**6. Feeding algae to fish**

The utilization of *Spirulina* (*Arthrospir platensis*) as a growth and immunity enhancer for Nile tilapia, *Oreochromis niloticus*, was investigated (L.). Fish have been shown to benefit from *Spirulina*'s growth-promoting properties. Although *Spirulina* supplementation boosted protein deposition in the fish body, especially when fed a 1.25–5.0 g/kg diet, there were no significant differences in fish survival among the three treatments. When fish were provided a *Spirulina* supplement, their physiological indicators improved. With increased *Spirulina* levels in fish diets, total fish mortality 10 days after IP injection with *A. hydrophila* and its count following incubation with fish serum decreased. When fish were given 5.0–10.0 g *Spirulina*/kg, the lowest mortality and bacterial levels were observed. These findings suggest that *Spirulina* supplementation is effective in preventing disease in tilapia culture, with an optimum dose of *Spirulina* in the diet of 5.0 to 10.0 g per kg of food [68].

In the study of Al-Koye [69], replacing fishmeal with 10% *Spirulina* had a positive impact on all growth metrics, including weight gain, daily growth rate, specific growth rate, relative growth rate, and productivity, particularly food efficiency ratio and survival. The protein content of the fish carcass was also affected, as was the lipid content of the fish diet, and had a significant impact on blood parameters. Different *Ulva* levels in the diet of [70] were utilized, and the fish maintained at 10, 15, and 20% nutritional *Ulva* had the most significant (P 0.05) values of protein efficiency ratio (PER), protein production value (PPV percent), and energy retention (ER percent). Green seaweeds (*Ulva* sp.) could thus be added to the diet of red tilapia

(*Oreochromis* sp.) at a rate of 15% to boost growth performance without affecting feed efficiency or survival rate.

The goal of [71]'s study was to see how a diet including *Ulva lactuca*, green macroalgae, affected the growth, feed consumption, and body composition of African catfish *Clarias gariepinus*. Weight gain, specific growth rate, and feed consumption all showed significant differences. Overall, the experiment revealed that African catfishfed diets containing 20% and 30% *U. lactuca* had poorer growth and feed utilization than the control group and fish-fed diets containing 10% *U. lactuca*. Güroy et al. [72] demonstrated that adding dietary low-level *Ulva* meal to numerous fish species, including rainbow trout *Oncorhynchus mykiss* and tilapia *O. niloticus* [73], improved growth performance and lipid deposition.

The effects of two algal meals (*Cystoseira barbata* or *Ulva rigida*) on feed consumption, development, and nutrient usage in young Nile tilapia, *O. niloticus*, were examined in a 12-week feeding experiment. The fish fed the 5% Cystoseira diet, control diet, and 5% *Ulva* diet gained the most weight (156%, 151%, and 150%, respectively), but the values were not significantly different (P > 0.05) from the other treatments, with the exception of the fish fed the 15% *Ulva* diet (P 0.05), which gained the least weight. 15% of the diet is made up of fish. The feed change ratio of *Ulva* meal was poor (FCR). At the maximal supplementation level of 15%, protein and energy utilization contribute to a decline in the groups fed algal meals. Carcass lipid levels fell as *Ulva* meal concentrations increased, but carcass lipid levels increased as Cystoseira meal concentrations increased (P 0.05). According to the findings, *C. barbata* or *U. rigida* meals could be utilized in tilapia diets in small amounts [74].

Rybak et al. [75] investigated the ability of freshwater *Ulva* species (*Ulva*ceae, Chlorophyta) to serve as metal bioindicators in rivers and lakes. From June to August 2010, researchers looked at changes in heavy metal (Ni, Cd, and Pb) and alkaline earth metal (Ca and Mg) concentrations in freshwater *Ulva* thalli. Ni was detected in the highest concentration among the heavy metals studied in thalli, whereas Cd was found in the lowest concentration. Metal concentrations in macroalgae, water, and sediment had statistically significant connections. *Ulva* populations from freshwater habitats were more efficient in bioaccumulating nickel than those from marine ecosystems.
