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Title: Biomanipulation of fishes for midge larvae control in Singapore’s reservoirs
Authors: Jeffrey Kwik1, Rayson Lim1, Jia Huan Liew1, E-wen Low2, Joy Lu2, Darren
C.J. Yeo1 and Cheng Hua Sim2
Presenting Author: Michelle Cheng Hua Sim
Keywords: chironomids, fish biomanipulation, food-web, midge larvae control, trophic
structure
(1 Department of Biological Sciences, National University of Singapore, 14 Science Drive 4,
Singapore 117543;
2
PUB, Singapore’s national water agency, 40 Scotts Road, Environment Building, #15-01,
Singapore 228231)
Introduction (185 words)
Non-biting midges, also known as chironomids (family: chironomidae), are small flying insects
with aquatic larvae that commonly exist in and near water bodies. They are highly adaptable
insects, with over 5,000 species recorded worldwide. Fifty-seven species of chironomids were
recorded in 3 freshwater reservoirs that we studied. The life stages of chironomids include: (i)
terrestrial adult midges that lay eggs in water bodies (especially on emergent structures in the
water body); (ii) aquatic larvae that burrow into the sediment where they undergo larval
development; and (iii) free-swimming pupae that occupy the water surface and eventually
emerge as flying adults. Occasionally, these flying adults can emerge in large numbers, causing
a nuisance to people residing around such water bodies. To address this issue, we test
biomanipulation at the freshwater fish community level as a sustainable ecological control of
midge populations in the long run. This was done by conducting small-scale (spatially) cage
exclosure experiments in several reservoirs to better understand the effects of excluding fish
species of different sizes from community-level trophic interactions (i.e., food webs).
Specifically, the impact on benthic chironomid larvae populations was analysed.
Materials and Methods (218 words)
In-situ exclusion experiments were conducted to test the effect of excluding fishes of various
trophic guilds from a controlled system (Mitchell et al., 2000; Dugdale et al., 2006).
Exclosures (2 m × 2 m × 2 m) that consisted of three replicates of four treatments (Table 1)
were deployed at 1-1.5 m depths in four reservoirs. These treatments aimed at excluding apex
predators (top of the food web) and meso predators (mid-tier of the food web).
Table 1. Control and treatment setups (three replicates per treatment)
Treatment 1
Treatment 2
Treatment 3
Treatments Control
No mesh
100 x 100 mm
50 x 50 mm
1 x 1 mm
Mesh size
Status quo
Excludes apex
Excludes mesoExcludes snails,
Effects
predators
and apex
meso- and apex
predators
predators
Within each exclosure, potential differences at the base of the food web were measured by
analysing benthic community diversity and composition by deploying artificial invertebrate
colonisers monthly over three months. The effects of various treatments on benthic
communities were investigated by fitting linear mixed effects models to the data, using a nested
design to account for temporal fluctuations. Calculated evidence ratios based on modelled
Akaike Information criterion (AICc) values (Burnham and Anderson 2002) with a value greater
than 1 show a greater likelihood that the exclosure treatments have an observable effect.
Results and Discussion (200 words)
All treatments had an effect on chironomid larvae abundances (evidence ratio > 1). However,
the effects of excluding predators on chironomid larvae populations could be classified into
two types (Figure 1). Reservoirs 1 and 4 had higher mean chironomid larvae abundance in
treatment groups compared to the control, with chironomid larvae abundance increasing with
decreasing mesh size treatments. An opposite trend, with the exception of treatment 3, was
observed for Reservoir 2 and 3 where chironomid larvae abundance fell with decreasing mesh
size. For treatment 3, chironomid larvae abundance was the highest across all treatments for all
reservoirs because treatment 3 excluded small insectivorous fishes and predatory-invertebrates,
thus completely alleviating predation pressure on chironomids.
The likely reasons for the different results observed for treatments 1 and 2 between the two
reservoir groups are as follows:
i)
Reservoirs 1 and 4: progressive size-exclusion of benthic, insectivorous fish
(which occur across all size classes e.g. 50-220mm) might have reduced the direct
predation pressure on chironomids.
ii)
Reservoirs 2 and 3: exclusion of piscivorous fishes and/or predators of predatoryinvertebrates, resulting in the increased predation pressure on chironomids by
small fishes such as gobies or predatory-invertebrates such as nymphs of
Odonates (size 5-50 mm).
Figure 1. Two different impacts of piscivorous and insectivorous fishes on chironomid larvae
population size.
Conclusions (113 words)
The exclusion experiments appear to work differently in different reservoirs (owing to inherent
distinctiveness of aquatic communities in each reservoir). Hence, while the results reflect that
biomanipulation is a potentially viable approach for chironomid larvae control, for effective
biomanipulation, the exact (trophic level) for manipulation/regulation should be adjusted to
match the trophic levels (or taxa types) found in each individual reservoir, depending on the
community structure, which is unique for each reservoir.
With these results, NUS and PUB will be able to conduct more trials to determine the extent
of predatory fish removal or insectivorous fish introduction in various reservoirs to control
chironomid larvae in reservoirs in which midge emergence is a concern.
References
Burnham, K. P. and D. R. Anderson. 2002. Model Selection and Multimodel Inference: A
Practical Information-Theoretic Approach.
Dugdale, T. M., Hicks, B. J., De Winton, M., and A. Taumoepeau. 2006. Fish exclosures versus
/intensive fishing to restore charophytes in a shallow New Zealand lake. Aquatic Conservation:
Marine and Freshwater Ecosystems 16(2): 193–202.
Mitchell, J. S., Bailey, R. C., and R. W. Knapton. 2000. Effects of predation by fish and
wintering ducks on dreissenid mussels at Nanticoke, Lake Erie. Ecoscience 7(4): 398-409.