The recent fish die-offs at Clear Lake pose more questions than there are answers at this point. For example, are the die-offs because of introduced chemicals or from other factors? Most fish kills are the result of a reduced amount of oxygen in the water. Algae blooms and hot weather can lower the dissolved oxygen (DO) significantly. Fish, like humans, require food, habitat and oxygen to survive. Whereas humans get their oxygen from the air around us, fish get theirs from the water. It’s called dissolved oxygen (DO). DO is measured in milligrams per liter. The higher the number the more oxygen in the water. DO levels can range from 1 to 18. Trout and other cold water fish require a DO reading of at least 8. Bass, crappie and bluegill need no less than 5 to be comfortable. When the DO drops below 5 some fish will die. If it drops below 3, just about all the fish will die. The amount of DO measured where the recent fish die-off took place was more than 7, which means it is highly unlikely the fish died from a lack of oxygen. Another factor was that many of the dead fish were adults. In most cases where the DO levels drop in an area, the adults swim away and it’s the juveniles that die.
Oxygen is actually a gas made up of molecules and when a human breathes in the molecules they go to our lungs, which extract the oxygen and transfer it to our blood. Gills serve as lungs for fish. The gills have a rich blood supply and are made up of a small type of filament. When a fish opens its mouth and inhales, the water flows over the gills where the oxygen is extracted and transferred to the blood. When there is little or no oxygen in the water the fish either have to move to a new area or die. Since the die-off occurred where there was plenty of oxygen, that would suggest something else killed the fish. That something could be the introduction of a chemical or chemicals. What is confusing is that the die-offs occurred in two separate areas about three miles apart. Several biologists suggest that a chemical was put in the water. It could have been a chemical that was dumped into one or more of the storm drains flowing into the lake.
A new wrinkle in the fish die-offs is that two dead otters were discovered near the dead fish. It’s rare to find one dead otter and even rarer to have two dead otters in the same area. What killed them is unknown. Otters are mammals and they breathe the same as humans, so it wouldn’t be a lack of oxygen that killed them. Fish die-offs are not uncommon at Clear Lake. Down through the years there have been die-offs of large fish, sometimes into the thousands and almost always involving carp or threadfin shad. Only a few of the die-offs have claimed bass, crappie or bluegill. A thousand dead fish won’t have any impact on the overall population at Clear Lake, which holds tens of thousands of fish. Hundreds die a natural death each day. However, it’s a bigger problem if the die-off involves a chemical pollutant and is human caused. If that is the case, authorities need to get to the bottom of the problem immediately. The lake is the lifeblood of the county and anything that happens is often reflected in a drop in tourism. The one positive thing about the die-offs is that they were reported by local residents, which means locals are very protective of our most precious natural resource. After all, it’s one of the oldest lakes in the world and we must take every measure to protect it.
