In this ten-month grow-out culture trial, a mixed diet proved to be better than a single diet, whether using natural or formulated diets. Abalone were significantly larger and heavier when fed mixed diets, as observed from the early months of culture until harvest. This effect, when feeding abalone with mixed diets, had already been reported in other species of Haliotis. Haliotis fulgens, grown under commercial culture conditions for 329 days, had the highest growth rate in terms of length and weight when fed with mixed diets, then with seaweeds, and lastly, formulated diet (Durazo-Beltrán et al. 2003). In a study by Naidoo et al. (2006), Haliotis midae fed with mixed diets also showed the best growth. In a feeding experiment comparing the performance of two natural diets versus a formulated feed on weaning red abalone Haliotis rufescens, the highest growth was obtained with Porphyra columbina (3.3 mm month− 1), followed by the mixed diet of P. columbina and formulated diet, then Macrocystis pyrifera, and lastly, the formulated diet (Hernández et al. 2009). Consistent with the result of the present study, the findings of these different feeding experiments on different species of Haliotis seemed to promote the use of mixed diets but not the sole use of formulated diet. According to Mercer et al. (1993), generally, high balanced levels of protein (> 15%), lipid (3–5%), and carbohydrate (20–30%) are essential for the optimal growth performance of abalone. Feeding abalone with seaweeds or formulated diet only may not be enough to meet their nutritional requirements for optimal growth, but a mix of these diets could, hence better growth may be attained using both diets. And even with the use of all-natural diets, mixing them improved the growth rate of H. asinina (Capinpin et al. 2015), Haliotis diversicolor (Wang et al. 2010), and H. midae (Robertson-Andersson et al. 2011) compared with feeding them monospecific diet.
Survival was consistently highest one month from the start of culture until harvest in abalone fed mixed diet, followed by those fed seaweeds, and lastly, by formulated diet. These results concur with the findings of Durazo-Beltrán et al. (2003), where H. fulgens showed a significantly highest survival of 53.7% after 329 days of culture when fed mixed seaweeds and formulated diets. However, contradictory to the result of the present study, they observed higher survival in abalone fed with formulated diet than those fed seaweeds. In a study on H. rufescens that also lasted for ten months, survival was significantly lowest in abalone fed artificial feed using a feeder (94.2%), almost similar in other treatment diets; highest in those fed with mixed M. pyrifera and artificial feed (99.1%) and mixed Gracilaria chilensis and artificial feed (98.8%) (Venegas et al. 2016). In another feeding trial for H. rufescens that lasted for 162 days, survival was lowest in those fed formulated diet only (78.3%), followed by those fed macroalgae (86.6%), and highest in a combination of these two (90%) (Kemp et al. 2015). The effect of mixed diets on better survival observed in H. asinina in the present study corroborates the results of other studies on other species of Haliotis.
Parallel to the result of the present study, previous studies have shown that abalone fed natural food or seaweeds had higher FCR than those fed formulated diet. In a 90-day feeding trial for H. asinina, those fed seaweeds had an FCR of 6.98, while only 1.5–2.3 for those fed formulated diets (Bautista-Teruel and Millamena 1999). In a ten-month feeding trial in tanks on H. rufescens, FCR was highest with the mixed natural food of M. pyrifera (16.0) and G. chilensis (12.7) diet, whereas lowest using artificial feed with (1.4) and without a feeder (1.6) (Venegas et al. 2016). Moreover, extended culture periods also increase FCR, especially when using natural food as feed. Capinpin et al. (1999), when using Gracilariopsis sp., reported an FCR of 12.58–15.54 in H. asinina cultured for 150 days, while 19.63–22.57 at 180 days. In a 90–180 day culture of the same species, using the same feed, FCR ranged 7.56–22.57, with higher FCR during longer culture duration (Encena et al. 2013). Surprisingly, however, in just 90 days of feeding H. asinina with seaweeds, Fermin and Buen (2000) obtained a very high FCR of 31–40. FCR could greatly vary, especially when natural food is used. However, the supplementation of natural foods can help improve the FCR. A 62-day feeding experiment on H. asinina revealed that Gracilaria sp. supplemented with probiotics (Bacillus amyloliquefaciens MA228, Enterobacter ludwigii MA208, and Pediococcus acidilactici MA160) resulted in significantly lower FCR compared with the unsupplemented seaweed feed (Amin et al. 2020). Abalone, as natural grazers, feed on a mixed species of algae and other plants growing in their natural environment. Given a monospecific diet, their feed intake may vary depending on the culture conditions or the sufficiency of nutrients in the diet needed to promote growth or support reproduction.
Abalone fed formulated diet developed a distinct turquoise shell color, while those fed seaweeds and mixed diets retained their natural color. The light blue-green shell color in H. asinina fed formulated diet had been reported earlier by Capinpin and Corre (1996) and Bautista-Teruel and Millamena (1999). Feeding abalone with formulated diet to change the shell color was eventually adopted as a tagging mechanism for sea ranching and stock enhancement (Gallardo et al. 2003; Okuzawa et al. 2008; Lebata-Ramos et al. 2013; Salayo et al. 2020). However, not all formulated diets will change the color of the abalone shell. In Lebata-Ramos et al. (2021), only the unrefined diet formulation of Bautista-Teruel et al. (2016) resulted in a turquoise abalone shell, while the refined formulation, which has Spirulina spp. and sodium alginate, did not cause any distinct changes on the natural shell color of the abalone. Aside from the presence of some feed ingredients, a certain amount or a daily ration of such may be required for the shell color to change. In the present experiment, using mixed diets of formulated feed and natural food, given at alternating rations every other day, maintained the natural shell color of H. asinina.
Temperature is among the most potent abiotic factors influencing marine mollusks (Kong et al. 2017). In the present study, survival of abalone, regardless of treatment diets, showed a significantly-moderate positive correlation with temperature. Although an increasing temperature can be detrimental to any organism, the water where H. asinina were cultured and showed a positive correlation with their survival had a narrow temperature range of 26.00–33.00°C. Even temperate species, like the Haliotis diversicolor supertexta, have the ability to tolerate temperatures as high as 36.8°C when maintained at 30°C or as low as 3.5°C when maintained at 20°C (Chen and Chen 1999). Hence, for the tropical species H. asinina, temperatures at low to mid 30s°C may be optimum for survival. Also, the lower temperatures during the culture period were recorded during days of heavy rains, which had caused the water to become silted and resulted in more abalone deaths.