Why are some smallmouth so dark?

Tuffs Lab: September 24, 2015

Why are some smallmouth so dark?

A few years ago, a good friend in the fishing world asked me why some Smallmouth Bass are so dark. At the time, I didn’t have a good answer, but it is something that I have continued to try and learn more about. This past summer, some of the footage collected by my field crew provided some interesting insights into this issue.

The images associated with this post show the same fish, which was encountered guarding a nest. At the time, the field crew first saw the fish, it was almost jet black, as are many of the males guarding nests on this end of Lake Ontario. Interestingly, after the crew got in the water and set up a camera to monitor the behavior of the fish for our research, it had turned a much paler shade of brown (see 1st images). Over the next couple of hours, with the crew gone and only the camera present, the fish then returned to being almost jet black (see later images). The fish stayed in the same location throughout this time and a slightly damaged fin on the left side confirmed it was the same fish. Clearly, this shows that it is not that some Smallmouth are always black, but rather that individual fish can exhibit striking color changes at different times.

As a passionate Smallmouth angler, I have also noticed this with individuals that I have caught and put in my live well. Although a fish may be black when you first see it in the water, it is often much paler when you remove it later from your live well.

So, what is happening with these fish? According to the scientific literature on this issue, these types of color changes can actually provide important messages (often to other individuals of the same species) about the status of the fish displaying a certain color. In fact, a recent scientific paper on this topic explains that coloration can be used to convey information about a variety of things such as social status (e.g. dominant fish), sexual activity, stress responses and aggressiveness vs boldness.

At least for myself, this explanation seems to fit with observations made on the water. Smallmouth that are very black are relatively common on nests in the early season. Later in the year, dark individuals are still frequently observed, but these may often be dominant, or more aggressive individuals in groups of lighter colored fish. If the fish is disturbed on the nest, or caught by an angler and placed in the live well, the change in the “mood” of the fish probably contributes to the color change and it turns lighter.

For those interested in the mechanism, specialized cells in the skin called “chromophores” contain the machinery that provide fish with this ability. There also seems to be a variety of ways that this mechanism can be regulated, including via hormones, the nervous system and even at the level of the cell itself in response to changing environments (i.e. to provide better matching with changing backgrounds).

We hope you find this type of thing as interesting as we do and we welcome your thoughts.

The Testament of a Fisherman

By: Robert Traver 1964

I fish because I love to, because I love the environs where trout are found, which are invariably beautiful, and hate the environs where crowds of people are found, which are invariably ugly;  

• Because of all the television commercials, cocktail parties, and assorted social posturing I thus escape;

• Because, in a world where most men seem to spend their lives doing things they hate, my fishing is at once an endless source of delight and an act of small rebellion;  

• Because trout do not lie or cheat and cannot be bought or bribed or impressed by power, but respond only to quietude and humility and endless patience;  

• Because I suspect that men are going along this way for the last time, and I for one don’t want to waste the trip; because mercifully there are no telephones on trout waters; • Because only in the woods can I find solitude without loneliness;  

• Because bourbon out of an old tin cup always tastes better out there;  

• Because maybe one day I will catch a mermaid;  

And, finally, not because I regard fishing as being so terribly important but because I suspect that so many of the other concerns of men are equally unimportant – and not nearly so much fun.

Find out more about LIB