2 University of Chinese Academy of Sciences, Beijing 100049, China;
3 College of Fisheries, Guangdong Ocean University, Zhanjiang 524088, China;
4 Key Laboratory of Sustainable Fisheries and Environmental Protection for Lakes of Northern Jiangsu, Huai'an Research Center, Institute of Hydrobiology, Chinese Academy of Sciences, Huai'an 223002, China
Intra-specific variability in life history traits has been reported in many fish species, e.g., Oreochromis niloticus (Duponchelle and Panfili, 1998), Etheostoma lynceum (Heins et al., 2004), several European freshwater fish species (Blanck and Lamouroux, 2007) and Neogobius melanostomus (Gutowsky and Fox, 2012). On one hand, species with wide range distributions often show large variation in life history traits, e.g., Perca fluviatilis (Heibo et al., 2005), Pseudorasbora parva (Gozlan et al., 2010), Neogobius melanostomus (Kornis et al., 2012) and Oreochromis mossambicus (Gutowsky and Fox, 2012). Such large geographic scale variation is usually a result of evolutionary process involving long-term variability of the environment (Southwood, 1977, 1988; Townsend and Hildrew, 1994). On the other hand, some fish species have the capacity to response to short-term environment changes (Duponchelle et al., 1999; Lopes et al., 2000; de MPhenotypic plasticity in fish life-history traits in tworona et al., 2009), and populations successfully colonizing a new/changed environment could potentially shift their life history traits to facilitate establishment and expansion (Fox et al., 2007; Feiner et al., 2012; Gutowsky and Fox, 2012). Many studies have been conducted regarding life history traits of invasive fish species, which colonize a new environment by expanding their distribution area or after artificial introduction (Rosecchi et al., 2001; Tomeček et al., 2007; Amundsen et al., 2012). Such invasive species usually show high plasticity and dynamics in life history traits (Vila-Gispert et al., 2005; Joanna et al., 2011; Russell et al., 2012), responding to changes in biotic and abiotic conditions, e.g., an increase in population and subsequent resource limitation (Bøhn et al., 2004). Typically, many of the invasive species were observed switching to more-opportunistic traits during the initial stages of population establishment, e.g., elevated juvenile growth, earlier sexual maturation, and increased reproductive effort (Fox et al., 2007; Amundsen et al., 2012; Copp et al., 2016). However, there are few data on responses of resident populations to the consequences of a rapid change in the environment (Lopes et al., 2000). The formation of artificial reservoirs constitutes a situation in which such responses for resident fish populations can be observed.
Dams have been constructed in many river systems for multiple purposes, e.g., flood control, water supply, navigation, irrigation and the generation of hydroelectric power (Nilsson et al., 2005; Zarfl et al., 2015). The impoundment of the reservoirs rapidly changes the original fluvial habitats into semi-lentic or lentic environments (Zhong and Power, 1996; Agostinho et al., 2008). Fish assemblages change dramatically in association with such habitat changes. Specialists adapting to lotic habitats may not be able to complete their life history process due to the lack of essential habitats, consequently dramatically declining in abundance and even facing extinction (Rahel, 2002; Park et al., 2003). On the other hand, the impoundment of reservoirs inundates large areas of land, creating a large water volume for fishes (Miranda, 2001; Wang et al., 2013). The decomposition of terrestrial organic matter releases a large amount of nutrients, thus increasing the productivity of plankton; submerged organic matter itself can also be directly utilized as food by fishes and many other aquatic biota, some of which in turn serve as food for fishes (Sugunan, 2000; Deng et al., 2015). Generalists that adapt to the lentic habitat will take the advantage of the habitat changes and available resources and achieve their potential for colonization (Agostinho et al., 2008; Liew et al., 2016). These species could be viewed as colonizers and may possess capability to modify their life history traits in response to such habitat changes, thereby might facilitate an increase in population.
The Three Gorges Dam (TGD) is located in the Yichang section in the main channel of the Changjiang (Yangtze) River, China (Fig. 1). It was completed in 2009; however, impoundment and the regulation of the flow of the Changjiang River began in 2003. Impoundment altered the river channel for approximately 600 km upstream, forming the Three Gorges Reservoir (TGR) (Fu et al., 2010). The spawning grounds and specialized habitats of riverine fishes (with some of them endemic, e.g., Procypris rabaudi, Rhinogobio cylindricus) have been destroyed. As a result, these species have declined in abundance dramatically (e.g., Coreius heterodon, Coreius guichenoti), with some of them even potentially extinct (e.g., Leptobotia elongata, Acipenser dabryanus) (Park et al., 2003; Liu et al., 2012a; Xia et al., 2016). On the other hand, many other species adapted to lentic and slow-flowing habitats (e.g., Hemiculter bleekeri, Pelteobagrus nitidus, Siniperca kneri) have been favoured by such habitat changes and have increased in population size rapidly (Zhao et al., 2015; Chen, 2016). Knowledge of the life history traits of these species may provide fundamental information for understanding strategies involved in colonizing changed environments (Winemiller, 1989; Winemiller and Rose, 1992); such knowledge also contributes essentially to sustainable fishery management in reservoirs (King and McFarlane, 2003).
Siniperca kneri (Garman) is a piscivorous species that is distributed widely in East Asia and is a commercially very important species with high market value in China (Gao, 1991; Han et al., 1996; Li et al., 2014). It mainly inhabits freshwater lakes or river sections with slow flow (Gao, 1991; Cui and Li, 2005). S. kneri rarely occurred in fishermen's catches in the upper reach of the Changjiang River before the impoundment of the TGR (He, 1990; Duan et al., 2002). The abundance of S. kneri has increased dramatically in the TGR since the impoundment, especially in some tributaries, e.g., the Xiangxi River, Daning River, and Xiaojiang River (Shao et al., 2006; Li et al., 2011; Yang et al., 2013; Zhao et al., 2015). In the Xiangxi River, S. kneri has frequently occurred in commercial catches since 2005 (Shao et al., 2006); it increased gradually thereafter and became one of the dominant species by 2012 and 2013 (Zhao et al., 2015). We suggested that the S. kneri population in the TGR might shift to more-opportunistic life history traits to fascinate this abundance increase. In the present study, the life history traits, e.g., age, growth and reproductive biology, were investigated for S. kneri in the Xiangxi River of the TGR. As there was no previous research into the life history traits of S. kneri in the TGR area before impoundment, we could not directly test this suggested shift of life history traits by comparing the current population to the historic one. However, the life history traits related to age, growth and reproductive characteristics of S. kneri have previously been studied in some reservoirs (Ye et al., 1986; Xie, 1995; Liu et al., 2012b) and rivers (Chen et al., 2003; Wang et al., 2006) in China. Thus, the results of the present study were compared to those of other S. kneri populations reported in the literatures.2 MATERIAL AND METHOD 2.1 Fish sampling
The Xiangxi River flows into the TGR on the left side, approximately 32 km upstream from the TGD (Fig. 1). Specimens of S. kneri were collected in the lower reach of the Xiangxi River at Xiangxi, Xiakou, and Gaoyang (Fig. 1). The sampling locations at Xiangxi and Xiakou were in the flooded area of the TGR with a lentic environment throughout the year. The sampling location at Gaoyang was in the transitional area when the water level of the TGR was at 145 m during June through September. This location was usually lentic in the other months of the year when the water level of the TGR was higher. The fish were collected using experimental gill nets (Hubert, 1996). Each net consisted of six panels with different mesh sizes, i.e., 2.2 cm, 2.6 cm, 3.0 cm, 6.0 cm, 7.5 cm and 10.0 cm bar-mesh. Each panel was 2 m in depth and 60 m in length, and then each net was 360 m in total length. Sampling was carried out monthly from September 2012 to January 2014, except for October 2012 and January and February 2013 (Table 1). October 2012 was the time when the water level of the TGR was increasing from 145 m to 175 m. Following this inundation process, many floating debris, mainly tree branches, became scattered on the surface of the Xiangxi River. Therefore, gill nets could not be set, and we did not sample in that month. We did sample in January and February 2013 but failed to catch any fish. The water level in the TGR was at its highest peak in that period (175 m). Additionally, as the water temperature was lowest in this period of the year, fish usually moved to deep water. It might be a reason that fish could not be caught efficiently by gill nets in that period. The nets were usually set before dusk (18:00 to 20:00 depending on different seasons), deployed for approximately 12 h, and the samples were taken the next morning. The fish were kept on ice immediately after sampling and were taken to the laboratory for further analysis. Water temperature was measured daily four times a day (at 2:00 a.m., 8:00 a.m., 2:00 p.m. and 8:00 p.m., respectively) at 0.5 m depth below the water surface at Xiakou using a waterproof temperature recorder, HOBO UA-002-08 (Onset Computer Corporation, Bourne, MA, USA) during September 2012 through January 2014.2.2 Reproduction
Each fish was measured for standard length (Ls, 1 mm) and weighed for wet body weight (Ww, 0.1 g). The fish were then dissected, and sex was determined from the gonads. For females, the gonads were removed and weighed (Wg, 0.001 g). After all the internal organs were removed, the fish were weighed to obtain the clean weight (Wc, 0.1 g). The gonadosomatic index (GSI) was calculated for each female by
Preliminary observations showed that there was no difference in developmental stage or fecundity between the left and right ovaries. The left ovary was preserved in Bouin's solution for 24 h and then transferred to 70% ethanol for further gonad histological analysis following Hinton (1990). A sixstage scale was used to classify the maturity of the gonads based on histological analysis (Crim and Glebe, 1990).
Fecundity was determined from the right ovaries of mature females (gonads at stage Ⅳ or Ⅴ), evenly covering the size range of fish collected. Approximately 0.3 g of oocytes was removed from the middle and each of the two ends of the ovary, mixed and weighed (Wo, 0.001 g), and preserved in a 5% formalin solution. All the eggs, including mature oocytes and those at earlier development stages, were counted (n) under a dissecting microscope. The absolute fecundity (AF) was calculated for each individual by
The relative fecundity (RF) was calculated for each individual by
Sagittal otoliths were extracted from each fish, and usually only the left otolith was used for age and growth analyses. The otoliths were processed following a standard procedure to produce a transverse plane section (He et al., 2008). Some otoliths were broken or over-polished. Only otoliths successfully processed were further analyzed. Putative annuli were observed in the transverse plane as opaque/translucent zones under a microscope with transmitted light and were located as the transition from an opaque zone to the next translucent zone (Fig. 2a). The putative annuli were most distinguishable on both sides of the sulcus acusticus on the proximal side. Annual growth increments and the otolith radius were measured along an axis from the core through the proximalsulcus acusticus part on the dorsal side of the transverse plane section (Fig. 2b). For otoliths with one or more complete opaque/translucent zones, the margin increment ratio (MIR) was calculated as
where R is the otolith radius, Rn the radius of the last complete zone, and Rn-1 is the radius of the penultimate completed zone (Beckman et al., 1990; Haas and Recksiek, 1995). For otoliths without a completed putative annulus, the margin increment width (MIW) was measured as the distance from the core to the margin end, equal to the otolith radius. Monthly changes in the size of the margin increment were analysed to validate the annual periodicity of the putative annuli.
Each fish was assigned to an age class. When a fish was caught in the calendar year of its birth, it was assigned to age 0; if it was caught in the year following the calendar year of its birth, it was assigned to age 1, etc. (Devries and Frie, 1996). To construct the Von Bertalanffy growth function, each fish was also assigned a more precise age by fixing its birth date as 1 May, the middle of the main spawning season (see Result), and taking the date of capture into consideration (Stewart and Hughes, 2007). For a fish caught during April through June with a translucent margin zone outside of the last annulus, its age was calculated as
where A is the age in years, N is the number of annuli counted, and Dc is the number of days from its nominated birth date to capture. Otherwise, age was calculated as
The Ls-at-age data were then fitted to the Von Bertalanffy growth function (VBGF),
where Lt is the Ls-at-age t, L∞ is the asymptotic Ls, K is the coefficient of growth, and t0 is the age at which the length is theoretically zero. The VBGFs for males and females were compared using the analysis of residual sums of squares (ARSS) method (Chen et al., 1992).3 RESULT 3.1 Water temperature
Water temperature varied from 12.4℃ to 33.7℃ with a mean (±SD) of 23.2±5.8℃. It showed a clear seasonal pattern with the lowest value in February (12.7±0.1℃), increased gradually through June (26.7±1.4℃), remained high in July (30.1±1.1℃) and August (30.1±1.3℃), and decreased gradually from September onward (Fig. 3).3.2 Age and growth
A total of 798 individuals were collected with more than 30 individuals in each sampling month. The fish varied from 72 mm to 355 mm Ls and 7.1 g to 1 347.5 g Ww (Table 1). Otoliths were successfully processed for 649 individuals, including 356 females (72–355 mm; Ls) and 293 males (79–345 mm; Ls). The MIW increased gradually from September to the following May. During June through August, all fish analysed had at least one complete opaque/translucent zone (Fig. 4a). The MIR decreased gradually from April to June. Subsequently, it increased gradually until the following April (Fig. 4b). Thus, annuli were identified as a shift from an opaque zone to the next translucent zone, which occurred between April and June.
The observed ages varied from 0 to 4 years, i.e., 5 age classes. Individuals of age 1 and 2 accounted for 56.9% and 31.0% of the harvest, respectively (Table 2). The ARSS analysis indicated that there was no significant difference in the Ls-at-age relationship between males and females (F = < F0.05, 3, 4=6.59, P>0.05). The Ls-at-age relationship for both sexes combined was fitted as:
The GSI values for stages Ⅰ, Ⅱ and Ⅲ partly overlapped, varying from 0.001 to 0.021. The GSI values at stages Ⅳ and Ⅴ varied from 0.023 to 0.234. Females with GSI values greater than 0.02 were generally mature (Fig. 5). Histological examination showed that the gonads of females in January were at stages Ⅱ or Ⅲ. Mature females (gonads at stage Ⅳ or Ⅴ) appeared in March (with a percentage of 27%), increased in April (82%) and May (82%), and then decreased in June (42%) and July (36%). No mature females were observed in August 2013 to January 2014, the last month of fish collection (Fig. 6). Thus, S. kneri spawned during March through July in 2013 when water temperatures varied from 15.4℃ to 30.1℃; spawning peaked in April and May when water temperature ranged from 20.4℃ to 22.8℃.
The mature females aged from 1 to 4 years, and individuals of age 1 (27.6%) and age 2 (67.9%) were dominant. The smallest mature female was at age 1 with an Ls of 156 mm and a Ww of 108.0 g. The AF varied from 2 690 to 106 909 eggs per individual with a mean (±SD) of 40 056±26 410. It increased linearly with Ls by the function AF=609.3Ls-95 742 (n=99, R2=0.77, P < 0.05), and varied with ages (Fig. 7). The RF varied from 28 to 272 eggs/g with a mean of 140±53 eggs/g (Table 2).4 DISCUSSION
Our results reveal the age structure, growth, and reproductive characteristics of S. kneri in the Xiangxi River of the TGR. We compared our results with the available data of other populations of S. kneri reported in literatures (Table 2). The Ls at age 1 in the TGR tended larger than that of the three compared populations (i.e., in Sandaohe Reservoir, Xinfengjiang Reservoir, and North River), but smaller than that in the Guishi Reservoir (Fig. 8); females in the TGR were mature at age 1, which is younger than the age of maturity in the Xinfengjiang Reservoir and the North River (age 2 or 3) (Table 2); the smallest mature female observed in the TGR was smaller than that in the Xinfengjiang Reservoir (178 mm Ls); and the RF in the TGR was much higher than that in the Xinfengjiang Reservoir (Table 2). Thus, the population of S. kneri in the TGR tended to have faster growth in the first year, a younger age and smaller size at first maturation, and higher fecundity.
In the triangular life history strategy framework, the life history traits of S. kneri in the TGR tended to be more "opportunistic" compared to the other populations (Winemiller and Rose, 1992). Such "opportunistic" life history traits benefit rapid population increase and are favoured during the initial stages of population colonization (Fox et al., 2007). The three compared reservoirs were impounded in late 1950s, and their ages at the time when S. kneri was investigated were 24 years (Xinfengjiang Reservoir), 31 years (Sandaohe Reservoir) and 50 years (Guishi Reservoir), respectively (Table 2). The age of the TGR at the time of our investigation (10 years) was much younger than these compared reservoirs. Thus, the population of S. kneri in the TGR was at the earlier stage of colonization compared to the other three reservoir populations. S. kneri has increased in abundance dramatically in the TGR after impoundment (Gao et al., 2010; Li et al., 2011; Yang et al., 2013; Zhao et al., 2015). Opportunistic-tended life history traits had been previously observed in many invasive populations of fishes during the early stages of colonization and establishment when moved to a new environment (Fox et al., 2007; Amundsen et al., 2012; Copp et al., 2016). In the present study, such opportunistic-tended life history traits were also observed for the resident population of S. kneri in the TGR during the early stage of impoundment, which may benefit establishment of the population, similar to many invasive species at the early stages of invasion.
The nutrition status and productivity of a reservoir are usually higher in the first several years of impoundment, which then decrease and reach a stable status (Jackson and Hecky, 1980). Accordingly, the fish abundance shows a similar fluctuation pattern: increases in the early stage and decreases thereafter, eventually reaching a stable level (Agostinho et al., 2016). We observed faster growth of S. kneri in the first year in the TGR than in most of the other waters. This might reflect higher prey availability at the early stage of impoundment of the TGR. S. kneri is a piscivorous fish, feeding on living prey fish and shrimps throughout its life (Han et al., 1996; Li et al., 2014). In the TGR, its diet was mainly composed of small-sized fishes, e.g., Hemiculter bleekeri, Saurogobio dabryi and Pseudobrama simony, and shrimps e.g., Macrobrachium nipponense and Exopalaemon modestus (unpublished). These prey fishes and shrimps were observed to be dominant and highly abundant in the TGR (Zhao et al., 2015; Chen, 2016). Increased nutritional concentrations and productivity had been observed in the TGR following the impoundment (Ye et al., 2007; Yang et al., 2010), which might contribute to the increase in abundance of the prey fishes and shrimps for S. kneri. However, there is no available data about nutrition status and prey fish abundance in the other waters. Further investigation should be carried out monitoring fluctuations in nutrient concentration, productivity, fi sh abundance, and life history patterns when the reservoir getting elder.
The age structure and growth of S. kneri in the Guishi Reservoir were unique among all the compared populations: the largest Ls at each of the different ages and the highest proportion of age 0 and 1 individuals in the harvest. The largest Ls at different ages suggested that the growth of this population was faster than that of all the other populations throughout the lifespan, probably reflecting higher prey availability. It was reported that fishery resources, including S. kneri, had been undergoing substantial overexploitation in the Guishi Reservoir (Liu et al., 2010). Potentially low abundance of S. kneri and other piscivorous fishes might cause a high growth rate of this species in the Guishi Reservoir.
Both scales and otoliths are very commonly used to age fish (Campana, 2001; Santana et al., 2006). The choice of which hard part for age determination requires validation of ring deposition combined with comparative study of aging techniques across hard parts within a species (Barnes and Power, 1984; Robillard and Marsden, 1996; Maceina and Sammons, 2006). Scales have been traditionally most commonly applied in fish age determination as they can be nonlethally removed, and are more convenient and easy to process (Busacker et al., 1990). Age determination using otoliths is suggested more reliable than scales in some cases (Jones, 1992; Campana and Thorrold, 2001; Santana et al., 2006). While, in many species, age determination from otoliths and scales agree well with each other (Boxrucker, 1986; Kruse et al., 1993). Scales were used to determine the age of S. kneri in all the previous studies (Ye et al., 1986; Xie, 1995; Liu et al., 2012b). In addition, opercula are another commonly used structure to determine age in species of the genus Siniperca, e.g., Siniperca scherzeri and Siniperca chuatsi (Wu et al., 1996; Chen et al., 2003). Annual periodicity of annuli in neither scale nor opercula was not validated for Siniperca species. In the present study, we found periodic opaque/ translucent growth zones in the transverse plane section of the sagittal otoliths of S. kneri (Fig. 2a) and validated their annual periodicity. We defined the shift from an opaque zone to the next translucent zone as an annulus, so age could be estimated by counting the shifts. Our results provided the first validated process of age determination for S. kneri and other species of Siniperca. Appearance of annual mark in scales of S. kneri had been well described in previous studies (Ye et al., 1986; Xie, 1995; Liu et al., 2012b). Our preliminary observation showed that age determination from scales agrees basically to that from sagittal otoliths (unpublished data). Thus, we suggest that age and growth determined in the previous researches and the present study could be compared.
Various environmental factors and biological events cause annulus formation on hard structures of fish (Fabré and Saint-Paul, 1998; Pilling et al., 2007). The shift from an opaque zone to the next translucent zone in the otoliths of S. kneri occurred during April and June. The opaque zone was formed in winter and early spring when the growth of the fish was slow and gonad development required a large energy investment; the translucent zone was formed during summer and autumn when growth was fast. Our results showed that the period of annulus formation also highly overlapped with the spawning season for S. kneri in the TGR (from March to July). The environmental and physiological cues inducing the formation of annuli in the otoliths of S. kneri need to be further investigated.
Our results regarding the age, growth and reproduction of S. kneri provided valuable knowledge supporting the fishery management of this species in the TGR. The domination of age-1 and -2 individuals in the harvest of S. kneri in the TGR suggested that many of these fish will not have spawned, or at best only once. It is necessary to set a minimum size of fishery catch on this population to make sure that the individuals will reproduce once or more before harvested. The spawning of S. kneri in the TGR occurred during March and July and peaked in April and May. The season of banned fishing in the TGR was from February through April before 2016, which covered only a part of its spawning season. The season of banned fishing in the TGR was adjusted to March through June in 2016. This adjustment may promote the protection of its spawning. Further studies should be designed based on the data obtained in the present study to assess the resource status of S. kneri in the TGR and to thus develop a proper strategy for the sustainable management of this population.5 CONCLUSION
In the present study, we investigated the age, growth and reproductive biology of S. kneri in the TGR. When compared to other populations of S. kneri evaluated in previous studies, the population in the TGR tended to grow faster in the first year, become mature at a younger age, and have greater fecundity. These opportunistic-tending life history traits may facilitate a rapid increase of the S. kneri populations, thus allowing this species to colonize the TGR. Meanwhile, our results regarding the age structure, growth and reproductive biology of S. kneri provided fundamental information for stock assessment and sustainable management strategies for this commercially important fishery resource. Further studies should be designed to investigate the life history traits and plasticity of other species in the TGR. It is also very important to investigate potential shifts in the life history traits of fish species associated with long-term ecosystem succession in the TGR.6 DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.7 ACKNOWLEDGEMENT
Field sampling was technically supported by Three Gorges Reservoir Ecosystems Experimental Station.
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