Fish and experimental protocol
Three-hundred-fifty market-size (755–3865 g) common carp Cyprinus carpio were obtained from six sources at various times of year to for effects of variation in rearing conditions. The weight of collected carp corresponded to the weight of carp normally delivered to the market. Fish were obtained from the Faculty of Fisheries and Protection of Waters of the University of South Bohemia in Ceske Budejovice (FFPW USB), Vodnany and the fisheries Chlumec nad Cidlinou, Blatna, Hodonin, Klatovy, Lnare, and Tabor. Ten fish were collected from each fishery at the spring (March/April), summer (June/July), and autumn harvests (October/November) in 2018 and 2019. Carp were transported live to the laboratory of the FFPW, killed by a blow to the head, weighed, measured, and filleted. Two fillets, one with skin removed, from each fish were individually vacuum packed, immediately frozen, and stored at − 32 °C until chemical analysis.
Ethics approval
All the methods used in the present study followed relevant guidelines and regulations. Also, the competent authority (Ethical Committee for the Protection of Animals in Research of the University of South Bohemia, FFPW Vodnany) approved the fish sampling and protocols of the present study and reporting herein follows the recommendations in the ARRIVE guidelines.
Chemical analysis
Seven-hundred carp fillets were analysed for basic nutritional composition, dry matter, protein, fat, and ash. All samples were homogenized by grinding before analysis.
The determination of dry matter followed ISO 1442:1997 Meat and meat products—Determination of moisture content (Reference method)26. The homogenized samples were dried with sand to constant weight at 103 ± 2 °C in a laboratory oven (Memmert UE 500, Memmert GmbH + Co. KG, Germany).
The determination of ash was based on the standard ISO 936:1998 Meat and meat products—Determination of total ash27. The homogenized samples were burned in a muffle furnace (Nabertherm A11/HR, Nabertherm GmbH, Germany) at 550 ± 25 °C to a grey-white colour.
The determination of total fat was based on the standard ISO 1443:1973 Meat and meat products—Determination of total fat content28. The homogenized samples were hydrolysed by hydrochloric acid, and fat was extracted by light petroleum in SOXTEC 2050 (FOSS Headquarters, Denmark).
The determination of nitrogen used the Kjeldahl method based on the standard method ISO 937:1978 Meat and meat products—Determination of nitrogen content (Reference method)29. The homogenized samples were digested by sulphuric acid and a catalyser in a KjelROC Digestor 20 (OPSIS AB, Sweden) digestion unit at 420 ± 10 °C. Organically bound nitrogen was measured on the KJELTEC 8400 with KJELTEC sampler 8420 (FOSS Headquarters, Denmark). Calculation of protein content from nitrogen used the conversion factor for meat of 6.25.
All analysis of dry matter, ash, and total fat were performed in duplicate and analysis of nitrogen (protein) was performed in triplicate for each sample.
Calculation of fat-free nitrogen (Nff) in g/100 g used the formula24:
$$ N_{ff} = frac{{100 times N { }}}{{100 – F { }}}. $$
This formula was applied to nitrogen (N) and fat (F) content for all samples, providing a fat-free nitrogen value for each sample.
Fish meat content calculated based on nitrogen factor Nf (total fillet) in g/100 g used the formula9:
$$ Fish ;content_{Nf} = frac{N times 100}{{N_{f} }}. $$
Fish meat content calculated based on fat-free nitrogen factor (Nff) and DCC (defatted carp content) in g/100 g used formulas11:
$$ Fishc; content_{Nff} = DCC + F, $$
$$ DCC = frac{N times 100}{{N_{ff} }}. $$
Statistical analysis
Kolmogorov–Smirnov and Bartlett’s tests were applied to assess normal distribution data and the homoscedasticity of variance, respectively. A two-way ANOVA and Tukey’s test was conducted to analyse effects of season, weight, fishery, and difference between fillets with and without skin. The significance level was set at p < 0.05. Data were expressed as the mean ± SD values and range. Analysis was performed using STATISTICA v. 12.0 for Windows (STATSOFT, Inc.).
Redundancy analysis (RDA) with functional traits as response variables and company, sampling year and season as categorial (explanatory) data was applied to describe the differences among sample distribution. The ordination plots were produced using Canoco, Windows release, 5.10 version (Biometris, the Netherlands, and Petr Šmilauer, Czech Republic).
Source: Ecology - nature.com
