Welfare of Atlantic salmon postsmolts in closed-containment production systems, using a function-based approach

In salmon culture, there are concerns related to sea lice, escapees, and high fish mortality (~20%). This has prompted use of closed-containment aquaculture systems (CCS) in sea, or land-based. This technology, producing larger fish in CCS before stocking in sea may reduce sea-phase mortality and sea lice pressures on wild salmon. However, such systems have higher investment costs and to compensate, a higher production intensity is assumed, with fish densities to 100 kg/m3. Acute stress for the fish may occur during such density or crowding events. It is not known if chronic exposure to the CCS environment will affect the ability of the fish to handle additional acute stressors. The main objective of SalmoFutura was thus to develop new function-based tools for welfare documentation and assessment of Atlantic salmon in closed-containment systems, including use of acute challenge stress tests (ACT). We first did an experiment to find the optimal time to sample the fish, after a stress event, that can tell best about how the fish react. In this trial, the fish were kept either at low or high fish density, and then subjected to an additional crowding stress event (ACT) at 300 kg/m3. We found that the magnitude of change was greatest at 0h, 1h and 24h after the stress event. We also found that the fish may have different energy needs to cope with such stress, depending on if they were kept in low or high fish density before the stress. We also found that levels of the hormone a-MSH were higher when fish were kept in high- versus low fish density, and this hormone may be more useful than cortisol to describe long-term effects of stress in fish. In a second experiment we studied salmon reared in two types of CCS: a recirculating aquaculture system (RAS) at 12 ppt salinity in the water and a flow-through system (FT) at full (32 ppt) sea salinity. Each system included experimental groups at either low (20-40 kg/m3) or at high (80-115 kg/m3) fish densities, to generate different stress levels. The fish kept at the high density became chronically stressed, and in RAS the high density group showed depressed growth for up to 2.5 months. After this initial period however, the fish in RAS adapted to the high density and showed signs of increased growth rate. In the FT fish however, the desired density in the chronically stressed high-density group was achieved only much later, after 2.5 months of the experiment, due to high mortality at the start of the experiment for this group (28% after one month). The survival rate among chronically stressed fish (high-density) raised in RAS was 98%, while it was only 71% for chronically stressed from the FT system. Hence, the FT fish did not adapt as quickly as the RAS fish to high density. Another important part of SalmoFutura was to develop new welfare indicators. We focused on skin, an important tissue in fish for physiological regulation and defense against disease. The high production intensities in CCS may compromise skin health and lead to more pathogen transmission and disease. Thus, it is important to identify factors affecting wound healing, detection methods and healing stimulants. We found that fish kept in FT in the second experiment above, showed a stronger regulation of genes in skin after an additional crowding stress event (ACT). This suggests that the FTS fish have an increased cost of recovery from the severe crowding stress when compared to fish reared in RAS. A wound healing model was also developed and two minor experiments done. In the first experiment the capacity of wound healing was compared between unstressed (low density) and stressed fish (high density) over a period of 57 days. The results from gene expression (microarrays) demonstrated that there were differences in the wound healing response in the stressed and unstressed fish during all the investigated time points. Further, our results showed that the major differences between the two groups took place in the first 14 days after wounding. We managed through SalmoFutura to understand more of how welfare indicators in salmon kept in closed-containment systems are affected by stress and environment. Furthermore, we developed new welfare indicators, in particular for skin, and discovered new mechanisms on how the skin wound healing process occur. We have substantially improved our toolbox and pipeline for welfare analysis. It now includes several new gene specific assays, histology and immunohistochemistry. In addition, we have managed to establish a wound healing model for salmon that will be highly relevant in future evaluation and understanding of skin ulcer development in salmon, a particularly relevant issue for rearing in intensive closed-containment systems. The findings in SalmoFutura will be transferred to user partners and to new projects, and the work is being continued in the CtrlAQUA SFI centre.

To enable increased production, Norwegian aquaculture is dependent upon environmentally sound production platforms. New technologies are being considered for salmon postsmolts such as closed-containment systems in sea and on land. However, the consequence s for fish welfare in such systems must first be known. SalmoFutura aims to generate knowledge and tools to document welfare of postsmolts in closed-containment systems. We will use new welfare tools to evaluate how such systems affects the ability of p ostsmolts to respond to new challenges. While homeostasis can be maintained under chronic mild stress, and difficult to detect, an additional challenge can push the animal over to allostatic overload, and compromise normal physiological response and welfa re. SalmoFutura aims to characterize and develop indicators and a Welfare Index, that can predict poor welfare before they happen in large scale. First we will characterize temporal profiles of welfare indicators following acute challenges tests (ACT), th ereby finding the optimal post-ACT sample time to maximize the scope of indicator responses in the second trial. Here, selected size-classes that are critical for welfare, will first be subjected to chronic stressors, followed by an ACT. Welfare will then be assessed as the ability of the fish to produce normal physiological and neural responses. We will study three organs in particular, skin, gill, and brain. Regarding skin, industry reports that closed-containment systems may increase the risk of wounds . Hence, skin integrity and gill physiology will be prioritized. Regarding brain, recent studies have shown that previous environmental experiences provide memory-based mechanisms to deal with challenges, integrating physiological stress and cognitive abi lities. Together, these physiological and mental robustness indicators will provide important information about the robustness of the fish towards future challenges in closed-containment systems.