Living in a hypoxic world: A review of the impacts of hypoxia on aquaculture

Effects of hypoxia on the reproductive endocrine axis of the pejerrey (Odontesthes bonariensis)

Recently, hypoxic areas have been identified in water bodies of the Pampas region due to human activity. The objective of this work was to study the effect of low concentrations of dissolved oxygen (hypoxia) on the reproductive endocrine axis of a pampas fish (Odontesthes bonariensis). Groups of 8 males and 8 females were subjected to severe hypoxia (2-3 mg l-1) and normoxia (7-9 mg l-1) in 3000 l tanks by duplicate during the reproductive season (spring). After 21 days, 4 males and 4 females from each tank were sacrificed, and blood was drawn to measure estradiol (E2) and testosterone (T). The brain, pituitary gland and a portion of the gonads were extracted and processed to measure the expression of: gnrh1, cyp19a1b, fshβ, lhβ, fshr, lhcgr and cyp19a1a. From the second experimental week, no spawning was found in the hypoxic females, while at the end of the treatment period no male released sperm. Fish under hypoxic conditions showed signs of gonadal regression, reduction of GSI and plasma levels of sex steroids. Furthermore, the expression of gnrh1 in both sexes, cyp19a1b and fshr in males and only fshβ and cyp19a1a in females decreased in comparison with normoxic fish. After 40 days under normal conditions, signs of reproductive recovery were observed in the treated fish. The results obtained demonstrated that hypoxia generated an inhibition of some components of the pejerrey's reproductive endocrine axis, but the effect was reversible.

Enhancing water quality monitoring through the integration of deep learning neural networks and fuzzy method

The escalating growth of the global population has led to degraded water quality, particularly in seawater environments. Water quality monitoring is crucial to understanding the dynamic changes and implementing effective management strategies. In this study, water samples from the southwestern regions of Iran were spatially analyzed in a GIS environment using geostatistical methods. Subsequently, a water quality map was generated employing large and small fuzzy membership functions. Additionally, advanced prediction models using neural networks were employed to forecast future water pollution trends. Fuzzy method results indicated higher pollution levels in the northern regions of the study area compared to the southern parts. Furthermore, the water quality prediction models demonstrated that the LSTM model exhibited superior predictive performance (R2 = 0.93, RMSE = 0.007). The findings also underscore the impact of urbanization, power plant construction (2010 to 2020), and inadequate urban wastewater management on water pollution in the studied region.

Long-term hypoxia-induced physiological response in turbot Scophthalmus maximus L

Hypoxia affects fish's survival, growth, and physiological metabolism processes. In this study, turbot plasma glucose and cortisol contents, hepatic glycolysis (hexokinase [HK], phosphofructokinase [PFK], pyruvate kinase [PK]) and lipolysis (fatty acid synthetase [FAS], lipoprotein lipase [LPL]) enzyme activities, anti-oxidant enzyme (superoxide dismutase [SOD], catalase [CAT], glutathione peroxidase [GSH-Px]) activities, malondialdehyde (MDA), lactate and glycogen contents, gill histological parameters (lamellar length [SLL], width [SLW], interlamellar distance [ID]), respiratory frequency (RF), the proportion of the secondary lamellae available for gas exchange (PAGE), and hifs (hif-1α, hif-2α, hif-3α) expression were determined during long-term hypoxia and reoxygenation. Results showed that long-term hypoxia (3.34 ± 0.17 mg L-1) significantly elevated plasma cortisol and glucose contents; increased hepatic HK, PK, PFK, FAS, and LPL activity; decreased hepatic glycogen, lactate contents, and lipid drop numbers; and caused changes of hepatocyte (vacuolation, pyknotic, and lytic nucleus) after treatment for 4 weeks. Hepatic SOD, CAT, GSH-Px activity, and MDA contents; lamellar perimeter, SLL, ID, RF, and PAGE; and hepatic hif-1α, hif-2α, and hif-3α manifested similar results. Meanwhile, hif-1α is significantly higher than hif-2α, and hif-3α. Interestingly, females and males demonstrated no sex dimorphism significantly different from the above parameters (except hepatic FAS, LPL activity, and lipid drop number) under hypoxia. The above parameters recovered to normal levels after reoxygenation treatment for 4 weeks. Thus, long-term hypoxia promotes turbot hepatic glycogenolysis and lipolysis, induces oxidative damage and stimulates hepatic antioxidant capacity, and alters gill morphology to satisfy insufficient energy demand and alleviate potential damage, while hif-1α plays critical roles in the above physiological process.

Impact of anthropogenic global hypoxia on the physiological response of bivalves

Dissolved oxygen (DO) is an important parameter that affects the biology, physiology, and immunology of aquatic animals. In recent decades, DO levels in the global oceans have sharply decreased, partly due to an increase in atmospheric carbon dioxide, temperature, and anthropogenic nutrient loads. Although there have been many reports on the effects of hypoxia on the survival, growth, behavior, and immunity of bivalves, this information has not been well organized. Therefore, this article provides a comprehensive review of the effects of hypoxia on bivalves. In general, hypoxia negatively impacts the food consumption rate and assimilation efficiency, as well as increasing respiration rates in many bivalves. As a result, it reduces the energy allocation for bivalve growth, shell formation, and reproduction. In severe cases, prolonged exposure to hypoxia can result in mass mortality in bivalves. Moreover, hypoxia also has adverse effects on the immunity and response of bivalves to predators, including decreased burial depths, sensitivity to predators, impairment of byssus production, and negatively impacts on the integrity, strength, and composition of bivalve shells. The tolerance of bivalves to hypoxia largely depends on size and species, with larger bivalves being more susceptible to hypoxia and intertidal species being relatively more tolerant to hypoxia. The information in this article is very useful for elucidating the current research status of hypoxia on bivalves and determining future research directions.

Wetland Destruction in a Headwater River Leads to Disturbing Decline of In-stream Nitrogen Removal

Wetlands have long been recognized as efficient nitrogen (N) processing systems. While widespread interest is in constructing wetlands to mitigate N pollution, there is a dearth of information about the environmental consequences following wetland dismantlement. This study elucidated the changing trajectories of water quality and N removal capacity in a headwater river that initially contained a series of constructed wetlands but later underwent wetland destruction. An estimated 17% surge in total N concentration has been reported since the wetlands' destruction. This adverse trend is primarily attributed to a weakened in-stream N removal capacity, which was reduced to a mere 25% of the levels observed when the wetlands were operational. Further analysis confirms that the presence of wetlands actively shapes desirable environmental settings for N processing. In stark contrast, wetland destruction leads to unfavorable environmental conditions, which not only restrain in-stream anaerobic metabolisms but also trigger algal proliferation and biological N fixation. Collectively, this research provides compelling evidence of the detrimental consequences associated with wetland destruction, emphasizing the need for remedial strategies to mitigate these negative effects.