From the April 2011 issue of Susquehanna Fishing Magazine:http://susquehannafishing.com/By William MilheimI have always been interested in the natural sciences. I must admit, some of it is boring, but when it relates to fishing I always perk up and listen. In the past few months we’ve discussed weather and water temperature, and this month we’ll be looking into dissolved oxygen and its role in our fishing success. Again we’ll use smallmouth bass as our target fish. They are popular and I do most of my guiding for them.
Dissolved oxygen is created by diffusion from the surrounding air –
aeration from tumbling water, such as rapids and falls, and is also a by-product of photosynthesis. Oxygen levels also can be reduced through over fertilization of aquatic plants by run-off from farm fields containing phosphates and nitrates (the ingredients in fertilizers). Under these conditions, the numbers and size of aquatic plants increase. Then, if the weather becomes cloudy for several days, respiring plants will use much of the available dissolved oxygen. When these plants die, they become food for bacteria, which in turn multiply and use large amounts of oxygen.
Dissolved oxygen is measured in parts per million. Numerous scientific studies suggest that 4-5 parts per million (ppm) of dissolved oxygen is the minimum amount that will support a large, diverse fish population. The dissolved oxygen level in good fishing waters generally averages about 9.0 parts per million (ppm).
Now that the science portion is over, we can see how dissolved oxygen affects our river. We know now that after a good rainstorm the water gets muddy and dissolved oxygen levels are naturally low. When the water warms to the seventies aquatic plants are growing using dissolved oxygen. As the water temperature continues to rise it will lower the dissolved oxygen level.
It’s easy to find the dissolved oxygen rates in the river by searching the internet for USGS references to the Susquehanna River. Then find the part of the river you fish. It will give dissolved oxygen, flow rate, and PH.
How can we use our new gained knowledge in our favor to increase our fishing success? Be flexible in your location. Areas that held smallmouth in the spring might not hold fish in the summer, because the dissolved oxygen is too low. Last summer (2010) was a great example. Low water conditions most of the summer into the fall meant that the focus was on fast moving water, where the dissolved oxygen levels are higher than slow moving water. For most of the fishing season on the North Branch, dissolved oxygen isn’t much of a concern until the water warms in the summer months. Fishing deep water or slow-moving deep water where you would think the smallmouth would be just isn’t the case. Move to fast moving water or a deeper pool that is fed by rapids. These areas will hold large numbers of smallmouth.
We know now that the dissolved oxygen fluctuates from day to day and certain conditions, such as water temp, stained water, and the demand they can put on dissolved oxygen levels. I always get the question, “when will fall bite start?” Many factors need to come into play; first the water temperature needs to drop and the dissolved oxygen level needs to rise. All the aquatic vegetation is starting to die off, all the leaves that drop in the river need to deteriorate. The demand on dissolved oxygen for decomposition of vegetation is very high.
Next time you go fishing, consider the affects of dissolved oxygen. The key is to fish in spots that provide the best possible areas where high levels of dissolved oxygen will concentrate the smallmouth.
Next issue we will look into the lunar table and how this will affect your day fishing.
Bill Milheim has been fishing and guiding the North Brach of the Susquehanna River for over 25 years.
