Introduction: The sustainable expansion of coastal aquaculture faces significant challenges due to the critical depletion of freshwater resources and rising environmental constraints. In northern regions and coastal ecosystems, utilizing alternative water bodies such as brackish water has emerged as an eco-friendly alternative for high-value finfish production. Species like Caspian kutum (Rutilus frisii) hold immense economic value but require optimized, bio-secure, and intensive husbandry protocols to thrive in variable salinities. Traditionally, stocking density has been a major operational bottleneck affecting physiological stress, feed conversion, and overall survival. Recent paradigms emphasize that incorporating functional feed additives—such as immune stimulants, probiotics, and prebiotics—can substantially mitigate density-dependent stressors, enhance mucosal immunity, and optimize metabolic pathways. This study evaluates the multi-phasic growth dynamics, feed utilization efficiency, and survivability of finfish under semi-intensive concrete tank systems, providing a scalable model for brackish water aquaculture development.

Material and Methods: A rigorous field-based, semi-commercial trial was executed over an 18-month timeline (from September 2023 to December 2024) at a specialized concrete tank facility located in Choobar, Astara County, Guilan Province, Iran, utilizing brackish water directly drawn from the Caspian Sea basin with a stable salinity range of 10–12 ppt. The experimental architecture followed a Completely Randomized Design (CRD) with three replicates per treatment, split across two distinct, consecutive growth phases: Phase I evaluated initial fingerling rearing over a 3-month duration under high-density constraints of 300 and 450 fish per concrete tank, while Phase II spanned a 12-month pre-grow-out period to evaluate extended production metrics at lower densities of 150 and 225 fish per tank. To counteract potential crowding stress, diets were supplemented with standard baselines of functional prebiotics and encapsulated multi-strain probiotics acting as potent immune stimulants, while critical water quality parameters were meticulously kept within strict physiological ranges (temperature: 16–24 °C, dissolved oxygen > 8.5 mg/L, and pH: 7.9–8.3) to periodically compute the Specific Growth Rate (SGR), Weight Gain Percentage (WG%), Condition Factor (CF), and Feed Conversion Ratio (FCR).

Results and Discussion: Statistical analyses revealed that stocking density exerts an incredibly profound influence (P < 0.05) on growth dynamics and nutritional efficiency, showing that fingerlings in Phase I registered weight gain percentages of 250% and 226% for the 300 and 450 fish/tank cohorts, respectively, while the benefits of lower stocking configurations became highly pronounced during the prolonged 12-month pre-grow-out phase. Specifically, fish reared at the lower density of 150 individuals per tank achieved an exceptional weight gain of 1370% paired with a remarkably efficient Feed Conversion Ratio (FCR) of 1.3, whereas those subjected to the higher density of 225 individuals exhibited a significantly lower weight gain of 1280% alongside an elevated FCR of approximately 1.5. Furthermore, SGR and CF values were consistently superior in the lower-density cohorts (P < 0.05), and the strategic inclusion of dietary prebiotics and probiotics successfully bolstered the specimens' physiological defenses, as evidenced by a remarkably high survival rate ranging between 94% and 97% across all groups, proving that functional immune stimulants can successfully alleviate overcrowding anomalies as the average final weight comfortably reached target metrics of 140–150 grams.

Conclusion: This study demonstrates that optimizing stocking densities alongside proactive water quality management and functional nutrition is key to maximizing aquaculture yields in non-conventional water systems. The strategic implementation of lower stocking densities significantly enhances biomass production, improves feed conversion efficiency, and suppresses metabolic exhaustion. Furthermore, utilizing brackish water resources paired with gut-modulating dietary supplements offers a sustainable, highly viable, and commercially scalable protocol for the modern aquaculture industry in coastal territories.