Fishing Down Or Fishing up in Chinese Freshwater Lakes

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Fisheries Management and Ecology, 2014, 21, 374–382

Fishing down or ﬁshing up in Chinese freshwater lakes Y. WANG

School of Nature Conservation, Beijing Forestry University, Beijing, China

J. XU

Donghu Experimental Station of Lake Ecosystems, State Key Laboratory of Freshwater Ecology and Biotechnology of China, Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, China

X. YU

Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China

G. LEI

School of Nature Conservation, Beijing Forestry University, Beijing, China

Abstract Changes in mean trophic level (MTL) of catches have been widely used to reflect the impact of industrial fisheries on aquatic ecosystems because this measure represents the relative abundance of fished species across the trophic level spectrum. In this study, fisheries data from six important freshwater lakes at the middle-lower Yangtze River and Huaihe River reach of Southern China from 1949 to 2009 were used to evaluate changes in catch MTL. After fishery markets opened at 1985, fish catches increased significantly in all the lakes. Lakes Poyang and Dongting, which were dominated by omnivores and connected to the Yangtze River, showed no significant change in catch MTL before and after 1985. Catch MTL in lakes Taihu and Hongze increased significantly due to an increase in the proportion of pelagic zooplanktivorous. Catches in Lake Chaohu were dominated by zooplankton-feeding lake anchovy, Coilia ectenes Temminck & Schlegel and icefish, Neosalanx taihuensis Chen, while Lake Donghu was dominated by phytoplanktivorous carps. Due to low biodiversity, catch MTL of these two lakes showed no significant change before and after 1985. Both fisheries-based and human activities-based drivers influenced the structure and catch MTL of fisheries in Chinese freshwater lakes.

KEYWORDS: biodiversity, catch trends, food web, inland capture ﬁsheries, mean trophic level.

fisheries production in 2010 (FAO 2012). However, Introduction despite the recorded increase, inland fisheries resources Inland capture fisheries provide an important source of are facing threats from anthropogenic activities, includ- income and protein for many people around the world. ing water abstraction, habitat degradation, land drainage, Annual global capture from inland waters has increased dam construction, pollution and eutrophication, and since 2000, with a total production of 11.2 million ton- unsustainable fishery management (Welcomme et al. nes in 2010 (FAO 2012). This increase is attributed to 2010). Overexploitation of the world’s inland fisheries the rising catches in Asian and African countries. China has become a concern (Pauly et al. 2002; Hilborn et al. accounted for 20.4% of the world’s inland capture 2003; Allan et al. 2005). Welcomme et al. (2010)

Correspondence: Guangchun Lei, School of Nature Conservation, Beijing Forestry University, Beijing 10083, China (e-mail: [email protected])

374 doi: 10.1111/fme.12088 © 2014 John Wiley & Sons Ltd CATCH MEAN TROPHIC LEVEL OF CHINESE LAKES 375 estimated that inland capture fisheries in Asia and Africa Huaihe River reach of Southern China from 1949 to are intensely exploited resulting in probable decreased 2009 were used to examine catch MTL as an indicator potential for further expansion. Consequently, fishery of overfishing in Chinese lakes. The aim was to deter- resources are at risk (Welcomme et al. 2010). In China, mine (1) whether the MTL of capture fisheries has chan- freshwater fish capture production has remained stable at ged in the last 60 years, and (2) how the fishery approximately 2 million tonnes per year since 2006, resource composition influences the MTL of Chinese indicating that the growth of wild fishery resources has freshwater lakes before and after 1985, when the Chi- approached its upper limit (Li 2013). nese government opened fish markets. Since 1985, the Meta-analysis of available data is an important tool national circulation of aquatic products has no longer for the evaluation of threats and opportunities facing fish been managed by the government. This led to an populations and to ensure sustainable management of increase in fishing effort and the exploitation of a greater inland capture fisheries. Pauly et al. (1998) were the first number of aquatic systems and resources. The results of to document the decline in the mean trophic level this study are expected to provide information for Chi- (MTL) of fishery catches around the world since 1950. nese inland fishery management and aquatic ecosystem The term ‘fishing down the food web’ is symptomatic of conservation. overfishing, unsustainable harvest and unintended eco- logical changes induced by the removal of species from Materials and methods high trophic levels (Branch et al. 2010; Welcomme et al. 2010). MTL provides an integrated view of the Lake Poyang (lake area: 2933 km2, 21.69 m a.s.l), Lake organisation of trophic structure and is the primary mar- Dongting (lake area: 2625 km2, 33.5 m a.s.l.), Lake Taihu ine index chosen by the Convention on Biological (lake area: 2425 km2, 3.14 m a.s.l.), Lake Hongze (lake Diversity to measure global biodiversity (Butchart et al. area: 1597 km2, 12.5 m a.s.l.) and Lake Chaohu (lake 2010). Many studies report the status of marine area: 769.5 km2, 8.37 m a.s.l.) are the five largest fresh- ecosystem catch MTL (Pauly et al. 1998; Essington water lakes in South China, and Lake Donghu (lake area: et al. 2006; Branch et al. 2010; Hornborg et al. 2013). 32 km2) is the largest city lake located on the Southern Inland fisheries are complex and largely influenced by Yangtze River, Hubei Province (Fig. 1). The combined large-scale economic activities and overexploitation of total area of lakes Poyang, Dongting, Taihu and Chaohu aquatic resources, and reports that focus on inland fisher- is 55.9% of the total lake area of the middle-lower Yan- ies are few (Welcomme et al. 2010). gtze River. The area of Lake Hongze is approximately Heavy fishing pressure has been demonstrated to 32.7% of the total lake area of the middle-lower Huaihe reduce the abundance of desired species and affect the River (Dou & Jiang 2003). Hydrological alterations fish population or community structure in terms of size caused by the construction of embankments or sluice and species (Welcomme 1999; Welcomme et al. 2010). gates in the 1950s disconnected lakes Taihu, Hongze, Better understanding of the importance of inland fisher- Chaohu and Donghu from the mainstream of the river. All ies resources may influence the direction of general these are shallow lakes that have become eutrophic (Yang development policies for aquatic systems. Transition et al. 2010). Although lakes Poyang and Dongting are the from long-lived, high trophic level and piscivorous fish only two lakes that connect with the Yangtze River, they towards short-lived, low trophic level invertebrates and have become eutrophic, with nutrients in the water col- planktivorous pelagic fish has been reported in some umns varying during the wet and dry seasons. Currently, freshwater fisheries in the northern hemisphere (Hilborn there are 107 and 103 species of fish in lakes Poyang and et al. 2003). There are few studies on trophic level Dongting, 97 and 66 species in lakes Taihu and Hongze changes in China’s inland capture fisheries. (Dou & Jiang 2003), 54 species in Lake Chaohu (Guo Chinese inland capture fisheries are multispecies and et al. 2007) and 39 species in Lake Donghu. Flood con- multigear in nature, thus, applying standard assessment trol, fisheries, shipping and recreation are the most impor- models and concepts of overfishing only in lakes that tant ecosystem services provided by these lakes. have a limited number of exploited species is inappropri- Inland fisheries in these lakes were defined as the cap- ate. Catch data of Chinese freshwater lakes are rarely ture of wild stocks of freshwater fish. Unlike in aquacul- reported at the species level; thus, fishery landings are ture, the main targeted species for inland fisheries are generally grouped according to the economic value of grass carp, Ctenopharyogodon idella Val.; silver carp, fish based on adult size, making MTL assessment chal- Hypophthalmichtys molitrix (Val.); and common carp, lenging (Zhong & Power 1997; Welcomme 2011). In Cyprinus carpio L. The economically important species this study, capture fisheries data from six important for capture fisheries in subtropical Chinese freshwater freshwater lakes in the middle-lower Yangtze River and lakes vary.

Figure 1. Map showing the locations of the six freshwater lakes in China.

In this study, the fishery catch was divided into seven All statistical analyses were performed using SPSS groups: phytoplanktivores [H. molitrix and Aristichthy v. 19.0 (IBM, Armonk, NY, USA). nobilis (Richardson)]; herbivores [Ctenopharyngodon idellus, Megalobrama amblycephala Yih and Parabr- Results amis pekinensis (Basilewsky)]; piscivores [Culter alburnus Basilewsky, Siniperca chuatsi (Basilewsky), Long-term trends of the Chinese lake fishery catch Elopichthys bambusa (Richardson), Silurus asotus Linnaeus, Culterichthys erythropterus (Basilewsky) and Annual fish catches fluctuated from 1949 to 1984 in Pelteobagrus eupogon (Boulenger)]; omnivores [Caras- both the connected and disconnected lakes. Except for sius auratus (L.), C. carpio and Mylopharyngodon pic- those in Lake Donghu, fishery products in the other five eus (Richardson)]; lake anchovy [Coilia ectenes lakes peaked in the 1950s, followed by a decline in the (Temminck & Schlegel)]; icefish [Neosalanx taihuensis 1960s, stagnated in the 1970s and increased again in the (Chen)]; and other small fishes [Hemiculter bleekeri 1980s. After 1985, the catches increased rapidly (ANOVA, Warpachowski, Saurogobio dabryi Bleeker, Rhodeus all P < 0.001) (Fig. 2). For Lake Poyang and Lake ocellatus (Kner), Sarcocheilichthys nigripinnis (Gunther),€ Dongting, fishery landings had another peak in the Saurogobio dumerili Bleeker, Hemibarbus maculatus 1990s, followed by a steady decrease during the 2000s Bleeker, Hemiramphus intermedius Cantor and Abbotti- (Fig. 2). na rivularis (Basilewsky)]. Data on catches were obtained from the fisheries management bureaus of the Variations in MTL and fish assemblages six lakes and from fishery survey reports. MTL was estimated as suggested in Pauly et al. The total catches of connected lakes Poyang and Dong- (1998): ting were dominated by omnivorous fishes, namely P C. carpio and C. auratus (averages 50.7 6.4% and TLijYij MTL ¼ P 43.8 14.9%, respectively). The proportion of omni- Yij vores increased significantly after 1985 in these lakes. On the same timescale, the proportion of two small where MTL is the mean trophic level of catches in year zooplanktivorous fishes, lake anchovy and icefish, j, Yij is the catch of species i in year j, and TLi is the tro- decreased (Table 1). The proportion of lake anchovy in phic level of species or functional group i. The trophic lakes Taihu and Hongze increased significantly from fi level estimates for Chinese subtropical lake sh were 42.8 10.7% to 50.6 13.5% (P < 0.001) and from based on stable nitrogen isotope results according to 10.7 13.8% to 31.5 1.0% (P = 0.012), respectively Zhang et al. (2013).

(a) (b)

(e) (f)

Figure 2. Trends of ﬁshery catches in (a) Lake Poyang, (b) Lake Dongting, (c) Lake Taihu, (d) Lake Hongze, (e) Lake Chaohu and (f) Lake Don- ghu for the period of 1949–2009.

(Table 1), and accounted for the largest proportion of with the MTL of Lake Poyang (r2 = 0.928, F = 74.942, the catch after 1985. Except for the group of small P < 0.001, n = 14), Lake Taihu (r2 = 0.296, F = 4.896, fishes, all the other six groups in Lake Chaohu catches P = 0.031, n = 52) and Lake Hongze (r2 = 0.671, changed significantly (all P < 0.05), with increased pro- F = 7.380, P=0.024, n = 11). portions of planktivores and lake anchovy (Table 1). Lake Donghu fisheries have been dominated by H. Discussion molitrix and A. nobilis since the 1970s. The average proportion has increased continuously from Catch MTL is an indicator of changes in the ecosystem, 84.7 5.6% to 97.5 0.4% of the total fish yield but its usefulness has been questioned because catches after 1985, and the proportions of both phytoplankti- are influenced by changes in economics, management, vores (P < 0.001) and herbivores increased significantly fishing technology and exploitation patterns (Branch (P = 0.004) (Table 1). et al. 2010; Stergiou & Tsikliras 2011). The negative The average MTL of Lake Donghu catches during the trend in global MTL observed at the end of the 1990s is past 60 years was below 3.0, whereas those of the other no longer supported by data obtained during the two five lakes were above 3.0 (Fig. 3). Catch MTL in Lake most recent decades (Branch et al. 2010). In this study, Poyang (ANOVA, P = 0.074) and Lake Dongting (ANOVA, catch MTL of lakes Taihu (Spearman’s r = 0.580, P = 0.852) showed no significant changes after 1985 P < 0.001, n = 55) and Hongze (Spearman’s r = 0.779, (Fig. 3). For disconnected lakes, such as Lake Taihu and P < 0.001, n = 16) showed an increasing trend, whereas Lake Hongze, the catch MTL increased significantly those of the other four Chinese lakes showed no signifi- after 1985 (ANOVA, P = 0.007; and ANOVA, P = 0.008, cant change before and after 1985 (all P > 0.05). Inland respectively), whereas the catch MTL of Lake Chaohu fisheries are distinct from marine fisheries in their nature (ANOVA, P = 0.158) and Lake Donghu (ANOVA, and in the range of internal (fisheries-based) and external P = 0.897) did not change significantly (Fig. 3). The (natural- and human ecosystem-based) influencing proportions of piscivorous fish were positively correlated drivers (Welcomme et al. 2010).

Table 1. Fisheries composition before and after 1985 of six Chinese freshwater lakes

Poyang Dongting Taihu Hongze Chaohu Donghu

Before After Before After Before After Before After Before After Before After 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 (n = 3) (n = 11) (n = 2) (n = 11) (n = 32) (n = 26) (n = 12) (n = 4) (n = 18) (n = 19) (n = 6) (n = 6)

Phytoplanktivores (%) Mean 8.4 5.4 11.0 7.0 9.7 6.6 4.5 6.5 10.4 2.4 84.7 97.5 SD 4.3 4.4 11.3 10.4 3.3 3.5 3.9 1.7 7.3 0.8 5.6 0.4 F 1.129 0.252 11.588 ** 0.911 22.413 *** 31.338 *** Herbivores (%) Mean 9.9 4.6 4.0 2.8 4.1 4.4 2.0 2.0 6.0 15.2 6.8 0.2 SD 2.5 2.6 1.4 1.2 2.4 6.5 2.0 0.4 1.6 6.2 4.4 0.1 F 10.240 ** 1.524 0.048 0.002 38.363 *** 13.627 ** Omnivores (%) Mean 43.0 52.8 23.0 47.6 10.9 7.3 20.1 23.6 5.1 1.2 1.8 0.8 SD 2.0 5.4 14.1 12.0 5.7 3.3 17.5 2.4 3.6 0.4 1.3 0.3 F 8.981 * 6.847 * 8.347 ** 0.155 22.413 *** 3.368 Piscivores (%) Mean 15.2 22.9 18.0 27.9 3.1 3.0 7.0 0.7 15.9 7.5 3.5 0.0 SD 1.9 6.6 7.1 6.7 1.7 2.8 7.7 0.2 5.3 1.3 3.4 0.0 F 3.900 3.616 0.005 2.555 45.662 *** 6.479 ** Lake anchovy (%) Mean 4.6 0.4 20.0 1.1 42.8 50.6 10.7 31.3 54.5 68.7 SD 4.8 1.0 21.2 3.6 10.7 13.5 13.8 1.0 14.6 4.0 F 9.208 ** 11.512 ** 6.050 ** 8.411 * 16.737 *** Iceﬁsh (%) Mean 0.3 7.1 6.6 1.6 17.1 8.1 4.7 SD 0.4 5.1 4.4 1.7 1.0 2.1 1.9 F 0.180 290.688 *** 26.394 *** Other small ﬁshes (%) Mean 18.9 13.9 23.3 13.6 22.3 21.5 54.1 18.8 0.0 0.3 3.4 1.5 SD 6.3 7.0 13.1 10.1 7.9 9.9 28.1 1.9 0.0 0.4 3.7 0.4 F 1.207 1.461 0.114 6.062 * 12.002 *** 1.607

***P < 0.001; **P < 0.01; *P < 0.05.

High fishing pressure caused the populations of large anchovy and icefish (Li et al. 2010). In addition, there predatory fishes, such as C. erythropterus, C. alburnus was some evidence that the mean size of harvested spe- and E. bambusa, to collapse early in 1960s in discon- cies decreased, although comprehensive data sets do not nected lakes (Chen et al. 2009b; Li et al. 2009, 2010; exist to demonstrate this. For example, the mean body Zhang et al. 2012). As the production of piscivorous length of one of the main species lake anchovy species shrinks sharply in disconnected lakes, small, decreased from 87.6 mm in 1983 to 80.4 mm in 2003 shorter-lived insectivorous and zooplanktivorous species and 74.0 mm in 2007 in Lakes Taihu (Tang 1987; Liu increase rapidly, altering species composition, abun- et al. 2008; Mao et al. 2011). The relative proportion dance and ecology of the fish communities. Thus, of piscivores in the catch was significantly related to recent increases in the abundance of lake anchovy and MTL, although this was relatively slight (i.e. after other fishes could be the result of increased fishing 1985, piscivorous accounted for 3.0 2.8% in Lake effort on the large cultures of their predators (Chen Taihu, 0.7 0.2% in Lake Hongze). The estimated tro- et al. 2009b; Li et al. 2009, 2010; Zhang 2010). Conse- phic position of lake anchovy was 3.4 (Zhang et al. quently, there is little potential for further increase in 2013), higher than the yearly catch MTL; thus the catch. This phenomenon is demonstrated by the less increased proportion of this small zooplanktivorous fish complex food web structure of Lake Taihu in the 1990s had a significant influence on catch MTL. Thus, catch and the decrease in the biomass of large predators from MTL could increase because of the increase in the the 1960s to 1990s, despite the increase in the biomass numbers of small zooplanktivorous species producing a of small species, particularly zooplankton-feeding lake false fishing up.

(a) (b)

(e) (f)

Figure 3. MTL (circle) and the percentage of piscivore (cross) in (a) Lake Poyang, (b) Lake Dongting, (c) Lake Taihu, (d) Lake Hongze, (e) Lake Chaohu and (f) Lake Donghu from 1949 to 2009.

In Lake Chaohu, the fish assemblage making up the the disconnected lakes. Qian et al. (2002) and Liao catch changed completely after 1985, with a significant et al. (2002) reported an increase in the percentage of decrease in piscivores, omnivores and phytoplanktivores, small species in the fish landings in lakes Poyang and along with a significant increase in herbivores, such as Dongting. Moreover, the mean size and age of all H. maculatus, P. pekinensis, M. amblycephala and lake species declined (Liao et al. 2002; Qian et al. 2002). anchovy. Zhang et al. (2012) reported that catch MTL They suggested that the increases in catch in lakes Poy- in Lake Chaohu declined steadily since the 1970s ang and Dongting in the 1990s were not signs of pros- because of increased fishing associated with the perity, as illegal fishing gear with meshes smaller than increased contribution of small fishes (Zhang et al. 1 cm and a huge electric fishing trawl were used to col- 2012). In this study, changes in Lake Chaohu catch lect fishes of all sizes. For example, the proportion of MTL before and after 1985 were not significant. These small benthic piscivores S. asotus and P. eupogon results are similar to those of Fortibuoni et al. (2010), increased in fishery landings. The absence of significant who observed that the change in the overall MTL was changes in catch MTL could also be due to the overall not significantly due to the balancing effect of changes fishing pressure that led to the simultaneous exploitation in fishery trophic level composition. In Lake Donghu, of all trophic levels. Declines in catch MTL were not the composition of the catch has been limited to silver observed in these lakes, possibly because the initial fish- carp and bighead carp since the 1970s (Yang et al. ing down of freshwater food webs would lead to 2005), so the catch MTL of Lake Donghu also showed increase in catch and subsequently to a phase transition no significant change before and after 1985. associated with stagnating or declining catches. There- The connected lakes Poyang and Dongting have more fore, production of the fish assemblage as a whole abundant macrophytes for spawning and feeding of responds, resulting in the same level of catch over a omnivorous species, such as C. carpio and C. auratus. considerable range of fishing pressure. When catches Thus, the fish assemblages were different from those of from most trophic levels increase, the risk of fishery

collapse intensifies even when MTL trends are stable or Fishery management problems increasing. A field survey in Lake Dongting showed that A steady or increasing catch in Chinese lakes has involved overfishing was a serious issue evident from the lack of the catching of smaller fishes, which are now regarded as larger and older fishes (Ru & Liu 2013). Fishing down the main harvest, to compensate for the decreasing catch the fish assemblage might have no effect on MTL of large fishes. Increased fishing pressure can largely be because in inland waters, the multispecies multigear nat- attributed to the increase in the number of fishers using ure of the fishery results in the capture of most species more efficient and readily available fishing gears. The per- and sizes of fish. Welcomme (1999) suggested that the centage of small species within the catch increases. Mean- mean length of the fish assemblage could be an indica- while, historical data from fisheries in lakes Dongting and tor of fishery condition. The mesh size of gear used in Poyang show that the mean size and age of both predatory these lakes is very small even though the dominant fish and small fishes declined over time. For example, the in the catches were of different sizes. In lakes Poyang, mean body length of M. piceus and C. idella decreased Dongting and Hongze, omnivores were dominant; the from 417 and 362 mm in 1997 to 271 and 254 mm in Lake Taihu and Lake Chaohu fisheries were dominated 2011 in Poyang Lake, (Qian et al. 2002; Jiang et al. by lake anchovy, and in Lake Donghu, phytoplankti- 2013). Declines in mean lengths of all commercially vores with lowest average catch MTL were most abun- important species have been registered in lakes Dongting dant. Therefore, the fishing-down phenomenon could (Appendix 1) and Poyang (Appendix 2) by other authors. not be purely attributed to length selection caused by These unselective fishery phenomena occurred in all six mesh size in these Chinese lakes. Certain types of envi- lakes (Chen et al. 2009a). This vicious cycle will result in ronmental pressure would reinforce the effects of decreases in the number of fishes after a temporary increased fishing pressure. Sethi et al. (2010) reported increase and will lead to an eventual collapse in the aqua- that MTL could be sensitive not only to fishery-induced tic ecosystem. changes at the ecosystem level but also to economic Activities such as establishing artificial fish habitat and technological factors. The inland aquatic ecosystem and releasing hatchery reared juvenile fishes into rivers in China has been largely influenced by large-scale eco- and lakes, forbidding illegal fishing and setting up natu- nomic activities (Chen et al. 2012). Hydrological altera- ral reserves have been implemented to protect inland tions are the largest external threat to fish biodiversity fisheries resources in China (Chen et al. 2012), but the in these freshwater lakes (Fu et al. 2003; Ru & Liu effective implementation of these measures needs 2013). Construction of embankments and sluice gates improvement. One such measure is closing the lakes to has raised lake water levels and increased the pelagic fishing during the breeding season (Lake Poyang, from area, denying migratory fishes access to the feeding and 20 March to 20 June; Lake Dongting, from 20 March to breeding grounds that are necessary for their survival 30 June; Lake Taihu, from 1 February to 31 August; (FAO 1985). After the river–lake connections of lakes Lake Hongze, from 1 February to 1 June; and Lake Cha- Taihu, Hongze, Chaohu and Donghu were cut in the ohu, from 1 February to 31 July). However, these short 1950s, the abundance of migratory fishes, such as periods may only protect fish with short lifespans: the M. piceus, C. idellus, H. molitrix and A. nobilis, long-lived species are not given enough time to recover. decreased sharply. These species could only be supple- Although fishery management activities, such as estab- mented by stocking juvenile fishes (Yang et al. 2005; lishing fishing size limits, fishing gear regulations, closed Chen et al. 2009a; Li et al. 2009; Zhang 2010; Zhang areas and seasons, and harvest limits, have been pro- et al. 2012). As there are no overbank flows, sediment posed in some basins (Chen et al. 2012), fishery man- and pollution concentrated in the lake lead to an imbal- agement bureaus need to establish total harvest limits ance of whole-ecosystem’s primary production, that is, a and improve the management system. loss of benthic production, and a shift towards pelagic primary production, as noted in other ecosystems affected by eutrophication (Zhang et al. 2012). Eutro- Acknowledgments phication accompanied by the disappearance of macro- This research was supported by the Fundamental Research phytes, eliminated the critical habitats for S. chuatsi, Funds for the Central Universities (NO. BLX2012023) C. carpio and C. auratus, and gradually enabled lake and National Natural Science Foundation of China (Grant anchovy to become the dominant species (Guo et al. Nos. 41301077 and 31370473). We thank anonymous ref- 2007). Zhang et al. (2012) also reported that both envi- erees for providing valuable comments on the manuscript. ronmental changes, jointly with fishing, may have We appreciate the efforts of Dr. Robin Welcomme for caused cascading effects, which makes the system less English editing on the final draft of this manuscript. resistant to disturbance (Zhang et al. 2012).

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