WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2025 23 results were observed when replacing menhaden fishmeal with PAPs in fingerling diets. Some of the studies presented no differences in production performance, and others showed a suppressed growth for catfish offered feeds formulated with animal-derived ingredients (e.g., meat and bone meal). The processing conditions, storage, and nutrient profile of PAPs can largely affect the production performance of any fish species when replacing high-quality fishmeal. The manufacturing processes of these ingredients have greatly advanced since most of these studies were conducted, ensuring more consistent and better-preserved PAP ingredients. And, it is interesting to revisit the idea of completely eliminating the forage fish-derived fishmeal from catfish feeds. Iron Supplementation and Catfish Anemia Idiopathic catfish anemia has been an ongoing issue affecting U.S. catfish farms for decades. It was first documented by Dr. Tom Lovell in 1983 as “no blood” disease, and it typically manifests in early spring or late fall. Normal packed cell volumes (PCVs) in commercially raised catfish typically range from 20–40%, with higher values observed in summer. However, anemic fish often exhibit critically low PCVs of 10% or less, lethargy, and respiratory distress under otherwise adequate dissolved oxygen concentrations. Larger, food-sized fish are most affected by this disease, which is also characterized by low circulating iron, decreased iron levels in the liver, prolonged clotting time, pathological accumulation of fat in the liver resulting in a pale liver that often floats (fatty liver) and lightcolored skin, gills and viscera, leading to the industry-coined term ‘white lip’ (Figure 4). Early evidence suggested that anemia was related to feed, although feed analyses indicated diets were nutritionally complete. Feed-related anemia has been documented in channel catfish in Alabama and Georgia and possibly in Louisiana. For example, in 1983, feed-related anemia was reported on 39 of 166 catfish farms in west-central Alabama, with weekly mortality rates reaching 5%. Losses ceased when fish were temporarily offered feed from a different manufacturer. Hepatic lipidosis (fatty liver) is the most prominent characteristic of fish suffering from feed-related anemia (Dr. Lester Khoo, personal communication). Possible causes of this form of anemia include feed contaminants from ingredients or possibly microbial degradation of nutrients, which can interfere with red blood cell production. Differently from feed-related anemia, iron-responsive anemia is typically observed from farms in the delta region of Mississippi and Arkansas. While both forms of anemia appear to be associated with the lack of red blood cell production, their etiologies are unknown. The pathology of fish suffering from iron-responsive anemia is similar to feed-related anemia but lacks a fatty liver, suggesting different causes for this disease. Following iron supplementation, research has demonstrated that intraperitoneal and dietary iron supplements promote normal production of red blood cells (Camus et al., 2014). Historically, iron sulphate supplementation in catfish feeds has been a standard practice when this element can mitigate gossypol toxicity derived from cottonseed meal. With the anemia issues, it has been normal to supplement a bit more of this inorganic source of iron as a prophylactic measure to improve red blood cell formation and prevent anemia. When anemia cases are not feed-related, the supplementation of iron has been shown to improve the production of erythrocytes in diseased fish, relabeling the disease in these conditions as “iron-responsive anemia”. Several investigations evaluating different iron concentrations and sources have been conducted, especially in the 1990’s and early 2000’s, and no adverse effects were observed for channel catfish. Experiments in our laboratory, however, showed that elevated concentrations of dietary iron sulfate can be a double-edged sword; whereas higher hematocrits could be observed even in healthy catfish, an increase in disease susceptibility was observed when fish were challenged with the bacteria Edwardsiella ictaluri, the causative agent of enteric septicemia of catfish (Yamamoto et al., 2024a; Yamamoto et al., 2024c). During an infection, fish have physiological mechanisms to restrict the circulation of iron and deprive the pathogen of this nutrient. Our findings hypothesized that elevated doses of iron sulfate in the diet may bypass these limiting mechanisms and allow the bacteria to thrive during an infection. Until more research is developed on this front, and we better understand what is causing this mysterious disease, it is extremely important to use iron-fortified feeds as a remediate approach for catfish anemia instead as a prophylactic measure. Especially when bacterial diseases, which are more frequent in the warmer months, may be able to take advantage of these supplemental doses and possibly aggravate losses during an outbreak. Intestinal Microbiota and Feed Additives With the continued development of high-throughput sequencing technologies, it has become more affordable to survey the intestinal bacterial community in fish nutrition studies. This has been a useful tool to understand possible shifts in the gut microbiota. For example, when feed supplements included in the feeds might promote the proliferation of beneficial bacteria, or the other way around, if alternative ingredients can disrupt gut bacterial communities. The intestinal microbiota can symbiotically interact with their host by aiding the digestive function and gastric development, maintaining intestinal mucosal barriers, teaching the immune system during the developmental stages, and conferring increased disease resistance. Bacterial diseases comprise most losses for catfish farmers, and these pathogens use a fecal/oral route of infection, with a predilection for intestinal tissue. In pond culture, the gastrointestinal tract of farmed catfish is continuously exposed to a variety of opportunistic microorganisms and toxins that can naturally occur in the nutrient-rich pond water, increasing their susceptibility to intestinal diseases. In this sense, dietary probiotics can be a useful prophylactic measure to FIGURE 4. (A) Channel catfish fingerling exhibiting feed-related anemia. (B) Hybrid catfish diagnosed with idiopathic catfish anemia. Both cases (A and B) were animals from commercial facilities submitted to the MSU CVM Aquatic Research and Diagnostic Laboratory (photos courtesy of Dr. Lester Khoo). (C) Blood samples from an anemia outbreak at the Thad Cochran National Warmwater Aquaculture Center: on the left, anemic hybrid catfish; on the right, a healthy hybrid catfish (personal repository). (CONTINUED ON PAGE 24)
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