SUNDAY 22 JANUARY, 2017 – UWI TODAY
7
RESEARCH IN ACTION
Have you ever heard
of the world-famous local fish called
Poecilia reticulata
? Youmay know it as the guppy, canal fish,
seven colours or millions. Many know this tiny fish from
hours of childhood fishing pleasure, or as an aquarium pet.
The Trinidadian guppy is famous because it is sold in many
countries as an aquarium fish, it is the focus of research
by many scientists abroad, and it has been introduced to
many countries to control mosquitoes since it eats mosquito
larvae.
Mosquitoes are a nuisance and threaten human health
because they are biting insects that feed on human blood,
and they are vectors, or carriers, of several life-threatening
diseases such as dengue, yellow fever andmalaria.The recent
spread of these and other mosquito-borne diseases, such as
Chikungunya and Zika, spurred renewed efforts to control
mosquitoes. One popular method is to introduce larvae-
eating fishes, such as guppies, to water where mosquitoes
breed in the hope that they will eventually reduce the
number of adults.
As recently as 2013, guppies were released into fresh
water in Pakistan to fight a dengue epidemic. In 2014, school
children and volunteers joined the
Guppy Campaign
, a
programme that released guppies into puddles as a means
of fighting malaria. And in 2015 and 2016, they were used
in Brazil to control the spread of Zika and dengue. Perhaps
because of the familiarity and ease of using this approach,
there were reports of people independently releasing guppies
in disease-affected countries. A search for “mosquito
control guppy” in Google produces hundreds of articles
and websites promoting this approach.
But among these you will also find an article recently
published by my colleagues and I, casting doubt on the
effectiveness and wisdom of using this strategy.
Guppy Campaign
volunteers use a simple effective
strategy to convince others that guppies will eat mosquito
larvae – jars of guppies eatingmosquito larvae.The problem
with this, and similar experiments showing that guppies
eat mosquitoes is that the fish had no other food available.
There is strong evidence that guppies prefer other foods.
In experiments in which guppies were given mosquito
larvae and other foods, the guppies ate more of the other
foods. The faeces of guppies caught in the wild showed that
guppies in nature ate even fewer mosquito larvae than in
the experiments. Here in Trinidad, we observed that guppies
feed extensively on mosquitoes when they are in planters
of stagnant water, but not when they are in moving waters
or their natural settings. They also eat fewer mosquitoes
in polluted water, probably because of the wider variety of
foods available.
So, it is not that guppies do not eat mosquito larvae –
they do – but it seems that they much prefer to eat other
things, and will only eat a lot of mosquitoes when other
foods are scarce. In other words, guppies may not be as
effective in controlling mosquitoes as we think.
So, why may introducing guppies to guppy-free areas
not be wise?
Chances are that the introduced guppies will be released
into the wild or escape, for example, through flooding, or
young guppies can hitch a ride elsewhere on birds or other
animals. Guppy entry to any site is highly likely to result
in the fish establishing a population that thrives in its new
home. Guppies have become established in at least 69
countries outside of its native range, and use for mosquito
control is implicated in about 60% of these cases. Escapees
Male Guppy, collected in Tobago waters.
PHOTO: RONNY LUNDKVIST
Dr Dawn Phillip is a Lecturer in the Department of Life Sciences, Faculty of Science and Technology, UWI St Augustine
can easily establish new populations because of the very
characteristics that make them attractive for pets, research,
andmosquito control – they are hardy little fishes that easily
adapt to new conditions, reproduce often, give birth to
young fish, and grow and start reproducing quickly.
Guppies can tolerate and quickly evolve in a wide
range of conditions. They are found in small ditches,
drains, ponds, streams and lakes, in clear or muddy, fresh
or brackish water, in temperate to tropical countries. They
can be found in polluted water, for example, the drains
along the Priority Bus Route, sewers and oil-contaminated
streams. A mated female can store sperm for months, and
use them to fertilise several batches of her eggs. Females
collected from the wild are almost always pregnant, and this,
plus their high adaptability, means that a single pregnant
female has more than an 80% chance of starting her own
successful population if introduced to a new environment,
producing her first batch of young about 28 days after the
eggs are fertilised.
Guppies alter the streams into which they are
introduced. Guppies introduced to previously guppy-free
parts of streams in T&T competed with resident species
for resources, eventually reducing the diversity of local
fishes to become the most abundant species. They changed
the biology of the remaining fish species, by altering their
reproduction, growth, survival, and hence density (that is
,
the number of individuals in an area).
In countries beyond their native range, the effects of
guppy introduction may be more pronounced. In Hawaii,
poeciliids
(guppies and their cousins), introduced since the
1920s, were found to be 10–30 times more numerous than
native fishes. Some native fishes have disappeared from
these areas. Guppies also increased the amount of benthic
biofilms (the slippery coating on rocks, plants and any solid
surfaces in water, that are composed of diverse and complex
community composed of algae, bacteria, fungi and other
microorganisms embedded in a complex organic matrix).
They also increased the abundance of benthic invertebrates
such as insect larvae and worms, but this was uneven across
species so that some actually decreased while others became
more abundant. At least one alien invasive insect became
more abundant in the presence of guppies.
Because each species affects its environment in different
ways, these changes at introduced sites translate into other
changes in the way these stream ecosystems function. One
important feature is the cycling of nutrients such as carbon,
nitrogen and phosphorus, as this helps ensure that these
important building blocks for organisms are available in
forms that they can use. Guppies have increased available
dissolved nitrogen by up to eight times compared to areas
without them, and total organic carbon by up to five times.
What does this mean? Consider the biofilm as an
example: biofilm communities play critical roles in aquatic
environments. They help cycle nutrients, and supplying
energy and organic matter to other stream organisms. They
are sensitive and respond to changes in nutrient availability,
generally increasing in thickness and extent in response to
increases in nutrients. In extreme cases, they can develop
those unpleasant green or grey-green mats commonly seen
in neighbourhood drains.
So what should be done to control mosquitoes? We
recommend against using guppies on a large scale, as
they may be ineffective, and have significant risks to local
ecosystems and their biodiversity. Instead, more effective
methods (for example, mosquito nets or window screens)
should be used. Where guppies are deployed, this should be
done inwell-controlled settings, and be carefullymonitored.
There is a lot of information on biological control that can
be used for guidance on best practice, but we strongly
recommend that research into the use of guppies to control
mosquitoes needs tomerge the medical, health and ecology
and evolutionary sciences.
CONTROLLING THE
BIO-CONTROL
B Y D AW N P H I L L I P
“So, it is not that guppies do not eat mosquito larvae – they do – but it
seems that they much prefer to eat other things, and will only eat a lot of
mosquitoes when other foods are scarce. In other words, guppies may not be
as effective in controlling mosquitoes as we think.”
