WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2025 71 Beyond Nutrition: How Feed Quality Makes or Breaks Water Quality Feed is the single largest operational cost in aquaculture. In Africa, feeding practices can have additional challenges for fish welfare. Issues of nutritional adequacy and the physical quality of the feed are common. Many small-scale farmers rely on locally available, low-cost ingredients like maize bran or kitchen waste. While affordable, these often lack the essential proteins, amino acids, and micronutrients required for robust health, leading to weakened immunity and poor growth (Tacon and Metian 2015; Munguti et al. 2021). Furthermore, poor storage can lead to mycotoxin contamination or rancidity, introducing toxins that cause physiological stress (Diana et al. 2013). The feed’s physical properties can be as important as its nutritional profile. The pellets’ physical characteristics directly impact the environment and the fish. For instance, durable tilapia pellets with appropriate bulk density (typically 0.3–0.6 g/mL) maintain their integrity, allowing for more efficient consumption. In contrast, poorly formulated pellets produce “fines” that remain uneaten. Feeds with poor water stability disintegrate rapidly, leaching nutrients into the water before fish can consume them. For surface-feeding tilapia, a pellet that sinks too quickly is inaccessible, leading to poor feed intake and heightened competition. Uneaten feed and waste, along with increased faecal matter from poorly digested feed, accumulate at the pond bottom, increasing the biological oxygen demand and degrading the environment for the current and subsequent production cycles (Fudge et al. 2021). This drives up ammonia and phosphate levels, which can fuel algal blooms and lead to dangerous night-time oxygen depletion (Figure 2), creating a chronically stressful hypoxic environment (Shrimp Care 2020; Silva et al. 2019). Improving nutritional welfare, therefore, requires an integrated approach. This involves creating affordable, high-quality feed formulations using local ingredients, training farmers on proper feed storage techniques, establishing consistent feeding schedules, and, importantly, educating farmers on how to choose feeds based on physical qualities such as density, durability, water stability, floatability, and pellet size, rather than solely focusing on price. The Unseen Enemy: Managing Water Quality in a Changing Climate A fish’s environment is its life support system. In African aquaculture, maintaining water quality is a constant battle against invisible threats, exacerbated by resource limitations and a changing climate. The three primary challenges are low oxygen, ammonia toxicity, and extreme temperatures.Dissolved oxygen (DO) is the most critical water quality parameter. Levels below 4 mg/L induce stress, and below 2 mg/L can be lethal, causing fish to gasp at the surface in a desperate struggle for air (Abdel-Tawwab et al. 2019). Hypoxia (low oxygen) forces a metabolic shift to inefficient anaerobic pathways, leading to lactic acid build-up and suppressing critical functions like immunity and growth (Mustapha 2014; Ashley 2007). Maintaining adequate DO is a significant challenge in areas where access to reliable electricity and mechanical aeration is limited, particularly for smallholders. Ammonia, primarily from fish waste and decomposing feed, accumulates rapidly in stagnant, overstocked ponds. The un-ionized form (NH3) is highly toxic, capable of damaging gill tissues, reducing oxygen uptake, and disrupting nervous system function (Randall and Tsui 2002). Its toxicity increases exponentially with rising water temperature and pH, a dangerous combination common in tropical aquaculture systems (Boyd and Tucker 2012). Chronic exposure to sub-lethal levels of ammonia keeps fish in a state of physiological distress and decreases their resilience to diseases (Soler et al. 2021; Xu et al. 2021) (Figure 3). African aquaculture operates in a uniquely challenging thermal environment. Shallow earthen ponds can experience dramatic diurnal temperature swings of 10–15°C, subjecting fish to repeated thermal shock (Mugwanya 2022). Fish cannot regulate their internal temperature, and rapid temperature changes impair enzyme function, weaken immune responses, and stress the cardiovascular system (Wedemeyer 1996). Furthermore, warmer water holds less oxygen while simultaneously increasing the fish’s metabolic demand for it, creating a precarious balance that can easily result in catastrophe (Knight 2022). Climate change, with its associated droughts and unpredictable rainfall, may intensify these fluctuations and increase (CONTINUED ON PAGE 72) FIGURE 2. Uneaten feed and waste drive up ammonia and phosphate levels, which can fuel algal blooms and lead to dangerous night-time oxygen depletion. Photo courtesy SARNISSA. FIGURE 3. Chronic exposure to sub-lethal levels of ammonia keeps fish in a state of physiological distress and decreases their resilience to diseases. Photo courtesy SARNISSA.
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