Out of Their Element: What Makes Gulf Sturgeon So Jumpy?

Out of Their Element: What Makes Gulf Sturgeon So Jumpy?

The jumping of Gulf sturgeon (Acipenser oxyrinchus desotoi) on the Suwannee River is well publicized and is widely known among Floridians. Some jumping sturgeon have even caused injuries to unfortunate boaters who get in the way of these armored aerial acrobats. Sturgeon jump most often in the early morning, and each fish averages one jump per day. Jumping is not confined to large individuals either, as fish as small as 1 foot long will jump. Most jumping activity occurs from June to early July, when some reaches of the river can experience as many as 6 to 10 jumps per minute! The most intense jumping occurs when the water level is low, before the summer convective storm season sets in. Yes, the fact that these massive timeless fish jump is not the question here. They jump. We know. What’s not well publicized is why exactly these sturgeon feel the need to jump in the first place.

Gulf sturgeon are not trying to shed ectoparasites as many believe, nor are they trying to capture prey. The jumping is also unrelated to spawning. They are not reacting to the presence of boats.

They loiter in the deep dark pools of the Suwannee River and in other Gulf Slope rivers from April through September and slowly digest the 20 percent of body fat they put on during the previous 6 months of easy living, and easy feeding, in the Gulf of Mexico. When in the Suwannee they seldom feed, so put that fishing pole away.

Resting in the sluggish deep recesses of the Suwannee, Gulf sturgeon are conserving energy. But just hovering above the river bottom requires neutral buoyancy. Similar to how a SCUBA diver adjusts his BC, these fish must add air to the swim bladder. Air slowly leaks out of the swim bladder and is absorbed in the surrounding tissues. To keep from scraping along the bottom and to swim more efficiently in the sluggish current, the swim bladder must be refilled an average of once daily. Since the swim bladder in Gulf sturgeon is connected to the digestive tract by a duct, the bladder can be recharged by gulping air. But how do fishes, being totally aquatic as they are, find any atmospheric air to gulp? Well, we have nearly reached our answer as to why they jump.

Any SCUBA diver knows that you can’t descend easily through the water column with a BC full of air, even with dive weights. By the same principle, a sturgeon can’t just rise gently to the surface, take a big gulp of air, and glide back down into the depths. Did I mention that while sturgeon are lollygagging in the river, they are also chatting up a storm? Call them gabby or chatty, Gulf sturgeon really know how to bend an ear (or otolith, in this case). These fish communicate with one another via a series of snapping sounds. Jumping high up into the air and then falling powerfully back into the water, with a loud report, and swimming directly to the bottom, serves two purposes. The main purpose is to allow the fish to gulp air and yet have enough momentum to reach the river bottom immediately afterward (despite the increased buoyancy from the added air). The secondary purpose may be to communicate. Immediately prior to jumping, these fish emit a series of snapping sounds, and following the belly-flop-style smack when entering the water, the sturgeon emit more snapping sounds. The role jumping plays in sturgeon communication, if any, is far from clear and would be very difficult to prove scientifically. However, it seems probable that these fish are communicating with one another using not just snapping sounds (which have been recorded by U.S. Geological Survey scientists and heard by SCUBA divers), but perhaps also by the splash following a jump. So, the next time you encounter a jumping sturgeon while enjoying a lazy day on the Suwannee, ask it why it jumps. Listen carefully to its answer and don’t be surprised if its response sounds something like “snap, snap-snap, snaaap, snaaap, snap”.

Source and Suggested Reading:

Sulak, K. 2013. Catching air – those magnificent jumping Suwannee sturgeons. American Currents 38(2):23–25.

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