Salinity, Temperature, and Their Interactions with Fish Physiology

Why Salinity and Temperature Matter in Aquaculture

Every fish, shrimp, or shellfish you raise lives inside its water column the way a land animal lives inside its air. Salinity and temperature are the two variables that most directly shape whether that water supports life or stresses it. In a recirculating aquaculture system (RAS), you control both variables deliberately. Understanding the physiology behind them is what separates a technician who reacts to problems from one who prevents them.

Salinity: What It Is and How We Measure It

Salinity is the concentration of dissolved salts in water, traditionally expressed in parts per thousand (ppt), which is numerically equal to practical salinity units (PSU). Full-strength ocean water is approximately 35 ppt. Brackish water typically ranges from about 0.5 to 30 ppt. Freshwater is below 0.5 ppt.

Three instruments appear in today's demonstration, and each works on a different principle.

Optical refractometer — Light bends (refracts) when it passes from one medium to another. Dissolved salts increase water's refractive index, so a refractometer reads salinity by measuring how much the sample bends a beam of light. Optical refractometers are inexpensive, require no batteries, and are calibrated with distilled water. Their resolution is typically 1 ppt, which is adequate for general husbandry but not for high-precision research. Always zero the instrument with distilled water before each session.

Conductivity-based salinity probe or bench meter — Dissolved salts carry electrical charge, so saltier water conducts electricity better. The probe passes a small current through the sample and calculates salinity from the measured conductivity and temperature. Resolution is commonly 0.1 ppt or better. Temperature compensation is automatic on most modern units, but always verify the probe's temperature sensor is also accurate.

Multiparameter sonde (e.g., YSI Pro or equivalent) — A sonde packages a conductivity cell, a thermistor, a dissolved oxygen sensor, and often a pH electrode into one submersible housing. It logs data and can be deployed in situ. For high-precision marine RAS work, a properly calibrated conductivity-based sonde is the preferred method because it offers the best resolution, direct in-water measurement, and simultaneous logging of related parameters.

Each method has uncertainty. Record your readings to the instrument's stated resolution and note the manufacturer's accuracy specification. A difference of 1–2 ppt between an optical refractometer and a conductivity meter on the same sample is not unusual; it does not mean either instrument is broken — it reflects the difference in resolution and measurement principle.

Temperature and Fish Metabolic Rate

Fish are ectotherms: their body temperature matches their water temperature, and their metabolic rate rises and falls with it. A useful rule of thumb from aquaculture physiology is that for many species, metabolic rate roughly doubles for every 10 °C rise in temperature within the tolerable range. This relationship is described by the Q₁₀ coefficient.

Higher metabolic rate means higher oxygen demand. A tank of tilapia at 28 °C may consume more than twice the dissolved oxygen per hour as the same biomass at 18 °C. This is why summer aeration and oxygenation requirements spike, and why dissolved oxygen crashes are more common on hot days. If you plot fish oxygen consumption rate against temperature using tabular data from a standard reference (such as Stickney's Aquaculture: An Introductory Text), you will see a curve that rises steeply through the preferred temperature range.

Temperature also changes the toxicity of common aquaculture stressors. Ammonia equilibrium shifts with temperature: warmer water holds a higher proportion of un-ionized ammonia (NH₃), which is the toxic form. Pesticide and heavy metal toxicity similarly increase with temperature. This means your acceptable ammonia threshold must be lower in summer than in winter for the same species.

Osmotic Stress and Salinity Tolerance

Fish regulate the concentration of their internal fluids through osmoregulation. Euryhaline species (such as salmon and tilapia) tolerate a wide salinity range. Stenohaline species (such as many reef fish) require stable, species-appropriate salinity. When ambient salinity changes faster than a fish can compensate, it expends extra energy on osmoregulation, immune function drops, and susceptibility to pathogens rises.

The safe rate for salinity change in most marine RAS species is generally cited as no more than 1–2 ppt per hour for acclimated fish, though this varies by species. Always research the specific tolerance of the animals in your system. Watch for stress behaviors: increased surface respiration, rapid opercular (gill cover) movement, unusual swimming posture, loss of schooling behavior, or fish hovering near inflow pipes.

Adjusting Salinity in a Demonstration Tank

To raise salinity, you add synthetic sea salt mix. Never add dry salt directly to a tank containing animals — always pre-dissolve in a bucket of system water first. Calculate the mass of salt needed before you touch the bag. Check your math with the instructor before you add anything. Add the dissolved salt slowly, wait for complete mixing (at least 5–10 minutes with circulation running), measure, and repeat until you reach the target.

Safety Points

• Synthetic sea salt mixes can irritate eyes and mucous membranes. Wear splash goggles and nitrile gloves when handling dry salt or concentrated solutions.
• Instrument probes contain fragile sensors. Rinse with deionized water after use and store per the manufacturer's instructions.
• Sondes may have lithium batteries. Do not submerge the battery compartment beyond the rated depth.
• When working around tanks, watch for wet floors. Non-slip footwear is required in the lab.
• If a fish shows acute distress at any point during the salinity adjustment, stop the addition immediately, increase aeration, and notify the instructor.