Species traits determined different responses to “zero-growth” policy in China’s marine fisheries
Total catch control regulation does not lead to the recovery of fisheries and the maintenance of community functionTo contain the decline of wild capture fisheries by overfishing, a series of management regulations have been in place in China to mitigate the fishing impacts as much as possible and maintain sustainable stocks. The “zero-growth” policy is one of the most outstanding representatives. The results showed certain achievements after the implementation of the policy. Simulating the status without the “zero-growth” policy, B/Bmsy fell below 0.5 by 2010 and close to zero by 2019, indicating the impossibility for recovery. However, the policy is not enough for fishery recovery and community health, failing to stop the degradation of fishery resources. Under the implementation of the “zero-growth” policy, B/Bmsy was in a healthy state in 1998, fell below 1 for the first time in 2003, and dropped to 0.52 in 2019, accompanying by F/Fmsy as 1.60. If fishing pressure were maintained at the level of 2019 (F = 1.56 Fmsy), the resource would decline to the depletion state by 2030 (B/Bmsy close to zero, F/Fmsy = 3.64, catch = 35 T). Therefore, a great degree of negative production growth as well as the strict implementation is extremely important. A rapid reduction in the catch control under 0.5 Fmsy scenario would expect to achieve a quick recovery with B/Bmsy over 1 in 2025. Nevertheless, a significant reduction in production would lead to the decline of fishery economics, livelihood difficulties for fishermen and a series of derivative social problems28. An alternative of 1.0 Fmsy would be feasible, under which B/Bmsy could rise to 1 by 2030 with a production of 11.64 MT, close to MSY.The “zero growth” policy faces some inherent challenges, at least from the point of view of ensuring the sustainable use of individual species stocks. Attention should be also paid at the catch quota control of individual species. Because the variation of the intrinsic growth rate of different species, the B is dynamic, and the F changes with the change of B. In a constant production, r-strategic species could remain a higher B/Bmsy than 1 even at a large proportion in catch, but K-strategic species did not show the same fortune. The control of total catch volume rather than individual species could not prevent the community structure from becoming fragile, with the exhaustion of high-trophic species and the decrease of mean trophic level.Individual species have different responses to overfishing that highly associated with their biological characteristicsHigh trophic level species can be sensitive to overfishing, and difficult to rebuild stocks after collapseHairtails Trichiurus spp. are the largest contribution group to China marine capture fisheries, at 0.90 MT about 8.3% of the total production in 20202. They are carnivorous and aggressive with a mean trophic level of 4.4, mainly feeding on fishes in the adult stage, and Mysidacea and Euphausiacea in the juvenile stage29,30. The spawning seasons of Trichiurus spp. are mainly from April to June, and from September to November in Chinese waters31.China coastal areas are excellent foraging and spawning grounds for Trichiurus spp, sustaining a large stock size. If the “zero-growth” policy was not implemented since 1999, the resources of Trichiurus spp. would be exhausted by 2027, having no possibility to recovery at 1.0 Fmsy. Although the total fisheries production has been controlled, and the fishing moratorium period partly covered the spawning seasons of Trichiurus spp., their resource continuous declined into a “destroying” state in 2007, due to the time-lag effect of fishing on high trophic level predators characterized by long population doubling time-consuming32. Under intensive fishing pressure, Trichiurus spp. have showed astonishing fisheries-induced adaption33 by reducing the age and size of maturity, which effectively alleviates the decline rate of B value, resulting the maintenance of Trichiurus spp. capture production. Under the rebuilding scenario of fishing pressure as 1.0 Fmsy, Trichiurus spp. B/Bmsy rose to 0.87 by 2030, lower than the recovery rate of national total capture fisheries, suggesting the recovery rate of high trophic level species could be slow34. Furthermore, in this study fisheries rebuilding only considers the responses of species to fishing pressure, irrespective of a series of factors sensitive to high trophic level species such as pollution and climate change, which indicated a longer period is needed for resource recovery.Middle trophic level species seems non sensitive to total catch control policyAs a representative of middle trophic level species, L. polyactis performed different from Trichiurus spp. Under high fishing pressure. It forms spawning and over-wintering aggregations between nearshore and offshore waters, as well as vertical migration, rising at dusk and falling at dawn35. The spawning season is from mid-February to early May, prior to the national fishing moratorium, indicating young juveniles are in effective protection rather than spawning stock. In the 1950s, L. polyactis was one of the few important species in domestic marine capture fisheries in Chinese waters, producing more than 100,000 T annually5. The catch volumes then showed a downward trend and fell significantly to less than 50,000 T in the 1960–1980s. After 3 decades low catch volumes, the annual capture production rebounded significantly to more than 200,000 T and maintained at such high levels for 2 decades5, showing high resilience to overfishing.Despite many concerns on the risk of resource exhaustion of L. polyactis stocks5,36, official statistics showed that the annual catch remains high. The L. polyactis production broke through 150,000 tons in 1995, and was above 300,000 tons after 2005. There is likely to have a large offshore stock of L. polyactis, which gradually joined the catch under increasing fishing efforts offshore. Furthermore, the L. polyactis stocks can be resilience to high pressure for several reasons: (1) its miscellaneous diet makes them be able to receive sufficient food sources; (2) size and age at sexual maturity reduced37,38; and (3) the over consumption of top predators relieves the prey pressure on middle trophic level species, such as L. polyactis, snappers, and flatfishes. A good job is the difficulties of artificial propagation and seedling breeding of small yellow croaker were broken for the first time in 201539 and the whole artificial cultivation was successfully realized in 2020 (https://www.chinanews.com.cn/cj/2020/07-02/9227715.shtml), which would effectively alleviate the market demand and wild stock sustain of small yellow croaker.Pelagic small fish stocks may not recovery quickly as early cognitionSmall pelagic fishes enjoy assembling in large schools of tens of thousands of individuals, and are more vulnerable to predators. Species S. sagax mainly filter plankton with a low trophic level about 2.8. It spawns in May–June, with high fecundity (an absolute fecundity of 30,000–100,000 pelagic eggs) and fast growth, and has short generation time of 1.4 years40. S. sagax shows strong phototropy, and can be caught using light purse seine, gill net, and fixed net fishing at night41,42.In 1989, the biomass of S. sagax was about twice of Bmsy. With the decreasing capture production of traditional economic fishes, S. sagax became a target species using specific fishing methods43, resulted in catch increase accompanied with B/Bmsy decline into a state of extremely unhealthy in 2019. Recovery of small pelagic species stocks would be delayed by the total catch control policy, mainly because the removal of large numbers of predator species left more opportunities for their feeding objects44. Resource rebuilding of S. sagax was not as quick as expected, as small pelagic species had to endure increasing predation pressure from the recovery of high-trophic species under the total catch control. At 1.0 Fmsy scenario, B/Bmsy would be only 0.88 by 2030, in need of a longer period to healthy state.Well-planned restocking can enhance resource recoverySwimming crab P. trituberculatus has high reproductive capacity, with a female can release two to three batches of eggs during a breeding season, and a batch contains about 1–6 million eggs45. Under the complementary of existing management measures and restocking programmes, the production of P. trituberculatus was kept in a certain amount close to a healthy state, and there is not an urgent need for its stock rebuilding. Since the 1990s, restocking of hatchery-produced larvae of P. trituberculatus has been promoted in coastal waters of China. Large-scaled restocking programmes were documented: 33 million larvae were released into the Yellow Sea by Shandong Province in June 2013 (http://hyj.shandong.gov.cn/xwzx/sjdt/201311/t20131120_507389.html); 50.3 million larvae with carapace width over 6 mm were released in the northern Yellow Sea by Liaoning Province in June 2020 (http://nync.ln.gov.cn/fwzx/zxdt/202007/t20200707_3902016.html); 16.1 million larvae were released into the East China Sea by Daishan County of Zhejiang Province in June 2021 (http://www.daishan.gov.cn/art/2021/6/8/art_1383064_59012675.html). What should be of concern is when, where, and how many seedlings are released46,47,48, to maximumly utilize the environmental resources without encroaching on the benefits of other species.Short-living species can be resilience to overfishingThe main cephalopod species in Chinese fisheries are Sepiella maindroni, mainly distributes in the East China Sea35 and Sepia esculenta, mainly distributes in the Bohai Sea, the Yellow Sea and the East China Sea49. As a 1-year lifespan species with fast growth rate, S. maindroni forms spawning migration from deep water to shallow nearshore bays in spring, partly within the fishing moratorium period. Due to the positive phototaxis, the cuttlefishes can be captured by light seining. Sepiella esculenta was the most important cephalopod economically in the northern coastal seas and one of the four major fisheries in the Bohai Sea and the Yellow Sea until the 1970s50. The abundance of this species has been greatly reduced with continuous fishing pressures and dwindling spawning grounds51.Total catch control and fishing moratorium showed significant output on the short lifespan cuttlefishes. Without the implementation of the “zero-growth” policy, the cuttlefishes resources would have been exhausted by 2015 and impossible to rebuild. According to the current state of resources, by 2030 the cuttlefish stocks can be recovered under the 1.0 Fmsy scenario. Moreover, the extent of cuttlefishes stock recovery relies on food supply.Ways to sustain fisheriesThe conflict between rising demand for fishery products and declining resources under multiple pressures including overfishing, climate change, and marine pollution has put heavy pressures at a global scale52. Chinese government has undertaken serious reforms to effectively replan the fishery industry.The effective recovery and rational utilization of resources depend on the support by sufficient reliable data. China started fishery statistics right after the foundation of the People’s Republic of China, completed by MOA (1949–2017) and MARA (since 2018). However, the statistical dataset has been questioned internationally53. According to the explanation by FAO54, before 2000s, especially from 1979 to the late 1990s, as the central government raced to meet the increasing demand for seafood and to grow the domestic production, the local governments had frequently overreported their local catch. In addition, fishermen may falsely claim to increase their production for surplus compensation, after the government introduced fishing subsidies. On the contrary, the production might have been underreported since the early 2000s55,56, which could be attributed to the existence of a large number of “black ships” (fishing vessels without relevant legal permits). Moreover, the lack of professionals in the early period and inaccurate knowledge of species identification by fishermen also lead to data uncertainty. Reasonable fisheries data should be consistent with the species functional traits and life history characteristics. However, in the actual fishing activities, the intentional and high-intensity selective fishing of species may greatly deviate the catch data from the data predicted by models. The Chinese government has been trying to improve the statistical system, including data coefficient adjustment, training of fishermen and professional, and supervision of statistical authorities5. In this study, selected objects are inshore species: the species are familiar to fishermen; the fishing vessel supervision is in place; the data collection is relatively rational and complete; all these are conducive to the reliability of the results.The zero-growth policy, which has been implemented since 1999, is an important measure in the history of marine fishery development and management in China. That is, the total catch of marine fisheries in the current year cannot be higher than that of the previous year. However, the “12th Five-year Plan” for national fishery development (2011–2015) issued by the Ministry of Agriculture canceled the mandatory targets of controlling the production but to encourage more catches of marine fisheries (http://www.moa.gov.cn/gk/ghjh_1/201110/t20111017_2357716.htm). In 2013, the State Council published the first state-level marine fishery development document as “Several Advices on Promoting Marine Sustainable and Healthy Development”, incorporating marine fishery development into the strategy of building a maritime power (http://www.gov.cn/zwgk/2013-06/25/content_2433577.htm). This policy shift was clearly reflected in the significant increase in the national annual catch from 12 to 14 MT. Until the “13th Five-year Plan” for national fishery development (2016–2020) issued in 2016, the zero-even negative-growth policy was revalidated, and the volume of annual output control was clearly proposed as 8–10 MT57, which was determined by multiplying the fishing coefficient by the total stock size derived from the assessment of surveys on the zoning of fisheries and the supplementary survey of marine biological resources in the exclusive economic zone and the continental shelf7. To achieve the target of keeping fishing capacity at a high level of sustainability, significant reductions in fishing pressures over a period of time are required, as well as rational updates of control policies.Many policies were introduced together or around the same time as the “zero-growth” policy, such as summer fishing moratorium, fishing license system, and fishing fuel subsides. However, the achievements are far from satisfactory. The fishing fuel subsidy policy together with the license system induced the direct fishing vessel construction boom which resulted in fewer but bigger and more powerful fishing vessels. Fishing moratorium is the most promising policy, by leaving enough time and space for fish to successfully reproduce. However, the truth is that, right after the fishing closure season, almost all fishing vessels immediately rush into the sea and fishermen try their best to fish as much as possible within the gears and engine power permission of their fishing licenses, attempting to earn a year’s income in a short period of 2–4 months. As a result of such high fishing effort, the achievements of seasonal fishing bans were largely offset and resource densities fell to low levels after autumn. The number of legally binding standards for mesh size is not enough, only 6 at present of at least 40 fishing target species and over 10 fishing gears, leaving many fishing gears and fish species outside the regulation of existing standards6,58. Ideally, standards of mesh size should be updated corresponding to the changes of species traits, however, it is a challenge because the main fishing mode is multiple species fishery by bottom trawling. Moreover, species in China seas are diverse, and the spawning period of different species may not fall into the fishing closure season5. The lack of specificity to sufficiently cover all the species may result an unbalance of community composition. Another system “Double Control” aims to limit both the numbers of fishing vessels and the total power. Unfortunately, the inspections of fishing vessels and their power are not very strict, due to the need of developing local economy and guaranteeing the fishermen’s income, e.g., under a nominal power mask the low-power engines have been replaced by high-power engines, some fishing vessels do not have the fishing licenses28. The limitation of the license number and engine power also stimulate the technological improvement for more catch7.The structure adjustment of fisheries composition is the main management measure at present. The high degree of self-sufficiency in fishery products in China has been achieved through overfishing of domestic fishery resources, resulting in the rapid depletion of fisheries in China’s coastal waters59. Aquaculture, accounting for more than 70% of China’s total fisheries production2, is identified as a successful way. Accompanying by aquaculture development, a series of problems also arise, particularly, the demand of low-value/trash fish and fish meal that significantly drives further expansion of capture fisheries60. Cooperation with other countries to promote regional aquaculture may be an alternative way to meeting global growing demand for seafood and combating overfishing61,62. Seeking resources from the high seas and EEZs of other countries is also a choice, of course, on the premise of taking full account of ecology, maritime, and food security of other countries63,64,65.In addition, this study pointed out a new focus for fisheries management, in which differences in species biological traits, including species vulnerability, population multiplication, and resilience to environmental pressures, should be given full consideration. On this basis, more detailed and targeted management schemes are supposed to propose to achieve the dual purpose of recoverable fisheries resource and balanced species composition, so as to become a truly sustainable fishery. In short, the effective implementation of various management measures is an indispensable guarantee. More
