Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

Winter Fish

Tuesday, February 4, 2014

This is not the time one usually thinks of fish. It is 15 ℉ outside. There is a foot of snow on the ground and the river is completely frozen over. None the less, it is the fish that I am thinking of. As I look out across the river, I wonder what life is like under the ice. It must be very different than in the summer. For one thing, the water is cold - just above freezing. Also, the days are dark, with very little light filtering through the snow. How does this affect the life of the fish?

When the temperature drops, cold-blooded animals, like fish, slow down. The muscles take longer to react and life takes on a quality of slow motion. This sluggishness affects smaller fish to a greater degree than larger fish - since the smaller fish loose whatever heat they generate heat more rapidly than the larger ones. I picture the small fish remaining fairly motionless throughout the winter while the larger fish are able to do some slow cruising.
I would expect the fish to seek out the warmer pockets of water. As water cools in the winter, it becomes more dense, sinking to the bottom of the river However, once it reaches 39℉, its density begins to decrease. This means that the water at 39℉, the most dense water, is on the bottom while the colder, less dense water is above. So by sitting along the bottom of the river, the fish remain in the warmest pockets of water.

With a covering of snow on the ice, very little light must filter through to the waters below. It is a world of perpetual nighttime with nothing to see but shadows.

So how does all of this affect the fish? Are they sitting on the bottom in a state of suspended animation, content to rest there until the ice thaws and their world of water warms to a more conducive temperature? Is their metabolism so slow in the cold that they don’t need to feed? Or are the bigger fish hunting the little fish as usual, but all done in slow motion? Is it too dark to see predators? If they do detect a predator, do they have the energy to avoid it. Or are they even aware of the danger, not realizing that they are about to become someone’s dinner?

No matter what the fish are capable of in the cold, they do have predators. I realize this as I look down at the ground on the bank by the river. Here I find the scat of the river otters. The scat is composed mostly of fish scales and bones. This is why I have been thinking of fish on this winter day. The river otters have clearly been preying on the fish under the ice.
Any small hole in the ice can serve as an entryway to the world below. River otters, being warm-blooded mammals, can operate at full speed. The cold does not affect them as it does a cold-blooded fish. A river otter is a fast, efficient swimmer. It can stay submerged for up to 4 minutes. When it needs to come up for a breath, it can make a quick trip to the undersurface of the ice where it will find a pocket of air, and then back down to hunt for more fish. What chance does a cold-slowed fish have of evading capture? I expect their best chance of survival is concealment and luck. Find a place to hide in the muck or in some dense vegetation and hope that the river otter passes you by.

The river otters in my river have chosen the bank by my house as one of their latrine sites.scales The latrine is a place where the river otters leave the water to groom, leave scent marks and to defecate. In the twenty years of living by the river, I have seen the otters only twice. Yet every year, I have observed their scat. This year, I decided to take a closer look at it. In it, I found a wonderful array of fish bones and scales. The scales were mostly small - ranging 1/16th to 3/16th of an inch in size, indicating that it is mostly the smaller fish the otters are feeding on. Is this because the larger fish are more active than the smaller fish. Being larger, they will retain more heat and may be better equipped to evade capture. There were, however, some larger scales in the scat, indicating that not all of the bigger fish are able to escape.
Once I had collected the scat, I wanted to see if I could determine which types of fish were represented. For this I needed an identification key to fish scales. Searching online, I found the “Guide to the Identification of Scales of the Inland Fishes of Northeastern North America” by Robert A. Daniels. killifishThis book provides a scientific key that allows one to attempt to identify the fish in question to the Family level. I say that it allows one to “attempt to identify”, because, like many scientific keys, matching the description in the key to the sample at hand is not always easy. For example, the scales on the tail of the fish may be a different size and a little different shape from those under the fins. A scale may be slightly damaged or have a growth defect. Matching all of these scales with the one “idealized” scale in the key can often be challenging.
perchThe key only identifies a fish to the Family level. For instance, the sunfish family, Centrarchidae, contains 16 species in the northeast, including pumpkinseeds, rock bass, largemouth bass, crappies and others. The key only allows me to identify them as a “sunfish”
Regardless of these limitations, I have been having fun trying to identifying my scales. Although I may not be certain that I have identified them correctly, I am certain that there are fish in the river on this winter day and that whether they are aware of it or not, they are being preyed upon by the river otters.

The scales in this story have been stained red for better viewing.