Effects of coeliacomesenteric blood flow reduction on intestinal barrier function in rainbow trout Oncorhynchus mykiss

Aquaculture potential of Atlantic wolffish (Anarhichas lupus): stress and physiological responses to acute handling

The Atlantic wolffish (Anarhichas lupus) is a cold-water species with the potential to diversify aquaculture in Northern countries. Few studies have investigated the stress physiology of Atlantic wolffish, and the current knowledge on stress in wolffish species is largely derived from the closely related spotted wolffish (Anarhichas minor). In the current study, Atlantic wolffish were exposed to handling stress mimicking common husbandry conditions in aquaculture such as repeated air exposure and net-chasing. Samples were taken prior to stress exposure (pre-stress; control) as well as 5- and 24-h post-stress. A series of primary and secondary acute stress response parameters were assessed: plasma cortisol, glucose and lactate levels, hematological indices (hemoglobin, Hb; hematocrit, Hct; mean corpuscular hemoglobin concentration, MCHC), and osmoregulatory capacity through plasma osmolality and gill Na+/K+ATPase (NKA) activity. Other secondary stress responses with implications for fish health and welfare are intestinal integrity and transport functions. These parameters were assessed using the Ussing chamber technique. The cortisol peak values were low in comparison to other fish species studied after acute handling stress and occurred as late as 24 h post-stress, suggesting that Atlantic wolffish is a slow and low cortisol responder. Plasma glucose remained stable, whereas lactate concentrations significantly decreased between 5 and 24 h after stress. There was no effect on pH, Hb, or Hct, although a significant increase in MCHC was found after 5 h and 24 h, originating from a small increase in Hb. This result suggests a minor increase in Hb synthesis after stress exposure. The intestinal integrity and transport functions as well as gill NKA-activity remained unchanged after stress exposure. In conclusion, Atlantic wolffish appears to exhibit a relatively moderate stress response, characterized by a slow and low primary stress response and minimal secondary effects following husbandry-related acute stress. These findings contribute to the understanding of the species' potential for development as a candidate for marine, cold-water aquaculture.

Insights into thermal sensitivity: Effects of elevated temperature on growth, metabolic rate, and stress responses in Atlantic wolffish (Anarhichas lupus)

The Atlantic wolffish (Anarhichas lupus) is a cold-water fish with potential for aquaculture diversification. To unveil the mechanisms underlying the compromised growth in Atlantic wolffish when reared at higher temperatures, we investigated the relationship between temperature, growth rate, aerobic capacity, stress biomarkers, and gut barrier function. Juveniles acclimated to 10°C were maintained at 10°C (control) or exposed to 15°C for either 24 h (acute exposure) or 50 days (chronic exposure). Fish exposed to 15°C exhibited reduced growth, higher standard, and maximum metabolic rates compared to those at 10°C. In the chronically exposed group at 15°C, metabolic rates were lower than those of acutely exposed fish. The absolute aerobic scope exhibited no significant variation in temperatures; however, the factorial scope showed a notable reduction at 15°C in both acute and chronic exposed groups, aligning with a correlated decrease in individual growth rates. Chronic warming led to increased plasma glucose levels, indicating energy mobilization, but cortisol levels were unaffected. Furthermore, chronic warming resulted in reduced intestinal barrier function, as evidenced by increased ion permeability and a negative potential in the serosa layer. We conclude that warming elevates metabolic rates while reducing intestinal barrier function, thus increasing energy expenditure, collectively, limiting energy available for growth at this temperature from increased allostatic load. Thus, juvenile wolffish maintaining their aerobic scope under thermal stress experience slower growth. This research provides insights for improving the welfare and resilience of wolffish in aquaculture at elevated temperatures and understanding their response to increased environmental temperatures.

Do extreme postprandial levels of oxygen, carbon dioxide, and ammonia in the digestive tract equilibrate with the bloodstream in the freshwater rainbow trout (Oncorhynchus mykiss)?

The gastrointestinal tract (GIT) lumen of teleosts harbors extreme conditions, especially after feeding: high PCO₂ (20-115 Torr), total ammonia (415-3710 μM), PNH₃ (79-1760 μTorr in the intestine), and virtual anoxia (PO₂ < 1 Torr). These levels could be dangerous if they were to equilibrate with the bloodstream. Thus, we investigated the potential equilibration of O₂, CO₂, and ammonia across the GIT epithelia in freshwater rainbow trout by monitoring postprandial arterial and venous blood gases in vivo and in situ. In vivo blood was sampled from the indwelling catheters in the dorsal aorta (DA) and subintestinal vein (SIV) draining the posterior intestine in the fasting state and at 4 to 48 h following catheter-feeding. To investigate possible ammonia absorption in the anterior part of the GIT, blood was sampled from the DA, SIV and hepatic portal vein (HPV) from anaesthetized fish in situ following voluntary feeding. We found minimal equilibration of all three gases between the GIT lumen and the SIV blood, with the latter maintaining pre-feeding levels (PO₂ = 25-49 Torr, PCO₂ = 6-8 Torr, and total ammonia = 117-134 μM and PNH₃ = 13-30 μTorr at 48 h post-feeding). In contrast to the SIV, we found that the HPV total ammonia more than doubled 24 h after feeding (128 to 297 μM), indicative of absorption in the anterior GIT. Overall, the GIT epithelia of trout, although specialized for absorption, prevent dangerous levels of PO₂, PCO₂ and ammonia from equilibrating with the blood circulation.

Remote physiological monitoring provides unique insights on the cardiovascular performance and stress responses of freely swimming rainbow trout in aquaculture

Investigating the mechanisms that fish employ to maintain homeostasis in their everyday life requires measurements of physiological and behavioural responses in the field. With multivariate bio-loggers, we continuously measured gastrointestinal blood flow (GBF), heart rate, activity and body temperature in rainbow trout (Oncorhynchus mykiss) swimming freely amongst ~5000 conspecifics in a sea cage. Our findings clearly demonstrate that while both acute aquaculture-related stress and spontaneous activity resulted in transient reductions in GBF (i.e. reductions of up to 65%), recovery from stressful handling practices subsequently involved a substantial and prolonged gastrointestinal hyperemia far beyond the level observed prior to the stressor. The gastrointestinal hyperemia may be necessary to repair the damage to the gastrointestinal tract caused by acute stress. Furthermore, heart rate responses to acute stress or voluntary activity differed depending on the individual’s physiological state. Stressed fish (i.e. mean heart rates >70 beats min⁻¹) exhibited a bradycardic response to acute stress or activity, whereas fish with mean heart rates <60 beats min⁻¹ instead demonstrated strong tachycardic responses. Remote monitoring of physiological and behavioural variables using bio-loggers can provide unique insights into ‘real-life’ responses of animals, which can largely differ from the responses observed in confined laboratory settings.

Rainbow Trout Maintain Intestinal Transport and Barrier Functions Following Exposure to Polystyrene Microplastics

Ingestion has been proposed as a prominent exposure route for plastic debris in aquatic organisms, including fish. While the consequences of ingestion of large plastic litter are mostly understood, the impacts resulting from ingestion of microplastics (MPs) are largely unknown. We designed a study that aimed to assess impacts of MPs on fish intestinal physiology and examined integrity of extrinsic, physical and immunological barriers. Rainbow trout were exposed to polystyrene (PS) MPs (100-400 μm) via feed for a period of 4 weeks. Fish were fed four types of diets: control, diets containing virgin PS particles, or particles exposed to two different environmental matrices (sewage or harbor effluent). Extrinsic barrier disturbance in intestinal tissue was evaluated via histology. The paracellular permeability toward ions and molecules was examined using Ussing chambers and mRNA expression analysis of tight junction proteins. Active transport was monitored as transepithelial potential difference, short-circuits current and uptake rate of amino acid ³H-lysine. Immune status parameters were measured through mRNA expression level of cytokines, lysozyme activity, and hematological analysis of immune cells. We could not show that PS MPs induced inflammatory responses or acted as physical or chemical hazards upon ingestion. No measurable effects were exerted on fish intestinal permeability, active transport or electrophysiology.