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Article

Feeding Selectivity and Diet Shift of Protosalanx chinensis during Spring in Lake Dalong, Northeastern China

1
Heilongjiang River Fisheries Research Institute of Chinese Academy of Fishery Sciences, Harbin 150070, China
2
Scientific Observing and Experimental Station of Fishery Resources and Environment in Heilongjiang River Basin, Ministry of Agriculture and Rural Affairs, Harbin 150070, China
3
National Agricultural Experimental Station for Fishery Resources and Environment, Fuyuan 156500, China
4
College of Fisheries and Life Science, Shanghai Ocean University, Shanghai 201306, China
*
Author to whom correspondence should be addressed.
Water 2023, 15(10), 1847; https://doi.org/10.3390/w15101847
Submission received: 24 March 2023 / Revised: 5 May 2023 / Accepted: 10 May 2023 / Published: 12 May 2023
(This article belongs to the Special Issue Ecology of Freshwater Fishes)

Abstract

:
Ontogenetic niche shifts have played an important role in the life history and ecological functions of fish. The clearhead icefish (Protosalanx chinensis, Basilewsky, 1855) is a small, pelagic, and commercially important fish that mainly feeds on zooplankton and will transition to feeding on fish when prey fish are available, though its life span is only about one year. In fact, we know little about the food selectivity and diet shifting of P. chinensis before its transition to feeding on fish. To reveal the food selectivity of P. chinensis before its transition to feeding on fish, the gut contents and environmental zooplankton community were investigated simultaneously in Lake Dalong of Northeastern China from April to June 2021. The results showed that P. chinensis experienced a diet shift from copepoda to cladocera during spring in Lake Dalong. From April to early June, both the size of cladocera and copepoda in guts increased as the size of P. chinensis increased. However, the favorite category changed to the smaller cladocera in late June, when the density of the smaller cladocera was rather high relatively. Considering June was the critical period for P. chinensis to prepare for transitioning to feeding on fish, the food resources availability must be seriously considered for sustainable aquaculture of P. chinensis. It was hypothesized that there was a trade-off of feeding selectivity between the size and density of the prey.

1. Introduction

Many piscivorous fishes, including freshwater and marine fish, experience an ontogenetic diet shift of transitioning from non-fish prey to fish prey over time [1,2]. Transitioning to piscivory behavior results in decreased mortality, increased growth and reproductive fitness, and then enhanced lifetime fitness [3,4,5]. Individuals of the same cohort usually transition to piscivory asynchronously, and the individuals that transition early to piscivory are characterized by early birth and fast growth, and the food resources availability is also important to achieve a higher growth rate before transition to piscivory [6,7,8,9]. The diet of fish before a transition to piscivory, though of not being well known, is a key prerequisite [10]. Meanwhile, the diet shift of fish also drastically occurs during the zooplanktivorous larvae and juvenile stage, probably corresponding with the increased feeding ability resulting from enlargement of the mouth, increased swimming ability, and development of sense organs [11,12]. Exploring the diet shift of a dominant fish in a certain community is essential to deepening the knowledge concerning the prey–predator interactions and the role of habitat in the feeding habits of the fish, all of which is important to understanding trophic network dynamics and managing fisheries sustainably [13,14,15].
The clearhead icefish (Protosalanx chinensis, Basilewsky, 1855) is a pelagic, euryhaline fish species of Salangids native to the coasts of the East China Sea, Yellow Sea, and Bohai Sea, as well as the adjacent rivers and lakes that connect with them [16,17]. It is a short-lived small fish that can reach a maximum size of 225 mm standard length with a life span of approximately one year [18]. P. chinensis exhibits a semelparous reproductive strategy, with spawning primarily taking place during the months of December and January [19]. Despite the small size, this fish is precious and of high commercial value and has been traditionally exploited in China; the main historical fishing grounds were the large shallow lakes in the lower reaches of the Yangtze River basin, such as Lake Taihu [20]. In the past several decades, however, native P. chinensis populations have been degraded markedly and continuously due to overfishing and habitat degradation both in freshwaters and estuaries [21]. Fortunately, transplanting efforts, which have been underway since 1987 due to the economic and germplasm relevance of P. chinensis, have resulted in widespread introductions into many reservoirs and lakes, including brackish waters in northern inland China [22,23]. Nevertheless, the stock collapsed in Lake Daihai after the first harvest of the first case of successfully transplanted P. chinensis in China [23], and almost all the transplanted stocks exhibited likewise dramatic fluctuation, which limited the commercial viability of the industry of P. chinensis over the years [19]. Based on the investigations, we found that the population characteristics in the stages of early-stage juvenile and reproductive adult were the critical aspects affecting the population dynamic and that the “self-thinning” phenomenon only occurred in early-stage juveniles before a diet shift in high-density stocks [24,25]. After their introduction into Lake Dalong in 1997, as of 2005, P. chinensis became the most important commercial fish of this lake, which has made Lake Dalong an important ranching watery and germplasm bank of P. chinensis. Ranching in Lake Dalong, which is no exception, has also experienced similar population dynamics and “self-thinning”.
“Self-thinning” refers to the process of mortality among young organisms, which often results from the competitive stresses derived from a high density of the same species [26], and competition for food resources is an important source of intra-specific stresses in high-density populations. In the wild populations, early-stage juvenile P. chinensis feed mainly on zooplankton, and larger young-stage individuals can switch to fish and shrimp when the food resources are available, but smaller young-stage individuals cannot transition to piscivory in a lifetime due to the absence of food resources [20,24]. In the artificially stocked high-density populations, “self-thinning” happened in early-stage juveniles, and the populations collapsed before a transition to feed on fish and shrimp. There is no doubt that relative prey abundance should be an important factor to the population dynamics [25]. In addition, the prey size commonly increases as the predator size increases [27], and the size of prey should be considered in investigating the feeding habits of fish and evaluating the prey abundance. Zooplankton has been reported as the main food of early-stage juveniles P. chinensis [20,24,28], but it is not well understood which categories and sizes of zooplankton are more important through this period.
The present study aims to investigate the ontogenetic variations in prey preference of different sizes and categories of P. chinensis during the critical period before transitioning to feed on fish and shrimp to deepen the knowledge on the food resources requirements. This is essential to improve both native population conservation and restoration and to ensure sustainable high yield in transplanted stocks management.

2. Materials and Methods

2.1. Study Area

Lake Dalong is a shallow alkaline lake located in the lower reaches of the Nen River (N 46°40′–46°47′, E 124°19′–124°26′), northeastern China. As a northern lake, its ice-free period is from mid-April through mid-November. The long-term average water level area is about 110 km2 with an average depth of 3.5 m. Specimens were sampled from the northwestern part of Lake Dalong near the locality of Hefa Village of Duerbote County (Figure 1).

2.2. Fish Sampling

Clearhead icefish (P. chinensis) were sampled on April 21, May 20, June 4 (early June, E-Jun.) and June 22 (late June, L-Jun.) 2021. One cone trawler net (length, 5 m; diameter, 1 m; mesh size, 1 × 1.2 mm) was used for sampling fish in April and May, and another cone trawler net (length, 5 m; diameter, 1 m; mesh size, 2.0 × 1.5 mm) was used for sampling fish in June. The body lengths of collected P. chinensis samples were measured to the nearest 1 mm in the laboratory to analyze the size relationship between the prey and P. chinensis. It is possible to determine whether the gut is empty superficially because the body of P. chinensis is translucent, and 30 specimens with non-empty guts were selected in each sample (for a total number of 120 sampled specimens) and fixed in 10% buffered formaldehyde for at least one week. Then the specimens were rinsed three times and transferred to 75% ethanol for prey determination.

2.3. Prey Sampling

We sampled zooplankton 4 times on the same days and sites as the fish sampling from April to late June and collected double samples at each sampling to obtain the average data. For each sample, 5 L lake water was collected from a level 0.5 m under the surface, and the other 5 L lake water was collected from the level 0.5 m above the bottom and filtered through a shallow-water plankton net (length 50 cm, diameter 20 cm, mesh size 63 μm) on board, and the target zooplankton were sieved out. Subsequently, these collected plankton net samples were stored in 60 mL PE bottles, fixed with 5% buffered formaldehyde solution (final concentration), and transported to the laboratory for further analysis.

2.4. Gut Content and Prey Source Analysis

Some of the sampled guts contained fully digested and unidentifiable prey, and 71 guts with identifiable prey were analyzed. The middle section of the guts with undigested prey were cut, and the gut contents were removed to a slide. All zooplankton samples from water column and gut contents were examined and measured under a microscope system (Olympus cellsens Entry 1.18, Tokyo, Japan), and the body length of each item was measured to the nearest 1 μm when possible. The biomass of cladocera was calculated via the individual average weight of the same genus, and that of copepoda was calculated via the body length–weight relationship [29].

2.5. Data Analysis

Frequency of Occurrence (FO,F%), Weight Percentage (W%), Number Percentage (N%), Index of Relative Importance (IRI), and Index of Relative Importance Percentage (IRI%) of each prey item were calculated as follows:
F% = Os/Od × 100%,
W% = Ws/Wd × 100%,
N% = Ns/Nd × 100%,
IRI = F% × (W% + N%),
IRI% = (IRI/ΣIRI) × 100%,
where Os is the counted number of guts with a specific prey item, Od is the counted number of guts with diet, Ws is the total weight of a specific prey item, Wd is the total weight of all prey items, Ns is the total number of a specific prey item, and Nd is the total number of all prey items.
General patterns have been found in fish morphology–diet relations, and the scaling has been reliable [27]. To detect the food selectivity of P. chinensis on the main diet of different sizes, both cladocera and copepoda sampled in water and guts were classified into size classes by 200 μm intervals of body length.
Food selectivity of P. chinensis, which was estimated using the Ivlev index E [30], was calculated as follows:
E = (riPi)/(ri + Pi),
where ri is the relative proportion of item i in the diet of P. chinensis, and Pi is the relative proportion of item i in all the environment zooplankton items. The E range is −1.0–1.0. According to Cotonnec [31], −0.25 < E < +0.25 indicates non-selective feeding, E ≥ +0.25 indicates a preference, and E ≤ −0.25 indicates discrimination against particular prey items.
In order to reveal zooplankton size changes in the guts of P. chinensis, the single sample K–S (Kolmogorov–Smirnov) test was used to determine whether the body length of zooplankton in the guts in each month was a normal distribution, and then the Mann–Whitney U test was used to compare the body length of copepoda and cladocera in the guts of P. chinensis in successive samplings. These statistical analyses were conducted using SPSS Statistics 26.0 (IBM, Armonk, NY, USA), and p < 0.05 was considered significant. The community structure dynamics of zooplankton in the lake and the analysis of gut content and feeding selectivity of P. chinensis were conducted using Microsoft Excel 2019 (Microsoft, Seattle, WA, USA). Scatter diagrams were plotted using Graphpad prism 8.0 (Graphpad software, San Diego, CA, USA).

3. Results

3.1. Size Groups of Fish Sampled and Number of Guts Analyzed

There were 120 specimens of P. chinensis collected in total. The standard body length of the samples ranged 11–79 mm, and the samples were divided into 6 groups based on 10 mm interval in body length: 10–19 mm, 20–29 mm, 30–39 mm, 40–49 mm, 60–69 mm, and 70–79 mm (Table 1). The number of samples in different size classes and the number of guts with identifiable content are shown in Table 1.

3.2. Zooplankton Community Dynamics during Spring

Four categories of zooplankton (copepoda, cladocera, nauplii, and rotifer) were found during the spring. The overall density ranged 26.25–266.25 ind./L, peaking in late June and bottoming out in early June. Rotifer density peaked in late June and bottomed out in April. Nauplii density peaked in May and bottomed out in early June. Copepoda density peaked in May and bottomed out in early June. Cladocera density peaked in May and bottomed out in April (Figure 2).
Zooplankton biomass ranged 0.42–1.72 mg/L, peaking in May and bottoming out in early June. Cladocera biomass peaked in May and bottomed out in early June. Copepoda biomass peaked in late June and bottomed out in early June. Nauplii biomass peaked in May and bottomed out in early June. Rotifer biomass peaked in late June and bottomed out in April (Figure 3).
Among cladocera and copepoda, the smallest individual found was a cladocera of 207.57 μm in body length, and the biggest one was a cladocera of 1601.13 μm. Therefore, 7 size classes were defined: I, 200–400 μm; II, 400–600 μm; III, 600–800 μm; IV, 800–1000 μm; V, 1000–1200 μm; VI, 1200–1400 μm; and VII, 1400–1601.13 μm.
Both cladocera and copepoda were sorted into the seven defined classes. In April, all cladocera individuals were longer than 1200 μm, while copepoda ranged 400–1000 μm and were mainly in the 600–800 μm class. In May, cladocera ranged 200–1500 μm and were mainly in the 600–800 μm class, while copepoda also ranged 200–1500 μm but were mainly in the 400–600 μm class. In early June, cladocera ranged 200–1400 μm and were mainly in the 800–1000 μm class, while copepoda ranged 200–1200 μm and were mainly in the 400–600 μm class. In late June, cladocera ranged 200–1000 μm and were mainly in the 200–400 μm class, while copepoda ranged 200–400 μm and 800–1400 μm and were mainly in the 200–400 μm and 800–1200 μm classes (Figure 4).

3.3. Diet Composition

Among all the samples, cladocera and copepoda were dominant prey categories. No matter in terms of FO, number, or weight, cladocera was the most important prey category, and copepoda was the second most important prey category (Table 2).
In April, no cladocera were found in the guts; only copepoda occurred with smaller nauplii, and nauplii stopped occurring after May. Cladocera and copepoda occurred in every gut in May. FO of copepoda decreased to 50% starting in early June, and no copepoda were found in the guts in late June, while cladocera occurred in every gut in June (Figure 5).
The mass percentage, quantity percentage, and IRI percentage of different prey categories showed the same trends with FO, i.e., copepoda declined and cladocera increased in the diet of P. chinensis as time went on. The IRI percentage of cladocera began to exceed that of copepoda, reaching 55.9% in May, and cladocera became the sole dominant prey category in early June (Figure 6).
Sizes of copepoda in guts were relatively concentrated, and 90.94% of them fell in the 500–1000 μm classes in body length, though the average size of copepoda increased along with the growth of P. chinensis during the spring. In May, P. chinensis ingested a broader spectrum of copepoda in size than in April, and the average body length of copepoda also increased significantly (p < 0.05, Z = 2.173). In early June, the average body length of copepoda also increased significantly compared to May (p < 0.05, Z = 2.579), but copepoda had by then become unimportant as a food source for P. chinensis (Figure 7a).
Cladocera in guts were mainly concentrated in two size classes. The class of 300–500 μm accounted for 54.12%, and the class of 900–1200 μm accounted for 23.53%. In early June, the body length of gut cladocera decreased significantly compared to May (p < 0.05, Z = −2.966), and the decrease in late June was greater than that in early June (p < 0.05, Z = −12.284), showing a downward trend overall (Figure 7b).

3.4. Feeding Selectivity

In April, P. chinensis preferred to feed on copepoda and avoid cladocera and nauplii based on both quantity and mass data (Figure 8). In May, P. chinensis actively selected copepoda and avoided nauplii with random selection of cladocera based on both quantity and mass data. In early June, nauplii were avoided, and copepoda were randomly selected based on both quantity and mass data, while there was an active selection for cladocera based on quantity data. In late June, cladocera were actively selected, and nauplii and copepoda were avoided based on both quantity and mass data. We obtained almost the same results based on quantity or mass of categories in each sampling (Figure 8).
In terms of number, the copepoda class of 400–600 μm was actively selected, and the copepoda classes of 600–1000 μm and cladocera (longer than 1200 μm) were avoided in April. The copepoda classes of 600–1200 μm and the cladocera classes of 1000–1400 μm were actively selected, and other categories and classes were randomly selected in May. The copepoda class of 1000–1200 μm and the cladocera class of 1000–1200 μm were actively selected in early June. However, only the cladocera classes of 200–800 μm were actively selected in late June, during which all other categories and classes were avoided (Table 3).
The selectivity of size based on mass was almost in accordance with that based on quantity, except that copepoda of 600–1000 μm were randomly selected in April, copepoda of 600–800 μm and cladocera of 1000–1200 μm were randomly selected in May, all copepoda classes were randomly selected in early June, and cladocera of 600–800 μm were randomly selected in late June (Table 3).

4. Discussion

4.1. Zooplankton Categories and Feeding Selectivity of P. chinensis

This research provided the first report of P. chinensis feeding selectivity on the categories of zooplankton. There is almost no space between the two adjacent gill rakers of P. chinensis fry less than 10 mm that mainly feed on rotifer [25,32]. In April, the body length of P. chinensis ranged 11–19 mm in Lake Dalong, and the corresponding gill raker space was less than 70 μm [32]; therefore, copepoda were the favorite food of P. chinensis due to the moderate size, while cladocera were too big, and nauplii were too small, though nauplii also occurred in the guts. In May, the body length of P. chinensis ranged 20–39 mm, and the corresponding gill raker space was less than 90 μm [32], and the mouth size also increased. Compared to April, May showed increases in both the total density and biomass of zooplankton, and smaller cladocera occurred in the water. However, copepoda were still the sole favorite food, even though cladocera commonly occurred in the guts; there were no longer any nauplii found in the guts, which meant nauplii were too small to be retained by the gill rakers. In early June, compared to May, both total density and biomass of zooplankton decreased, with the density of copepoda in particular decreasing sharply, and cladocera became the sole favorite food. Rotifer density increased dramatically in late June, but only cladocera occurred in the guts, which meant rotifers were also too small for the diet of P. chinensis. In the spring of Lake Taihu, copepoda comprised the dominant categories, and the FO of copepoda in the guts of P. chinensis increased before decreasing while the FO of cladocera experienced a process of decreasing, increasing, and decreasing. The FO of copepoda and cladocera showed the same decreasing trend since summer, when both cladocera and copepoda were dominant categories in the water column, and the total number of cladocera in guts was slightly higher than copepoda throughout the year, though the abundance of copepoda was slightly higher than that of cladocera in the water column [20,33]. In Lake Xingkai, cladocera were more important than copepoda in the diet of P. chinensis even though the copepoda biomass was about eight times as much as the cladocera biomass in the water column [24,34]. Therefore, P. chinensis prefer cladocera to copepoda to a certain extent, which is similar to Japanese smelt Hypomesus nipponensis, although Japanese smelt has a broader feeding spectrum [35,36]. In the P. chinensis populations of Lake Xingkai and LakeTaihu, the categories of copepoda and cladocera were both the most important prey throughout the spring [20,24], but this research indicated the diet of P. chinensis shifted from copepoda to cladocera, which was related to the prey resource changes in Lake Dalong.

4.2. Size Correlation between P. chinensis and the Prey

This research also provided the first report of P. chinensis feeding selectivity on the size of zooplankton. With a diet shift in fish, the prey size usually increases as the fish grows, provided that the new prey is readily available [27]. In this study, P. chinensis in the size class of 10–20 mm preferred to feed on copepoda in the size class of 400–600 μm. In addition, P. chinensis in the size class of 20–40 mm preferred to feed on copepoda in the size classes of 600–1200 μm and on cladocera in the size classes of 1000–1400 μm, and they could catch the biggest zooplankton in this water, which meant the size of their mouth was big enough to prey on any large-size zooplankton since May. Nevertheless, P. chinensis in the size class of 40–50 mm preferred to feed on both copepoda and cladocera in the size class of 1000–1200 μm, and P. chinensis in the size class of 60–80 mm preferred to feed on cladocera in the size class of 200–800 μm, which meant they would rather prey on smaller prey of high density than on larger prey of low density. There appears to be a trade-off of feeding selectivity between the size and density of the prey; this subject needs further study.

4.3. Top-Down Effects of P. chinensis Predation and Fishery Management

In general, zooplankton biomass increases as the water temperature increases [37]. However, the zooplankton biomass in June was lower than that in May, and it was the lowest in early June during the spring in Lake Dalong. These unusual dynamics of zooplankton can probably be attributed to the predation of aquatic animals, and the sharp decreasing of macro-zooplankton biomass and dramatic increasing of small cladocera and rotifer biomasses are probably related to the feeding selectivity of P. chinensis. Obviously, there was a bottleneck of prey resources of P. chinensis in June, which was the period in which P. chinensis transitioned to feed on fish and shrimp and when the self-thinning occurred. Therefore, effective measures must be taken to manipulate the population density of P. chinensis and the abundance of macro-zooplankton in June to prevent that “self-thinning” from occurring and the fishery population from collapsing.

5. Conclusions

This research provided the first report of P. chinensis feeding selectivity on the categories and sizes of zooplankton. We found that there was a diet shift from copepoda to cladocera in spring and that P. chinensis preferred cladocera to copepoda after that point. It also was observed that P. chinensis would rather prey on smaller prey of high density than on larger prey of low density, and it was hypothesized that there was a trade-off of feeding selectivity between the size and density of the prey. This research suggested that the prey resources of P. chinensis must be addressed in June to avoid “self-thinning” from occurring in fishery management.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w15101847/s1, Figure S1: Body length distribution of copepoda and cladocera in Lake Dalong during spring 2021 (mass percentage).

Author Contributions

Sampling, methodology, statistical analysis, writing—preparation of the original draft, H.Z.; proposing the idea of the study, methodology, writing—review and editing, supervision, funding acquisition, F.T.; sampling and zooplankton identification, Z.L.; sampling, measurements, W.L.; sampling, mapping, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by “Project of Research and Development on Applied Technology of Heilongjiang Province, grand number (GA20B202)” and “The Central-level Non-profit Scientific Research Institutes Special Fund of China, grand number 2020TD07” and “Project serving for the Ministry of Agriculture and Rural Affairs of China, grand number A120401”.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article and its Supplementary Materials.

Acknowledgments

We express our sincere gratitude to Changzhou Yangtze River Aquatic Product Co., Ltd. and her workers for the support in sampling.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location of sampling area in Lake Dalong.
Figure 1. Location of sampling area in Lake Dalong.
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Figure 2. Density variation of zooplankton in Lake Dalong during spring 2021.
Figure 2. Density variation of zooplankton in Lake Dalong during spring 2021.
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Figure 3. Biomass variation of zooplankton in Lake Dalong during spring 2021.
Figure 3. Biomass variation of zooplankton in Lake Dalong during spring 2021.
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Figure 4. Body length distribution of copepoda and cladocera in Lake Dalong during spring 2021.
Figure 4. Body length distribution of copepoda and cladocera in Lake Dalong during spring 2021.
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Figure 5. Changes of occurrences of prey categories in the diet of P. chinensis in Lake Dalong during spring 2021.
Figure 5. Changes of occurrences of prey categories in the diet of P. chinensis in Lake Dalong during spring 2021.
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Figure 6. Changes of W%, N%, and IRI% of prey categories in the diet of P. chinensis in Lake Dalong during spring 2021.
Figure 6. Changes of W%, N%, and IRI% of prey categories in the diet of P. chinensis in Lake Dalong during spring 2021.
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Figure 7. The size relationships between the prey and P. chinensis in Lake Dalong during spring 2021. (a) The size relationships between copepoda and P. chinensis and (b) the size relationships between cladocera and P. chinensis.
Figure 7. The size relationships between the prey and P. chinensis in Lake Dalong during spring 2021. (a) The size relationships between copepoda and P. chinensis and (b) the size relationships between cladocera and P. chinensis.
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Figure 8. Ivlev index for food categories of P. chinensis in Lake Dalong during spring 2021.
Figure 8. Ivlev index for food categories of P. chinensis in Lake Dalong during spring 2021.
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Table 1. The number of samples and guts analyzed for diet determination of P. chinensis in Dalong Lake in each sample.
Table 1. The number of samples and guts analyzed for diet determination of P. chinensis in Dalong Lake in each sample.
Size CLASS (mm)AprilMay Early JuneLate June
10–1930 (10)
20–2913 (10)
30–3917 (15)
40–4930 (15)
60–699 (6)
70–7921 (15)
Note: The numbers outside the parentheses represent the number of samples, and those inside the parentheses represent the number of guts with identifiable content; “—” indicates determination is absent.
Table 2. The importance of prey categories in the diet of P. chinensis in Lake Dalong during spring 2021.
Table 2. The importance of prey categories in the diet of P. chinensis in Lake Dalong during spring 2021.
Occurrence Percentage (F%)Quantity Percentage (N%)Weight Percentage (W%)IRIIRI%
Cladocera84.5158.6276.3211403.7877.67
Copepoda50.7041.0823.573277.7622.32
Nauplius1.410.310.110.59--
Note: “--” indicates that the value is less than 0.01.
Table 3. Ivlev index for copepoda and cladocera in different size classes.
Table 3. Ivlev index for copepoda and cladocera in different size classes.
Size ClassesIIIIIIIVVVIVII
Ivlev (Quantity)
Copepoda
Apr.0.00 0.62 −0.25 −0.45 0.00 0.00 0.00
May−0.62 −0.06 0.34 0.81 0.44 −0.62 −0.62
E-Jun.−1.00 −0.86 −0.24 −0.13 0.30 0.00 0.00
L-Jun.−1.00 0.00 0.00 −1.00 −1.00 −1.00 0.00
Cladocera
Apr.0.00 0.00 0.00 0.00 0.00 −1.00 −1.00
May−0.76 −0.04 −0.84 −0.51 0.30 0.35 0.24
E-Jun.−1.00 −0.66 0.04 0.23 0.65 0.21 0.00
L-Jun.0.25 0.83 0.29 −0.63 0.00 0.00 0.00
Ivlev (Mass)
Copepoda
Apr.0.00 0.84 0.19 0.00 0.00 0.00 0.00
May−0.63 −0.15 0.21 0.78 0.36 −0.68 −0.68
E-Jun.−1.00 −0.90 −0.10 −0.33 0.10 0.00 0.00
L-Jun.−1.00 0.00 0.00 −1.00 −1.00 −1.00 0.00
Cladocera
Apr.0.00 0.00 0.00 0.00 0.00 −1.00 −1.00
May−0.79 −0.13 −0.85 −0.58 0.21 0.43 0.14
E-Jun.−1.00 −0.76 −0.20 0.03 0.51 0.01 0.00
L-Jun.0.37 0.87 0.23 −0.55 0.00 0.00 0.00
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Zeng, H.; Tang, F.; Li, Z.; Lu, W.; Zheng, Y. Feeding Selectivity and Diet Shift of Protosalanx chinensis during Spring in Lake Dalong, Northeastern China. Water 2023, 15, 1847. https://doi.org/10.3390/w15101847

AMA Style

Zeng H, Tang F, Li Z, Lu W, Zheng Y. Feeding Selectivity and Diet Shift of Protosalanx chinensis during Spring in Lake Dalong, Northeastern China. Water. 2023; 15(10):1847. https://doi.org/10.3390/w15101847

Chicago/Turabian Style

Zeng, Haoyu, Fujiang Tang, Zhe Li, Wanqiao Lu, and Yi Zheng. 2023. "Feeding Selectivity and Diet Shift of Protosalanx chinensis during Spring in Lake Dalong, Northeastern China" Water 15, no. 10: 1847. https://doi.org/10.3390/w15101847

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