Supplementary Information Table of Contents Supplementary Information

Supplementary Information Table of Contents Supplementary Information ...... 1 SI 1. Supplementary Information for Methods ...... 3 Study Area: Discriminating four water bodies ...... 3 Fishery indices: Explanation and data collection ...... 3 Collecting data for species trophic levels and calculating mean trophic level (MTL) ...... 6 Figure S1.1 China’s four seas (Bohai Sea, Yellow Sea, East China Sea, and South China Sea) and its claimed Exclusive Economic Zone...... 8 Table S1.1 Summary of the 95 taxa (e.g., species/genus/family) in the catch of China’s bottom-trawl fisheries in its claimed exclusive economic zone. Each taxon was reconstructed by the Sea Around Us Project (SAUP). We show trophic level (TL) and maximum length (ML), with the latter based on FishBase...... 9 References cited ...... 14 SI 2. Supplementary Information for Results ...... 19 Defining four development eras for China’s BTF from 1950 to 2018 ...... 19 Table S2.1. China’s landings (tonnage) from its own claimed EEZ and other EEZs over the four eras from 1950 to 2014. The EEZs in bold texts are the top 18 EEZs where China’s BTs shared a significant proportion (> 10%) of all BTF landings from 1985 to 2018...... 21 Figure S2.1 Timeseries of a) total landed value from three areas (all areas, China’s four seas, and China’s claimed EEZ), and b) the percentage of landed value...... 22 Figure S2.2 Ratios between (i) annual catch and the highest annual catch (red line) and (ii) catch used directly for human consumption and all catch (blue line) of eight categories of stock assemblages: a) large fish, b) medium fish, c) small-to-medium fish, d) cephalopods, e) shrimps, f) crabs & lobsters, g) jellyfish, and h) small fish & other invertebrates. Data source: Sea Around Us (www.seaaroundus.org)...... 23 Figure S2.3 Ratios between (i) annual catch and the maximum catch (i.e., highest annual catch; red line) and (ii) catch used directly for human consumption and all catch (blue line) of 12 major fishery stocks (with the represented major assemblage shown in Figure S2.5). Data source: Sea Around Us (www.seaaroundus.org)...... 24 Figure S2.4 Timeseries of mean price of eight stock assemblage groups. Dotted lines and the grey texts next to them refer to relevant policies or events. Red texts (e.g., Shrimp farming) are annotations. The cyan bands indicate the different development eras. The results should be interpreted with caution as the estimate prices for Chinese fishery stocks by Sea Around Us were based on records from other Asian countries...... 25 Figure S2.5 Catches of five major large-fish stocks and the entire large fish assemblage...... 26 Figure S2.6 Catches of five major medium-fish stocks and the entire medium fish assemblage...... 27

Figure S2.7 Catches of two major shrimp stocks and the entire shrimp assemblage...... 28 Figure S2.8 Catches of two major cephalopod stocks and the entire cephalopod assemblage...... 29 Figure S2.9 Catches of two major crab stocks and the entire crab & lobsters assemblage...... 30 Figure S2.10 Catches of five major stocks of the small fish & other invertebrates, and all small fish & other invertebrates...... 31 Figure S2.11 Stacked landings (Mt) by bottom trawlers (BTs) in each of the 13th to 24th EEZ where China’s BTs shared 21% to 5% of the landings from 1985 to 2018...... 32 Figure S2.12 Stacked landings (Mt) by bottom trawlers (BTs) in each of the 25th to 31st EEZ where China’s BTs shared 5% to 0.1% of the landings from 1985 to 2018...... 33 References cited ...... 34

SI 1. Supplementary Information for Methods

Study Area: Discriminating four water bodies

China’s claimed EEZ vs. distant waters Here, China’s claimed EEZ mainly includes five areas: (i) EEZ under the administration of China, (ii) EEZ under the administration of Taiwan (province of China), (iii) EEZ (i.e., Diaoyu Islands) disputed among PRC and Japan, (iv) EEZ surrounded the Paracel Islands (i.e. Xisha Islands), and (v) EEZ surrounded the Spratly Islands (i.e. Nansha Islands) (Fig. S1.1). These EEZ regions for China are used by the Sea Around Us (SAUP) as a spatial unit to reconstruct yield statistics (e.g., catch and landed value) 1, and our yield data were reconstructed based on the SAUP database. Correspondingly, we use the term “distant waters” in referring to waters beyond China’s claimed EEZ as described above. We use United Nations terminology in referring to Taiwan (province of China).

China’s four seas (C4S) vs. distant waters beyond C4S The term “China’s four seas” (C4S) alludes to four seas (Bohai Sea, Yellow Sea, East China Sea, and South China Sea) that surround China’s continental land mass, embracing China’s claimed EEZ and reaching beyond it (Fig. S1.1). The total area of the four seas amounts to 4.8 million km2; and 32% of the area is the continental shelf within the 200-m isobath, where most fishery resources are concentrated 2,3. We used a collection of EEZ subregions (defined by SAUP) within C4S to represent China’s seas when reconstructing catch by China’s BTF from C4S 4, given that the boundaries of these water are vague. Notably, China considers fisheries in the four seas as ‘domestic fisheries’ – even though some are outside its claimed EEZ – and those beyond as ‘distant-water fisheries’, as used in the yearbooks (i.e., CFSYs) 5,6. Before China’s establishment of its EEZ (in 1996), there were no clear national boundaries between China and other neighboring countries on the four seas. Even after the establishment of the EEZ regime, China disputed EEZs with Japan, South Korea, and some nations in the South (e.g., Vietnam). After 1996, China’s fishery scientists and governments continued to consider fisheries within the four seas as ‘domestic fisheries’ (Dr. Z. CHEN & Dr. X. SHAN, Chinese Academy of Fisheries Science; Nov. 2019, personal communications). We therefore interpreted the capacity data for ‘domestic fisheries’ and ‘distant-water’ fisheries from China as statistics for C4S and waters beyond C4S, respectively. Fishery indices: Explanation and data collection

Total & mean capacity We used two measures to represent total fishing capacity, deriving mean capacity from integrating them: (i) total number of vessels and (ii) total horsepower. As such data were not available for BTF in China prior to 2003, we collected data from local sources to reconstruct these timeseries for China’s BTF 4.

Total yield & fishing efficiency

We reconstructed (i) total catch (by tonnage) and (ii) total landed value of the catch (by 2010 real USD) based on the annual catch and landed values estimated by SAUP, which included data specifically for BTs (available from 1950 – 2014) 4,7. We then used annual yield and fishing effort measures to create two estimates of fishing efficiency: (iii) catch per unit effort (CPUE) 8,9 and (iv) landed value per unit effort (VPUE). We decided that the effort could be measured with the gross engine horsepower of vessels deployed in each year in China’s BTF. Fishing effort is usually measured by total fishing capacity multiplied by total fishing time, and both are difficult to measure precisely. Instead, researchers commonly used gross engine horsepower multiplied by total days spent on fishing in a year as a surrogate 10. However, using total days to measure fishing time is also misleading; fishers may change their fishing time in a day in response to fish availability or weather conditions. Here, we used the catch and landed value per unit horsepower per year to represent CUPE and VPUE respectively because we lacked the data to estimate fishing time precisely in China. We assumed that annual fishing time might be not hugely impacted by China’s summer moratoria (started in 1980) for at least four reasons. First, although we knew China enacted summer moratorium policy for BTF in 1980, such a policy was not well implemented by local governments especially before 1995 11,12. Second, China’s moratoria generally synchronize with monsoon seasons when fishing time was naturally much lower even before the moratorium policy was in place. Third, studies have suggested that fishing effort actually intensifies significantly after these moratoria; boats fish much longer in a day or trip, to a level that might actually counteract the reduced fishing time during the moratorium 13,14. Fourth, over its history, China’s summer-moratorium regime was not consistent (in terms of dates or duration) across the four seas or trawl types, even in the same year; Chinese trawlers in one area could allocate fishing efforts to another area, where moratoria were temporarily absent, without much compromise in fishing time 12,15.

Fishery health indices We used four metrics to evaluate fishery health: (i) mean trophic level (MTL) 16, (ii) mean trophic level of the catch directly consumed by humans (MTLh), (iii) fishing-in-balance index (FIBI) 17, and (iv) log-relative-price index (LRPI) 18. The MTL is often used to reflect the trophic level of marine food web, and a decline in MTL usually signifies fishery depletion (a.k.a., fishing down/through the food web) 16,19. However, MTL can be influenced by various mechanisms (e.g., changes in fishing technology and targeting patterns), and a rise on MTL might simply result from increase in overfishing on juveniles of high-TL species 20–23. Therefore, we created a new index, i.e., MTLh, which measures the mean trophic level of the catch that were directly consumed by humans. We compared these two trophic indices to monitor sustainability of China’s BTF. The trophic level of each species or taxon was primarily derived from Chinese peer-reviewed literature. We then drew missing information from FishBase (www.fishbase.org) and SeaLifeBase (www.sealifebase.org: see next section). The human-consumption ratio was derived from estimates of end-use types of global marine capture fisheries by the SAUP 24. The FIBI is usually used to reflect fishing expansion (FIBI increase) or contraction (FIBI decrease) and is deducted by comparing catch after a change in MTL with the potential yield because of the change 17. Here we assumed that energy transfer efficiency between two trophic levels was 10% 17,25. The LRPI represents the log-transformed slope of a linear relationship between price and trophic level. In a healthy fishery, species at

4 higher trophic levels generally obtain higher prices, thus generating a positive LRPI value. In contrast, a bell-shape trajectory is common as a fishery develops18.

Catch composition We were curious about the contribution (%) of each assemblage (e.g., large fish) to the total catch or the landed value (i.e. stock-assemblage dominance). We used these two dominance indices to monitor changes in catch composition and to help us explain the change on MTL in China’s claimed EEZ 26. Here, we sorted the BTF catch in China’s claimed EEZ into seven assemblages based on the functional-group categories in the SAUP database 7. However, we did correct the categories of five taxa in the SAUP database due to the maximum lengths of the taxa did not match with the categories assigned by SAUP (Table S1.1). For instance, we found that sand eels (Genus Ammodytes) were categorized as ‘Large reef assoc. fish (ML >=90 cm)’ in SAUP database. In China, this genus was primarily represented by A. personatus, a small-sized fish (ML < 30 cm) in the Yellow Sea 11,27. We therefore considered this genus as ‘small fish’.

Collecting data for species trophic levels and calculating mean trophic level (MTL) We used a variety of data sources to derive the trophic level for each species (or other non- specific taxa) that was listed under ‘scientific_name’ in the catch data from SAUP (Table S1.1). This amounted to a total of 95 different taxa (including 49 species and 46 higher level taxa, Table S1.1). We searched Chinese peer-reviewed publications and thesis/dissertations (hereafter, Chinese academic literature) to find trophic levels for these species (or taxa) in the China Knowledge Resource Integrated Database (a.k.a., CNKI, http://cnki.net/, 1950 – 2019), using the following settings: Full text contains the scientific name (i.e., Latin name) or common name (of the species in Chinese) and ‘营养级’ (i.e., trophic level). Different synonyms and common names were used each time in order to cover the whole possibility of published data. Trophic level records from Chinese studies were primarily based on the stable (nitrogen or carbon) isotope technique, with a smaller fraction based on food items or both 28.

We followed five rules to estimate the trophic level for each taxon.

1. We preferred locally sourced estimates (i.e., Chinas academic literature) to those from FishBase or SeaLifeBase, as better representing the taxa in Chinese waters. We only used data from FishBase or SeaLifeBase for taxa that lacked studies from China. 2. If a species lacked any records on trophic level, we used information of its closest relatives as the surrogate. 3. When we found multiple studies for a species, we averaged the trophic levels first within the same study (e.g., multiple estimates over different time periods or using different methods), and then averaged the estimates among different studies. By doing so, we avoided weighting studies with more than one estimate. 4. For a genus (or a higher taxon), we first explored which species within this taxon were found in China’s marine waters based on (i) FishBase (selected by country = China) 29, (ii) SeaLifeBase (selected by country = China) 30, (iii) a Chinese fisheries publication summarized fishery stocks from the four seas 11, (iv) Marine Fishes in China (a taxonomic book) 31, and (v) Chinese academic literature from CNKI. We then identified the dominant species (or most common species) found in China’s BTF based on the collected data. We used the mean trophic level of these species (N = 1 to 10 based on data availability) to represent the trophic level of the focal taxon 11,25. 5. For ‘Marine fishes not identified’ in SAUP dataset, we used the mean of the trophic levels of all fish taxa in the dataset as the proxy. For ‘Miscellaneous aquatic invertebrates’ and ‘Miscellaneous marine crustaceans’, we employed estimates from a study based on the Ecopath model for the estuary of the Pearl River (in the South China Sea) 32. We applied the function by Pauly et al. (1998) to calculate the MTL of all catches of China’s 16 BTF and the adjusted MTLh for human consumption in each year . We also downloaded the MTL of all landings from China’s EEZs from SAUP website (http://www.seaaroundus.org/data/#/marine-trophic-index). We were interested in comparing these three indices over the history. To examine structure changes on the trajectory of MTLs, we detected break points in segmented linear regressions between MTL and year based on the ‘Bellman principle of optimality’ (function ‘breakpoints’ in r package ‘strucchange’)34. This optimization algorithm detected the minimum number of breakpoints (or segments) based on the lowest Bayesian information criterion (BIC) score. We chose three as the minimum

6 number of data points for each linear regression segment and allowed variable regression slopes for different segments.

We also considered two issues in estimating MTL. 1. The MTL of a large area could be distorted by gradual fishing expansion from inshore to offshore waters, hiding the decline on MTL in initial waters 33. To address this problem, researchers have proposed using the regional MTL to detect possible fishing expansion nodes 33. Given that we found no such expansion node, in line with other findings in this area 33, we were able to just use the original MTL calculation. 2. It has been argued that trophic level of any given species could vary with change in body size (e.g. juvenile vs. adult) which may itself arise from fishing pressure 21. To account for this effect, studies have proposed an adjusted formula to calculate MTL based on a set of parameters such as asymptotic length, length at first capture, fishing mortality, and water temperature 21. However, we could not make this adjustment for the many data-poor species in our study. In any case, a similar study on China’s fisheries from 1979 to 2014 in the East China Sea indicates that although the adjusted MTL for body size was slightly lower than the non-adjusted MTL, their trends were very similar 25.

Figure S1.1 China’s four seas (Bohai Sea, Yellow Sea, East China Sea, and South China Sea) and its claimed Exclusive Economic Zone.

Table S1.1 Summary of the 95 taxa (e.g., species/genus/family) in the catch of China’s bottom-trawl fisheries in its claimed exclusive economic zone. Each taxon was reconstructed by the Sea Around Us Project (SAUP). We show trophic level (TL) and maximum length (ML), with the latter based on FishBase.

Taxon in SAUP Common name Chinese name Functional group TL ML Reference for estimating TL

35,36 Acanthopagrus schlegelii Blackhead seabream 牛尾鱼/黑鲷 Medium fish 4.54 50 Averaged from two studies (4.756, 4.33) 36 Acetes japonicus Akiami paste shrimp 日本毛虾 Shrimps 2.93 2.4 Proxy: Acetes chinensis 29 Ammodytes Sand eels 玉筋鱼 Small fish (Large fish, SAUP)* 3.10 15 Species: Ammodytes personatus Apostichopus japonicus Japanese sea 刺参/仿刺参 Other demersal invertebrates 2.57 30 Averaged from three studies (2.490, 2.438, cucumber 2.408; 2.25; 3.25) 37–39

40 Arca Arks, turkey wings 魁蛤属 Other demersal invertebrates 2.00 9 Proxy: Scapharca subcrenata 41,42 Arctoscopus japonicus Japanese sandfish 发光鲷 Medium fish 2.87 30 Averaged from two studies (2.4; 3.33) (Small fish, SAUP)* Ariidae Sea catfishes, coblers 海鲶科 medium fish 3.69 - Averaged from two major species（3.49, Netuma thalassina; 3.89, Arius sinensis) 29 36,40,43 Atrobucca nibe Blackmouth croaker 黑姑鱼 medium fish 3.56 45 Proxy: Nihea albiflora (3.72, 3.24, 3.715) Berycidae Alfonsinos, redfishes 金眼鲷科 medium fish 3.81 51 Averaged from two major species (3.81, Beryx splendens； 3.81, Centroberyx affinis) 29 Bivalvia Clams 双壳纲 Other demersal invertebrates 2.00 9 Proxy: Scapharca subcrenata 毛蚶 40 41,42 Brachyura Marine crabs 短尾下目 Crabs & lobsters 2.60 9.5 Averaged from two studies (2.58; 2.6; 2.651)

Cephalopoda Squids, cuttlefishes, 头足类 Cephalopods 2.97 - Averaged from seven cephalopod species: octopuses Uroteuthis edulis (2.62) 41, U. duvaucelii (2.85) 41, Sepia aculeata (2.88) 41, Amphioctopus fangsiao (2.04) 41；Sthenoeuthis oualaniensis (3.3, 2.9) 44,45; Loligo beka (3.7) 46；A. fangsiao (3.2) 46；Octopus variabilis (3.0) 46 47 Chanos chanos Milkfish 虱目鱼，遮目鱼 Large fish 2.40 180 Charybdis Whirlpool swimming 虫寻 Crabs & lobsters 2.67 - Averaged from two species (2.73, Charybdis crabs japonica; 2.6, Charybdis cruciate) 41 Chelidonichthys kumu Bluefin gurnard 绿鳍鱼 medium fish 3.31 60 Averaged from three studies (2.95; 4.3；2.69) 48– 50

30 Chionoecetes Tanner, snow crabs 雪蟹 Crabs & lobsters 3.54 Chionoecetes opilio 47 Chirocentrus dorab Dorab wolfherring 宝刀鱼 Large fish 3.40 100 Conger myriaster Whitespotted conger 星康吉鳗 medium fish 3.62 100 Averaged from three studies (3.5; 3.5; 3.857) 36,43,46

51 Decapoda Crabs, lobsters, 十足目 Crabs & lobsters 2.50 shrimps 37,42 Echinoidea Sea urchins, sea 海胆类 Other demersal invertebrates 2.40 Averaged from two studies (2.4, 2. 047) hedgehogs Epinephelus Seabasses, hinds 石斑鱼属 medium fish 3.90 Averaged from two species: Epinephelus fasciatus (3.77)，E. akaara (3.81) 52 Fenneropenaeus Chinese white shrimp, 中国对虾 Shrimps 2.55 15.4 Averaged from three studies（2.32, 2.33, 3.0） chinensis fleshy prawn 46,53,54

Gadiformes Cods 鳕形目 medium fish 3.75 Averaged from three major species (3.3, Bregmaceros macclellandii; 3.5, Lotella phycis; 4.2, Physiculus japonicus) 11,55 49 Gadus macrocephalus Pacific cod 大头鳕，太平洋 Large fish 4.38 119 Averaged from two estimates (4.39, 4.37) 鳕鱼

42 Gastropoda Sea snails 腹足纲 Other demersal invertebrates 2.20

Gobiidae Gobies 虾虎鱼 Small fish 3.47 Averaged from three studies (3.1, Gobiidae; 3.62,Ｒhinogobius pflaumi; 3.69, Amblychaeturichthys hexanema）43,56,57

Haliotidae Abalones, ear shells 鲍鱼 Other demersal invertebrates 2.00 Species: Haliotis discus hannai, from two studies 37,58 (2.0, 2) Holothuroidea Sea cucumbers 海参 Other demersal invertebrates 2.57 Averaged from three studies (2.490, 2.438, 2.408, Apostichopus japonicus；2.25, Apostichopus japonicu; 3.25, Apostichopus japonicu) 37–39 Larimichthys crocea Great yellow croaker 大黄鱼 medium fish 3.16 80 Averaged from two estimates (3.0，3.31) 59 Larimichthys polyactis Yellow croaker 小黄鱼 medium fish 3.65 40 Averaged from two studies (3.65, 3.75, 3.78) 43,49 Lates calcarifer Barramundi 尖吻鲈，花鲈 Large fish 3.97 200 Averaged from six studies (3.153, 4.41, 3.722, 3.52, 4.57, 4.45) 32,39,48,49,60 40,60 Liza haematocheila So-iny mullet 鮻鱼 medium fish 2.95 80 Averaged from two studies (2.89, 3) 53 Loliginidae Japanese squid 日本枪乌贼 Cephalopods 2.72 12 29 Lutjanidae Bigeye snapper 黄笛鲷 Medium fish 4.10 35 Species: Lutjanus lutjanus

Marine fishes not Marine fishes nei Medium fish 2.95 29 identified 36 Marsupenaeus japonicus Kuruma prawn 日本囊对虾 Shrimps 3.19 19 29 Menidae Moonfishes 眼镜鱼 Medium fish 3.50 30 Species: Mene maculate 43 Meretrix lusoria Japanese hard clam 文蛤 Other demersal invertebrates 2.70 5

Metapenaeus Indo-Pacific prawns 新对虾 Shrimps 2.44 Averaged from two species (2.54, M. affinis; 2.33, M. ensis) 54 61 Metapenaeus joyneri Shiba shrimp 周氏新对虾 Shrimps 1.64 11

Miichthys miiuy Mi-iuy croaker 鮸鱼 Medium fish 3.70 70 Averaged from three studies (4.34, 4.4, 2.77, 42,46,50 3.3) 32 Miscellaneous aquatic Aquatic invertebrates 其他无脊椎动物 Other demersal invertebrates 2.47 invertebrates 32 Miscellaneous marine Marine crabs, shrimps, 虾蟹杂渔获 Crabs & lobsters 2.50 crustaceans lobsters nei Mizuhopecten yessoensis Yesso scallop 虾夷扇贝 Other demersal invertebrates 2.33 Averaged from two studies (2.625, 2.12, 2.276, 37,39 2.3) Mollusca Clams, seasnails, 软体动物 Other demersal invertebrates 2.55 Averaged from three estimates (2.10, 2.12, squids, octopuses 2.184, 2.818, 2.69) 32,36,37 Monacanthidae Filefishes 马面鲀类 Medium fish 3.35 36 Averaged from two major species (3.4, Thamnaconus modestus；3.3, Thamnaconus hypagyreus) 42 36,46 Mugil cephalus Flathead grey mullet 鲻鱼 Large fish 3.41 100 Averaged from two studies (4.01, 2.8)

Mullidae Goatfishes 须鲷科 Medium fish 3.44 Averaged from three studies (3.65, 2.5, Upeneus bensasi; 3.8, U. japonicus) 41,42,62 43 Muraenesox cinereus Daggertooth pike 海鳗 Large fish 3.86 220 conger Mytilidae Sea mussels 贻贝科 Other demersal invertebrates 2.11 Averaged from two species (1.64, Mytilus coruscus; 2.24, M. galloprovincialis; 2.9, M. galloprovincialis) 36,38,39 36 Mytilus coruscus Korean mussel 厚壳贻贝 Other demersal invertebrates 1.64 25

Nemipterus Threadfin breams 金线鲷类 Small fish 2.50 Species: Nemipterus virgatus, averaged from two studies (2 .2; 2.8) 41,63

Nemipterus virgatus Golden threadfin 金线鱼 Medium fish 2.50 35 Averaged from two studies (2 .2, 2.8）41,63 bream

40,43 Nibea mitsukurii Honnibe croaker 黄姑鱼 Medium fish 3.48 75 Averaged from two studies (3.24, 3.715)

Octopoda Octopuses, argonauts 章鱼 Cephalopods 3.10 Averaged from two relatives (3.2, Amphioctopus 46 fangsiao; 3.0, Octopus variabilis) Pagrus auratus Silver seabream 金赤鲷 Large fish 3.60 130

Pampus Silver pomfret 鲳属 Medium fish 2.92 Averaged from two species (2.95, Pampus argenteus; 2.88, P. chinensis) 59 48,59 Pampus argenteus Silver pomfrets 银鲳 Medium fish 2.95 60 Averaged from two studies (2.91, 2.99) 30 Panulirus longipes Longlegged spiny 长足龙虾 Shrimps 3.33 30 Proxy: Panulirus homarus lobster Paralichthys olivaceus Bastard halibut 牙鲆,褐牙鲆 Large fish 3.43 103 Averaged from four estimates (2.91, 3.1, 3.8, 3.9) 36,64 29 Parastromateus niger Black pomfret 黑鲳 Medium fish 2.90 75 32 Penaeus monodon Giant tiger prawn 斑节对虾，墨吉 Shrimps 2.67 33.6 对虾 54 Penaeus penicillatus Redtail prawn 长毛对虾 Shrimps 2.53 19

41,42 Pennahia argentata Silver croaker 白姑鱼 Medium fish 3.32 40 Averaged from two studies (4.03; 2.6)

Perciformes Perch-likes 鲈形目 Medium fish 3.04 Averaged from four major families (Scaridae, Sciaenidae, Serranidae, Sparidae) here included in the table 41,59 Platycephalus indicus Bartail flathead 印度牛尾鱼,鲬 Medium fish 2.80 100 Averaged from two estimates (2.5, 2.91, 3.0) Pleuronectiformes Flatfishes 鲽形目 Medium fish 3.73 > 30 Averaged from three species (3.2, Cynoglossus semilaevis; 3.89, Cleisthenes herzensteini; 4.1, Platichthys bicoloratus) 36,49 Portunus pelagicus Japanese blue crab 三疣梭子蟹 Crabs & lobsters 2.79 20 Averaged from two studies（2.37, 3.2）46,53 Portunus trituberculatus Gazami crab 三疣梭子蟹 Crabs & lobsters 3.29 15 Averaged from two studies（3.37, 3.2）42,43 Priacanthus Red bigeye 短尾大眼鲷 Medium fish 3.07 30 Averaged from three studies (2.6, 4.06, 2.56） macracanthus 41,42,45 Psenopsis anomala Pacific rudderfish 刺鲳 Medium fish (Small fish, 3.15 30 Averaged from three studies（4, 2.5, 2.96） SAUP)* 41,42,59 48 Pseudopleuronectes Yellow striped flounder 尖吻黄盖鲽 Medium fish 2.54 50 Proxy: Pseudopleuronectes yokohamae herzensteini 60,65 Ruditapes philippinarum Japanese carpet shell 杂色蛤/菲律宾蛤 Other demersal invertebrates 2.25 8 Averaged from two studies (2.48, 2.02) 仔 45 Ruvettus pretiosus Oilfish 棘鳞蛇鲭 Large fish 3.38 300 Proxy: Lepidocybium flavobrunneum 41,66 SAUPrida tumbil Greater lizardfish 多齿蛇鲻 Medium fish 3.30 60 Averaged from two studies (3.4, 3.2) 41,42 SAUPrida undosquamis Brushtooth lizardfish 花斑蛇鲻 Medium fish 3.81 50 Averaged from two studies (4.21, 3.4)

Scaridae Parrotfishes 鹦哥鱼科 Medium fish 2.09 Averaged from eight species (2.7, Bolbometopon muricatum; 2.0, Calotomus spinidens; 2.0, Hipposcarus longiceps; 2.0, Leptoscarus vaigiensis; 2.0, Scarus dimidiatus; 2.0, Scarus ferrugineus; 2.0, Scarus ghobban; 2.0, Scarus 29 scaber) 32,65 Sciaenidae Drums, croakers 石首鱼科 Medium fish 3.50 Averaged from two studies (3.779, 3.226) 30 Scylla serrata Indo-Pacific swamp 锯缘青蟹 Crabs & lobsters 3.17 28 crab

Scyllaridae Slipper lobsters 蝉虾类 Crabs & lobsters 2.74 Proxy: Scyllarides squammosus, averaged from 67 estimates of two islands of Hawaii Scyphozoa True jellyfishes 水母类 Jellyfish 2.90 Averaged from three studies (2.989, 2.968, 43,68 2.357, 3.28) 53 Sepiidae Golden cuttlefish 乌贼科 Cephalopods 2.86 Species: Sepia esculenta Seriola Amberjacks 鰤属 Large fish 2.94 160 Averaged from two major species（2.74, Seriola rivoliana, 3.03, Seriola dumerili, 3.06, Seriola Aureovittata）45,48

65 Serranidae Basses, groupers, 鲽科 Medium fish 3.92 hinds

Sillago Smelt-whitings 鱚属 Medium fish 3.42 Averaged from two species (3.56, Sillago 36,41,50 japonica; 2.5, 3.5, 3.85, S. sihama) Sparidae Porgies, seabreams 鲷科 Medium fish 3.59 Averaged from four species (3.332, Acanthopagrus latus; 3.680, 4.756, A. schlegelii; 4.33, Sparus macrocephlus; 3.19, Paerargyrops 32,36,68 edita) Sphyraenidae Barracudas 金梭鱼科 Medium fish 3.90 Averaged from three species (3.53, Sphyraena barracuda; 4.49, S. japonica; 3.5, 3.04, 4.53, S. 42,45,59,60 pinguis) Squillidae Squilla mantis shrimps 虾蛄科 (口虾蛄) Shrimps 2.78 Averaged from four studies（2.38, 2 .21, 2.91, 3.6) 36,41,46,53 36,41,59 Stephanolepis cirrhifer Thread-sail filefish 丝背细鳞鲀 Medium fish (Small fish, 2.67 30 Averaged from three studies (2.1, 2.8, 3.1) SAUP)* Tetraodontidae Puffers, tobies 河鲀类 Medium fish (Large fish, 3.50 80 Averaged from two species（3.6, Takifugu SAUP)* rubripes; 3.4, T. septentrionalis）29

Teuthida Squids 枪形目（鱿鱼） Cephalopods 2.74 Averaged from two species (2.62, Loligo chinensis; 2.85, L. duvaucelii) 41 53 Todarodes pacificus Japanese flying squid 太平洋褶柔鱼、 Cephalopods 2.97 50 太平洋柔鱼 53 Trachysalambria White-hair rough 鹰爪对虾 Shrimps 2.33 8.1 curvirostris shrimp, southern rough shrimp Trichiurus lepturus Large-head hairtail 带鱼 Large fish 3.74 234 Averaged from five estimates (3.7, 3.7, 3.46, 3.81, 4.05) 40,42 36,69 Turbo cornutus Horned turban 角蝾螺 Other demersal invertebrates 2.00 7 Averaged from two estimates (2.07, 2.12) *The category in the bracket is the original functional group as identified by the Sea Around Us Project (SAUP).

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SI 2. Supplementary Information for Results

Defining four development eras for China’s BTF from 1950 to 2018 Based on literature review and data analyses, we identified four development eras for China’s BTF: (i) First Era (1950 – 1963, E1); (ii) Second Era (1964 – 1978, E2); (iii) Third Era (1979 – 1996, E3); and (iv) Fourth Era (1997 – 2018, E4). We split E1 and E2 based on technology advances that started in early 1960s when a variety of motorized trawlers were developed in China 1,2. First, rapid motorization replaced sailboats and improved cruising ability and flexibility 3. Second, the use of synthetic fibers (polyethylene fiber) reduced drag resistance of trawls 1. These two factors facilitated the emergence of single trawlers in the South China Sea. China then developed bottom trawling, benthopelagic trawling, and pelagic trawling along the coast, with the first two being the dominant practices 2. Some fishery indices also changed markedly, including CPUE and catch composition. We split E2 and E3 based on economic reforms that started in late 1978 and had dramatic effects upon many fishery indices, including fishing capacity, yield, mean trophic level, and catch composition. In Dec. 1978, China launched the ‘economic reforms’ policy. Fishing vessels were gradually privatized and seafood prices were liberalized 4. Meanwhile, pair trawlers had become less profitable after the mid-1970s 5, because of the depletion of benthic high-value fish stocks (e.g., great yellow croakers) in China’s waters 6. Many pair trawlers were forced to switch targets to lower-valued fish species (e.g., black scrapers) 6. Given the depletion of large fish species that prey on shrimps, shrimps became relatively more abundant 7. In response, small single trawlers (i.e., otter and beam trawlers), which were designed to catch shrimps in inshore shallow waters, became popular across China 5,8,9. Over time, shrimp trawlers gradually became larger with more powerful engines and carried more nets (up to 20 nets per boat). The cod-end mesh sizes were very small (10 – 25 mm stretched), entrapping larger volumes of bycatch (50 – 95% by biomass) and over a hundred non-targeted species 7,9. Chinese trawlers also started to use newly available materials and technologies (e.g., toughened glasses, fish finders, electrical stimuli), which further intensified fishing pressures in China’s seas 1. For instance, in 1995, there were 1900 shrimp trawlers using electrical stimuli alone in Zhoushan, the largest oceanic fishing city of Zhejiang province 10. As a result, most of the catch of some commercial fish species (e.g., L. polyactis) was juveniles, signifying overfishing 11. China then issued various policies to reduce biomass trawling 12. However, the increasing demand of fishmeal and feed for aquaculture and animal farming provided an economic incentive to encourage such reduction fisheries, ignoring the policies 13,14. We split E3 and E4 based on when China ratified UNCLOS (1996) and created its Double Control management measure (1997). Both policy measures effected evident shifts in many fishery indices, including fishing capacity, yield, fishing-in-balance index, etc. Soon after China ratified UNCLOS in 1996, it signed bilateral fishery agreements with Japan in 1997 (effective in 2000), South Korea in 1998 (effective in 2001), and Vietnam in 2000 (effective in 2004), as interim solutions in addressing fishery and EEZ disputes. Every year since then, quotas have been allocated to each nation to fish in the co-managed zones defined by the agreements. All these changes shrunk a large portion of traditional fishing grounds for China’s BTF, driving them inshore. Such a concentration of BTF fostered more intense biomass trawling in China’s EEZ 15, to supply marine protein for feed in China’s extensive aquaculture and animal farming 14,16.

Given the greater competition and lower profits, some fishers stopped fishing. Others started to conduct illegal trawling – using electric pulses and/or trawling during summer moratoria – which became a serious challenge in managing China’s BTF 4,17. In response, China enacted more policies to reduce capacity of its marine capture fisheries (primarily BTF), including input control (e.g., vessel scrapping 2003), output control (e.g., Zero Growth 1999) and law enforcement actions (e.g., combating illegal fishing during moratoria) 12,17,18. China began to provide fuel subsidies to its fisheries after increases in crude oil price in early 2000s (especially 2005) but then reduced them from 2015 as oil price declined 12,19.

Table S2.1. China’s landings (tonnage) from its own claimed EEZ and other EEZs over the four eras from 1950 to 2014. The EEZs in bold texts are the top 18 EEZs where China’s BTs shared a significant proportion (> 10%) of all BTF landings from 1985 to 2018.

Proportion (%) of China’s EEZ E1 E2 E3 E4 Total catch in each foreign EEZ China 160073 445867 1744019 3514712 5864672 - Congo (ex-Zaire) 0 0 1198 11345 12543 83 Russia (Far East) 0 0 263703 614949 878652 77 Côte d'Ivoire 0 0 5966 73864 79830 69 Korea (South) 2160 32771 208445 1061697 1305073 40 Togo 0 0 2191 15725 17915 39 Congo, R. of 0 0 0 25468 25468 37 Cameroon 0 0 1728 9871 11600 34 Iran (Persian Gulf) 0 0 34485 34668 69152 32 Guinea-Bissau 0 0 40376 78816 119191 26 Gabon 0 0 7204 20025 27229 24 Liberia 0 0 1662 22020 23682 24 Japan (main islands) 12804 116116 185613 297965 612498 23 Benin 0 0 56 1871 1927 21 Equatorial Guinea 0 0 169 5008 5177 20 Sierra Leone 0 0 12557 2281 14838 19 Angola 0 0 2600 76368 78968 17 Korea (North, Yellow 119 1830 550 4645 7145 15 Sea) Nigeria 0 0 15614 14048 29662 10 Guinea 0 0 27305 50708 78013 9 Philippines 0 0 19847 37235 57082 9 Namibia 0 0 2150 37458 39608 8 Ghana 0 0 1600 2061 3661 6 Senegal 0 0 3457 22430 25887 6 Morocco 0 0 28932 123217 152149 5 Vietnam 1580 14122 0 91050 106751 3 Indonesia 0 0 35240 17823 53063 2 Mauritania 0 0 0 214 214 1 Pakistan 0 0 0 2306 2306 1 India (mainland) 0 0 3479 0 3479 0.3 Falkland Isl. (UK) 0 0 0 123 123 0.1 USA (Alaska, 0 0 3157 0 3157 0.1% Subarctic)

Figure S2.1 Timeseries of a) total landed value from three areas (all areas, China’s four seas, and China’s claimed EEZ), and b) the percentage of landed value.

Figure S2.2 Ratios between (i) annual catch and the highest annual catch (red line) and (ii) catch used directly for human consumption and all catch (blue line) of eight categories of stock assemblages: a) large fish, b) medium fish, c) small-to-medium fish, d) cephalopods, e) shrimps, f) crabs & lobsters, g) jellyfish, and h) small fish & other invertebrates. Data source: Sea Around Us (www.seaaroundus.org).

Figure S2.3 Ratios between (i) annual catch and the maximum catch (i.e., highest annual catch; red line) and (ii) catch used directly for human consumption and all catch (blue line) of 12 major fishery stocks (with the represented major assemblage shown in Figure S2.5). Data source: Sea Around Us (www.seaaroundus.org).

Figure S2.4 Timeseries of mean price of eight stock assemblage groups. Dotted lines and the grey texts next to them refer to relevant policies or events. Red texts (e.g., Shrimp farming) are annotations. The cyan bands indicate the different development eras. The results should be interpreted with caution as the estimate prices for Chinese fishery stocks by Sea Around Us were based on records from other Asian countries.

Figure S2.5 Catches of five major large-fish stocks and the entire large fish assemblage.

Figure S2.6 Catches of five major medium-fish stocks and the entire medium fish assemblage.

Figure S2.7 Catches of two major shrimp stocks and the entire shrimp assemblage.

Figure S2.8 Catches of two major cephalopod stocks and the entire cephalopod assemblage.

Figure S2.9 Catches of two major crab stocks and the entire crab & lobsters assemblage.

Figure S2.10 Catches of five major stocks of the small fish & other invertebrates, and all small fish & other invertebrates.

Figure S2.11 Stacked landings (Mt) by bottom trawlers (BTs) in each of the 13th to 24th EEZ where China’s BTs shared 21% to 5% of the landings from 1985 to 2018.

Figure S2.12 Stacked landings (Mt) by bottom trawlers (BTs) in each of the 25th to 31st EEZ where China’s BTs shared 5% to 0.1% of the landings from 1985 to 2018.

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