Iron is an often-overlooked trace mineral critical for the health and productivity of all animals.
Adequate iron levels are essential for the synthesis of hemoglobin, the component within red blood cells which binds oxygen and carries it to tissues. Hemoglobin also supplies oxygen to support the key metabolic pathways that drive energy production, growth and overall physiological function, including stress response and immune function, producing antibodies critical for combatting infections.
Low hemoglobin may lead to anemia, causing reduced appetite, lower feed efficiency and poor overall performance of the animal. As iron plays an important role in immune response, severe anemia can lead to increased disease incidence, high mortality and costly economic losses.
Iron is also essential for the function of myoglobin, a protein that stores and transports oxygen within muscle tissue. Fish myoglobin plays a crucial role in supporting aerobic metabolism, especially in muscles that require a lot of oxygen, like those in the tail or in active swimming fish.
Anemia in Aquatic Species on the Rise?
Anemia is becoming increasingly common in farmed fish, often resulting from nutritional deficiencies and/or infections and infestations caused by various pathogens. A recent study showed that up to 13% of Scotland’s farmed salmon are facing anemia annually.1 Microcytic anemia in fish is commonly associated with iron deficiency, but it can also result from infections or parasitic infestations that cause blood loss, such as hemorrhage induced by blood-sucking parasites.
Without adequate iron, fish may become more vulnerable to infectious diseases, and the risk of disease outbreaks in aquaculture systems increases significantly.
The trend toward intensive aquaculture—with high stocking densities and confined environments—exacerbates this issue. These stressful conditions can impair nutrient absorption and immune function, making nutritional deficiencies like iron deficiency more impactful.
What Are the Signs of Iron Deficiency in Fish?
The symptoms of iron deficiency and anemia in fish are much the same as they are in other species — low hemoglobin levels and reduced hematocrit (percentage of red blood cells in blood). The results are exhibited by reduced swimming activity, reduced appetite, slow growth, increased susceptibility to infections and disease, as well as pale skin and gills.
In channel catfish studies, a noticeable reduction in appetite, growth and feed efficiency, along with increased mortality has been observed.
Causes of Anemia in Aquatic Species
There are several factors that can lead to anemia in aquatic species:
Dietary Iron Deficiency
- Replacement of fish meal with alternative ingredients lower in iron content and bioavailability requires proper iron balancing to avoid nutritional deficiency and the associated negative impacts on the performance and health of fish.
- Non-nutritional Anemia
- Viral infections: Erythrocytic necrosis viruses alter red blood cell metabolism and increase cell lysis. Infectious salmon anemia virus causes hemorrhages in vital organs.
- Bacterial infections: Bacteria like Aeromonas hydrophila produce hemolysins that damage red blood cells. Various Aeromonas species cause hemorrhagic anemia in multiple fish species.
- External parasites: Parasites such as Argulus sp., Lepeophtherirus salmonis and Sparicotyle chrysophrii can cause blood loss through feeding on the host’s surface skin.
- Internal parasites such as Trypanosoma danilewskyi e Trypanoplasma borreli can digest the host’s hemoglobin.
- Environmental factors such as poor water quality or algae blooms causing blood loss due to the bleeding of the gills. High nitrite levels in water disrupt oxygen transport in the blood.
Iron deficiency in aquatic species is influenced by multiple factors, one of which is the increasing use of plant-based ingredients in aquaculture diets. Since the early 1990s, the industry has shifted toward sustainable and cost-effective alternatives to fish meal, such as soy and other plant proteins. While this transition supports environmental and economic goals, it may also contribute to reduced iron bioavailability. Plant-derived iron is generally less efficiently absorbed than the iron found in animal-based sources. Additionally, compounds like phytates and polyphenols, which are common in plant materials, can further inhibit iron absorption.
Environmental Change Compounds Impact of Anemia
Ocean water temperatures are rising. Since the early 1900s, global sea surface temperatures have risen at an average rate of 0.14°F (-17.7°C) per decade — nearly 1.7°F (-16.83°C) in total, with 2023 being the warmest year ever recorded.2
This spells trouble for the aquaculture industry.
Warmer ocean temperatures affect aquatic populations by increasing their metabolism. A higher metabolism rate means that fish not only need more oxygen for effective metabolic function, but their food requirements are raised in order to sustain energy levels, maintain growth, and ensure reproductive performance. Prolonged exposure to warm temperatures can also trigger a stress response, such as the release of cortisol, which weakens the immune system and leaves fish vulnerable to disease and mortality.
As waters heat up, common aquatic parasites also begin to thrive. Parasite infestations become more intense and longer in duration, creating additional pressure on aquatic species that can lead to greater disease incidence, and poorer overall performance, among other issues.
These challenges make the occurrence of anemia even more detrimental.
Preventing Anemia in Aquaculture
Preventing anemia is important to ensure abundant and efficient production of high-quality protein for consumers around the globe.
Balancing diets of farm-raised fish with adequate levels of readily available iron is necessary to achieve this goal. Studies suggest that iron sources, such as iron chelates, may be more effective in improving hematological health and oxygen transport in fish.
Zinpro iron is more easily absorbed and utilized by animals compared to inorganic iron sources. Zinpro iron also has increased stability and less reactivity in the digestive tract, minimizing interference with other nutrients. Zinpro® Availa® Fe e Zinpro® ProPath® Fe provide the highest biological efficacy of any commercial iron source on the market. This means more Zinpro iron is effectively absorbed and metabolically available in the animal’s organs, tissues and enzyme systems. Effective absorption helps mitigate anemia and plays a role in antioxidant defense both physiological responses that help the animal resist stressful situations that are common in production conditions.
Balancing Diets to Prevent Anemia
For more than 50 years, Zinpro has been supported the nutritional needs of animals. Producers working with all species have successfully optimized production and minimized mortality and associated losses with the help of Zinpro experts.
Knowing why iron is crucial, is the first step in improving the iron nutrition of aquatic animals. The next move is to observe how leading producers are already applying this knowledge. In an upcoming blog, we’ll share firsthand producer experience and the positive production outcomes and economic benefits of formulating diets with advanced iron sources such as Zinpro® Performance Minerals®.
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1Anemia in salmon aquaculture: Scotland as a case study. https://europepmc.org/article/MED/35039692
2Climate Change Indicators: Sea Surface Temperature
https://www.epa.gov/climate-indicators/climate-change-indicators-sea-surface-temperature
