Date Published: January 17, 2017
Publisher: Public Library of Science
Author(s): Fernando Barreto-Curiel, Ulfert Focken, Louis R. D’Abramo, María Teresa Viana, Andrea Motta.
Fish starvation is defined as food deprivation for a long period of time, such that physiological processes become confined to basal metabolism. Starvation provides insights in physiological processes without interference from unknown factors in digestion and nutrient absorption occurring in fed state. Juveniles of amberjack Seriola lalandi were isotopically equilibrated to a formulated diet for 60 days. One treatment consisted of fish that continued to be fed and fish in the other treatment were not fed for 35 days. The isotopic signatures prior to the beginning of and after the starvation period, for fish in the starvation and control treatments, were analysed for lipid content, fatty acid composition and isotopic analysis of bulk (EA-IRMS) and of amino acids (compound specific isotope analysis, CSIA). There were three replicates for the starvation group. Fatty acid content in muscle and liver tissue before and after starvation was determined to calculate percent change. Results showed that crude lipid was the most used source of energy in most cases; the PUFAs and LC-PUFAs were highly conserved. According to the protein signature in bulk (δ15N) and per amino acid (δ13C and δ15N), in muscle tissue, protein synthesis did not appear to occur substantially during starvation, whereas in liver, increases in δ13C and δ15N indicate that protein turnover occurred, probably for metabolic routing to energy-yielding processes. As a result, isotopic values of δ15N in muscle tissue do not change, whereas CSIA net change occurred in the liver tissue. During the study period of 35 days, muscle protein was largely conserved, being neither replenished from amino acid pools in the plasma and liver nor catabolized.
To understand the efficient use of amino acids by growing fish, several approaches have been investigated to formulate diets for fish in aquaculture . The most common method of estimating requirements is through dose-response curves using a minimum of four treatments with graded levels of high quality protein or amino acids . However, the gap between what is ingested compared to what is retained, as growth remains large, and any reduction in the amount of dietary amino acids will have economic benefits. One approach for improving protein efficiency of dietary formulations is to mimic the proportional composition of amino acids contained in the muscle tissue to assure the presence of all essential amino acids . However, to meet the exact requirements to generate data that can potentially be use to the formulation of efficient feeds, knowledge of the bioavailability of each dietary amino acid to be absorbed and retained is needed . The bioavailability could be indirectly estimated through the determination of the apparent digestibility of the dietary essential amino acids. However, in aquatic organisms, leaching of water-soluble nutrients from both feed and faeces always exists, thereby contributing to inaccuracy in the determination of the amounts of those available amino acids that are actually absorbed. Also, apart from those amino acids retained for anabolic processes (growth, i.e. protein deposition), there is also need to determine those amounts required to meet the demands of metabolic processes. Therefore, the amounts of at least some essential dietary amino acids are commonly underestimated .
Animal handling was according to our institutional ethical standards (UABC), and approved and supervises by our Institutional Ethical Commission. Juvenile of Seriola lalandi were used for the experimental procedure conducted under the certified laboratory for fish nutrition experimental laboratory, and all animal work performed here have been conducted according to the ethics statement from the University (UABC) in accordance to international guidelines. This study was approved and supervised by the ethics commission from the Instituto de Investigaciones Oceanológicas (IIO, UABC).
After the adaptation phase of 60 days, the average individual weight of S. lalandi juveniles was 160.0 ± 10.0 g. After 35 days of starvation, average weight loss was 25.9 ± 9.0% (Table 2) and lipid content in the muscle and liver tissues of fish decreased by 56% and 52%, respectively, as compared to the control group of fish. Average absolute individual wet weight losses for the muscle and liver tissue of fish in the starved treatment were 4.2 ± 0. 4g and 12.7 ± 1.6 g respectively, compared to values of 9.7 ± 0.3 and 26.8 ± 0.6 for muscle and liver, respectively, of fish in the control group (Table 2). The proportional fatty acids compositions of muscle and liver tissues of fed and starved fish are presented in Table 3. According to determined changes in relative content that occurred under starvation, C12:0, C14:0 and C16:0, as well as mono-saturated fatty acids, C16:1n7, C18:1n9 and 18:1n7 were preferentially used in both muscle (except 12:0) and liver tissue, presumably as sources of energy. All were significant changes except for 16:0 in the liver tissue and 18:7n-7 in the muscle tissue. Significant net change decreases of 17.9 mg/ g (18.4%) of muscle tissue and 26.5 mg/g (31.0%) of liver tissue occurred for the C18:2n6 PUFA. For C18:3n3 a relative decrease of 4.4 mg/g (12.0%) in muscle tissue occurred, whereas a significant increase of 57.2 mg/g (525.5%) in liver tissue was observed. The LC-PUFAs such as arachidonic acid (C20:4n6, ARA) resulted in a relative increase of 23.2 mg in each of the muscle (124.8%) and liver (81.4%) tissues. Eicospentaenoic acid (C20:5n3, EPA) decreased in muscle (4.4 mg/g, 7.1%) and significantly decreased in the liver (–22.3g/g, 37.5%), whereas decosahexanoic (C22:6n3, DHA) significantly increased by 128.0 mg/ g (89.7%) in muscle tissue and 86.0 mg /g (50.5%) in liver tissue. In general, standardized amounts of saturated and monounsaturated fatty acids were exhausted, both in muscle and liver tissues, whereas, with some exceptions, the PUFAs and LC-PUFAs were preferentially conserved.
Fish were presumed to have equilibrated to their diet after being fed the control diet for 60 days, as supported by the observations of Badillo-Zapata et al. , before being used in the experimental procedure. Equilibrium was maintained throughout the experimental procedure as confirmed by the bulk analysis for δ13C and δ15N from samples taken from muscle and liver tissues of fish in the control group at selected times (Figs 1 and 2). These samples were analysed as a whole (without lipid removal); therefore, the δ13C measured is derived from AA, carbohydrates and crude lipid, whereas the δ15N is derived from protein only.