1. Why Acid Strength Alone Does Not Decide a Fish's Fate
Even normal rain is slightly acidic, with a pH of about 5.6, because carbon dioxide from the air dissolves in it. Acid rain caused by human emissions of sulfur dioxide and nitrogen oxides is usually between pH 4.2 and 4.4 [1]. pH 7 is neutral, and a lower number means stronger acidity. Because the scale is logarithmic, a difference of 1 means the acid concentration is 10 times higher. Therefore, pH 3.5 has a hydrogen ion concentration about 32 times that of pH 5 (a calculation: 10 to the power of 1.5).
Yet, Lake Usori at Osorezan has a pH of about 3.5 due to volcanic sources, and dace live there [5]. In contrast, the EPA states that at pH 5, many fish eggs do not hatch, and adult fish may die at even lower pH levels, leading to lakes with no fish [2]. The pH 5 threshold is a rough guide that changes depending on the fish species and conditions; it is not a universal line for all fish. So, what exactly does acidic water do to a fish’s body?
2. How Acid Rain Carries Aluminum from the Soil into Lakes
When acidic rainwater passes through the soil, it dissolves aluminum from clay particles, which then flows into rivers and lakes [2]. Aluminum is a metal that can be harmful to both animals and plants. In acidic lakes, fish are exposed to two threats at once: the water itself is acidic, and it also contains aluminum released from the soil [2].
Where does a fish’s body first touch this water? The answer is the gills, the first part of the body the water touches.
3. How Fish Lose Salt Through Their Gills in Acidic Water
Fish gills do more than help with breathing; they also help maintain the balance of salts (sodium and chloride) inside the body. Studies report that in acidic water, hydrogen ions reduce the uptake of sodium and chloride at the gills and increase the amount leaving the body, causing a loss of salts [4]. When aluminum is present, this loss can get worse under certain conditions [4].
In a study of Atlantic salmon rivers, researchers found that at points with the lowest pH (5.4 to 5.6) and the highest inorganic aluminum (50 to 80 micrograms per liter), chloride levels in the blood dropped significantly. The activity of an enzyme in the gills that moves salts in and out decreased, and aluminum levels in the gills increased [3]. These numbers come from one specific study of salmon rivers and are not universal limits for all fish. So, how do dace in the strongly acidic Lake Usori use their gills to survive?
4. What Researchers Have Studied in the Gills of Dace in Strongly Acidic Lake Usori
Lake Usori is strongly acidic due to volcanic influence, with a pH of about 3.5. The fact that dace live and grow there was the starting point for research [5]. In the study, researchers investigated the high induction of ion transporters, channels, and related molecules in the salt cells (cells that move salts in and out) of the dace’s gills [5].
It seems natural to think that the system that keeps sodium and chloride in the body is working strongly in this fish. But this description alone does not tell us which molecules contribute, or how much. Strong claims such as "the only fish in the world" or "the only fish in the lake" cannot be supported by the materials used here, so we do not use them. Tolerance to acid differs greatly from species to species, and this is the background to the mismatch between pH 3.5 and pH 5. So, if the causes are reduced, do the fish that could not endure return?
5. Did Fish Return to American Rivers After Emissions Dropped?
In the United States, emission reductions have progressed. Nationwide, sulfur dioxide decreased by 94% and nitrogen oxides by 87% between 1990 and 2021. In rain, sulfate ions decreased by 88% and nitrate ions by 70% [7].
In 42 rivers in the Adirondack Mountains, researchers compared the period of strong acid impact (1979–1984) with the recovery period (2014–2021). The number of sites where brook trout were found expanded from 20 to 33 [6]. On average across 40 sites, the number of fish species increased by 112%, density by 236%, and weight by 66% (about 2.1 times, 3.4 times, and 1.7 times, respectively). The pH of the river water rose by 0.33, which corresponds to a nearly halved acid concentration [6].
However, this study does not say that "everything returned." The density and weight of brook trout remained largely unchanged during this period [6]. Why do fish not return easily even when the water improves?
6. Why Living Things Return Slowly After Lake Water Recovers
In the Killarney region near Sudbury, Canada, smelter smoke (sulfur dioxide) made lakes acidic by the late 1970s, reducing fish, amphibians, and birds. Later, smelter sulfur dioxide emissions dropped by more than 90% over 30 years, and pH levels rose in many lakes [8].
According to a research summary from this region, biological recovery was limited, and crustacean plankton communities only recovered to a moderate degree [8]. This summary discusses plankton, not fish. There is a time lag between water quality improving and living things returning. Lakes in Sweden have also generally become less acidic since the 1980s, but recovery is slow. Roach (a freshwater fish in the carp family) sensitive to acid have been recorded returning to some chemically recovered lakes [9]. However, it is difficult to assess biological recovery due to limited pre-acidification data and the effects of climate change [9].
Reducing emissions is effective. On the other hand, it takes time for fish and lake life to return, making continued observation important.
7. How You Can Investigate Water Acidity Yourself
If you want to investigate this yourself, you can try these steps. First, ask your science teacher if you can use pH test strips. With an adult’s help, you can measure and compare the pH of tap water or water collected after rain to understand the pH of water around you. Second, if you visit a nearby river or pond, observe what kinds of fish are visible. Do not get too close to the water’s edge, and always go with an adult. For official information, the Ministry of the Environment’s long-term monitoring reports on acid rain contain records of pH changes in Japanese lakes [10]. The Adirondack study can be verified on the USGS paper page.