The short answer is that there are menhaden dying, and only time will tell if the current fish kill will be major or minor. While it can be amazing for birdwatching, it isn’t the most optimal situation for the impoundment or the fish in it. We talked with Lori Sheridan Wilson, Dewees Ecologist, to see what is happening and why.
Why fish kills happen: The dissolved oxygen decreases to the point that fish can no longer survive. Menhaden are the canaries in the coal mine; they are the first to go when the O2 drops. As the water heats up, it can only support so much dissolved ogygen, which decreases as the temperature rises. In addition, the hot, dry summer has increased the salinity as well. Once the menhaden die, the decomposition process also uses oxygen, which decreases the available O2 for everyone. In tidal waters, the inflow and outflow of tides allows the fish to find water they are comfortable in. But an impounded wetland is an inexact science. Many impounded wetlands in South Carolina are experiencing fish kills this summer.
Does rain help? Sometimes. Monday night we had a sudden storm which dumped a lot of cold water into the impoundment. Theoretically this should help all the variables– dissolved O2, temperature, and salinity. But when it is a sudden downpour, things may go awry. In this case, there were layers of basically dead water in the impoundment. The water, devoid of usable oxygen, was isolated into dead zones, usually at the bottom of the impoundment. When the sudden downpour resulted in an influx of cold water, it sank to the bottom, mixing the oxygenated water with the dead water, decreasing the average dissolved oxygen in the water. A slow steady rain (like we had last night) is better for gently raising the O2 levels and lowering both temperature and salinity.
How do we try to fix the problem? Last year, when we know the dissolved O2 had reached critical levels, we basically drained the impoundment and the incoming tide flushed fresh water in. (For more on last year’s episode, click the links at the end of this post) This year, the rice trunk has had a problem keeping the water in. (Despite last week’s repairs, a new leak has resulted from a new rotten board. Lori has engaged divers to check it out and possibly fix the problem.) While we could let more water out, the future tides for the next few days won’t be high enough to flood the impoundment with lots of water, so we could exacerbate the problem even further. So the current strategy is to keep trying to get fresh water in, keep monitoring dissolved O2 levels, and try to remove the already dead fish, so they don’t consume oxygen as they decompose.
So yesterday, we watched/helped Lori and intern Lisa as they scooped buckets of dead menhaden out of the impoundment.
Menhaden make great crabbing bait– we bagged a bunch of relatively fresh ones and stored them for bait in the intern’s freezer. In addition, there is a pump at the drab dock again to improve circulation there.
We know that last year, at this point, there were lots of dying mullet, swimming on their sides to catch some air, breathing with distress. Larger flounders and red drum were gasping at the edges of the impoundment. This year, we haven’t seen distress in the larger fish yet, and the rain/cooler temps/ oxygenation should help the situation. We’ll update with more information when we have it.