Abstract
The substitution of fish resources as ingredients for aquafeeds by those based on vegetable sources is needed to ensure aquaculture sustainability in the future. It is known that Senegalese sole (Solea senegalensis) accepts high dietary content of plant ingredients without altering growth or flesh quality parameters. However, scarce information is available regarding the long-term impact of vegetable diets (combining the inclusion of both vegetable protein and oils) on the stress response and immunity of this fish species. This study aims to evaluate the concomitant effect of the extended use of vegetable protein-based diets with fish oil (FO) replacement (0, 50 or 100%) by vegetable oils (VO), on the response to acute (10 min) or prolonged (4 days) stress, induced by thermal shock. Plasma levels of cortisol, glucose and lactate as well as hepatic levels of glucose, glycogen and lactate were evaluated as primary and secondary responses to stress, 6 and 18 months after feeding the experimental diets (6 and 18 MAF). The brain monoaminergic activity in telencephalon and hypothalamus, and non-specific immune parameters were also evaluated. As expected, thermal shock induced an increase in values of plasma parameters related to stress, which was more evident in acute than in prolonged stress. Stress also affected lactate levels in the liver and the values of the alternative complement pathway-ACH50 in the plasma. Dietary substitution of FO induced an effect per se on some parameters such as decreased hepatic glucose and glycogen levels and peroxidase activity in plasma as well enhanced serotonergic activity in brain of non-stressed fish. The results obtained in some parameters indicate that there is an interaction between the use of vegetable diets with the physiological response to thermal stress, as is the case of the hepatic lactate, serotonergic neurotransmission in brain, and the activity of ACH50 in plasma. These results suggest that the inclusion of VO in plant protein based diets point to a slightly inhibited stress response, more evident for an acute than a prolonged stress.
Highlights
Over the last few years, a big effort has been directed to investigate the use of ingredients from vegetable sources that could substitute fishmeal (FM) and fish oil (FO) as the main constituents used in the formulation of diets for fish [1,2,3,4,5,6,7]
Cortisol release induced by stress or by ACTH stimulation is affected by the use of vegetable oils in fish which could be attributed to the different PUFA contents
The results reported so far are contradictory, as illustrated by the effects on parameters related to humoral immunity such as lysozyme activity or the alternative complement activity in serum in marine fish species like gilthead sea bream, Atlantic cod, European Sea bass or grouper [15,16,26,27,28,29,30,31,32]
Summary
Over the last few years, a big effort has been directed to investigate the use of ingredients from vegetable sources that could substitute fishmeal (FM) and fish oil (FO) as the main constituents used in the formulation of diets for fish [1,2,3,4,5,6,7]. The inclusion of vegetable ingredients in aquafeeds produces modifications in the fatty acid composition of the diets, inducing n-3/n-6 ratio imbalances which may affect the physiological response to stress in fish as has been reported in previous studies [15,16,17]. Cortisol release induced by stress or by ACTH stimulation is affected by the use of vegetable oils in fish which could be attributed to the different PUFA contents. A modulation of stress-related gene expression by vegetable oils has been reported in larvae and interrenal cells of European sea bass [17,23] and in the liver of gilthead sea bream [24]
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