survey, and overall golden cuttlefish egg density was higher at Maando, which averaged 2,501 eggs per square metre, roughly twice the 1,258 eggs per square metre recorded at Eunjeom. In contrast, the mean attachment area was higher at Eunjeom, at 49.76 percent, than at Maando, at 22.54 percent. Attached eggs were first observed in Week 1 and increased gradually until after Week 4, when a partial decrease was seen at Maando due to hatching before numbers rose again. Maando showed consistently high attachment throughout, while Eunjeom exhibited a steep increase in attachment area after Week 7. Bigfin reef squid eggs were observed on the bamboo structures exclusively at Maando from the first survey, and appeared on the netting structures only from Week 8 onward in small quantities. The squid eggs appeared as protective ovisacs each containing 5 to 7 fertilized eggs; because exact counts underwater were difficult, a minimum of five eggs per ovisac was used for estimation.

To estimate juvenile recruitment, the team ran hatching experiments. One hundred initially attached golden cuttlefish eggs, collected on 2 May 2024, were transported to a laboratory and monitored at a constant water temperature of 20 degrees Celsius. Hatching was first observed on 28 May, a time to hatching of 27 days. Head formation appeared on 12 May, corresponding to 10 days of cell division, and by 20 May, after 18 days of development, the embryos had begun to resemble adult cuttlefish, with the yolk gradually absorbed until hatching. Golden cuttlefish eggs showed a 92 percent hatching rate and bigfin reef squid eggs 85 percent. Eggs were also collected from gillnets and traps used over a 7-day period, from 25 April to 2 May 2024, by a single fishery worker, stored in an underwater cage and tested under the same conditions; their hatching rates were 92 percent for golden cuttlefish, 89 percent for bigfin reef squid and 91 percent for Japanese spineless cuttlefish.

The arithmetic of loss and rescue is stark. A total of 257,168 golden cuttlefish eggs were recorded across the netting structures and fishing traps, of which an estimated 236,595 individuals hatched. For bigfin reef squid, 27,915 attached eggs across netting structures, bamboo structures and traps yielded an estimated 23,883 hatchlings. Japanese spineless cuttlefish were observed only on fishing traps, with 960 attached eggs and an estimated 874 hatched individuals, bringing the total estimated recruitment through the artificial spawning grounds to 261,351 juvenile cephalopods. The study also counted 8,180 cephalopod eggs, comprising 3,350 golden cuttlefish eggs, 960 Japanese spineless cuttlefish eggs and 3,870 bigfin reef squid eggs, collected from the gillnets and traps of a single fisher in just 7 days of fishing. Given the 20 to 50 day embryonic development period and the large number of fishermen at work, the authors argue that numerous eggs are likely discarded annually in South Korea, as has been documented in the Gulf of Morbihan, and suggest that collecting these eggs and relocating them to artificial spawning grounds could reduce the loss of cephalopod stocks.

The authors are careful about what their data can and cannot show. They suggest that the difference in egg density between the two sites may reflect changes in selectivity influenced by environmental factors such as water temperature, salinity, flow velocity and suspended matter, and by biological factors such as distribution density, but they state that further research with precise environmental data is required to clarify these patterns, and that the delayed surge in attachment at Eunjeom also needs investigation. They likewise note that the survival rates of recruited juveniles should be evaluated in future studies by linking survival to hatching rates, and that because different species showed distinct substrate preferences, artificial spawning grounds should test various materials, shapes and structural arrangements to quantitatively analyse selectivity among cephalopod species. What the study does establish, they conclude, is evidence of the potential to conserve and restore golden cuttlefish stocks through artificial spawning grounds and the collection of eggs discarded during fishing activities, providing baseline data for stock recovery and for the development of stock management models for demersal spawning cephalopods.

Subject of Research: Artificial spawning grounds for golden cuttlefish egg attachment and cephalopod resource recovery in coastal South Korea

Article Title: Resource recovery through the creation of an artificial Acanthosepion esculentum spawning ground in Namhae-gun, Gyeongsangnam-do, South Korea

Article References: Bai, J. I., Hwang, S. I., Kim, J. H., Lee, J. Y., Kim, D. U., Han, M. I., & Choi, Y. W. (2026). Resource recovery through the creation of an artificial Acanthosepion esculentum spawning ground in Namhae-gun, Gyeongsangnam-do, South Korea. BMC Marine Science, 1(1), Article 4. https://doi.org/10.1186/s44479-026-00003-2

Image Credits: AI Generated

DOI: 10.1186/s44479-026-00003-2

Keywords: golden cuttlefish, Acanthosepion esculentum, artificial spawning grounds, cephalopod conservation, egg attachment, hatching rate, bigfin reef squid, fishing gear, resource recovery, South Korea, marine biology, stock restoration