The Effects of Different Feeding Rates on Growth Performance and Stomach Volume in Rainbow Trout ( Oncorhynchus mykiss )

In this study, the effects of various daily feeding ratios on the growth, stomach volume and meat composition in rainbow trout (Oncorhynchus mykiss) were investigated. The feeding experiment was conducted in 450 L volume fiberglass tanks with 3 × 3 factorial patterns. During the experiment, while food were given to the fish at the first group in the 0.5% (I) of ratio of their live weight, the second group at the level of 2% (II) food were given to the fish, at the third group at the level of 6% (III) food. The initial weight and size values of the groups were 76.16±0.41 g and 19.11±1.63 cm, respectively. The final weight and size values reached up to 128.89±34.21 g, 25.09±2.37 cm; 236.05±89.32 g, 24.78±2.22 cm; and 238.91±86.67 g, 21.65±1.64 cm, for groups I, II, and III respectively. The best growth performance in terms of weight was obtained in group III, while the best growth performance in terms of size was obtained in groups I and II (P < 0.05). The best feed conversion ratio was determined in the low feeding group I (P < 0.05). At the end of experiment, three of the experimental groups were different from each other. The highest stomach volume was measured in group III (P < 0.05). In conclusion, the 6% feeding ratio increased the growth and significantly increased the stomach volume, however decreased the feed conversion ratio.


Introduction
Feeding expenses in sea and freshwater rainbow trout (Oncorhynchus mykiss) aquaculture comprise 60-70% of the total costs of consumables in the business.This clearly exhibits the importance of feed and feeding in the aquaculture business (Bureau et al., 2006;Bascinar et al., 2008;Korkut, 2013;Karabulut et al., 2017).To decrease feeding expenses, it is necessary to determine the most suitable feeding strategies and to establish effective feeding protocols.Therefore studies on fish feeding have focused on both feed intake of the fish and fish growth.Recently, many studies have been conducted on feeding ratios, feeding frequency and compensatory growth (Kuyumcu, 2001;López et al., 2002;Keskin & Erdem, 2005;Bureau et al., 2006;Bascinar et al., 2008;Marimuthu et al., 2011;Guzel & Arvas, 2011;Al Zahrani et al., 2013;Abidi & Khan, 2014).Increased fish production can be accomplished by knowing the feeding characteristics of the fish that will be cultured and the digestion of the feed that the fish ingest (Horn, 1998;Ozdemir et al., 2002).In some aquaculture studies on feeding and digestion, stomach volume evaluations revealed experimental information that the increased amount of feed increased the stomach volume.However, this evidence was not sufficient to measure the stomach volume (Pirhonen & Farsman, 1998;Ogata & Shearer, 2000;Karatas, 2005).This information on stomach volume can help to explain the differences in feed intake and feed digestibility between individuals.The measurement of stomach volume in rainbow trout fish is carried out with high accuracy by killing the fish right after the feed ingestion and removing the stomach (Jobling et al., 2001).Nevertheless, stomach volume can be measured when the stomach of the fish is full, contains semi-digested food, or even is empty.In these cases, the measurements can be carried out by emptying the stomach of contents and determining the weight of the ingested feed or by injecting a material of measured volume into the stomach (Jobling et al., jas.ccsenet.

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At the end ppm).The and pylori quickly so could take Mercury u gloves, ma and dispos ntent lipid with Crude ash content was determined by incineration in a muffle furnace (550 °C for 12 h).To determine the stomach volume and growth performance of the fish taken randomly from each tank, meat composition analyses were conducted.Results are shown as the standard deviation of the mean.SPSS 22.0 package program was used to evaluate the findings of the present study.Differences between groups were determined by ANOVA test.When the difference was statistically significant, the Duncan multiple comparison test was used to determine the difference between the groups.

Results and Discussion
This study aimed to evaluate the differences seen in the stomach volume in rainbow trout fish as a result of different feeding densities.The average water temperature during the study was 10.50±0.91 °C (min.8 °C-max.13 °C), pH was 7.60±0.52and oxygen level was 8.57±0.77mg L -1 .
No rainbow trout fish died during the study.
At the end of the experiment final average weights of groups were detected as 128.86±34.21g (0.5%), 236.05±89.32g (2%) and 238.91±86.67g (6%), respectively.Tarim and Akyurt (1992), in their study on different feeding applications on brood rainbow trout, found that the highest average weight gain was determined to be in the 1.5% feeding group, whereas the lowest was determined in the 0.5% feeding group.According to Ustaoglu and Bircan (1998), Kuyumcu (2001), and Keskin and Erdem (2005), the highest weight gain in different feeding applications on rainbow trout was determined to be in the groups in which the fish were fed to satiation.Also, it was concluded that instead of high-ratio feeding, the fish should be fed in accordance with their needs.That result was in accordance with the results of the present study regarding the weight gain as a result of the 6% feeding ratio.In the evaluation values calculated at the end of the study, the best result was obtained in the 2% feeding group, group II, with 1.59; this was followed by the 0.5% feeding group, group I, with 0.80 and the 6% feeding group, group III, with 3.17.The data on feed conversion showed that the results were better than those obtained by Storebakken et al. (1981), Tarim and Akyurt (1992), Sahin (1994), Ustaoglu and Bircan (1998), Kuyumcu (2001), and Keskin and Erdem (2005), and the results obtained by Bureau et al. (2006) were similar to those obtained in Group I of the present study.
During the trial, the differences in the average weight gain, specific growth rate, and condition factor values between group I and group II and between group I and group III were statistically significant (P < 0.05), whereas the differences between group II and group III were statistically not significant.
Examining the feed conversion ratio analysis results, the differences between groups I, II and III were statistically significant (P < 0.05).In rainbow trout feeding, growth occurs when the feed conversion ratio is above a certain value; however, feed cost increases in such cases.The recommended feed consumption is the value wherein the feed conversion ratio is minimum; nonetheless, growth rate increases when feed consumption increases (De Silva & Anderson, 1995).In the present study, as the feed application was excessive in group III, the growth rate increased, but the feed conversion ratio also increased.At the end of the experiment, by making use the dissections results on fish hepatosomatic indices and viscerosomatic indices of fish have been determined.
According to the results of the analysis on the characteristics of meat composition, hepatosomatic indices was found statistically insignificant, while viscerosomatic indices were statistically significant (p < 0.05).At the end of the study, upon examination of the results obtained from dissections, viscerosomatic index values were parallel to those obtained by Ustaoglu (1996) and higher than those obtained by Guzel and Arvas (2011).Hepatosomatic index values were lower than those reported by Ustaoglu (1996) and Keskin and Erdem (2005) and similar to those obtained by Guzel and Arvas (2011).

Stomach Volume
At the end of the study, the fish were dissected for stomach volume measurements, stomach contents were emptied, and the stomachs were filled with mercury.Stomach volume was found to be 7.72±2.16mL in group I, while stomach volume values were 14.61±3.74mL and 16.01±5.64mL in group II and group III, respectively.According to the statistical analysis results; the difference between groups I -II and I -III were significant (P < 0.05), while that between groups II -III was not significant (P > 0.05).At the end of the study, stomach volume was found to be 7.72±2.16mL in group 0.5%, while stomach volume values were 14.61±3.74mL and 16.01±5.64mL in group 2% and group 6%, respectively.According to the information in the literature, as in other animals, stomach is the most important organ controlling the appetite in fish.Therefore, feed consumption occurs relative to the vacancy in the stomach.The time required for emptying the stomachs of the fish varies depending on water temperature, fish size, and the amount and quality of feed.In addition, it is associated with the gastric emptying phase, stomach stress rate, the surface area of the stomach, and the surface area of the consumed feed (Korkut, 2013).As a result of the study, it was demonstrated that the increase in the stomach volume of the fish was related to the amount of ingested feed and the size of the fish.

Meat Composition
Proximate analysis results of randomly sampled fish meat from the groups and their comparisons were given in the Table 2. Note.Values in the same row with no common superscripts are significantly different (P < 0.05).
In fish with different feeding ratio applications, crude protein analysis results were significantly different between groups I-II and I-III (P < 0.05).The lowest crude fat ratio was determined in group I.That was followed by group II and III, respectively.According to the analysis results, the difference between the groups I -III was not statistically significant (P < 0.05).At the and end of the experiment, by making use of the results of chemical analysis on the fish, the levels of crude protein, and crude fat fish meat were determined.Meat composition results obtained from all groups were lower than those obtained by Bureau et al. (2006) in their study on different feeding levels on rainbow trout, whereas their crude fat values were similar to those in group I and higher than those of the other groups.Crude fat values reported by Keskin and Erdem (2005) in rainbow trout were higher than those obtained in the present study, whereas crude protein values were similar.
It was determined in this study that different feeding ratios had significant effects on the growth, feed conversion, and visceral index ratios in addition to stomach volumes.In addition, the results obtained in this study were similar to those obtained in the previous studies.

Conclusions
The aim of this study was to reveal the effects of high density feeding rates on growth and stomach volumes in rainbow trout (Oncorhynchus mykiss).
At the end of the study, three different feeding ratios were positively affected the stomach volumes, and thus the visceral index values, showing that the stomach and internal organs of the fish fed with low feeding ratios had a proportionally lower mass.From that point of view, consumers clearly would benefit in the case of preferring fish produced with low feeding ratios.Although a lower growth performance was obtained in terms of specific growth rate, condition, which is one of the most important indicators affecting the market value of other species, show that the consumers may prefer the low feeding group.
In this study, it was determined that different feeding ratios had statistically significant effects on the growth, feed conversion, and stomach volumes of the fish.Growth, specific growth rate, and condition factors increased with the changes in the feeding ratios in addition to an increase in stomach volume.The results obtained as a result of different feeding ratios will contribute to the sustainability of the sector.Feed cost is the most important expense in rainbow trout aquaculture.In addition, release of excess fecal waste and inedible food to the environment may have negative consequences to the aquatic ecosystem.
Further studies will be based on that the recovery duration of the overextension of the digestive system, the determination of fish welfare throughout this process, and the environmentaly interaction.

Table 2 .
The results of meat composition analysis according to groups (X ±SH)