Strategy optimization of stock enhancement of golden cuttlefish, (Sepia esculenta) based on structural characteristics of reproductive and recruitment populations

Golden cuttlefish, Sepia esculenta, was one of the largest economic cephalopods in Chinese northern seas, with an annual yield of more than 1 000 tons until the 1970 s. Since then, with the continuous expansion of fishing activities and dwindling of spawning field, this species has greatly reduced its local abundance compared with the previous condition due to its biological sensitivity to the environment (one-year life cycle and low fecundity). For this situation, many measures have been adopted to boost the population to meet the needs of human consumption, such as the release of artificially hatched juveniles and restoration of spawning habitats. The primary objective of this study is to optimize the existing conservation strategies and methods of cuttlefish by exploring the structural characteristics of the cuttlefish population in Qingdao coastal waters. The brood amount and size of egg cells of adult cuttlefish in 2017 were calculated. The distribution of body length and spatiotemporal migration process of juveniles were analyzed by using continuous monitoring methods. Results showed a decreasing trend in the specification of golden cuttlefish that arrived at different stages during the grouping period (May- July). The brood amount and size of egg cells of breeding cuttlefish in the early-stage were significantly higher than those of middle- and later-stages. The fertilized eggs of cuttlefish are characterized by sinking and stickiness, thus attaching to the attachment, such as algae or reefs. For juvenile cuttlefish, they grow rapidly, and the dispersion level of body specifications increases with sampling time due to the long grouping period and spawning in batches habits of breeding cuttlefish. This behavior might be a reproductive strategy for avoiding intraspecific feeding competition and promoting the differentiation of spatial niche. Therefore, according to the actual brood amount and the quality of eggs in the collection periods of breeding cuttlefish, it is suggested that the utilization rate of the early-stage population should be increased to improve breeding efficiency and guarantee the supply of large-size juveniles. The placement of spawning substrates in the spawning ground (slow-flowing area, 15-20 m) of adult cuttlefish from May to July is an important measure of resource restoration. Based on the reproductive habits of breeding cuttlefish, background survey in food abundance of release’s water for juvenile cuttlefish is indispensable. To reduce the intraspecific predation pressure and improve the survival rate of released population during large-scale juveniles’ release, juveniles’ release in different periods should be taken into consideration. In order to further optimize the mode of resource restoration and reduce the cost of the proliferation of cuttlefish, we also suggest exploring the technology of releasing fertilized eggs.