The following individual was interviewed for today’s show. Our thanks to:

Dr. Grace Klein MacPhee
Associate Marine Research Scientist
Graduate School of Oceanography
University of Rhode Island
Narragansett Bay, RI

Interview:

ES: Please tell me a little about the study with winter flounder that you co-authored.

GKMP: Well, it was principally Aimee’s [Keller] study, and I helped her out with the Winter Flounder part because that’s my field of expertise. What the theory was, is that when winters are very warm, you have a lower flounder survival rate. And this had been noticed years ago from long term data sets that were done at nuclear power plants in Connecticut and also from long-term trial surveys in the bay. So it wasn’t a 100% new idea. It was a connection between numbers of flounder and the early-spring late-winter water temperatures. But trying to figure out why that was so was problematical because the changes in the temperature were small enough to where they don’t directly affect the flounder. They can tolerate several degrees change in temperature. So we knew it wasn’t just hot water. So our theory was that it might be causing some of their predators to emerge from their winter torpor earlier, and they would by emerging at the time that the flounder would be settling, so that there would be an increased predation on them. So that was one of our thesis. The other one was perhaps that it was affecting the winter-spring plankton bloom. And they had been noticing that the bloom has been decreased in the warmer years, and sometimes it doesn’t even occur. And the importance of this is that it provides food for them, the larval flounder, by way of increasing the phytoplankton, and then the zooplankton, and then the flounder, they eat the zooplankton. We thought that might be affecting them to. So we did this in some very large mesocosm, which are huge tanks that enclose about 13 cubic meters of water, they have a vent, and also they have a natural community at the bottom. We use water from the bay so you can get the communities that are in the bay. And you can control the temperature of it, so you can try an increase over a normal, and a decrease over normal, and a normal. And you can compare and see what the results are for the whole food chain. So when we did this, we noticed that also, there was more food production in the warmer tanks, there was less survival of the flounder. So, we pretty much eliminated the fact that it was a food problem. However, we did find that in the tanks without several of the important predators, that there was a decrease in the number of flounder. So that’s in a nutshell what we found.

ES:

GKMP: Well it affected the types of phytoplankton, but this didn’t seem to translate to flounder mortality. Well, what it did was, the flounder grew faster in the warmer tanks. They were bigger, but the survival was lower in the colder tanks. There was less of the phytoplankton and the zooplankton, but the flounder survived better, although they were smaller, they grew slower. So we said: well, we probably don’t think it’s the food, and it could be the predators. And one of the predators that was present in the tank was the sand shrimp. And this has been shown to prey upon places in the North Sea, Holland, and places that are closely related to the winter flounder, it’s another flat fish. And what they found that isn’t in warm winter is that they’re version of the sand shrimp is more active earlier and it eats up the place. So we thought that this might be going on with the flounder too. We have a student that’s working here now that’s looking at this for his dissertation.

ES: So basically the effect that the sand shrimp has is ?

GDMK: to lower flounder production.

ES: What are some of the factors that are contributing to warmer waters in Narragansett Bay?

GDMK: Well we’ve noticed that over the last forty years the temperatures have been slowly increasing. I don’t know if it’s global warming, or whatever it is, it’s a very slow rise in temperature, like 1 to 2 degrees, but it is a definite rise. Other than that, I think it’s weather related, climate related. We don’t have any power plants in our section of the bay that would contribute warm water. So it’s nothing that we can contribute to human interference.

ES: What is the significance of Narragansett Bay and of estuaries in general.

GDMK: Narragansett Bay is a very important nursery ground for winter flounder for the southern Rhode Island stock. Experiments were done, particularly by Saul Fela linking the production in Rhode Island sound with production in the salt ponds that grow along the coast and in the bays and in the estuaries. Flounder go to the same spawning areas year after year, and then return to deeper waters when they get bigger. There is some spawning off-shore, but the majority takes place in the shallower waters. So that’s the link between the estuaries and the salt ponds and the flounder stock off shore.

ES: Could you tell me a little more about sand shrimp in Narragansett Bay?

GKMP: Sand shrimp is a predator, and they eat crustaceans and they also eat fish larvae, especially when they’re settling. Because when flounder settle, they’re very small, and they’re very weak, and they don’t swim very well. So they tend to be an easy prey if the shrimp are active at that time. After a certain size, they’re not predated upon because they’re too big and they can escape. So there’s a short window of time when they’re vulnerable to predation.

ES: What is the relationship that winter flounder have with phytoplankton?

GKMP: Early on, it’s one of the first food items that they eat, along with other small organisms like **enids and things like that. They start off eating diatoms and dinoflaggaelates, but it doesn’t form a huge part of their diet. And then as they go along, they eat more of the rotifers and ***enids, and little mollusks, and then they finally work up to copepods. But the other connection is that the phytoplankton is food for the secondary trophic level, like the ***enids and the little copepods. So that’s the relationship. They eat it directly for maybe a week or two, and then it becomes food for their food.

ES: What is the relationship that winter flounder have with Narragansett Bay?

GKMP: It’s a fairly good sized spawning population that utilizes the upper bay. And it is one of the larger estuaries locally. They also spawn in salt ponds that are close to the coast, and they are sort of a semi-estuarine environment. But Narragansett Bay is a lot bigger. And there are reasonable numbers of flounder that are produced in the bay.

ES: What sort of impact are humans making to the bay?

GKMP: Well I think one of their biggest impact is coastal shore development, dredging and building close to the salt marshes, and things like that. The other impact it has locally is that our sewage system, it’s a combined drain water overflow. The sewage treatment plants are pretty good when it’s not too wet, but if there’s a strong rainfall, then sewage is washed out, because they’re combined with the drain, the sewage pipes. So that can be a problem too. And of course if you have an increased population, you have an increased problem with your sewage too, if it’s wet.

ES: So the main factor in winter flounder decline is predation.

GKMP: Yeah, and of course over-fishing is a factor too. But they have been working to control the fishing pressure in the bay. Everything kind of works together. It’s hard to say that one thing is the major factor.

ES: When did winter flounder start to decline?

GKMP: It has been decreasing since the seventies. So there’s been quite a decline. In some cases it’s as much as eighty percent or more for fisheries.

ES: I guess the results of your study rule out the effect that phytoplankton might have, whether too little in starving winter flounder larvae, or too much in causing hypoxia.

GKMP: Well, I don’t think it is. Also, the hypoxia tends to become more of a problem for the summer, for the summer species. Winter flounder is really a winter and an early spring species. By the time we do get hypoxia in the bay, in parts, the fish is better able to move around. The larvae don’t move very much in the place that they’re spawned, because they don’t swim very well. So they’re more vulnerable to anything that goes on. By the time summer rolls around they can move away from things like that.

ES: Are there other winter flounder predators in the bay?

GKMP: There’s a mud anemone that seems to be more active under warmer conditions. We had a graduate student that worked on that too. We don’t know what else might be encouraged. There might be other fish that are around at the time. It’s the time of year when there isn’t a lot moving around, and that’s one of the reasons why we think that the spawn at this time of year, it’s kind of an avoidance of predation. They lay eggs that sink. Most marine fish have floating eggs. Winter flounder’s eggs sink. So they’re on the bottom. They’re likely to be picked up by things that were on the bottom.

ES: What can humans do to reduce the decline in winter flounder populations?

GKMP: Well we can control the harvest rate. There are cycles in the temperature too, I mean it’s not a continuously rising temperature. There are cold winters and then there are warm winters. You can use the appearance of a warm winter to look a year or so ahead to controlling fishing pressure, so you won’t be able to harvest as many a year or so after a warm winter, based on the fact that there probably won’t be as many flounder there. You kind of let up if there’s a cold winter, and kind of look ahead and say that a year from now we should have a fair number of flounder. So it can be a tool for management. As far as the continually rising temperatures, there’s not much you can do about that.

ES: What kind of research are you doing now?

GKMP: Right now I’m doing an ichtyoplankton survey, so I’m doing a fish, egg, and larval survey in the bay. And I’m doing this with the Department of Environmental Management. They monitor the fish populations. One of their target species is winter flounder. And the other is ***tatog. And some of them are commercially important species. So they have an ongoing trial survey. They sample a number of adults, they can look at the number of potential spawners. They also sample a juvenile, so they get the newly settled fish from the year. So they wanted to complete the cycle by looking at the eggs and larvae to see the early stages. That’s what I’m doing right now.