What does heat stress mean for tilapias copper needs?

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No additional copper supplementation is needed for Nile tilapia raised in heat stress conditions, say researchers.

A team of Brazilian researchers sought to understand what amounts of copper were needed to raise healthy Nile tilapia in heat stress conditions.

The group published their results in the journal Aquaculture​.

“The present study was designed to evaluate the effect of graded levels of dietary inorganic copper on the growth, liver histopathology, and hematological and biochemical parameters of Nile tilapia evaluated before and after fish were subjected to heat/dissolved oxygen-induced stress (HDOIS),” ​said the researchers.

The group found that the highest levels of supplemental copper, 1,000 and 1,500mg Cu kg-1 left the largest concentrations in the liver, said the researchers. But fish getting the smallest amounts of the additive had reduced levels of iron in the liver.

“Therefore, considering the fish growth, health, ceruloplasmin activity and liver histopathology, it can be concluded that levels of available copper close to the required amount were sufficient to ensure the growth and health of Nile tilapia, even under the stress condition, whereas an excess of this mineral in the diet impaired the growth and health of this species,” ​they said.

Why copper and heat stress?​

Studies of the effects of dietary supplementation of copper have reported toxic effects or signs of deficiency. However, according to the available literature, there is a lack of information on the effects of dietary copper supplementation on fish subjected to stress, noted the authors.

In intensive system production, fish are constantly exposed to many stressors. Among these stressors, water temperature is considered the most important and can impair fish growth and health, they explained.

When fish are exposed to a stressor, energy normally expended for growth and reproduction will be used to maintain homeostasis, leading to a high susceptibility to disease (Tort, 2011​). "Therefore, it is important to understand how fish fed graded levels of copper respond under stress so that high mortality and economic losses can be avoided,"​ said the team.

Copper is needed for fish production, acting as a cofactor for several enzymes and aiding in iron metabolism through the ceruloplasmin enzyme, according to the Brazilian researchers. In plasma, copper helps deliver iron into circulation, they added

“Moreover, copper, along with iron and selenium, controls the activity of superoxide dismutase, catalase and glutathione peroxidase involved in the defense mechanisms against reactive oxygen species (Valko et al., 2005). ​The antioxidant effects of ceruloplasmin and metallothioneins appear to be the mechanisms by which copper protects cells against oxidative damage," ​reported the scientists.

Because copper plays an important role in fish metabolism, requirements for several species of fish, including salmon, channel catfish, hybrid tilapia and Nile tilapia have been established in previous studies, said the researchers.  

Copper sulfate also has been used as a controlling element to address algae and macrophytes in aquaculture setups, they said.

But excessive levels of copper in an aquaculture system or fish diets may cause damage to cells and result in lesions on the intestines, liver, kidney, gill and brain, they said. It also can reduce immune functioning and growth.   

Trial details​

In the first phase of the trial, 600 fish were give one of six diets for 120 days, said the researchers. Growth performance, feed intake, copper availability, liver histopathology and ceruloplasmin actions were tracked and the nitrogen retention efficiency (NRE) was established.

Diets included a commercially available supplemental copper sulfate at 0 (0Cu), 15.99 (4Cu), 23.99 (6Cu), 31.99 (8Cu), 3,998.4 (1,000Cu) and 5,997.6 (1,500Cu)mg kg-1 of the diet and were mixed to have 26% digestible protein and 12.56MJ digestible energy kg-1, they said.

Livers of sample fish were collected after the first phase of the experiment as was fecal matter, they said. Blood samples were taken after both phase one and phase two of the trial.

In the second phase fish physiological condition, hematological profile and enzyme activity were analyzed before and after a heat stress event, said the researchers. The stress event was created by increasing the water temperature to 32 degrees Celsius, which also reduced oxygen saturation.

Results ​

In phase one, final body weight, weight gain and feed intake were similar for fish getting 0, 4, 6 and 8 Cu, said the researchers. Fish getting 1,000 and 1,500mg Cu had reduced results in those areas.

In phase two, high copper levels altered hematological parameters before HDOIS, they said. “Considering fish growth, health, ceruloplasmin activity and liver histopathology, it can be concluded that levels of available copper near the requirement were sufficient to ensure the growth and health of Nile tilapia, even under a stress condition, whereas the excess of this mineral in the diet impaired the growth and health of this species,” ​they added.

“Fish fed 1,000 and 1,500 mg Cu kg− 1 showed the lowest values for RBC [red blood cell], Htc [hematocrit] and Hb [hemoglobin],”​ they said. “Copper supplementation at 1,500 Cu kg− 1 resulted in the lowest TPP [total plasma protein], and 1,000 and 1,500 mg Cu kg− 1 resulted in the lowest Glob [globulin].”​

“A comparison of the values before and after HDOIS revealed that fish fed a diet with no copper supplementation showed lower values for TPP and Glob after stress,”​ said the researchers. And, fish given the 8mg Cu diet had a larger amount of albumin and albumin:globulin ratio, they added.

Fish getting no supplemental copper had the least iron in their livers, said the researchers. “The copper concentration in the liver was dose dependent, and supplementation with 0–8 mg Cu kg− 1 did not alter the copper concentration in the liver,” ​they added.

Histopathological analysis did demonstrate changes in the liver of some fish, but results did not correspond to diet, they said. Enzyme activity could not be analyzed.