The Science Behind Lake Norman's Fish Kills
Fish kills have become an annual problem at Lake Norman. Since 2004, wildlife officials have reported frequent large kills of striped bass, or stripers, a popular species with fishermen. So far this year, they've counted over 800. To find out more about the biology behind these kills, WFAE's Duncan McFadyen called Lawrence Dorsey. He's the District 6 Fisheries Biologist for the North Carolina Wildlife Resources Commission.
MCFADYEN: So what is it about striped bass that makes them so attractive to fishermen? What makes them worth stocking?
DORSEY: Well, they're an open-water game fish that school. They fight very hard. They do have the potential in some situations to get a large size, and they're just another component of the fishery that's a little different from largemouth bass or crappy or catfish. That's really what provides that attraction---that open water nature that makes them a different species to go after. And as I said, they have a good ability to fight and provide a good angling experience that way.
MCFADYEN: And they can't reproduce in the lake?
DORSEY: That's correct. They actually do attempt to reproduce. But the eggs of striped bass need to be suspended in a current over about a 72 hour period. Because Lake Norman is impounded, those eggs are produced but they don't become viable; they can't be suspended because the water does not flow there. The [Catawba river] water becomes stagnant as it becomes a reservoir.
MCFADYEN: So how long have these kills been going on, and what's causing them?
DORSEY: We've really seen the kills on a frequent basis since 2004. And the root of the problem is a combination of a pocket of oxygen that's dissolving over times which has been occurring for quite a while. But the new variable that's probably driving the kills is there's a new fish species that's been introduced: river herring---an unauthorized introduction, they were not introduced by the Wildlife Commission--- that seem to congregate in this pocket of oxygen at the bottom of the water. Striped bass, who are naturally attracted to this forage fish, go down into this area and as this area, which over time dissolves and concentrates, the stripers are trapped and ultimately run out of oxygen and die.
MCFADYEN: What is a solution to this problem?
DORSEY: Hybrid striped bass are one avenue that we can consider going down and it's probably one that we're looking at right now. Hybrid striped bass are a combination between white bass and striped bass. The thinking there is that these fish may do better because they may have a little less strict temperature and oxygen requirements. But it's not one that we're ready to do right now, because there are other factors such as downstream escapement---what they might do downstream. There are several factors we're considering but we're certainly looking at that as an option to put into Lake Norman in addition to or instead of striped bass.
MCFADYEN: Do you have any plans to stop stocking striped bass in Lake Norman?
DORSEY: There are no plans now. We actually do our stocking in June of every year and we did our normal stocking of striped bass just a couple of months ago.
MCFADYEN: Are there any other fish you have to stock?
DORSEY: Not as a rule. Most fisheries and reservoirs across the state are self-sustaining and so they don't' require any supplemental stocking. As I mentioned earlier with striped bass, they're in a different category because they don't' reproduce and without a stocking from our agency there wouldn't be striped bass in our reservoirs.