Development phases. We found that the history of China’s BTF could be split into four eras:
(i) First Era (1950—1963, E1), characterized by fishing through the food web while moving offshore; (ii) Second Era (1964—1978, E2), characterized by diversification and more powerful vessels due to technology development; (iii) Third Era (1979—1996, E3), characterized by explosive growth after the economic reform and moving into distant waters beyond C4S; and (iv) Fourth Era (1997—2018, E4), characterized by fewer but more powerful vessels and growing distant-water fisheries (hereafter, DWF) See text for the explanation of the development phases in Supplementary Information SI 2.
Total fishing capacity. We showed that the total capacity of Chinese BTF was very low (in relative terms, < 1500 trawlers) in E1 and E2, but increased dramatically from E3 after China embarked on a policy of economic reform (1978) (Fig. 1a). The tally peaked at very large numbers (~ 70,000 trawlers, 8 GW) around 1997 (Fig. 1a)—the highest known for any nation—and then declined to ~ 30,000 vessels in 2018 (6 GW, Fig. 1a). China’s BTF capacity in DWF beyond C4S grew quickly in just two decades, once it had started in 1985 (45 trawlers per year, r2 = 0.99; Fig. 1b). By 2018, their capacity accumulated to 1500 vessels and 1.2 GW (Fig. 1b), equal to 20% of the gross engine power of all Chinese trawlers.
Mean fishing capacity: Within C4S, the horsepower per vessel (HpV) of Chinese BTF has grown consistently (from 65 to 198 kW) except for a shock from 1978 to 1980 (Fig. 1c), when a large number of small private trawlers emerged after economic reform34,35. In contrast, the HpV of China’s BTF beyond C4S fluctuated dramatically and peaked in mid–1990s (1500 kW); notably a new rise has occurred since 2013, as China’s fisheries moved towards fewer, more powerful boats32 (Fig. 1c). We found Chinese trawlers were generally more powerful than its other marine fishing vessels. With C4S, the ratio between HpV of BTF and of other Chinese marine fisheries increased dramatically from 1980 to early 1990s, but then plummeted before stabilizing at around 5.0 (Fig. 1d). Such a ratio in fishing fleets beyond C4S showed a similar shape of trajectory with a quick rise in early 1990s but then declined to around 1.0 by 2010 and stabilized (Fig. 1d).
Total catch: Total landings by Chinese BTF grew fastest in E3, and then stabilized after 1996 before a large new growth after 2013 (Fig. 2a). The total landed value showed a similar trajectory, but generally increased more continuously (Supplementary Fig. S2.1). The percentage of Chinese marine catch derived from BTF increased consistently in the first three eras (Fig. 2b). The proportion peaked around 60%, and then leveled off until a new rise after 2013 to around 70% (from 2015 - 2018, Fig. 2b). The proportion of BTF landings from China’s claimed EEZ gradually declined from ~ 98 to 70% by 1975, and then bounced back to 90% by 1984, before a new drop to ~ 40% (i.e., 60% were from distant waters; Fig. 2b). This indicates Chinese BTFs initially moved offshore, partly driven by the no-trawl zone policy (1955), and then moved inshore after the fishery agreement with Japan (effective in 1975)32. Chinese trawlers have, however, increasingly operated in distant waters since China started to develop DWF beyond C4S in 198532. The catch share from C4S then gradually declined from 100% in 1985 to 60% in 1997, then bounced back again slightly before dropping to ~ 55% after 2013 (Fig. 2b).
Fishing efficiency (CPUE & VPUE): Within C4S, catch per unit effort (CPUE) of Chinese BTF initially doubled from 1950 to 1954, peaking at 6.8 t / (kW · year), and then halved by 1962 (Fig. 2c). After 1962 CPUE was boosted again to 5.3, largely by technology development35 (Fig. 2c). But this rise did not last long and the CPUE plunged in 1970s to less than 1 t / (kW · year) by 1984 (Fig. 2c). After China implemented summer moratorium within C4S in 1981, the CPUE declined slower and then gradually rose to nearly 1.5 t / (kW · year) by 2018 (Fig. 2c). Beyond C4S, CPUE plummeted even more strikingly from 55 to 5 t / (kW · year) in just one decade from 1985 (when it was 118 times the value within C4S) to 1995 (when it became only 3 times the value within C4S) (Fig. 2d). Landed value per unit effort (VPUE) demonstrated similar trajectories in both analyses (Figs. 2c&d).
Mean trophic level (MTL) in China’s claimed EEZ. Our results suggest that China’s BTF in its claimed EEZ showed two signs of problematic fishing behavior (Fig. 3a). The first is fishing down/through the food web, which means increased representation of lower trophic levels in the catch36,37. This behavior was evident in China’s BTF and in its all marine fisheries within China’s claimed EEZ, during E1, E2, and part of E3 (Fig. 3a). For instance, we found the MTL of China’s BTF landings that were directly consumed by humans (hereafter, MTLℎ) declined at a rate of - 0.05 per decade from 1950 to 1971 (r2 = 0.73) and more dramatically between 1972 and 1976 (-0.28 per decade, r2 = 0.98, Fig. 3a). After a short period of increase between 1976 and 1981 (after the economic reform), the MTLℎ continued to decline quickly from 1981 to 1988, reaching the lowest record in its history (MTLℎ = 2.82), with humans eating lower on the food chain than in 1950 (MTLℎ = 3.12). The second behavior is greater biomass trawling after summer moratoria (started in 1981, but only implemented nationwide until 1995)32. This resulted in higher proportions of catches of juveniles from high tropic-level species (recruitment during the moratoria), driving the MTLs of both BTF landings and all marine landings returned to increase (Fig. 3a). However, MTLℎ did not follow a similar trajectory (of the two MTLs) and even declined from 1990 to 2006 (–0.02 per decade, r2 = 0.41), given that these juveniles were not directly consumed by humans (Supplementary Figs S2.2 & S2.3). Only after 2006, when China extended the moratorium (by one month) and launched programs to combat illegal fishing32, MTLℎ started to rise slightly (0.06 per decade, r2 = 0.80) and reached 2.92 by 2014 (the same level as in 1990). However, there remained a prominent gap between MTLℎ and MTL of all BTF landings (Fig. 3a).
Log-relative-price index (LRPI) in China’s claimed EEZ. The LRPI represents the log- transformed slope of a linear relationship between price and trophic level of the fisheries stocks38. In a healthy fishery, species at higher trophic levels generally hold higher prices and thus generate a positive LRPI value, while a bell-shape trajectory is common when high-valued fisheries stocks are gradually being depleted38. In China’s claimed EEZ, we found that the LRPI of all species (or fish species only) of China’s BTF generally followed a bell-shape trajectory (Fig. 3b), suggesting a gradual decline on high trophic-level fish stocks, in line with the MTL (Fig. 3a). However, it should be noted that the negative values in the early eras likely arose from overestimates of prices for shrimps and fishes at lower trophic levels in previous studies 39,40 (Supplementary Fig. S2.4).
Fishing-in-balance index (FIBI) in China’s claimed EEZ. The FIBI is commonly used to reflect fisheries’ geographic expansion (FIBI increase) or contraction (FIBI decrease). It goes up if catches increase faster than would be predicted by trophic level declines, and it goes down if catches fail to compensate for a decrease in trophic level25. We found China’s BTF in its claimed EEZ gradually expanded in the E1 and E2 at a consistent rate (Fig. 3c), that expansion then sped up in E3 (after economic reform) and slowed down in E4 (after China ratified UNCLOS in 1996 and started to reduce total fishing capacity)32. This trajectory is generally in line with China’s all marine fisheries estimated by the Sea Around Us Project (SAUP)41, although the latter varied more prominently especially in E3 (Fig. 3c).
Catch composition in China’s claimed EEZ. We found prominent shifts in the dominance of different stock assemblages over the four eras in China’s claimed EEZ, again providing strong indications of (i) fishing through food webs, (ii) biomass trawling, and (iii) fishing based on availability (Fig. 4, see more details in Supplementary Figs S2.5—S2.10). For instance, in E1, large fish are the dominant stock assemblage, followed by shrimps and medium-sized fish in terms of percentages of landings. However, dominance of large and medium fish gradually declined (though not depleted, Supplementary Figs S2.5&2.6) as other species at lower trophic levels (e.g. jellyfish) increased their share (Fig. 4a), suggesting fishing through the food web. In E4, the dominance of shrimps and jellyfish vanished after 2002. Since then, the catch shares of most assemblages have tended to converge between 10 and 20% (Fig. 4a), meaning that Chinese BTF has become less selective. The catch share of crabs & lobsters (mainly crabs) increased (Fig. 4a), likely because they became relatively more available (Supplementary Fig. S2.9). In contrast, the dominance measured by landed value showed a totally different picture (Fig. 4b); this was dominated by the high prices of shrimps with surprisingly high relative values of jellyfish during parts of E3 and E4.
Global contribution. We found China has greatly increased its share in global BTF, especially between 1985 and 1996, when its contribution to global landings rose from 4% to 21% (Fig. 5a). Three other developing nations in Asia (i.e. Indonesia, Vietnam, and Thailand) also rose to sit among the top five exploiters between 1994 and 2014 (in terms of mean contribution to the landings). In contrast, the top five fishing powers between 1950 and 1970 (i.e. USA, Spain, Russia, Portugal, and Japan) have generally shrunk their shares, with only the USA still found within the top 5 contributors between 1994 and 2014 (Fig. 5a). Notably, the global landings of BTF peaked at 37.6 Mt in 1989 and then generally declined, although the catch remained relatively high (> 25 Mt; Fig. 5b). Such a transition roughly matches with the decline on CPUE (and VPUE) of China’s BTF beyond C4S (Fig. 2d).
Global footprint. Although China’s BTF operated only within C4S in E1 and E2 (Figs. 6a&b), their footprints have gradually expanded to distant waters beyond C4S since 1985 (in E3&E4, Figs. 6c&d). Over history, their footprints were found in a total of 32 nations (including China; Supplementary Table S2.1) and active in 30 countries in E4 (vs. only five in E1 & E2 and 27 in E3; Fig. 6). In terms of overall contributions across all years, most of China’s BTF catch in distant waters has come from East Asia (Japan, South Korea, and North Korea; total of 50%), followed by Northern Pacific (Russia & USA; 23%), Africa (across the Atlantic coast, 20%), Southeast Asia (e.g. Vietnam, Philippines, Indonesia; 6%), and Middle East & South Asia (e.g. Iran, India, Pakistan; 2%) (Fig. 6e). From 1985 to 2018, China’s BTF took a significant share (>22% of overall BTF landings) in each of 12 nations’ EEZs (Table S2.1, Fig. 7, see China’s share in other nations in Supplementary Figs S2.11&2.12). These 12 nations together contributed 82% of China’s distant-water landings by its BTF between 1950 and 2018. Interestingly, China gradually took over from other nations (e.g. Spain, South Korea, Russia) in half of these EEZs (Fig. 7a,e,h,I,j,k) and was also dominant in EEZs that BTF from other foreign nations hardly touched (e.g. Côte d’Ivoire; Fig. 7b,c,d,f,g,l).