Research Article |
Corresponding author: Milena Radenković ( milena.radenkovic@pmf.kg.ac.rs ) Academic editor: Ken Longenecker
© 2022 Milena Radenković, Milica Stojković Piperac, Aleksandra Milošković, Nataša Kojadinović, Simona Đuretanović, Tijana Veličković, Marija Jakovljević, Marijana Nikolić, Vladica Simić.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Radenković M, Piperac MS, Milošković A, Kojadinović N, Đuretanović S, Veličković T, Jakovljević M, Nikolić M, Simić V (2022) Diet seasonality and food overlap of Perca fluviatilis (Actinopterygii: Perciformes: Percidae) and Rutilus rutilus (Actinopterygii: Cypriniformes: Cyprinidae) juveniles: A case study on Bovan Reservoir, Serbia. Acta Ichthyologica et Piscatoria 52(1): 77-90. https://doi.org/10.3897/aiep.52.78215
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European perch, Perca fluviatilis Linnaeus, 1758 and roach, Rutilus rutilus (Linnaeus, 1758) are the most common species present in mesotrophic and eutrophic lakes throughout Europe. Their biomass, especially in juvenile stages, contributes the most to the fish production of these ecosystems. In Bovan Reservoir, these two species constitute the bulk of the juvenile fish biomass. This study aimed to investigate the feeding composition of these two species in order to evaluate their niche overlap due to the availability of resources during different seasons. Traditional diet analysis indices and Kohonen artificial neural network (i.e., a self-organizing map, SOM) were used to investigate the diet of 158 individuals of both species and evaluate their food niche overlap. The indicator value (IndVal) was applied to identify indicator food categories based on which the contents of their alimentary tracts were grouped first into neurons and then into clusters on the SOM. Our results showed that juvenile fish used zooplankton and benthic prey in their diet. Roach often fed on nonanimal prey, while perch of age 0+ used fishes in the diet. Additionally, four clusters of neurons were isolated on the SOM output network. The distribution of perch and roach alimentary tracts in neurons indicated no high degree of competition between them. While diet analyses indices show which food category is generally important in specimensʼ diet, the SOM recognizes those specimens and arranges them together into the same or adjacent neurons based on dominant prey. Understanding fish feeding habits is critical for the development of conservation and management plans. Since Bovan is a eutrophic reservoir, our knowledge of fish feeding habits needs to be considered for stocking strategies in the future.
feeding overlap, IndVal index, perch, roach, self-organizing map
Dietary analysis has been used for decades in biological and ecological studies of different fish species (
Studies of diet in fish assemblages at a certain location allow us to recognize distinctive trophic guilds and make inferences about their structure, the degree of importance of the different trophic levels, and the relations among their components (
Perch, Perca fluviatilis Linnaeus, 1758, and roach, Rutilus rutilus (Linnaeus, 1758), are two fish species cohabiting the littoral zone in many European lakes (
Study area and fish sampling. Bovan is an artificial reservoir situated in the middle flow of the Sokobanjska Moravica River near the municipality of Aleksinac in southeast (43°38′46′′N, 021°42′28′′E) (Fig.
The field-work was conducted in May and September of 2011 and 2012. Fish were sampled using gillnets of mesh size 10 mm. For each analyzed fish, the total length (TL) was measured to the nearest mm and then weighted (W) to the nearest g. Studies of fish diet, feeding ecology, and food habits are carried out commonly through dissection and examination of alimentary tracts (
Alimentary tract content analysis. Shannon’s diversity index (H) was used to assess the prey diversity of the dietary contents in each fish species during all seasons. The index was calculated as
H = –Σ(pi)(lnpi)
where pi is the proportion of individuals belonging to the ith species relative to the total number of individual prey items recovered for a fish species (
To determinate the most important prey in the diet, the Prominence Value (PV) of the dietary component was calculated using the following formulas (
%PV = 100PV ∙ ΣPV–1
where %FO is the frequency of occurrence (the number of alimentary tracts containing each food item in relation to the total number of alimentary tracts with food), and %N is relative abundance (the number of individuals of each food item with respect to the total number of individuals). The vacuity index (%VI) was used to express a number of empty alimentary tracts (
To interpret the species’ feeding strategy, the
Pi = 100ΣSi ∙ ΣSti–1
where Pi is the prey-specific abundance of prey i; Si is the alimentary tract content (by number) comprised of prey i, and Sti is the total alimentary tract content in only those fish with prey i in their alimentary tracts. In the graph, prey items positioned in the upper part of the graph show a specialist feeding strategy of the fish, and those positioned in the lower part indicate a generalist feeding strategy of the fish. Besides, the diet specialization was estimated by the diet evenness index (E)
E = H ∙ Hmax–1
where Hmax = lnS, and S is the total number of preys in the sample. According to
Diet similarity among different species of fish, or the same species during different seasons, was assessed using Schoener’s overlap index (α). It was evaluated using the Prominence value (PV) of each food item (
α = 1 − 0.5(Σ|PVxi − PVyi|)
where PVxi is prominence values of food item i in species x, PVyi is prominence values of food item i in species y. The index has a minimum of 0 (no overlap), and a maximum of 1 (complete overlap). According to
Statistical data analysis. Analysis of alimentary tract content allows us to determine species’ diet composition and further understand their feeding habits and trophic role in the ecosystem (
The network structure of the SOM is composed of two layers, the input and output, each consisting of data processing units, i.e., neurons (
Since SOM is a visualization technique without any statistical indication, the indicator value (IndVal) by
Aij = Mij ∙ Mi−1
Fij = NATij ∙ NATj−1
IndVal ij = 100AijFij
where Mij is mean value of mass of food category (i) in the alimentary tracts of cluster (j), Mi is mean value of mass of food category (i), NATij is the relative frequency of occurrence of food category (i) in the alimentary tracts of cluster (j), NATj is the relative frequency of occurrence of all food categories of cluster (j), Ai is the relative abundance in percentage (%), and Fij is the relative frequency of occurrence in percentage (%) of food category (i) in the alimentary tracts of cluster (j).
The Monte Carlo significance test with 1000 permutations was applied to identify significant prey taxa with the use of PC-ORD statistical software (
A total number of 130 individuals, with 7.4–11.2 cm in TL, were used to examine diet composition. The number of analyzed specimens by season was as follows: 23 specimens for perch in spring 2011, 20 specimens in autumn 2011, then 17 specimens in spring 2012, and 12 specimens in autumn 2012. The number of analyzed specimens of roach was the same in the spring of both years (18 specimens), then in autumn of 2011 (15 specimens), and finally in the autumn of 2012 (7 specimens). Fish with empty alimentary tracts (28 individuals) were excluded (%VI = 17.72).
Values of the frequency of occurrence (%FO), relative abundance (%N), and prominence value (%PV) for each food category found in alimentary tracts of analyzed fish are presented in Tables
Assessment of diet composition of perch (Perca fluviatilis) and roach (Rutilus rutilus) collected in 2011 from Bovan Reservoir, Serbia, expressed as relative abundance (%N), frequency of occurrence (%FO), and prominence value (%PV) of food.
Taxon or group | Spring 2011 | Autumn 2011 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Perch | Roach | Perch | Roach | |||||||||
%N | %FO | %PV | %N | %FO | %PV | %N | %FO | %PV | %N | %FO | %PV | |
Protozoa | 2.06 | 26.08 | 1.22 | — | — | — | 0.83 | 20.00 | 0.43 | — | — | — |
Rhizopoda | — | — | — | 2.63 | 11.11 | 1.14 | — | — | — | 5.61 | 20.00 | 3.32 |
Rotatoria | 0.51 | 4.34 | 0.12 | 2.63 | 5.55 | 0.81 | — | — | — | 3.57 | 6.66 | 1.22 |
Bryozoa | 6.92 | 30.43 | 4.45 | — | — | — | 7.61 | 30.00 | 4.88 | 4.08 | 6.66 | 1.39 |
Hydracarina | 0.07 | 4.34 | 0.01 | — | — | — | 1.39 | 25.00 | 0.81 | — | — | — |
Ostracoda | 2.35 | 43.47 | 1.81 | 10.52 | 55.55 | 10.24 | 5.47 | 75.00 | 5.55 | 6.12 | 53.33 | 5.92 |
Anostraca | — | — | — | — | — | — | — | — | — | — | — | — |
Conchostraca | 0.88 | 13.04 | 0.37 | — | — | — | 0.18 | 5.00 | 0.04 | — | — | — |
Notostraca | — | — | — | — | — | — | — | — | — | — | — | — |
Cladocera | 0.22 | 4.34 | 0.05 | — | — | — | 0.37 | 5.00 | 0.09 | — | — | — |
Daphnia sp. | 5.15 | 26.08 | 3.07 | 14.73 | 88.88 | 18.14 | 1.11 | 10.00 | 0.41 | 11.73 | 86.66 | 14.46 |
Bosmina sp. | 6.84 | 73.91 | 6.86 | 25.78 | 88.88 | 31.75 | 8.72 | 80.00 | 9.14 | 24.48 | 93.33 | 31.32 |
Leptodora kindtii | 0.88 | 17.39 | 0.42 | — | — | — | 0.09 | 5.00 | 0.02 | — | — | — |
Calanoida (Copepoda) | 27.54 | 100.0 | 32.17 | 12.63 | 55.55 | 12.29 | 21.63 | 85.00 | 23.38 | 14.28 | 66.66 | 15.44 |
Cyclopoida (Copepoda) | 35.42 | 95.65 | 40.46 | 20.00 | 55.55 | 19.47 | 43.63 | 90.00 | 48.53 | 18.87 | 66.66 | 20.40 |
Isopoda | 0.07 | 4.34 | 0.01 | — | — | — | — | — | — | — | — | — |
Amphipoda | 5.59 | 73.91 | 5.61 | 1.57 | 5.55 | 0.48 | 4.82 | 65.00 | 4.55 | — | — | — |
Gammaridae | 0.07 | 4.34 | 0.01 | — | — | — | — | — | — | — | — | — |
Insecta (other) | — | — | — | — | — | — | 0.09 | 5.00 | 0.02 | — | — | — |
Diptera (other) | — | — | — | — | — | — | 0.27 | 5.00 | 0.07 | — | — | — |
Chironomidae | 3.97 | 34.78 | 2.73 | 1.05 | 11.11 | 0.45 | 3.24 | 25.00 | 1.89 | 0.51 | 6.66 | 0.17 |
Plecoptera | 0.58 | 8.69 | 0.19 | — | — | — | — | — | — | — | — | — |
Ephemeroptera | — | — | — | — | — | — | 0.18 | 5.00 | 0.04 | — | — | — |
Trichoptera | 0.07 | 4.34 | 0.01 | — | — | — | 0.09 | 5.00 | 0.02 | — | — | — |
Oligochaeta | 0.07 | 4.34 | 0.01 | 8.42 | 22.22 | 5.18 | 0.18 | 5.00 | 0.04 | 10.71 | 20.00 | 6.34 |
Fishes | 0.66 | 17.39 | 0.32 | — | — | — | — | — | — | — | — | — |
Detritus | — | 94.44 | — | — | 33.33 | — | — | — | — | — | 100.0 | — |
Assessment of diet composition of perch (Perca fluviatilis) and roach (Rutilus rutilus) collected in 2012 from Bovan Reservoir, Serbia, expressed as relative abundance (%N), frequency of occurrence (%FO), and prominence value (%PV) of food.
Taxon or group | Spring 2012 | Autumn 2012 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Perch | Roach | Perch | Roach | |||||||||
%N | %FO | %PV | %N | %FO | %PV | %N | %FO | %PV | %N | %FO | %PV | |
Protozoa | 2.73 | 35.29 | 1.84 | — | — | — | 0.83 | 25.00 | 0.48 | — | — | — |
Rhizopoda | — | — | — | 2.95 | 16.66 | 0.84 | — | — | — | — | — | — |
Rotatoria | 0.91 | 5.88 | 0.25 | 1.68 | 5.55 | 0.47 | — | — | — | — | — | — |
Bryozoa | 6.66 | 35.29 | 4.49 | 2.95 | 5.55 | 0.83 | 8.22 | 25.00 | 4.82 | — | — | — |
Hydracarina | 0.10 | 5.88 | 0.02 | — | — | — | 0.97 | 16.66 | 0.46 | — | — | — |
Ostracoda | 1.82 | 29.41 | 1.12 | 7.59 | 50.00 | 6.46 | 4.87 | 50.00 | 4.03 | 6.25 | 71.43 | 5.74 |
Anostraca | 0.10 | 5.88 | 0.02 | — | — | — | — | — | — | — | — | — |
Conchostraca | — | — | — | — | — | — | 0.69 | 8.33 | 0.23 | — | — | — |
Notostraca | — | — | — | — | — | — | 0.97 | 8.33 | 0.32 | — | — | — |
Cladocera | — | — | — | — | — | — | 1.11 | 8.33 | 0.37 | — | — | — |
Daphnia sp. | 3.23 | 17.64 | 1.54 | 18.98 | 94.44 | 22.21 | 2.08 | 8.33 | 0.70 | 16.66 | 100.0 | 18.11 |
Bosmina sp. | 4.54 | 70.58 | 4.33 | 18.98 | 94.44 | 22.21 | 5.29 | 83.33 | 5.66 | 36.45 | 100.0 | 39.62 |
Leptodora kindtii | 0.20 | 5.88 | 0.05 | — | — | — | — | — | — | — | — | — |
Calanoida (Copepoda) | 26.36 | 100 | 29.94 | 17.72 | 77.77 | 18.81 | 23.67 | 91.66 | 26.58 | 9.37 | 71.43 | 8.61 |
Cyclopoida (Copepoda) | 42.93 | 94.12 | 47.30 | 25.32 | 77.77 | 26.88 | 39.97 | 100.0 | 46.89 | 22.92 | 85.71 | 23.06 |
Isopoda | — | — | — | — | — | — | — | — | — | — | — | — |
Amphipoda | 5.85 | 94.12 | 6.44 | — | — | — | 5.57 | 83.33 | 5.96 | — | — | — |
Gammaridae | — | — | — | — | — | — | — | — | — | — | — | — |
Insecta (other) | 0.10 | 5.88 | 0.02 | — | — | — | — | — | — | — | — | — |
Diptera (other) | 0.10 | 5.88 | 0.02 | — | — | — | — | — | — | — | — | — |
Chironomidae | 4.04 | 29.41 | 2.48 | 1.68 | 11.11 | 0.67 | 4.45 | 33.33 | 3.01 | — | — | — |
Plecoptera | 0.30 | 5.88 | 0.08 | — | — | — | 0.83 | 8.33 | 0.28 | — | — | — |
Ephemeroptera | — | — | — | — | — | — | — | — | — | — | — | — |
Trichoptera | — | — | — | — | — | — | — | — | — | — | — | — |
Oligochaeta | — | — | — | 2.11 | 5.55 | 0.59 | 0.42 | 8.33 | 0.14 | 8.33 | 28.57 | 4.84 |
Fishes | — | — | — | — | — | — | — | — | — | — | — | — |
Detritus | — | — | — | — | 100.0 | — | — | — | — | — | 100.0 | — |
The most varied diet was recorded in perch caught in the spring of 2011 (H = 2.05), with even 21 different prey categories detected, while the perch caught in the autumn of 2012 had the least varied diet (15 different prey categories, H = 1.63). Organisms categorized as Protozoa, Bryozoa, Ostracoda, Bosmina sp. and Daphnia sp. cladocerans, Calanoida, and Cyclopoida copepods, then Amphipoda, and Chironomidae, were the most common prey of all perch, but their proportion in the diet varied from season to season. Calanoid copepods were present in all analyzed perch alimentary tracts caught in spring 2011 and 2012, while cyclopoid copepods were present in all analyzed perch samples caught in autumn 2012. Only perch specimens caught in the spring of 2011 used fish fry in their diet as well as detritus and isopod crustaceans. The similarity in the diet of the analyzed perch was suggested by the high values of Schoener’s overlap index (α from 0.87 to 0.95, Table
Schoener’s overlap index (α) for the whole sample of perch (Perca fluviatilis) and roach (Rutilus rutilus) collected in 2011 and 2012 from Bovan Reservoir, Serbia. The codes provided include P or R for fish species (perch or roach, respectively), the year (2011 and 2012) and the season (S for spring and A for autumn).
α | P2011S | R2011S | P2011A | R2011A | P2012S | R2012S | P2012A | R2012A |
---|---|---|---|---|---|---|---|---|
P2011S | — | 0.54 | 0.87 | 0.58 | 0.93 | 0.65 | 0.94 | 0.65 |
R2011S | — | 0.31 | 0.93 | 0.49 | 0.84 | 0.54 | 0.86 | |
P2011A | — | 0.61 | 0.89 | 0.68 | 0.95 | 0.54 | ||
R2011A | — | 0.56 | 0.84 | 0.57 | 0.83 | |||
P2012S | — | 0.61 | 0.93 | 0.46 | ||||
R2012S | — | 0.54 | 0.83 | |||||
P2012A | — | 0.50 |
Roach did not have a varied diet as perch, and, within species, they had quite a uniform diet during different seasons. Out of, in total, 12 identified prey categories in the diet of roach caught in spring 2011 and 2012, and in autumn 2011, there were as many as 11 prey categories (H = 1.75–1.9). Roach caught in autumn 2012 had the least diverse diet (seven prey categories, H = 1.55). Rhizopoda was the only prey present in the roach diet, but not in the perch diet. The most frequent food categories in the roach diet were members of the class Ostracoda, Calanoida, and Cyclopoida, as well as Daphnia sp. and Bosmina sp. (%FO ≥ 50 in all studied seasons) (Tables
The modified Costello graphic showed mostly a generalized feeding strategy in studied fish including some specimens that specialized on certain prey items (Fig.
Costello graph. Prey-specific abundance vs. frequency of occurrence the diet of perch (Perca fluviatilis) and roach (Rutilus rutilus) collected in 2011 and 2012 from Bovan Reservoir, Serbia. (A) perch spring 2011, (B) roach spring 2011, (C) perch autumn 2011, (D) roach autumn 2011, (E) perch spring 2012, (F) roach spring 2012, (G) perch autumn 2012, (H) roach autumn 2012. Rare preys are encircled.
Four clusters of neurons (A, B, C, and D) were isolated on the SOM output network (Fig.
The 130 alimentary tracts of perch (Perca fluviatilis) and roach (Rutilus rutilus) collected in 2011 and 2012 from Bovan Reservoir, Serbia, assigned to 49 (7 × 7) SOM output neurons within clusters A, B, C, and D. The code for each alimentary tract consists of one letter for the fish species (P or R), two digits for the year of sampling 11 (2011) or 12 (2012), one letter for sampling season (S = spring or A = autumn) and the ordinal number of the individual.
Significant IndVal values were recorded for 10 out of 26 food categories (Table
Relative frequency (%FO), relative abundance (%N), and indicator values (IndVal) for food categories of perch (Perca fluviatilis) and roach (Rutilus rutilus) collected in 2011 and 2012 from Bovan Reservoir, Serbia. The highest (at P ≤ 0.05) IndVal in a given cluster (A, B, C, D) are in bold (exact significance levels are presented in Fig.
Fish diet group | A | B | C | D | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
%FO | %N | IndVal | %FO | %N | IndVal | %FO | %N | IndVal | %FO | %N | IndVal | |
Protozoa | 4 | 3 | 0 | 0 | 0 | 0 | 43 | 73 | 32 | 18 | 23 | 4 |
Rhizopoda | 17 | 51 | 9 | 10 | 0 | 5 | 0 | 0 | 0 | 0 | 0 | 0 |
Rotatoria | 4 | 27 | 1 | 3 | 9 | 0 | 9 | 64 | 6 | 0 | 0 | 0 |
Bryozoa | 4 | 2 | 0 | 5 | 3 | 0 | 87 | 91 | 79 | 2 | 4 | 0 |
Hydracarina | 4 | 7 | 0 | 0 | 0 | 0 | 4 | 7 | 0 | 16 | 85 | 14 |
Ostracoda | 54 | 20 | 11 | 59 | 18 | 11 | 26 | 14 | 4 | 59 | 48 | 28 |
Anostraca | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 100 | 2 |
Conchostraca | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 11 | 100 | 11 |
Notostraca | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 100 | 2 |
Cladocera | 0 | 0 | 0 | 3 | 29 | 1 | 0 | 0 | 0 | 5 | 71 | 3 |
Daphnia sp. | 75 | 25 | 19 | 92 | 32 | 29 | 0 | 0 | 0 | 25 | 43 | 11 |
Bosmina sp. | 83 | 18 | 15 | 100 | 32 | 32 | 43 | 18 | 8 | 93 | 33 | 31 |
Leptodora kindtii | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 14 | 100 | 14 |
Calanoida | 17 | 1 | 0 | 95 | 10 | 9 | 96 | 43 | 42 | 98 | 46 | 45 |
Cyclopoida | 17 | 0 | 0 | 97 | 10 | 10 | 100 | 39 | 39 | 100 | 50 | 50 |
Isopoda | 0 | 0 | 0 | 0 | 0 | 0 | 4 | 100 | 4 | 0 | 0 | 0 |
Amphipoda | 8 | 5 | 0 | 5 | 1 | 0 | 83 | 45 | 37 | 77 | 48 | 37 |
Gammaridae | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 100 | 2 |
Insecta (other) | 4 | 65 | 3 | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 35 | 1 |
Diptera (other) | 4 | 5 | 3 | 0 | 0 | 0 | 4 | 74 | 1 | 0 | 21 | 0 |
Chironomidae | 21 | 5 | 1 | 0 | 0 | 0 | 57 | 74 | 42 | 20 | 21 | 4 |
Plecoptera | 0 | 0 | 0 | 0 | 0 | 0 | 13 | 97 | 13 | 2 | 3 | 0 |
Ephemeroptera | 0 | 0 | 0 | 0 | 0 | 0 | 4 | 100 | 4 | 0 | 0 | 0 |
Trichoptera | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 5 | 100 | 5 |
Oligochaeta | 38 | 85 | 32 | 3 | 9 | 0 | 0 | 0 | 0 | 7 | 6 | 0 |
Fishes | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 9 | 100 | 9 |
Distribution pattern for 26 food categories represented in the diet of perch (Perca fluviatilis) and roach (Rutilus rutilus) collected in 2011 and 2012 from Bovan Reservoir, Serbia. The shading is scaled independently for each food category. The shade of black for each food category is highly correlated with the values of the IndVal index. The degree of shading decrease is also indicated by a decline in the values of the IndVal index.
In this study, we have analyzed the food interactions between perch and roach juveniles. Although general food categories consumed by perch and roach were similar, each species had its own predominant prey items during different seasons. In general, perch changes diet during ontogeny by feeding on zooplankton, macroinvertebrates, and fish (
Zooplankton is the essential diet of fish fry (
In Bovan Reservoir, consumption of cladocerans was higher in roach than in perch and, in contrast, perch was more likely to feed on amphipods and copepods (Cyclopoida and Calanoida) than roach. This is also indicated by
The large cladoceran Leptodora kindtii is also an important food component in the roach and perch diet (
In general, our results showed that macroinvertebrates constituted a minor fraction of the food items found in the perch and roach alimentary tracts. The majority of juvenile perch fed on chironomids (
During the investigated seasons, detritus was also present in the diet of juvenile perch, but to a much lower extent than in the juvenile roach diet. It was possible to detect its presence in the diet but not to quantify it, except with frequency of occurrence, the values of which were high. The importance of detritus in the roach diet has been noted by
This study showed that the roach has better competitive abilities for cladocerans than juvenile perch. It results in a shift in feeding preferences of juvenile perch and thereafter increased competition with older perch and additionally decreased growth and recruitment to the piscivorous stage (
The modified Costello’s method suggests that some of the analyzed specimens specialized on certain types of prey, whereas the entire sample seems to have a generalized feeding strategy. This can be deduced from the fact that a few prey items have a high prey-specific abundance (%Pi) and low frequency of occurrence (%FO). Roach is considered a generalist feeder with the exception of specialization on Oligochaeta and Rotatoria. According to Costello’s graph, for some roach specimens, Oligochaeta were of great importance during the whole investigation, with the exception of autumn 2012 (%Pi < 50). The explanation for this is the dominance of Oligochaeta in Bovan Reservoir bottom fauna (
Due to the different degrees of digestion, information on the alimentary tractsʼ contents may consist of only general food categories (i.e., higher taxonomic levels) or may be identified to the lowest possible taxonomic level. If we decide to uniform the data and present the alimentary tractsʼ contents “roughly” or on the other hand in detail this would result in losing information on a large part of the alimentary tractsʼ content (
First, there were two groups of roach specimens assigned to clusters A and B, and two groups of perch specimens assigned to clusters C and D. Those in cluster A benefited from Oligochaeta, which were used during the whole study as reflected in significant IndVal. Specimens in cluster B during all study periods most often fed on cladocerans Bosmina sp. and Daphnia sp., which is proved by significant IndVal values. All perch and roach specimens from the most diverse cluster B had Bosmina sp. in their alimentary tracts. Perch assigned to cluster C focused on Chironomidae and zooplankton, including Protozoa and Bryozoa (IndVal significant only for cluster C), while those in cluster D ate mostly zooplankton. Also, it is visible in cluster C that no specimens consumed Daphnia sp. Copepods played an important role in the diet of perch, as indicated by significant IndVals. Additionally, each specimen distributed in clusters C and D had Cyclopoida in its alimentary tract. Protozoa, Bryozoa, Ostracoda, and Amphipoda are good examples of the advantage of self-organizing maps and IndVal in relation to traditional index Prominence value. IndVal for these groups is significant only for cluster C, only for cluster D, or both, while the Prominence value for these preys is low throughout the whole research. This distribution of specimens’ alimentary tracts in neurons indicates that there was no high degree of competition between perch and roach, and the segregation between them was strict. The value of Schoener’s niche overlap index found in this research was indicating an almost total diet overlap within the species, as also visually shown by the results obtained using self-organizing maps, where all roach and only six specimens of perch were classified into clusters A and B. All other specimens of perch were in clusters C and D. Low trophic overlap is expected for these two species that seem to use this strategy to allow their coexistence in high abundance in Bovan Reservoir. Seasonality significantly affected both species’ diet composition, indicating the different proportions of food resources between periods because similar food categories were present during all seasons, but IndVal singles out certain food categories as significant.
Self-organizing maps have proven to be most suitable for application over complex and nonlinear ecological data and are particularly suitable for application over large data sets (
This study shows the diet analysis based on traditional indices, which have been used for decades, and the diet analysis presented using self-organizing maps and IndVal. Comparing the results obtained in these two ways, the impression is that results are very similar or even identical. The high Prominence values and separation of certain preys on Costello’s graph (upper right corner) show which preys are dominant. This is confirmed by significant IndVal. Also, there are preys like Protozoa, Bryozoa, Ostracoda, Amphipoda, and Chironomidae that are positioned in the middle of Costello’s graph all the time, and the Prominence values are not particularly high or low. For these preys IndVal values are significant, and the specimens that consume them are together in a cluster on the SOM map, which means that these preys are important only for certain specimens, and not for the whole population. Oligochaeta are a good example, too. They are important prey for certain roach specimens based on Costello’s graph, and IndVal is significant for them. All these specimens are arranged in cluster A. Also, there are, in the perch diet, rare or unimportant preys, for which the Prominence values are low, and on a graph, they are in the lower-left corner. Consequently, these specimens are arranged in the same cluster, and IndVal values are insignificant. Likewise, the SOM output network visually shows the results of Schoener’s niche overlap index too, where the separation between species is clearly seen. It appears that the IndVal shows the same results as the Prominence value and Costello’s graph, while the SOM output network shows whether there is an overlap in diet between specimens or species, as do the Schoener’s niche overlap index.
Our results showed that juvenile fish used in diet both zooplankton and macrozoobenthos specimens; roach often fed on nonanimal prey, while perch of age 0+ also used fish in their diet. However, both species play an important role in the food web of ecosystems. Thus, the presented study provides a basis for further research on the feeding biology of these two species. Moreover, integrating these results with those previously published could be used to draw up a common strategy for managing the reservoir fish stock.
In summary, this study offers valuable insights into the dietary strategies of perch and roach. However, fish feeding analysis using self-organizing maps provides a more complete insight into the fish feeding habits, and thus the similarities and differences between them. Because as the distance in the network increases, the differences in models assigned to the neurons also increase. One neuron can contain data from several samples (i.e., specimens), and therefore there is certainly a high degree of their dietary similarity. In the end, it should be mentioned that with the identification of the alimentary tract contents, which is a complex and time-consuming process, especially in juveniles, self-organizing maps in combination with the IndVal index represents an adequate and time-saving analysis.
This work was supported by the Serbian Ministry of Education, Science and Technological Development (Agreement No. 451-03-68/2022-14/200122).