Milkfish (Chanos chanos) under stress: Contributing to fish welfare in tropical aquaculture by identifying and quantifying potential stressors

Abstract

Due to an ever-growing world population, the demand for aquatic food resources has increased constantly in the last decades and will continue to do so in the future. In order to meet the growing demand, aquaculture practices have intensified tremendously over the last decades. Current intensification of management practices is considered as a potentially severe threat to fish welfare and a sustainable production. Besides potential stressors due to the lack of optimization of management procedures, outdoor-based aquaculture systems are additionally prone to daily and seasonal environmental fluctuations. Therefore, cultured fish have to be able to cope with environmental fluctuations, such as changes in temperature and/or dissolved oxygen concentration, which could potentially cause stress. Stressful stimuli initiate an endocrine response in fish. For teleost fish, this includes among others, the release of cortisol via the hypothalamic-pituitary-interrenal axis. Furthermore, stress induces glucocorticoid-mediated changes in the fish’s energy metabolism (e.g. reallocation of energy resources) to provide energy in order to restore the homeostatic state, and subsequently to cope with the stressor. Several Asian countries, involving also several (sub-) tropical countries, are dominating the list of top producers in global aquaculture production. Thereby, milkfish (Chanos chanos) represents an important aquaculture species for some of these (sub-) tropical countries. As observed globally, milkfish aquaculture practices have been intensified markedly over the last decades. In particular, for Philippine milkfish aquaculture, it has been reported that these practices have led to associated negative consequences, such as a remarkable deterioration in water quality, which was linked, among others, to massive fish kill events. Surprisingly, until now, research on (chronic) stress or stress response of milkfish to different potential stressors under aquaculture conditions (environmental or management related) has received comparatively little attention. Therefore, the aim of this thesis was to identify and quantify potential environmental and management related stressors for milkfish, hereby intending to contribute to fish welfare in tropical aquaculture. For this purpose, different tools, from scale cortisol, (enzymatic) biomarkers to respiratory measurements, were used to assess the chronic stress response as well as metabolic implications for juvenile milkfish. Therefore, the thesis was divided into two parts: (i) the assessment of the stress-physiological response of juvenile milkfish when exposed to thermal and/or hypoxic stress under controlled laboratory conditions (Chapter II and III); and (ii) the quantification of chronic stress levels of milkfish cultured in commercial marine cage systems, thereby intending to identify prevailing stressors (Chapter IV). The outcomes of this thesis showed that even a comparatively mild increase in water temperature led to a significant chronic stress response in milkfish (increased scale cortisol; Chapter II). Furthermore, higher water temperature also significantly affected the metabolism of juvenile milkfish, demonstrated by enhanced metabolic rates and indications of reduced energy resources (Chapter III). Additionally, we could demonstrate that milkfish might be more susceptible to further stressors, such as decreasing dissolved oxygen concentrations, when exposed to thermal stress (Chapter III). Based on our results and considering anthropogenic induced climate change and warming as well as the strong temperature dependence of oxygen solubility, increasing water temperatures and hypoxic conditions should be considered as a future challenge for cultured milkfish. During our fieldwork in the most productive milkfish mariculture area of the Philippines, we could quantify chronic stress levels (scale cortisol) of milkfish cultured in commercial mariculture systems for the first time (Chapter IV). Thereby, we could report that milkfish experienced stressful conditions in commercial cage systems and are most likely exposed not only to a single but to a variety of chronically stressful stimuli over time (Chapter II vs Chapter IV). The outcomes of this thesis contribute to the foundational understanding and identification of prevailing stressors in milkfish mariculture. By aiming to pinpoint potential chronic stressors for milkfish, including changes in environmental conditions (e.g. increase in water temperature) as well as different aspects of typically applied management strategies (e.g. stocking density), the results and applied methods of this thesis can be used as an approach to optimize current culture conditions. This can consequently contribute to improve fish welfare and sustainability in milkfish mariculture as well as for other species cultured in (tropical) mariculture.