According to the United Nation’s Food and Agriculture Organization, South Africa is since 2004 the largest abalone producer outside Asia. Concurrently, the economic value of abalone mariculture for coastal communities has increased [1–3]. Abalone farming requires a high capital investment and relies on formulated diets to improve productivity by increasing abalone growth [1, 2, 4, 5] and food conversion ratio [2, 3, 6]. These diets are good alternatives to the natural diet of abalone, i.e., kelp and seaweeds, and they have been used in combination with macroalgal diets . Among the advantages of these pelletized diets is the fact that they can be modified to match the life stages and dietary needs of the farmed abalone  as well as the fact that they are more practical than a natural kelp or seaweed diet as they are available throughout the year [2, 8]. Some farms combine artificial diets and kelp in the production of the South African abalone, Haliotis midae , and other haliotids , but artificial diets are mostly used in the grow-out period of H. midae . Typical main ingredients in artificial diets are fish meal, defatted soybean meal and casein as protein sources [8, 11, 12]. A combination of fish meal and soybean meal can result in faster abalone growth than diets with fishmeal as the only main protein source [10, 13]. For example, Abfeed™ S34® (Marifeed Pty Ltd), the diet used in the present study, is a commercial feed for post-weaning of abalone containing fishmeal and soybean meal as the main sources of protein.
Animals fed Abfeed™ grew faster than animals fed kelp up to an age of two and a half years, after which growth was reduced . The onset of growth reduction corresponded to the age at which H. midae can reach sexual maturity under farm conditions , which may cause the channelling of metabolic energy into gonad development . This hypothesis is supported by differences in sexual maturation between wild and farmed H. midae [14, 16–18]. In captivity, 50% of male and female abalone reached sexual maturity at 18 months and two years, respectively . Wild abalone, however, attain sexual maturity at an age of approximately 7 years . Early sexual maturity has an impact on the farming of H. midae, as this species takes approximately four years from hatching to a marketable weight of 80 g.abalone-1 . Abalone are broadcast spawners with a high gonadal mass index in both males and females of up to 75 mm3.g-1 soft tissue . Therefore, uncontrolled sexual development and spawning may complicate growth forecasting, which is required for successful farm management, product quality and marketing of the animals.
The ingredients in artificial diets may play an additional role. H. midae raised on formulated feeds with fishmeal as the only protein source devoted more energy to gonad development than abalone fed diets that included both fishmeal and soybean meal . Soybean meal is rich in phytoestrogens, which are suspected to act as endocrine disruptors in H. midae  and other animal species [19–21]. However, Wu et al. (2019)  tested soybean meal in H. midae diets and could not confirm that the flavonoids in soybean meal are responsible for the hypothesized effect on gonad development. In other species, phytoestrogens have been linked to precocious genital development, decreased testosterone levels, altered activity levels, decreased prostate weight, decreased sperm quality, temporary or permanent infertility, abnormal lactation in mammals, an increased incidence of dystocia and feminization [20, 22–24]. In fish, soybean-based diets caused shifting of the sex-ratio towards females, decreased testosterone levels, reduced fertility through an effect on the number of eggs spawned and sperm quality, inadequate gametogenesis and a reduction of fertilization and hatch rate [25–28].
There is a paucity of published information on the effect of soybean meal on abalone gonad development in both males and females when compared to natural diets. For example, in feeding studies that were designed to analyse gonad development of H. midae, kelp was not included as a test diet [10, 13, 29]. Capinpin & Corre (1996)  suggested that growth was fastest in H. asinina fed artificial diets as compared to red algae, Gracilariopsis heteroclada, until the abalone reached a size at which energy was channelled into gonad development. The authors hypothesized that this was due to the relatively high protein and energy content of these diets, but gonad mass index (GBI) was not determined and no histological evidence was provided to support this hypothesis.
An additional factor that may influence gonad development in H. midae is stocking density and water flow rate. Abalone are broadcast spawners and can be triggered to release eggs and sperm by the presence of gametes in the water column. Spontaneous spawning events have been recorded in H. asinina . Counihan et al.  have suggested that high stocking densities in tanks may influence the frequency of spawning events in H. asinina due to the fact that abalone are closer together, which may result in males releasing gametes and associated sex pheromones. Thus, the number of abalone in a tank may influence gonad development or the frequency of spawning events. Furthermore, during spawning season and under favourable environmental farm conditions H. midae broodstock can reproduce approximately every six weeks , which suggests a fast recruitment of oocytes. Broodstock abalone are kept in a controlled environment at low population densities, which is different from the conditions in the grow-out tanks, with variable and generally high numbers of abalone and consequently fast water flow rates. The effect of stocking density on abalone reproductive development in a grow-out system remains to be tested.
The aim of this study was to use histological examination to compare the effect of a partial substitution of the fishmeal in the artificial diet with soybean meal on gonad development of both male and female H. midae. In addition, a treatment, in which abalone were fed kelp, the natural diet, was included. Abalone stocking intensity and water flow rate to the culture tanks were used as additional independent variables.