Pathogenesis of acute ulceration response (AUR) in hybrid striped bass

Explorative study on scale cortisol accumulation in wild caught common dab (Limanda limanda)

Background

Flatfish live in a diverse marine ecosystem that is changing due to natural variations and anthropogenic influences. These changes can evoke a stress response mainly resulting in production of the glucocorticoid cortisol, which mediates effects on various levels of biological organization. The finding that cortisol accumulates in fish scales, offering a retrospective view on cortisol production, provides opportunities to use this matrix for chronic stress assessment. The present study is the first to gather information on scale cortisol concentration in wild-caught common dab (Limanda limanda), based on a two-pronged approach using (1) field measurements and (2) a laboratory in vivo-study where wild-caught dab were fed by cortisol-spiked feed during 30 or 90 days to demonstrate the possible accumulation of cortisol in the scales and to evaluate its impact on fish health.

Results

Based on the field measurements, the average scale cortisol concentration in wild-caught fish was 0.0034 ± 0.0046 µg kg⁻¹ scale (n = 67). This indicates that wild common dab is indeed able to incorporate cortisol in the scales.

Based on the experimental data, the cortisol-fed fish showed an increased plasma cortisol concentration (80.16 ± 82.58 µg L⁻¹) compared to the control group (4.54 ± 9.57 µg L⁻¹) after 30 days of cortisol feeding. The increase in plasma cortisol concentration was positively correlated with an increased cortisol concentration in the scale after 30 days of cortisol-spiked feeding. This correlation was, however, no longer observed after 90 days of cortisol-spiked feeding. Interestingly, cortisol concentration of the scales on the pigmented side was significantly higher compared to the non-pigmented side. Some health parameters such as epidermal thickness, body condition and Ichthyobodo sp. parasitic infection showed a correlation with scale cortisol concentration after 30 days.

Conclusions

We have demonstrated that common dab is able to accumulate cortisol in its scales. This seems to occur proportionally to circulating concentrations of plasma cortisol in fish fed with cortisol supplemented feed after 30 days.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-022-03385-3.

Fin Erosion of Salmo salar (Linnaeus 1758) Infested with the Parasite Caligus rogercresseyi (Boxshall & Bravo 2000)

Simple Summary

Sea lice can generate wounds on fish and they are associated with the consumption of mucus, skin, and blood, inducing extensive epithelial erosion and inflammation. Knowledge related to the fin condition of fish with sea lice infections is scarce, limiting the range of parameters regarding their welfare status of fish. In the present study, the fin condition and two stress parameters were assessed in Caligus rogercresseyi infected fish. An association between fin erosion, increasing fish stress and parasite abundances was observed, suggesting that C. rogercresseyi infection is a possible cause of fin degeneration in Atlantic salmon.

Abstract

Fin condition is a simple indicator of fish welfare, which anticipates detrimental effects on fish in aquaculture systems. This study evaluated the fin condition of Salmo salar at different abundances of the parasite Caligus rogercresseyi. Fish were exposed to infestation with copepodids and the cohort was allowed to develop to the adult stage. The relative fin index was measured. Significant differences between infested and control fish for both pectoral and anal fins were observed. Moreover, there were significant negative relationships between fin condition and parasite abundances for pectoral, anal, and pelvic fins, suggesting that infestations with C. rogercresseyi could be a possible cause for fin damage in Atlantic salmon. Moreover, this damage was associated with increased stress levels, suggesting that damage can be related to physiological changes on infested fish. According to these results, pectoral fin assessments have the potential to provide information on the welfare of fish with C. rogercresseyi infestation. Determining the causes of poor fin development may improve fish welfare, even when infested by parasites.

High fish density delays wound healing in Atlantic salmon (Salmo salar)

In this study, we look closer at how high fish densities influence wound repair mechanisms in post-smolt Atlantic salmon. The fish were wounded with a 5 mm skin punch biopsy needle and stocked at two different densities, a high fish density (100 kg/m³) treatment and a low fish density treatment (20 kg/m³) serving as the control. The healing wounds were followed for 57 days with samples taken 1, 3, 7, 14, 36, 43 and 57 days post wounding. The transcriptomic analysis suggests that high fish density enhance inflammation and represses cell proliferation, tissue secretion and collagen synthesis in the healing wounds. The histological analysis further showed delayed epidermal and dermal repair in the high fish density treatment compared to control. The overall wound contraction was also altered by the treatment. In conclusion, high fish density enhances immune responses and delay tissue repair, which ultimately results in delayed wound healing.

Histopathology of culture-associated skin erosions and papillary hyperplasia of Atlantic halibut, Hippoglossus hippoglossus (L.)

Atlantic halibut, Hippoglossus hippoglossus (L.), that were cultured in tanks with a smooth bottom (gel-coated fibreglass) substrate developed papillary hyperplasia and skin erosions on the blind (ventral) side. No viruses, bacteria or parasites were observed in any sections of affected skin. Comparison of microscopic pathology with that of skin from normal, wild-caught halibut showed severe epidermal proliferation with foci of severe mucous cell hyperplasia. Both epidermal thickness and mucous cell density were significantly greater in fish held on a smooth substrate compared to skin of healthy fish. Spongiosis was present, especially at the base of the papillary, hyperplastic epidermis, and there was a chronic inflammatory infiltrate in the scale pockets composed of lymphocytes, histiocytes and erythrocytes. Skin erosions had various degrees of epidermal loss, in some cases to the basement membrane. A predisposing factor for the epidermal sloughing may have been related to the spongiosis, and the vacuolated, degenerated basal cells. When affected halibut were cultured for an additional 28, 62 or 97 days on a sand substrate, which is conducive to skin lesion healing, there was no apparent change in epidermal thickness over time. However, mucous cell density significantly increased from day 0 to 97 during the healing process.

The effect of tricaine on use of the fluorescein test for detecting skin and corneal ulcers in fish

Fluorescein has been used for rapid and sensitive detection of fish skin and corneal ulceration. Effective use of the fluorescein test requires knowledge of conditions that might cause misleading interpretations or otherwise interfere with test reliability. Examination of fish health and the clinical workup often require tricaine as one of the most commonly used anesthetics. However, tricaine may interfere with correct interpretation of the fluorescein test and might also cause significant fish injury. The effects of tricaine exposure sequence on the fidelity of the fluorescein test was studied in Pacific halibut Hippoglossus stenolepis, walleye pollock Theragra chalcogramma, and northern rock soles Lepidopsetta polyxystra by examining the fluorescence of experimentally induced epidermal wounding. Tricaine can quench fluorescence that is emitted by fluorescein retained in skin ulcers, causing a false-negative reaction. Thus, for the fluorescein test to work properly, it is important to avoid the exposure of fluorescein-treated and rinsed ulcers to tricaine. The effects of exposure to buffered versus unbuffered tricaine on epidermal and corneal integrity were studied in Nile tilapia Oreochromis niloticus and channel catfish Ictalurus punctatus subjected to the fluorescein test and histological examination. Fluorescein could detect not only ulcers but also areas with only a partial loss of epithelium (i.e., erosion). The use of unbuffered tricaine to anesthetize these fish caused serious epidermal and corneal damage. If fish are euthanized with unbuffered tricaine for clinical workup, this severe epidermal or corneal damage could be misinterpreted as an antemortem lesion, leading to misdiagnosis. Even in water with alkalinity exceeding 50 mg/L as CaCO3, it would seem prudent to always buffer tricaine with sodium bicarbonate to prevent a pH change that might lead to iatrogenic effects from unbuffered tricaine. Thus, current general recommendations suggesting that tricaine does not need to be buffered in waters with alkalinity greater than 50 mg/L might need to be modified.

Effect of substrate on progression and healing of skin erosions and epidermal papillomas of Atlantic halibut, Hippoglossus hippoglossus (L.)

Juvenile Atlantic halibut cultured on a smooth substrate often develop skin lesions on the blind (abocular) side, significantly reducing carcass value and causing economic losses. We discovered that fish not only developed skin erosions, but also extensive epidermal papillomas when held on a smooth substrate for 39 months. This is the first time that epidermal papillomas have been reported in Atlantic halibut. To determine whether substrate type affected the progression or healing of these lesions, fish with papillomas and skin erosions were moved to identical rearing units having either an irregular (sand, gravel or Netlon) substrate, or a smooth (gel-coated fibreglass) substrate. After 62 days, 42-50% of fish held on any of the three irregular substrates had a skin erosion area (SEA) that had either diminished or appeared to be completely healed via gross examination. In contrast, none of the fish held on the smooth substrate showed improvement and 75-100% had a more severe SEA. Changes in the papilloma area (PA) were less dramatic: none of the fish held on an irregular substrate had any clear change in PA, although there was a suggestion that the PA in some fish might becoming less severe. Similarly, none of the fish held on the smooth substrate had a change in PA, although there was a suggestion that the PA in some fish was becoming more severe. After 97 days, there was also a suggestion that specific growth rates were higher in fish held on an irregular substrate when compared with those held on the smooth substrate. Overall, these data further substantiate previous studies suggesting that an irregular substrate, including the commercially used Netlon, is best for rearing Atlantic halibut. While there was a clear relationship between substrate type and whether skin erosions or papillomas progressed or healed, the exact cause(s) of these two responses is uncertain. Future studies should focus on risk factors such as stress, burrowing behaviour and the microenvironment of the substrate.