I’ve been interested in the systems ecology of malaria since an undergraduate course in population dynamics introduced me to models of malarial dynamics that included the transmission of malaria to people by mosquitoes and vice versa. Malaria impacts mosquitoes too and to them, we’re the problem! When I learned this I had one of those zen ‘aha!’ moments where I felt a visceral connection to all living things.
That’s why a recent article on Planet Ark caught my attention. It reported on research in Kenya that showed raising tilapia in fish ponds could be a valuable a new biological weapon in the fight to control malaria, especially important because of increasing resistance in malarial mosquitoes to pesticide controls. It seems a win-win situation too, since tilapia are already raised in the area for food, which means it wouldn’t require a drastic change in local behaviour to adopt this method. Effective mosquito control, a valuable and sustainable source of food and income, relatively easy implementation, and no dangerous pesticides – who could ask for more? Intrigued, but always a sceptic, I downloaded the original paper for a more in-depth read (Howard et al, 2007, link below).
Though this story has been widely taken up by the press, the authors not that the larval control of mosquitoes by fish is not new and has been used for over 100 years. Tilapia, a native African fish has had known mosquito-control properties since 1917, but to their knowledge, there was no field data published on this topic yet. In another study in one particular region of Kenya they found alarming levels of mosquitoes in abandoned fishponds relative to working fishponds. The objective of this research, then, was to evaluate the long-term impact on malarial mosquitoes of introducing tilapia into abandoned fishponds. A much smaller scope than implied in the many news articles reporting on this story.
The experiment they report was not large. They used three abandoned fishponds in close proximity to one another, reserving one for a control. They then monitored them for 15 weeks before and after treatment. The treatment consisted of the addition of young tilapia to two of the ponds at a density of 2/m2. Other than the treatment, all ponds were maintained in the same manner over the before and after periods. Upon analysis, the researchers found a drastic drop in mosquito larvae in the treatment ponds, relative to the control pond, almost immediately after the addition of the fish. At the end of the experiment, they added tilapia to the control pond and observed another quick drop in larval mosquito densities, though they noted that without a control for this treatment they couldn’t rule out factors than fish causing that decrease. It’s too bad they didn’t remove all the fish from the former treatment ponds prior to adding fish to the control pond for a tidy "two-way trip"(control to treatment; treatment to control). I wonder why they didn’t?
So what did I think overall? Well, I found the paper interesting and I certainly learned a few things about malaria, mosquitoes and fish in Africa, but I was left with a couple of questions. The first thing I wondered was if mosquitoes just avoided the ponds with fish in them. The researchers did too, but they concluded that predation by the fish was indeed the cause of the observed mosquito declines because: 1) the fish are known to like mosquito larvae, 2) there were small increases in larval populations when the fish were breeding and not eating, and 3) in a separate study they found, " . . . no significant difference between the number of fish-present and fish-absent fishponds containing mosquito immatures (Howard et al, manuscript in review)."
Now, this last point confused me. After all, they were arguing that fish reduced the number of larval mosquitoes in the ponds in the experiment and they were doing the experiment after observing large differences in larval mosquito numbers between in-use and abandoned fishponds. I could guess that the argument hinges on the word "containing", that there were mosquito immatures in both types of pond, but less in the fish-present ponds. If so, I’m not so sure this point supports their argument, mosquitoes could still be avoiding the fish–present ponds when possible, but may be habitat limited so that some proportion of the total mosquito population is forced to accept the risk of using the fish-present ponds. This is one part of the paper that could be strengthened.
A second question I had was triggered by one of the benefits the authors list for using tilapia in mosquito control. They note that since tilapia are already farmed and eaten in the region of Kenya where their study took place, acceptance by local communities and administrative sectors of tilapia as a mosquito control tool should be no problem. This seems logical, but why are there so many abandoned fishponds in the region in the first place? I would think that if tilapia were so lucrative, desirable and accepted that all the available ponds in the region would already be filled with tilapia. It sounds like there may be some other ecological, social or economic barrier that needs to be addressed before tilapia can be considered a realistic mosquito control method in this region. If someone has some ideas on this, please let me know. Cheers.
That’s why a recent article on Planet Ark caught my attention. It reported on research in Kenya that showed raising tilapia in fish ponds could be a valuable a new biological weapon in the fight to control malaria, especially important because of increasing resistance in malarial mosquitoes to pesticide controls. It seems a win-win situation too, since tilapia are already raised in the area for food, which means it wouldn’t require a drastic change in local behaviour to adopt this method. Effective mosquito control, a valuable and sustainable source of food and income, relatively easy implementation, and no dangerous pesticides – who could ask for more? Intrigued, but always a sceptic, I downloaded the original paper for a more in-depth read (Howard et al, 2007, link below).
Though this story has been widely taken up by the press, the authors not that the larval control of mosquitoes by fish is not new and has been used for over 100 years. Tilapia, a native African fish has had known mosquito-control properties since 1917, but to their knowledge, there was no field data published on this topic yet. In another study in one particular region of Kenya they found alarming levels of mosquitoes in abandoned fishponds relative to working fishponds. The objective of this research, then, was to evaluate the long-term impact on malarial mosquitoes of introducing tilapia into abandoned fishponds. A much smaller scope than implied in the many news articles reporting on this story.The experiment they report was not large. They used three abandoned fishponds in close proximity to one another, reserving one for a control. They then monitored them for 15 weeks before and after treatment. The treatment consisted of the addition of young tilapia to two of the ponds at a density of 2/m2. Other than the treatment, all ponds were maintained in the same manner over the before and after periods. Upon analysis, the researchers found a drastic drop in mosquito larvae in the treatment ponds, relative to the control pond, almost immediately after the addition of the fish. At the end of the experiment, they added tilapia to the control pond and observed another quick drop in larval mosquito densities, though they noted that without a control for this treatment they couldn’t rule out factors than fish causing that decrease. It’s too bad they didn’t remove all the fish from the former treatment ponds prior to adding fish to the control pond for a tidy "two-way trip"(control to treatment; treatment to control). I wonder why they didn’t?
So what did I think overall? Well, I found the paper interesting and I certainly learned a few things about malaria, mosquitoes and fish in Africa, but I was left with a couple of questions. The first thing I wondered was if mosquitoes just avoided the ponds with fish in them. The researchers did too, but they concluded that predation by the fish was indeed the cause of the observed mosquito declines because: 1) the fish are known to like mosquito larvae, 2) there were small increases in larval populations when the fish were breeding and not eating, and 3) in a separate study they found, " . . . no significant difference between the number of fish-present and fish-absent fishponds containing mosquito immatures (Howard et al, manuscript in review)."
Now, this last point confused me. After all, they were arguing that fish reduced the number of larval mosquitoes in the ponds in the experiment and they were doing the experiment after observing large differences in larval mosquito numbers between in-use and abandoned fishponds. I could guess that the argument hinges on the word "containing", that there were mosquito immatures in both types of pond, but less in the fish-present ponds. If so, I’m not so sure this point supports their argument, mosquitoes could still be avoiding the fish–present ponds when possible, but may be habitat limited so that some proportion of the total mosquito population is forced to accept the risk of using the fish-present ponds. This is one part of the paper that could be strengthened.
A second question I had was triggered by one of the benefits the authors list for using tilapia in mosquito control. They note that since tilapia are already farmed and eaten in the region of Kenya where their study took place, acceptance by local communities and administrative sectors of tilapia as a mosquito control tool should be no problem. This seems logical, but why are there so many abandoned fishponds in the region in the first place? I would think that if tilapia were so lucrative, desirable and accepted that all the available ponds in the region would already be filled with tilapia. It sounds like there may be some other ecological, social or economic barrier that needs to be addressed before tilapia can be considered a realistic mosquito control method in this region. If someone has some ideas on this, please let me know. Cheers.
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