Effect of Different Water Salinities on the Larvae of the Blue Bream Ballerus ballerus (Linnaeus, 1758) during Rearing

**Background:** Inland waters are increasingly affected by salinization from anthropogenic sources including mining, industrial discharge, agricultural runoff, and road de-icing salts. These changes threaten freshwater organisms, particularly the early developmental stages of stenohaline cyprinid fish, which are highly sensitive to salinity fluctuations. The blue bream (Ballerus ballerus) is a rheophilic cyprinid species native to the Szczecin Lagoon and Dąbie Lake in the Lower Odra River Valley. Although its population is declining due to interspecies hybridization, limited data exist on how salinity affects its early life stages. This study aimed to determine the appropriate water salinity level at which blue bream larvae achieve satisfactory growth and survival.

**Methods:** Blue bream spawners were caught from Lake Dąbie during mid-May. Larvae were obtained from controlled reproduction and reared in four separate recirculating aquaculture systems (RASs) at salinities of 0 (control), 3, 5, and 7 ppt. Pro-Reef sea salt dissolved at 20 °C was used to achieve target salinities. Each experimental circuit consisted of three 25 dm³ aquaria plus a 200 dm³ filter aquarium with a protein skimmer and biological bed. Larvae at 5 days post hatching (DPH), with an average body length of 7.5 mm, were stocked at 40 indiv.·dm⁻³ (1000 larvae per aquarium). Water temperature was gradually increased from 20 °C to 25.0 ± 0.5 °C. Measurements of weight and total length were taken every four days (days 1, 5, 9, 13, 17, and 21) from 30 randomly sampled larvae per aquarium. Larvae were fed live Artemia sp. nauplii ad libitum three times daily. Dissolved oxygen ranged from 7.1–7.4 mg O₂ dm⁻³, pH from 7.1–7.3, and ammonia and nitrite were undetectable. Dead larvae were counted daily. Calculated parameters included survival rate, index of total length gain (ITL), specific growth rate (SGR), relative growth rate (RGR), survival and larval biomass (SBR), and relative biomass growth rate (RBR). Statistical differences were assessed using Duncan’s test at α = 0.05.

**Key Results:** The first dead larvae appeared on day 2 in the 7 ppt group and on day 3 in the 3 and 5 ppt groups. The lowest mortality was in the control (0 ppt). Final survival rates were: 98.6% in 0 ppt, 97.0% in 3 ppt, and 83.2% in 7 ppt. The best growth was observed at 3 ppt salinity: mean final weight 64.1 mg, mean total length 22.6 mm, and highest biomass of 62.2 g (2.5 g·dm⁻³). The control group (0 ppt) showed similar results: mean final weight not explicitly stated for 0 ppt alone but biomass was 61.6 g (2.5 g·dm⁻³). No statistically significant differences were found between the 0 ppt and 3 ppt groups for weight or biomass. At 5 ppt, larvae reached mean weight 52.4 mg and length 19.2 mm; at 7 ppt, mean weight 48.6 mg and length 19.0 mm. No statistically significant differences were noted between the 5 and 7 ppt groups. The lowest biomass was 40.4 g (1.6 g·dm⁻³) in the 7 ppt group.

**Clinical Implications:** This study demonstrates that blue bream larvae exhibit high tolerance to salinities of 3–7 ppt, with optimal growth and survival at 3 ppt. These findings are ecologically relevant given the increasing salinization of inland waters from anthropogenic activities such as mining and road salt runoff. The high survival rates (83.2–97.0%) across all tested salinities suggest that blue bream may be more resilient to moderate salinity increases than previously assumed for stenohaline cyprinids. The results also highlight the importance of local water chemistry—the use of hard tap water (24°n hardness) may have mitigated chloride toxicity. This information can inform conservation strategies and aquaculture protocols for this species, as well as risk assessments for freshwater ecosystems facing salinization pressure.