Corresponding author: Josef Velisek ( velisek@frov.jcu.cz ) Academic editor: Jolanta Kiełpińska
© 2021 Alena Honzlova, Helena Curdova, Lenka Schebestova, Alzbeta Stara, Josef Priborsky, Anna Koubova, Zdenka Svobodova, Josef Velisek.
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:
Honzlova A, Curdova H, Schebestova L, Stara A, Priborsky J, Koubova A, Svobodova Z, Velisek J (2021) A nitrogen factor for European pike-perch (Sander lucioperca), northern pike (Esox lucius), and sheatfish (Silurus glanis) fillets. Acta Ichthyologica et Piscatoria 51(2): 119-129. https://doi.org/10.3897/aiep.51.63281
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Measures for consumer protection against food adulteration and misleading labeling are integrated into EU legislation, including methods for detection of misleading practices. Verification of meat content is available for marine products but not for freshwater fish because of the lack of standard nitrogen factors. The aim of this study was to establish nitrogen factors for European pike-perch Sander lucioperca (Linnaeus, 1758), northern pike Esox lucius Linnaeus, 1758, and sheatfish Silurus glanis Linnaeus, 1758. The study involved analysis of 808 fillet samples obtained in spring (March–April) and autumn (October–November) harvest seasons, 2018–2019, from seven Czech Republic fish rearing facilities. Samples with and without skin were analyzed for nitrogen content, dry matter, protein, ash, and fat according to established ISO methods. The recommended nitrogen factor for European pike-perch with the skin is 3.28 ± 0.09 and without the skin is 3.21 ± 0.09; for northern pike with the skin is 3.18 ± 0.09 and without skin is 3.15 ± 0.09; and for sheatfish with skin is 2.73 ± 0.13 and without skin is 2.75 ± 0.12. The established nitrogen factors will enable analysis of meat content to ensure that consumers are purchasing correctly described and labeled fish products.
Adulteration, fish-food fraud, fish products, nitrogen factor, fish
Freshwater and marine fishes play a significant role in human nutrition as a source of protein and other nutrients (
The annual global per capita consumption of fish products increased from 9.0 kg in 1961 to 20.3 kg in 2017. In this period, fish consumption increased by 3.1 percentage points annually, more than the annual population growth rate of 1.6 percentage points (
Pond fish farming has a long tradition in the Czech Republic, dating to the 12th century with the oldest documented pond in Bohemia established in 1115. In the past ten years, the production of freshwater fish in the Czech Republic reached 20 400–21 800 tons per year, of which approximately 10% is represented by processed fish products for foreign and domestic markets (
Some farmed freshwater fish species popular with consumers are among the more costly foodstuffs, motivating fraud among producers. Meat, including fish meat, carries a high potential for economically motivated adulteration (
Consumer protection against adulteration and misleading food labeling is contained in EU legislation. Methods for detection of practices misleading the consumer for pork, chicken, and seafoods have been published in European legislation and in standards of Codex Alimentarius. These are primarily procedures to determine meat content or content of absorbed water (
The aim of this study was to establish nitrogen factors as determined by the Kjeldahl method (
One-hundred-three market-size European pike-perch (385–2025 g weight) were obtained from five Czech aquaculture facilities: The University of South Bohemia in Ceske Budejovice, Faculty of Fisheries and Protection of Waters (FFPW USB), Vodnany, and the Blatna, Hodonin, Klatovy, and Lnare fisheries. Two-hundred-seven market-size northern pike (445–3980 g) were obtained from seven Czech aquaculture facilities: FFPW USB and the Chlumec nad Cidlinou, Blatna, Hodonin, Klatovy, Lnare, and Tabor fisheries. Ninety-four market-size sheatfish (505–9265 g) were obtained from FFPW USB and the Chlumec nad Cidlinou, Blatna, Hodonin, and Klatovy fisheries. The location of the farms is shown in Fig.
The study was conducted according to the principles of the Ethical Committee for the Protection of Animals in Research of the University of South Bohemia, Faculty of Fisheries and Protection of Waters, Vodnany.
Samples of fish fillets with and without skin were analyzed for dry matter, protein, fat, and ash. After partial thawing to avoid loss of water and soluble protein fractions, samples were homogenized by grinding on the knife mill PULVERISETTE 11 (FRITSCH GmbH, Germany).
The determination of percentage of dry matter was based on the standard method
The determination of ash content was based on the standard
Total fat content was assessed based on the standard
The determination of protein content used the Kjeldahl method based on the standard method
Analyses of the dry matter, ash, and total fat were conducted in duplicate, and analysis of protein was performed in triplicate for each sample.
Kolmogorov–Smirnov and Bartlett’s tests were applied to assess normal distribution data and the homoscedasticity of the variance, respectively. A two-way ANOVA with a subsequent Tukey’s test was performed to test the effects of season, weight of fish, place of rearing, and the difference between fillets with and without skin. The significance level was set at P < 0.05. Data were expressed as mean ± SD (minimum–maximum). The analysis was performed using STATISTICA v.12.0 for Windows (STATSOFT, Inc.).
Principal component analysis (PCA) of the nitrogen and fat content in fillets with and without skin as response values was applied to describe differences in fish samples from different locations. The analysis was complemented with a redundancy analysis (RDA) with whole fish weight as an independent explanatory variable. The ordination plots were produced using Canoco for Windows v.5.10 (Biometris, The Netherlands and P. Šmilauer, Czech Republic).
The basic composition of fillets with and without skin of European pike-perch is given in Table
Live weight and length of European pike-perch (Sander lucioperca) and chemical composition of fillets with skin and without skin.
Company | Year | Season | Weight [g] x ± SD (min–max) | Total length [cm] x ± SD (min–max) | Fillet | Dry matter [g · 100 g–1] x ± SD (min–max) | Ash [g · 100 g–1] x ± SD (min–max) | Fat [g · 100 g–1] x ± SD (min–max) | Protein [g · 100 g–1] x ± SD (min–max) | N [g · 100 g–1] x ± SD (min–max) |
---|---|---|---|---|---|---|---|---|---|---|
FFPW USB Vodnany | 2018 | Spring | 1359.1 ± 274.9 (1015.0–2025.0) | 51.6 ± 3.1 (48.0–57.0) | with skin | 22.47 ± 0.33ab(A) (21.77–22.94) | 1.35 ± 0.62ab(A) (1.07–3.21) | 1.37 ± 0.50d(C) (0.83–2.59) | 20.66 ± 0.35a(A) (20.18–21.47) | 3.30 ± 0.06ab(A) (3.23–3.43) |
without skin | 21.86 ± 0.27ab(A) (19.89–20.58) | 1.26 ± 0.06ab(A) (1.16–1.40) | 0.69 ± 0.20c(B)* (0.24–0.97) | 20.22 ± 0.27a(A) (21.28–22.27) | 3.23 ± 0.03ab(A) (3.18–3.29) | |||||
2019 | Spring | 563.0 ± 119.6 (435.0–805.0) | 39.1 ± 2.4 (36.5–44.0) | with skin | 22.64 ± 0.26ab(A) (22.36–23.17) | 1.25 ± 0.04ab(A) (1.21–1.32) | 0.33 ± 0.08b(A) (0.17–0.42) | 20.68 ± 0.27a(A) (20.16–21.07) | 3.31 ± 0.04ab(A) (3.23–3.37) | |
without skin | 22.26 ± 0.15ab(A) (21.99–22.46) | 1.24 ± 0.03ab(A) (1.20–1.31) | 0.37 ± 0.10b(A) (0.16–0.52) | 20.37 ± 0.25a(A) (19.99–20.89) | 3.26 ± 0.04ab(A) (3.20–3.34) | |||||
Fishery Lnare | 2018 | Spring | 437.5 ± 98.9 (330.0–660.0) | 35.8 ± 2.1 (36.5–44.0) | with skin | 21.72 ± 0.33ab(A) (21.33–22.30) | 1.17 ± 0.04a(A) (1.11–1.24) | 0.14 ± 0.10a(A) (0.02–0.40) | 20.38 ± 0.39a(A) (19.78–21.08) | 3.26 ± 0.06ab(A) (3.16–3.37) |
without skin | 21.73 ± 0.25ab(A) (21.14–22.00) | 1.20 ± 0.03a(A) (1.13–1.24) | 0.15 ± 0.14a(A) (0.05–0.57) | 20.37 ± 0.31a(A) (19.76–20.81) | 3.26 ± 0.05ab(A) (3.16–3.33) | |||||
Autumn | 722.0 ± 78.0 (605.0–850.0) | 42.3 ± 1.0 (40.5–43.5) | with skin | 22.46 ± 0.54ab(A) (20.95–22.98) | 1.27 ± 0.04ab(AB) (1.17–1.31) | 0.44 ± 0.43b(B) (0.16–1.67) | 20.48 ± 0.58a(A) (19.00–21.00) | 3.28 ± 0.09ab(A) (3.04–3.36) | ||
without skin | 21.79 ± 0.56ab(A) (20.49–22.44) | 1.20 ± 0.12a(A) (0.95–1.48) | 0.29 ± 0.18b(B) (0.12–0.76) | 19.77 ± 0.47a(A) (18.67–20.27) | 3.17 ± 0.07ab(A) (2.99–3.24) | |||||
2019 | Spring | 562.0 ± 43.6 (500.0–650.0) | 38.1 ± 1.4 (36.5–41.0) | with skin | 21.91 ± 0.59ab(A) (20.62–22.92) | 1.37 ± 0.08b(B) (1.23–1.47) | 0.47 ± 0.19b(B) (0.32–0.89) | 20.18 ± 0.41a(A) (19.49–20.74) | 3.23 ± 0.07ab(A) (3.12–3.32) | |
without skin | 21.40 ± 0.81a(A) (19.52–22.82) | 1.38 ± 0.11b(B) (1.14–1.50) | 0.48 ± 0.25b(B) (0.26–1.13 | 19.75 ± 0.57a(A) (18.30–20.33) | 3.17 ± 0.09ab(A) (2.93–3.25) | |||||
Autumn | 1044.5 ± 146.0 (805.0–1305.0) | 46.2 ± 2.2 (43.0–48.5) | with skin | 22.71 ± 0.71ab(A) (21.03–23.63) | 1.22 ± 0.04ab(AB) (1.13–1.30) | 0.41 ± 0.14b(B) (0.20–0.63) | 20.90 ± 0.39a(A) (20.19–21.60) | 3.34 ± 0.06b(A) (3.23–3.46) | ||
without skin | 22.34 ± 0.68ab(A) (20.80–23.16) | 1.18 ± 0.05a(A) (1.10–1.29) | 0.34 ± 0.14b(B) (0.17–0.56) | 20.44 ± 0.31a(A) (19.75–20.96) | 3.27 ± 0.05ab(A) (3.16–3.35) | |||||
Fishery Blatna | 2018 | Autumn | 1230.0 ± 309.2 (765.0–1780.0) | 47.3 ± 3.4 (42.0–53.5) | with skin | 23.18 ± 0.42b(A) (22.55–23.77) | 1.23 ± 0.09ab(A) (1.16–1.47) | 0.75 ± 0.35c(C) (0.26–1.36) | 20.73 ± 0.40a(A) (20.03–21.60) | 3.32 ± 0.06b(A) (3.20–3.46) |
without skin | 22.54 ± 0.42ab(A) (22.04–23.28) | 1.28 ± 0.07ab(A) (1.20–1.48) | 0.54 ± 0.26b(B)* (0.24–1.11) | 20.21 ± 0.30a(A) (19.86–20.78) | 3.23 ± 0.05ab(A) (3.18–3.33) | |||||
2019 | Autumn | 627.5 ± 97.8 (385.0–815.0) | 39.1 ± 2.8 (32.0–48.0) | with skin | 23.07 ± 0.46b(A) (22.31–23.28) | 1.30 ± 0.04ab(A) (1.23–1.35) | 0.21 ± 0.04a(A)* (0.04–0.37) | 21.12 ± 0.29a(A) (20.69–21.67) | 3.38 ± 0.05b(A) (3.31–3.47) | |
without skin | 22.66 ± 0.43ab(A) (21.89–23.28) | 1.25 ± 0.06ab(A) (1.17–1.35) | 0.31 ± 0.11b(B) (0.12–0.48) | 20.63 ± 0.22a(A) (20.33–21.06) | 3.30 ± 0.04ab(A) (3.25–3.37) | |||||
Fishery Hodonin | 2019 | Spring | 1210.0 ± 386.9 (660.0–1780.0) | 49.3 ± 4.8 (41.0–54.5) | with skin | 22.04 ± 0.19ab(A) (21.72–22.30) | 1.17 ± 0.04a(A) (1.11–1.22) | 0.39 ± 0.18b(B) (0.21–0.72) | 20.50 ± 0.38a(A) (20.03–20.92) | 3.28 ± 0.06ab(A) (3.20–3.35) |
without skin | 21.46 ± 0.31a(A) (20.91–21.84) | 1.20 ± 0.03a(A) (1.15–1.24) | 0.26 ± 0.08b(B) (0.14–0.38) | 19.59 ± 0.33a(A) (19.29–20.22) | 3.13 ± 0.05a(A) (3.09–3.24) | |||||
Autumn | 1053.8 ± 112.0 (910.0–1270.0) | 46.3 ± 0.9 (45.0–48.0) | with skin | 21.97 ± 0.35ab(A) (21.45–22.55) | 1.24 ± 0.07ab(A) (1.11–1.30) | 0.28 ± 0.15b(B) (0.05–0.60) | 20.44 ± 0.35a(A) (19.86–21.03) | 3.27 ± 0.06ab(A) (3.18–3.36) | ||
without skin | 21.49 ± 0.29a(A) (21.12–22.0) | 1.27 ± 0.06ab(A) (1.16–1.36) | 0.21 ± 0.07a(A)* (0.10–0.29) | 19.83 ± 0.14a(A) (19.57–20.03) | 3.17 ± 0.02ab(A) (3.13–3.20 | |||||
Fishery Klatovy | 2018 | Autumn | 1292.0 ± 128.6 (1150.0–1535.0) | 49.3 ± 2.1 (46.5–53.0) | with skin | 21.76 ± 0.72ab(A) (20.71–22.83) | 1.22 ± 0.11ab(A) (1.05–1.43) | 0.73 ± 0.31c(B) (0.1–1.37) | 19.61 ± 0.61a(A) (18.47–20.88) | 3.17 ± 0.10ab(A) (2.95–3.34) |
without skin | 21.24 ± 0.73a(A) (20.20–22.38) | 1.29 ± 0.12ab(A) (1.07–1.49) | 0.56 ± 0.37b(A)* (0.21–1.57) | 19.05 ± 0.58a(A) (18.12–19.80) | 3.05 ± 0.09a(A) (2.90–3.17) |
The ash content was significantly lower (P ˂ 0.01) in European pike-perch fillets with skin from Lnare (spring 2018, 1.17 ± 0.04) and Hodonin (spring 2019, 1.17 ± 0.04) and fillets without skin from Lnare (spring 2018, 1.20 ± 0.03; autumn 2018, 1.20 ± 0.12; and autumn 2019, 1.18 ± 0.05) and Hodonin (spring 2019, 1.20 ± 0.03), compared to fillets with (1.3 ± 0.08) and without (1.38 ± 0.11) skin from Lnare in spring 2019. We found no significant differences (P ˃ 0.05) in ash content of European pike-perch fillets with and without skin within a sampling period.
The fat content was significantly lower (P ˂ 0.01) in European pike-perch fillets with (0.14 ± 0.10) and without (0.15 ± 0.14) skin from Lnare (spring 2018), with skin (0.21 ± 0.04) from Blatna (autumn 2019), and without skin (0.21 ± 0.07) from Hodonin (autumn 2019) compared to other groups. We found significant differences (P ˂ 0.01) in fat content of fillets with skin and those without skin in European pike-perch from FFPW USB in spring 2018 (1.37 ± 0.50 and 0.69 ± 0.20), Blatna in autumn 2018 (0.75 ± 0.35 and 0.54 ± 0.26) and 2019 (0.2 ± 0.04 and 0.31 ± 0.11), Klatovy in autumn 2018 (0.73 ± 0.31 and 0.56 ± 0.37), and Hodonin in autumn 2019 (0.28 ± 0.15 and 0.21 ± 0.07).
We found no significant differences (P ˃ 0.05) in protein content of European pike-perch from with respect to farm, season, year, or presence of skin.
The nitrogen content was significantly lower (P ˂ 0.01) in European pike-perch fillets without skin from Hodonin (spring 2019, 3.13 ± 0.05) and Klatovy (autumn 2018, 3.05 ± 0.09) than in those fillets with skin from Lnare in autumn 2019 (3.34 ± 0.06) and Blatna in autumn 2018 (3.32 ± 0.06) and 2019 (3.38 ± 0.05). We found no significant differences (P ˃ 0.05) in nitrogen content of European pike-perch fillets with and without skin in a single sampling period.
Principal component analysis extracted the first two axes explaining 74.5% of the total variance. It did not explain the differences between functional traits fillets with and without skin, but a negative correlation of nitrogen and fat content with fish weight was revealed (Fig.
Ordination plots of sample distribution after principal component analysis (PCA) of functional traits as response variables (A) and redundancy analysis (RDA) (B) of functional traits as response variables and weight of European pike-perch, Sander lucioperca as explanatory variable. N+skin = nitrogen concentration in fillet with skin, N-skin = nitrogen concentration in fillet without skin, Fat+skin = fat percentage in fillet with skin, Fat-skin = fat percentage in fillet without skin, Weight = body weight of individual fish; abbreviations used in legend: S = spring sampling, A = autumn sampling). The length of the arrow reflects the power of the variable to differentiate the samples.
The basic chemical composition of northern pike fillets with and without skin are given in Table
Live weight and length of northern pike (Esox lucius) and chemical composition of fillets with skin and without skin.
Company | Year | Season | Weight [g] x ± SD (min–max) | Total length [cm] x ± SD (min–max) | Fillet | Dry matter [g · 100 g–1] x ± SD (min–max) | Ash [g · 100 g–1] x ± SD (min–max) | Fat [g · 100 g–1] x ± SD (min–max) | Protein [g · 100 g–1] x ± SD (min–max) | N [g · 100 g–1] x ± SD (min–max) |
---|---|---|---|---|---|---|---|---|---|---|
FFPW USB Vodnany | 2018 | Spring | 1721.4 ± 469.1 (1260.0–2550) | 63.1 ± 4.5 (59.0–74.0) | with skin | 21.63 ± 1.03ab(A) (19.79–23.36) | 1.52 ± 0.45b(A) (1.17–2.60) | 1.13 ± 0.63c(B) (0.49–2.61) | 19.73 ± 0.61a(A) (18.59–20.52) | 3.16 ± 0.10ab(A) (2.97–3.28) |
without skin | 21.78 ± 1.12ab(A) (19.84–23.59) | 1.30 ± 0.14ab(A) (1.20–1.70) | 1.18 ± 0.40c(B) (0.67–1.78) | 19.79 ± 0.60a(A) (18.57–20.79) | 3.17 ± 0.10ab(A) (2.97–3.33) | |||||
2019 | Spring | 1609.0 ± 473.6 (945.0–2470.0) | 55.5 ± 4.0 (48.0–60.0) | with skin | 23.47 ± 0.33b(A) (22.84–24.05) | 1.27 ± 0.06ab(A) (1.17–1.40) | 0.72 ± 0.30b(A) (0.26–1.31) | 20.12 ± 0.26a(A) (19.66–20.49) | 3.21 ± 0.04ab(A) (3.15–3.28) | |
without skin | 23.19 ± 0.41b(A) (22.53–23.90) | 1.31 ± 0.07ab(A) (1.21–1.47) | 0.59 ± 0.22b(A) (0.27–0.99) | 20.00 ± 0.30a(A) (19.51–20.51) | 3.20 ± 0.05ab(A) (3.12–3.28) | |||||
Fishery Chlumec nad Cidlinou | 2018 | Spring | 1728.5 ± 464.81 (880.0–2415.0) | 64.9 ± 5.5 (53.0–72.0) | with skin | 21.29 ± 0.65a(A) (20.30–22.33) | 1.24 ± 0.15ab(A) (1.06–1.64) | 0.60 ± 0.36b(A) (0.18–1.48) | 19.24 ± 0.48a(A) (18.21–20.03) | 3.08 ± 0.08a(A) (2.91–3.20) |
without skin | 21.39 ± 0.96a(A) (20.27–23.40) | 1.24 ± 0.05ab(A) (1.13–1.32) | 0.61 ± 0.47b(A) (0.06–1.52) | 19.10 ± 0.37a(A) (18.37–19.63) | 3.06 ± 0.06a(A) (2.94–3.14) | |||||
Autumn | 1459.0 ± 457.4 (530.0–2405.0) | 57.1 ± 5.9 (43.0–67.5) | with skin | 22.10 ± 0.55ab(A) (20.96–22.99) | 1.17 ± 0.06a(A) (1.07–1.29) | 0.68 ± 0.19b(A)* (0.39–1.08) | 19.62 ± 0.47a(A) (18.65–20.34) | 3.14 ± 0.07ab(AB) (2.98–3.25) | ||
without skin | 22.24 ± 0.79ab(A) (20.80–23.20) | 1.24 ± 0.07ab(A) (1.14–1.34) | 0.91 ± 0.32b(B) (0.45–1.45) | 19.51 ± 0.52a(A) (18.33–19.99) | 3.12 ± 0.08ab(AB) (2.93–3.20) | |||||
2019 | Spring | 1125.0 ± 271.1 (615.0–1545.0) | 55.0 ± 4.3 (46.0–63.0) | with skin | 21.92 ± 0.90ab(A) (20.54–23.27) | 1.34 ± 0.09ab(A) (1.28–1.61) | 1.02 ± 0.39b(B) (0.46–1.78) | 19.56 ± 0.57a(A) (18.49–20.72) | 3.13 ± 0.09ab(AB) (2.96–3.31) | |
without skin | 21.64 ± 1.02ab(A) (20.00–23.24) | 1.35 ± 0.07ab(A) (1.26–1.51) | 0.96 ± 0.38b(B) (0.48–1.77) | 19.39 ± 0.58a(A) (18.38–20.64) | 3.10 ± 0.09ab(AB) (2.94–3.30) | |||||
Autumn | 1653.5 ± 278.5 (1240.0–2075.0) | 59.5 ± 3.6 (54.5–66.0) | with skin | 22.51 ± 0.51ab(A) (21.55–23.00) | 1.24 ± 0.02ab(A) (1.21–1.28) | 0.59 ± 0.20b(A) (0.25–0.95) | 20.13 ± 0.42a(A) (19.34–20.59) | 3.22 ± 0.07b(B) (3.09–3.29) | ||
without skin | 22.62 ± 0.54ab(A) (21.72–23.44) | 1.29 ± 0.04ab(A) (1.23–1.35) | 0.65 ± 0.26b(A) (0.38–1.05) | 20.00 ± 0.27a(A) (19.49–20.54) | 3.20 ± 0.04ab(AB) (3.12–3.29) | |||||
Fishery Blatna | 2018 | Autumn | 1143.3 ± 361.2 (685.0–1820.0) | 54.7 ± 4.4 (46.5–60.0) | with skin | 22.77 ± 0.41ab(A) (22.12–23.45) | 1.33 ± 0.10ab(A) (1.33–1.52) | 0.67 ± 0.20b(A) (0.38–1.01) | 20.03 ± 0.26a(A) (19.60–20.32) | 3.20 ± 0.04ab(A) (3.14–3.25) |
without skin | 22.68 ± 0.43ab(A) (21.99–23.31) | 1.37 ± 0.10ab(A) (1.20–1.55) | 0.71 ± 0.38b(A) (0.14–1.33) | 19.87 ± 0.18a(A) (19.60–20.12) | 3.18 ± 0.03ab(A) (3.14–3.22) | |||||
2019 | Autumn | 1141.5 ± 322.9 (675.0–1850.0) | 56.4 ± 4.31 (48.5–64.0) | with skin | 21.50 ± 0.78ab(A) (19.47–22.57) | 1.21 ± 0.04ab(A) (1.14–1.27) | 0.65 ± 0.29b(A) (0.17–1.06) | 19.35 ± 0.70a(A) (18.13–20.67) | 3.10 ± 0.11ab(A) (2.90–3.31) | |
without skin | 21.60 ± 0.95ab(A) (19.11–22.76) | 1.28 ± 0.05ab(A) (1.22–1.38) | 0.80 ± 0.42b(A) (0.25–1.88) | 19.13 ± 0.65a(A) (17.66–19.69) | 3.06 ± 0.11a(A) (2.83–3.15) | |||||
Fishery Hodonin | 2019 | Spring | 1378.0 ± 384.2 (865.0–2030.0) | 59.3 ± 5.8 (50.5–68.5) | with skin | 21.83 ± 0.70ab(A) (20.55–22.87) | 1.28 ± 0.05ab(A) (1.20–1.36) | 0.62 ± 0.19b(B) (0.21–0.88) | 19.74 ± 0.77a(A) (18.16–21.03) | 3.16 ± 0.12ab(A) (2.91–3.36) |
without skin | 21.62 ± 0.59ab(A) (20.30–22.19) | 1.28 ± 0.06ab(A) (1.19–1.38) | 0.53 ± 0.22b(B) (0.24–1.05) | 19.43 ± 0.61a(A) (18.21–20.33) | 3.11 ± 0.10ab(A) (2.91–3.26) | |||||
Autumn | 1450.6 ± 240.0 (1075.0–1790.0) | 59.8 ± 4.4 (52.0–67.5) | with skin | 21.79 ± 0.60ab(A) (20.62–22.58) | 1.31 ± 0.04ab(A) (1.27–1.42) | 0.34 ± 0.14a(A)* (0.11–0.57) | 20.23 ± 0.68a(A) (18.65–20.83) | 3.24 ± 0.11b(A) (2.98–3.33) | ||
without skin | 22.05 ± 0.81ab(A) (20.74–23.28) | 1.39 ± 0.05ab(A) (1.27–1.45) | 0.56 ± 0.29b(B) (0.17–1.10) | 20.00 ± 0.61a(A) (19.13–20.94) | 3.20 ± 0.10ab(A) (3.06–3.35) | |||||
Fishery Klatovy | 2018 | Spring | 1360.0 ± 551.1 (725.0–2795.0) | 57.1 ± 5.9 (48.5–70.0) | with skin | 22.26 ± 0.87ab(A) (20.98–23.92) | 1.28 ± 0.05ab(A) (1.18–1.35) | 0.64 ± 0.28b(B) (0.14–2.28) | 20.23 ± 0.44a(A) (19.44–20.80) | 3.24 ± 0.07b(A) (3.11–3.33) |
without skin | 22.04 ± 0.60ab(A) (21.11–23.21) | 1.29 ± 0.04ab(A) (1.22–1.36) | 0.39 ± 0.29a(A)* (0.12–1.19) | 20.13 ± 0.40a(A) (19.19–20.63) | 3.22 ± 0.06b(A) (3.07–3.30) | |||||
Autumn | 1537.5 ± 461.4 (510.0–2200.0) | 59.0 ± 7.4 (42.0–70.0) | with skin | 22.16 ± 1.19ab(A) (19.10–23.93) | 1.23 ± 0.31ab(A) (0.59–1.59) | 0.50 ± 0.17b(B) (0.14–0.68) | 19.75 ± 0.89a(A) (17.43–21.06) | 3.16 ± 0.14ab(A) (2.79–3.37) | ||
without skin | 22.08 ± 1.34ab(A) (18.48–23.38) | 1.30 ± 0.24ab(A) (0.82–1.68) | 0.62 ± 0.19b(B) (0.09–0.83) | 19.70 ± 0.81a(A) (17.53–20.68) | 3.15 ± 0.13ab(A) (2.80–3.31) | |||||
2019 | Autumn | 1375.0 ± 385.9 (755.0–2115.0) | 57.4 ± 4.9 (50.5–66.5) | with skin | 21.14 ± 0.62a(A) (19.77–21.93) | 1.26 ± 0.05ab(A) (1.18–1.34) | 0.28 ± 0.19a(A) (0.08–0.58) | 19.43 ± 0.47a(A) (18.70–20.22) | 3.11 ± 0.07ab(A) (2.99–3.24) | |
without skin | 21.35 ± 0.78a(A) (19.56–22.24) | 1.36 ± 0.10ab(A) (1.21–1.50) | 0.40 ± 0.26a(A) (0.08–0.90) | 19.44 ± 0.42a(A) (18.43–19.81) | 3.11 ± 0.07ab(A) (2.95–3.17) | |||||
Fishery Lnare | 2018 | Spring | 1322.5 ± 431.1 (615.0–1960.0) | 57.2 ± 7.8 (44.5–68.5) | with skin | 22.32 ± 0.94ab(A) (19.69–23.09) | 1.27 ± 0.10ab(A) (1.16–1.51) | 0.33 ± 0.19a(A)* (0.07–0.67) | 20.17 ± 0.54a(A) (19.16–21.07) | 3.23 ± 0.09b(A) (3.07–3.37) |
without skin | 22.61 ± 0.78ab(A) (20.59–23.37) | 1.32 ± 0.13ab(A) (1.18–1.67) | 0.63 ± 0.34b(B) (0.27–1.35) | 19.97 ± 0.60a(A) (18.48–20.83) | 3.20 ± 0.10ab(A) (2.96–3.33) | |||||
Autumn | 1327.0 ± 524.1 (805.0–2105.0) | 58.9 ± 7.5 (50.0–70.0) | with skin | 21.37 ± 0.92a(A) (19.44–22.80) | 1.35 ± 0.07ab(A) (1.25–1.50) | 0.45 ± 0.20a(A) (0.14–0.99) | 19.65 ± 0.42a(A) (19.14–20.54) | 3.13 ± 0.07ab(A) (3.06–3.29) | ||
without skin | 21.21 ± 0.95a(A) (19.15–22.73) | 1.35 ± 0.11ab(A) (1.09–1.53) | 0.38 ± 0.28a(A) (0.12–0.93) | 19.42 ± 0.56a(A) (18.35–20.52) | 3.11 ± 0.09ab(A) (2.94–3.28) | |||||
Fishery Lnare | 2019 | Spring | 981.0 ± 434.4 (445.0–1650.0) | 49.7 ± 6.2 (40.0–58.5) | with skin | 22.65 ± 0.71ab(A) (21.23–23.56) | 1.31 ± 0.05ab(A) (1.22–1.38) | 0.72 ± 0.30b(B) (0.22–1.27) | 19.48 ± 0.27a(A) (19.47–20.24) | 3.17 ± 0.04ab(A) (3.12–3.24) |
without skin | 22.72 ± 0.87ab(A) (21.00–23.86) | 1.37 ± 0.06ab(A) (1.29–1.49) | 0.71 ± 0.36b(B) (0.26–1.52) | 19.85 ± 0.49a(A) (19.07–20.66) | 3.18 ± 0.08ab(A) (3.05–3.31) | |||||
Autumn | 1136.0 ± 20.9.9 (715.0–1435.0) | 54.5 ± 3.01 (47.0–58.0) | with skin | 22.02 ± 0.68ab(A) (20.50–23.03) | 1.13 ± 0.12a(A) (0.99–1.28) | 0.41 ± 0.16a(A) (0.15–0.70) | 19.95 ± 0.34a(A) (19.51–20.46) | 3.19 ± 0.05ab(A) (3.12–3.27) | ||
without skin | 22.05 ± 0.61ab(A) (21.20–23.05) | 1.16 ± 0.12a(A) (0.99–1.32) | 0.44 ± 0.21a(A) (0.12–0.78) | 20.02 ± 0.29a(A) (19.44–20.45) | 3.20 ± 0.05ab(A) (3.11–3.27) | |||||
Fishery Tabor | 2018 | Spring | 937.0 ± 202.2 (685.0–1315.0) | 52.3 ± 4.0 (46.0–60.0) | with skin | 21.69 ± 0.58ab(A) (20.94–22.75) | 1.28 ± 0.06ab(A) (1.18–1.38) | 0.43 ± 0.27a(A) (0.06–1.00) | 19.82 ± 0.29a(A) (19.38–20.28) | 3.17 ± 0.05ab(A) (3.10–3.24) |
without skin | 21.80 ± 0.65ab(A) (20.69–22.71) | 1.34 ± 0.07ab(A) (1.22–1.48) | 0.51 ± 0.24ab(AB) (0.08–0.95) | 19.53 ± 0.36a(A) (18.74–20.04) | 3.12 ± 0.06ab(A) (3.00–3.21) | |||||
Autumn | 677.0 ± 80.7 (560.0–800.0) | 46.0 ± 1.6 (43.0–48.5) | with skin | 22.42 ± 0.45ab(A) (21.38–22.97) | 1.42 ± 0.06b(A) (1.35–1.57) | 0.47 ± 0.13ab(AB) (0.30–0.74) | 20.10 ± 0.18a(A) (19.83–20.44) | 3.22 ± 0.03b(A) (3.17–3.27) | ||
without skin | 22.57 ± 0.45ab(A) (21.46–23.03) | 1.38 ± 0.07ab(A) (1.30–1.50) | 0.55 ± 0.20ab(AB) (0.20–0.92) | 20.07 ± 0.31a(A) (19.48–20.58) | 3.22 ± 0.05b(A) (3.12–3.29) | |||||
2019 | Spring | 1565.0 ± 889.1 (760.0–3980.0) | 57.7 ± 9.7 (48.0–82.0) | with skin | 23.52 ± 0.82b(A) (22.50–25.00) | 1.26 ± 0.05ab(A) (1.18–1.34) | 0.91 ± 0.48c(B) (0.34–2.02) | 20.30 ± 0.43a(A) (19.36–20.87) | 3.25 ± 0.07b(A) (3.10–3.34) | |
without skin | 23.22 ± 0.74b(A) (21.92–24.39) | 1.26 ± 0.06ab(A) (1.16–1.36) | 0.74 ± 0.42b(B) (0.20–1.62) | 20.05 ± 0.41a(A) (19.45–20.81) | 3.22 ± 0.07b(A) (3.11–3.33) | |||||
Autumn | 981.0 ± 269.9 (540.0–1650.0) | 51.0 ± 4.2 (43.5–60.0) | with skin | 21.84 ± 0.38ab(A) (21.07–22.50) | 1.35 ± 0.13ab(A) (1.20–1.57) | 0.68 ± 0.22b(B) (0.27–1.13) | 19.94 ± 0.39a(A) (19.37–20.37) | 3.19 ± 0.06ab(A) (3.10–3.26) | ||
without skin | 21.86 ± 0.45ab(A) (20.94–22.57) | 1.37 ± 0.14ab(A) (1.22–1.63) | 0.79 ± 0.19b(B) (0.60–1.18) | 19.71 ± 0.28a(A) (19.38–20.16) | 3.15 ± 0.05ab(A) (3.10–3.23) |
The ash content was significantly lower (P ˂ 0.01) in northern pike fillets with skin (1.17 ± 0.06) from Chlumec nad Cidlinou (autumn 2018) and in fillets with (1.13 ± 0.12) and without (1.16 ± 0.12) skin from Lnare (autumn 2019) compared to fillets with skin from FFPW USB in spring 2018 (1.52 ± 0.45) and from Tabor in autumn 2018 (1.42 ± 0.06). We found no significant differences (P ˃ 0.05) in ash content of fillets with and without skin at a single sampling time.
The fat content was significantly lower (P ˂ 0.01) in northern pike fillets with skin from Klatovy (autumn 2019, 0.28 ± 0.19), Lnare (spring 2018, 0.33 ± 0.19; autumn 2018, 0.45 ± 0.20; and autumn 2019, 0.41 ± 0.16), and Tabor (spring 2018, 0.43 ± 0.27), and in northern pike fillets without skin from Lnare (autumn 2018, 0.38 ± 0.28 and autumn 2019, 0.44 ± 0.21) and Klatovy (spring 2018, 0.39 ± 0.29 and autumn 2019, 0.40 ± 0.26) compared to other groups. We found significant differences (P ˂ 0.01) between fillets with skin and fillets without skin in the fat content of northern pike from Klatovy in spring 2018 (0.64 ± 0.28 and 0.39 ± 0.29), Chlumec nad Cidlinou in autumn 2018 (0.68 ± 0.19 and 0.91 ± 0.32) and Fishery Lnare in spring 2018 (0.33 ± 0.19 and 0.63 ± 0.34).
No significant differences (P ˃ 0.05) were observed in protein content of northern pike fillets with respect to farm, season, year, and presence/absence of skin.
The nitrogen content was significantly lower (P ˂ 0.01) in northern pike fillets with (3.08 ± 0.08) and without (3.06 ± 0.06) skin from Chlumec nad Cidlinou (spring 2018) and fillets without skin (3.06 ± 0.11) from Blatna (autumn 2019) compared to fillets with skin from Chlumec nad Cidlinou (autumn 2019, 3.22 ± 0.07), Lnare (spring 2018, 3.23 ± 0.09), and fillets with and without skin from Tabor (autumn 2018, 3.22 ± 0.03 and 3.22 ± 0.05 and spring 2019, 3.25 ± 0.07 and 3.22 ± 0.07) and Klatovy (spring 2018, 3.24 ± 0.07 and 3.22 ± 0.06). We found no significant differences (P ˃ 0.05) in nitrogen content of northern pike fillets with and without skin at a single sampling.
The PCA distribution of samples in northern pike did not show separate clusters, and the samples were discriminated against both axes (Fig.
Ordination plots of sample distribution after principal component analysis (PCA) of functional traits as response variables (A) and redundancy analysis (RDA) (B) of functional traits as response variables and weight of northern pike, Esox lucius as explanatory variable. N+skin = nitrogen concentration in fillet with skin, N-skin = nitrogen concentration in fillet without skin, Fat+skin = fat percentage in fillet with skin, Fat-skin = fat percentage in fillet without skin, Weight = body weight of individual fish; abbreviations used in legend: S = spring sampling, A = autumn sampling). The length of the arrow reflects the power of the variable to differentiate the samples. Sample scores were limited to 90 points for better fit.
The basic chemical composition of sheatfish fillets with and without skin is provided in Table
Live weight and length of sheatfish (Silurus glanis) and chemical composition of fillets with skin and without skin.
Company | Year | Season | Weight [g] x ± SD (min–max) | Total length [cm] x ± SD (min–max) | Fillet | Dry matter [g · 100 g–1] x ± SD (min–max) | Ash [g · 100 g–1] x ± SD (min–max) | Fat [g · 100 g–1] x ± SD (min–max) | Protein [g · 100 g–1] x ± SD (min–max) | N [g · 100 g–1] x ± SD (min–max) |
---|---|---|---|---|---|---|---|---|---|---|
FFPW USB Vodnany | 2018 | Spring | 900.0 ± 76.9 (750.0–1000.0) | 50.5 ± 1.2 (48.0–52.0) | with skin | 19.07 ± 0.57a(A) (18.22–20.27) | 0.99 ± 0.01ab(A) (0.97–1.00) | 1.17 ± 0.36a(B) (16.42–17.61) | 17.03 ± 0.36a(A) (16.42–17.61) | 2.73 ± 0.06ab(A) (2.63–2.82) |
without skin | 18.84 ± 0.58a(A) (17.72–20.02) | 1.03 ± 0.04ab(A) (0.98–1.13) | 0.79 ± 0.31a(A)* (0.45–1.40) | 16.88 ± 0.51a(A) (15.69–17.60) | 2.70 ± 0.08ab(A) (2.51–2.82) | |||||
2019 | Spring | 744.0 ± 161.0 (505.0–985.0) | 48.8 ± 3.1 (44.0–53.0) | with skin | 19.91 ± 0.91a(A) (18.83–21.38) | 1.04 ± 0.74ab(A) (0.98–1.09) | 0.74 ± 0.54a(A) (0.16–1.79) | 17.52 ± 0.69a(A) (16.58–18.62) | 2.80 ± 0.11ab(A) (2.65–2.98) | |
without skin | 25.64 ± 2.49b(B)* (21.64–31.63) | 1.07 ± 0.04ab(A) (0.95–1.12) | 0.54 ± 0.48a(A) (0.16–1.48) | 17.52 ± 0.70a(A) (18.40–20.79) | 2.80 ± 0.11ab(A) (2.68–3.01) | |||||
Fishery Chlumec nad Cidlinou | 2018 | Autumn | 2027.0 ± 1170.9 (885.0–4115.0) | 59.95 ± 9.31 (48.0–75.5) | with skin | 24.82 ± 2.42b(A) (22.52–30.48) | 0.99 ± 0.07ab(AB) (0.89–1.07) | 6.61 ± 2.98b(A) (3.06–13.33) | 16.59 ± 0.58a(A) (15.71–17.67) | 2.66 ± 0.09ab(AB) (2.51–2.83) |
without skin | 24.82 ± 3.45b(A) (20.67–30.38) | 1.05 ± 0.08ab(B) (0.84–1.12) | 7.21 ± 4.00b(A) (3.08–15.02) | 16.35 ± 0.67a(A) (14.67–17.18) | 2.62 ± 0.11a(A) (2.35–2.75) | |||||
2019 | Spring | 3220.0 ± 426.3 (2435.0–3725.0) | 76.9 ± 3.6 (70.0–82.0) | with skin | 30.56 ± 2.01d(C) (26.27–32.55) | 0.90 ± 0.05a(A)* (0.81–0.98) | 13.41 ± 2.67c(B) (7.71–16.50) | 16.31 ± 0.61a(A) (15.73–17.66) | 2.62 ± 0.10a(A) (2.52–2.83) | |
without skin | 27.25 ± 2.22c(B)* (23.25–30.68) | 0.98 ± 0.07a(A) (0.83–1.06) | 9.33 ± 2.89c(B)* (4.76–13.93) | 17.33 ± 0.65a(A) (16.12–18.00) | 2.77 ± 0.01ab(B)* (2.58–2.88) | |||||
Autumn | 3892.5 ± 732.0 (2515.0–4815.0) | 77.8 ± 6.8 (55.5–91.0) | with skin | 25.48 ± 1.32b(A) (23.46–27.19) | 0.92 ± 0.06a(A) (0.82–1.06) | 7.60 ± 1.62b(A) (5.16–10.03) | 16.33 ± 0.62a(A) (14.95–17.22) | 2.61 ± 0.10a(A) (2.39–2.76) | ||
without skin | 24.24 ± 1.60b(A) (22.31–27.34) | 0.97 ± 0.09a(A) (0.85–1.15) | 5.93 ± 1.65b(A) (2.91–8.68) | 16.77 ± 0.44a(A) (16.05–17.66) | 2.68 ± 0.07a(A) (2.57–2.81) | |||||
Fishery Blatna | 2018 | Autumn | 2552.5 ± 1290.3 (1005.0–4870.0) | 72.0 ± 12.3 (55.5–91.0) | with skin | 21.97 ± 0.58a(A) (21.16–23.09) | 1.08 ± 0.13ab(AB) (0.76–1.29) | 1.83 ± 0.72a(A) (1.04–3.03) | 18.23 ± 0.32a(A) (17.60–18.67) | 2.92 ± 0.05b(A) (2.82–2.99) |
without skin | 21.15 ± 0.36a(A) (20.42–21.77) | 1.15 ± 0.05b(B) (1.08–1.26) | 1.20 ± 0.59a(A) (0.39–2.52) | 18.12 ± 0.36a(A) (17.60–18.62) | 2.90 ± 0.06b(A) (2.82–2.98) | |||||
2019 | Autumn | 2527.5 ± 1481.6 (515.0–6115.0) | 72.4 ± 11.01 (60.0–100.0) | with skin | 20.77 ± 2.06a(A) (19.11–26.50) | 0.95 ± 0.05a(A) (0.87–1.03) | 2.42 ± 2.34a(A) (0.81–9.05) | 17.26 ± 0.64a(A) (16.17–18.15) | 2.76 ± 0.10ab(A) (2.59–2.90) | |
without skin | 19.86 ± 1.81a(A) (18.57–24.94) | 1.01 ± 0.07ab(AB) (0.93–1.12) | 1.53 ± 2.16a(A)* (0.22–7.71) | 17.21 ± 0.33a(A) (16.65–17.59) | 2.75 ± 0.05ab(A) (2.66–2.81) | |||||
Fishery Hodonin | 2019 | Spring | 5892.9 ± 1055.4 (4805.0–7780.0) | 93.3 ± 6.8 (87.0–105.0) | with skin | 26.09 ± 3.05b(A) (21.57–30.37) | 0.92 ± 0.08a(A) (0.83–1.04) | 7.94 ± 3.79b(A) (2.51–13.69) | 17.35 ± 0.81a(A) (16.00–18.42) | 2.78 ± 0.13ab(A) (2.56–2.95) |
without skin | 25.17 ± 3.19b(A (21.14–29.78) | 0.98 ± 0.08a(A) (0.88–1.11) | 6.84 ± 3.78b(A) (2.39–12.97) | 17.52 ± 0.74a(A) (16.17–0.74) | 2.80 ± 0.12ab(A) (2.59–2.92) | |||||
Autumn | 4979.0 ± 1798.8 (3120.0–9265.0) | 83.5 ± 9.5 (72.0–101.0) | with skin | 27.07 ± 2.47b(A) (23.18–32.86) | 0.95 ± 0.05a(A) (0.83–1.01) | 8.95 ± 3.01b(A) (4.60–16.57) | 17.06 ± 0.62a(A) (15.80–18.04) | 2.73 ± 0.10ab(A) (2.53–2.89) | ||
without skin | 25.64 ± 2.49b(A) (21.64–31.63) | 1.03 ± 0.05ab(A) (0.95–1.13) | 7.11 ± 2.72b(A) (3.16–13.82) | 17.29 ± 0.54a(A) (16.21–18.08) | 2.77 ± 0.09ab(A) (2.59–2.89) | |||||
Fishery Klatovy | 2019 | Autumn | 5220.0 ± 2247.8 (1955.0–8615.0) | 86.7 ± 13.9 (64.0–105.0) | with skin | 26.66 ± 2.08b(A) (22.77–30.97) | 0.92 ± 0.05a(A) (0.84–1.00) | 8.63 ± 2.80b(A) (4.16–13.10) | 16.99 ± 0.72a(A) (15.61–18.51) | 2.72 ± 0.12ab(A) (2.50–2.96) |
without skin | 26.04 ± 2.44b(A) (22.14–31.28) | 0.98 ± 0.05a(A) (0.90–1.05) | 6.92 ± 2.41b(A)* (3.23–11.10) | 17.17 ± 0.46a(A) (16.17–17.74) | 2.75 ± 0.07ab(A) (2.59–2.84) |
The ash content was significantly lower (P ˂ 0.01) in sheatfish fillets without skin from Chlumec nad Cidlinou (spring 2019, 0.98 ± 0.07 and autumn 2019, 0.97 ± 0.09), Klatovy (autumn 2019, 0.98 ± 0.05), and Hodonin (spring 2019, 0.98 ± 0.08) and fillets with skin from Chlumec nad Cidlinou (spring 2019, 0.90 ± 0.05 and autumn 2019, 0.92 ± 0.06), Blatna (autumn 2019, 0.95 ± 0.05), Klatovy (autumn 2019, 0.92 ± 0.05), and Hodonin (spring 2019, 0.92 ± 0.08 and autumn 2019, 0.95 ± 0.05) compared to sheatfish fillets without skin (1.15 ± 0.05) from Blatna in autumn 2018. We found significant differences (P ˂ 0.01) in ash content of fillets with skin and those without skin from Chlumec nad Cidlinou in spring 2019 (0.90 ± 0.05 and 0.98 ± 0.07).
The fat content was significantly lower (P ˂ 0.01) in sheatfish fillets with skin from FFPW USB (spring 2018, 1.17 ± 0.36 and spring 2019, 0.74 ± 0.54) and Blatna (autumn 2018, 1.83 ± 0.72 and autumn 2019, 2.42 ± 2.34) and without skin from FFPW USB (spring 2018, 0.79 ± 0.31 and spring 2019, 0.54 ± 0.48) and Blatna (autumn 2018, 1.20 ± 0.59 and autumn 2019, 1.53 ± 2.16) compared to other groups. We found significant differences (P ˂ 0.01) in fat content of fillets with and without skin in sheatfish from FFPW USB in spring 2018 (1.17 ± 0.36 and 0.79 ± 0.31), Chlumec nad Cidlinou in spring 2019 (13.41 ± 2.67 and 9.33 ± 2.89), Blatna in autumn 2019 (2.42 ± 2.34 and 1.53 ± 2.16), and Klatovy in autumn 2019 (8.63 ± 2.80 and 6.92 ± 2.41).
There were no significant differences (P ˃ 0.05) in protein content of sheatfish fillets of different farms, seasons, years, or with/without skin.
The nitrogen content was significantly lower (P ˂ 0.01) in sheatfish fillets with skin from Chlumec nad Cidlinou (spring 2019, 2.62 ± 0.10 and autumn 2019, 2.61 ± 0.10), and fillets without skin from Chlumec nad Cidlinou (autumn 2018, 2.62 ± 0.11 and autumn 2019, 2.68 ± 0.07) compared to those with (2.92 ± 0.05) and without (2.90 ± 0.06) skin from Blatna in autumn 2018. Nitrogen content of fillets with skin (2.62 ± 0.10) significantly (P ˂ 0.01) differed from those without skin (2.77 ± 0.01) in sheatfish from Chlumec nad Cidlinou in spring 2019.
The first two axes of PCA in sheatfish explained 92.80% and the PC1 axis explained 62.9% of the total variance (Fig.
Ordination plots of sample distribution after principal component analysis (PCA) of functional traits as response variables (A) and redundancy analysis (RDA) (B) of functional traits as response variables and weight of sheatfish, Silurus glanis as explanatory variable. N+skin = nitrogen concentration in fillet with skin, N-skin = nitrogen concentration in fillet without skin, Fat+skin = fat percentage in fillet with skin, Fat-skin = fat percentage in fillet without skin, Weight = body weight of individual fish; abbreviations used in legend: S = spring sampling, A = autumn sampling). The length of the arrow reflects the power of the variable to differentiate the samples.
Nitrogen factor indicates the content of nitrogen determined by Kjeldahl (
Species | Fillet | Nitrogen factor (Kjeldahl) |
---|---|---|
European pike-perch (Sander lucioperca) | with skin | 3.28 ± 0.09 |
without skin | 3.21 ± 0.09 | |
Northern pike (Esox Lucius) | with skin | 3.18 ± 0.09 |
without skin | 3.15 ± 0.09 | |
Sheatfish (Silurus glanis) | with skin | 2.73 ± 0.13 |
without skin | 2.75 ± 0.12 |
Fillet samples with and without skin from European pike-perch, Sander lucioperca; northern pike, Esox lucius; sheatfish, Silurus glanis from several rearing locations and different harvest seasons and years were analyzed for dry matter, protein, fat, and ash content. To date, there are no established nitrogen factors determined by the Kjeldahl method for European pike-perch, northern pike, and sheatfish with and without skin.
The basic nutrient values obtained in this study are similar to those reported by
Obtained nitrogen factors, taking into account the fat content of fish with skin and without skin, for European pike-perch, northern pike, and sheatfish determined by Kjeldahl method are given in Table
The established nitrogen factors allow analysis of products from European pike-perch, northern pike, and sheatfish, in accordance with the EU legislation. Regulation (EU) No 1169/2011 of the European Parliament and of the Council on the provision of food information to consumers (EC 2011), including the requirement for a Quantitative Ingredients Declaration (QUID) label specifying the quantity of fish content. On the basis of recommended nitrogen factors is possible to calculate the value of QUID (
There are limitations in the use of nitrogen factors. They are calculated as mean values with standard deviations, and it is important to bear in mind the effects of season, weight, fishery location, and nutritional status on natural values and the analytical variability of their determination and to apply the recommended value of ± 10% to the factor (
Determined nitrogen factors for European pike-perch, northern pike, and sheatfish would help to ensure that consumers are buying correctly labeled or described fish products. The recommended nitrogen factor for European pike-perch with skin is 3.28 ± 0.09 and without the skin is 3.21 ± 0.09, for northern pike with skin is 3.18 ± 0.09 and without the skin is 3.15 ± 0.09, for sheatfish with skin 2.73 ± 0.13 and without the skin 2.75 ± 0.12. Codex Alimentarius recommend allowing ± 10% variation (
The study was financially supported by the Ministry of Agriculture of the Czech Republic via the project QK1810095.