The Dirt on Fish: Effects of Anthropogenic Sedimentation on Fish Behavior and
Nutrition
Student: Joe Sapp
Mentor: Ellinor Michel
Introduction
Anthropogenic sedimentation has various detrimental effects in many freshwater ecosystems.
Sedimentation is most often caused by erosion following deforestation or development of pristine areas that
expose the soil surface to precipitation and surface runoff. Accumulation of eroded sediments in the
benthic zone of aquatic ecosystems has a variety of adverse effects on the fauna (Cohen et al. 1993,
McIntyre et al. unpublished manuscript).
Lake Tanganyika is an African rift lake renowned for its species diversity, especially that of the cichlid
fishes. Lake Victoria, another great African rift lake has witnessed an alarming decline in species diversity
in recent years and this decline has at least partially been attributed to increases in fine sediments. In
Victoria, increased turbidity has relaxed behavioral mating barriers thus reducing species diversity
(Seehausen et al. 1997). Sediments alter aquatic environments by absorbing or releasing nutrients or
toxins, eliminating refugia and habitat for many rock-dwelling organisms that depend upon a
heterogeneous substrate, and reducing the zone of algal growth via a reduction in light penetration in turbid
waters (Alin et al. 1999). Cohen et al. (1993) and Alin et al. (1999) have demonstrated the effects of
sedimentation on species diversity in Lake Tanganyika using faunal censuses, but much work remains to be
done towards understanding how sedimentation affects both species diversity and ecosystem functions in
the littoral zone.
Cichlid fishes are possibly the most studied organism in Lake Tanganyika, yet there is precious little known
about their physiological and behavioral responses to sedimentation. Herbivorous fish are dependent upon
autotroph productivity, which may be reduced in turbid waters. Furthermore, benthic herbivores may not
be able to avoid ingesting fine sediments on the substrate and may experience dramatic reductions in
available feeding substrate in sedimented environments. Because of this, one would expect benthic
herbivores to be the trophic group most dramatically affected by sedimentation among the cichlid species
flock. However, McIntyre et al. (unpublished manuscript) conducted fish censuses and found no
significant effects of sedimentation on the abundance of two common genera of herbivorous cichlids. These
preliminary observations raise the question: Are herbivorous cichlids affected by sedimentation?
To investigate this question, I conducted a series of field surveys and experiments on benthic herbivorous
fishes. Fecal organic content was analyzed for Tropheus brichardi and Petrochromis 'moshi' at two
sedimented and two reference sites in the littoral zone near Kigoma, Tanzania along the eastern shore of
Lake Tanganyika. Both species are benthic herbivores found in abundance in the littoral zone and each has
a distinctly different feeding style: T. brichardi hunts and pecks for algae on rocks whereas P. 'moshi'
scrapes a relatively large surface area with sandpaper-like teeth (personal observation). Additionally, I
collected data on feeding frequency, territorial behaviors, and ammonium ion excretion for T. brichardi at
all four sites. Lastly, I manipulated sediment mass on large rocks in a pilot experiment at one sedimentdisturbed and one reference site.
Materials and Methods
Site descriptions
There is a spatial pairing of my chosen sites, which are arbitrarily named. Jakobsen's Beach (S 04o54.64' E
029o35.92'), a reference site, is the southern-most site. Approximately 3 km to the north is Hilltop, which is
impacted with fine sediments due to recent development on a lakeside bluff. Approximately 7 km north
along the shoreline is the second reference site, Cobralangabo which lies across a small bay (approximately
1 km) from the second sedimented site, Kalalangabo (S 04*50.97 E 029*36.502'). Kalangabo has perhaps
been sedimented for longer, as it is located near a fishing village where cassava fields have been created on
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the steep surrounding slopes. The substrates of Kalangabo, Cobralangabo and Jakobsen's are composed of
quartzite boulders and cobbles, while Hilltop's substrate consists of a combination of quartzite boulders,
cobbles, and conglomerates from the Manyuvo Red Bed Formation (A. Rivers, Nyanza 2001).
Ammonium Excretion
Eight T. brichardi were caught by a snorkeler using a wall of fine netting that did not entangle the fish.
Fish were captured on five different days at the four sites. Immediately after capture, each fish was placed
in a ziplock bag containing 2 liters of freshly-filtered (Gelman A/E, 1.0 um pore size) lake water from the
same site. Bags were shaken vigorously prior to fish occupancy to ensure that there was enough oxygen for
the fish. After an incubation period of 30-120 minutes, three 20 mL water samples were taken from the
occupied ziplocks and filtered through a syringe-mounted glass-fiber filter (Gelman A/E) into HDPE
bottles. The samples were placed on ice until processing in the lab and the fish remained in the bags for
fecal analysis. Later that same evening, the samples of filtered water were analyzed for ammonium ion
concentrations with a fluorometer (Turner Designs Aquafluor). Along with the fish from each site, a blank
sample was run from a ziploc bag that was incubated for 120 minutes without a fish present.
Fecal Organic Content
Fecal samples were obtained for the eight T. brichardi caught for ammonium excretion. Additionally, eight
P. 'moshi' were caught using the same method. Fish were caught at the same time of day at each site for
each species to ensure similar intestinal contents. T. brichardi were caught in the field between 12:46 PM
and 1:28 PM (range= 36 minutes) and P. 'moshi' were caught between 2:04 PM and 3:30 PM (range=84
minutes). When sufficient fecal matter had been generated, it was sub-sampled from the bottom of the bags
and filtered on a Gelman A/E 47mm glass fiber filter that had been precombusted (450oC for three hours)
and preweighed. After filtration, filters were visually examined for non-fecal objects such as scales and
mucus and such objects were removed with forceps. Fish were euthanized in the field and preserved for
stable isotope and fatty acid analysis and to obtain additional data such as mass and length. Filters were
dried for 48 hours in a drying oven and weighed. They were then burned in a muffle oven at 450oC for
three hours and reweighed to determine organic content.
Behavioral Observations
At each site, I observed 15 T. brichardi using a snorkel, dive mask, timex watch, dive slate, and a
hodgepodge of wetsuits. Fish were observed at depths ranging from 1.5 to 4 meters. Observations were
conducted in the following way: When a fish was located, I did a brief preliminary observation to allow the
fish to adjust to my presence and to get an idea of the fish's range. I recorded approximate depth in meters
and approximate length of the fish in centimeters. For the first 2 minutes of observation period, I recorded
the amount of time the fish spent swimming as opposed to being stationary. After this period, I observed
the same fish for an additional five minutes in order to record the number of times it nibbled the substrate
(bites), the number and approximate size of fish chased away by the focal fish (aggressions), and the
number and approximate size of the fish that chased the focal fish (victimizations). Finally, swimming
time was recorded for the same fish over a second 2 minute period, making 9 total minutes of observation
for each fish. Ambient data such as cloud cover, wave height, and time of day were estimated or calculated
after the observation period for each fish.
Pilot Sediment Manipulation
I selected four rocks with approximately 0.25 m2 of upward-facing surface area at the Jakobsen's Beach and
Hilltop sites. Rocks were chosen haphazardly along the 4 meter depth contour of the littoral zone to
minimize wave energy and photographed with a digital Canon Powershot S330 within an underwater
housing. At Jakobsen's Beach, relatively clean rocks were selected and at Hilltop relatively sedimented
rocks were selected. A series of pre-manipulation observations were done in which the rocks were observed
via snorkel for ten minutes in turn and then ten more minutes. The species and number of bites was
recorded for each fish that visited the rock. If a fish did not bite the focal rock, it was not recorded.
After these pre-manipulation observations, scuba divers removed fine sediments from 2 of the rocks at
Hilltop with a turkey baster. Sediment was also collected at Hilltop via scuba and transported to Jakobsen's,
where it was applied to two of the four focal rocks. Rocks were photographed immediately after
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manipulation and again after a 2 day incubation period. Fish activity on all 8 rocks was recorded in the
same manner as prior to manipulations.
Data analysis
Excretion, fecal, and behavioral data was analyzed using Systat version 7.0. Ammonium excretion data
were standardized for fish mass and incubation time, arcsin-square root transformed, and analyzed using
nested ANOVAs with sedimentation and site nested within sedimentation as factors. A similar model was
used in the analysis of the arcsin-square root transformed organic content of fish fecal material, with
species identity (nested within sites) as an additional factor in the model.
Behavioral data were analyzed using nested MANOVA of the effects of sedimentation and
site(sedimentation) on behavioral traits (bites, aggressions, victimizations, and swimming time). Follow-up
univariate ANOVAs were performed for each behavioral trait.
The pilot sediment manipulation data was assessed qualitatively.
Results
Ammonium Excretion
There was a significant effect of incubation time (F2,26=20.637, P<0.001) and fish mass (F1,26=9.299,
P=0.005) on NH4 excretion by Tropheus brichardi. No effects of sedimentation (F1,26=0.482, P=0.494) or
site (F2,26= 0.145, P=0.866) were detectable (Fig. 1).
Fecal Organic Contents
There were significant differences between sedimented and reference sites (F1,56=39.314, P<0.001) in the
organic content of fish feces (Fig. 2). There were also significant differences between sites within sediment
categories (F2,56=5.434, P=0.007) and a significant species effect within sites (F4,56=8.520, P<0.001; Fig. 2).
Pairwise comparisons of sites revealed that the references sites (Jakobsen’s Beach and Cobralangabo) were
statistically indistinguishable, and both differed significantly from both sediment-disturbed sites.
Kalalangabo was significantly different from the other impacted site (Hilltop; P=0.009).
Behavioral Observations
Multivariate analysis revealed significant effects of sedimentation (Wilk’s λ=0.739, F4,53=4.672, P=0.003)
and site within sediment categories (Wilk’s λ=0.568, F8,106=4.330, P<0.001) on the suite of behavioral traits
recorded for Tropheus brichardi. Sediment-disturbed sites were characterized by significantly lower bite
rates and greater aggression and swimming time (Fig. 3-5). Differences between sites within sediment
categories were also significant or nearly so for all behavioral variables (Table 1).
Pilot Sediment Manipulation
Due to the preliminary nature of the pilot experiment and the small number of replicates, the data was not
analyzed statistically. However, it was clear from examination of the data that no clear pattern in fish
grazing frequency was present. Before and after pictures were examined and indicated relatively good
retention of sediments on manipulated rocks at Jakobsen’s Beach. However, at Hilltop, one of the two
cleaned rocks seemed to have been recovered in sediment, perhaps due to the rock’s location below and in
close proximity to a sandy fish nest.
Discussion
Ammonium Excretion
Because the data shows significant effects of fish mass and incubation time, it reaffirms the importance of
keeping these factors constant in this type of study. It is possible that the variance in fish mass and highly
variable incubation time (due to adverse field conditions when collecting samples) confounded any pattern
caused by the variables of interest: site and sedimentation. Future studies should strive to minimize the
variance in incubation times and fish size.
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Fecal Organic Content
This data reveals a clear pattern based on site and treatment. Sedimentation was associated with a
significantly decrease in the amount of organic material in fish feces. This pattern is especially clear in the
case of T. brichardi. The feeding styles of the two species would suggest that P. ‘moshi’ would be most
affected by changes in sedimentation because its grazing method is much less selective than that of T.
brichardi. Perhaps this grazing method results in greater intake of inorganic material by P. ‘moshi’ even at
references sites relative to more selective foragers. T. brichardi had significantly greater fecal organic
content than P. ‘moshi’ though the relationship was reversed at Kalangabo. We do not know what could be
responsible for the pattern at Kalangabo.
Behavioral Observations
T. brichardi at reference sites spent less time swimming and more time eating than conspecifics at impacted
sites. Fish in impacted sites were involved in more territorial interactions, especially those in which the
focal fish was the aggressor. There is no general effect of sedimentation on densities of Tropheus or
Petrochromis (McIntyre et al. unpublished), so this is probably not a result of increased fish densities, but
rather habitat quality. Sedimentation reduces the availability of adequate territory, leading to greater
competition for remaining available territories. Fish are able to persist at similar densities at sedimented
sites by altering their behavior. As the quality of territories goes down, it becomes more and more
worthwhile for a fish to expend energy defending its territory. This explanation is supported by the fecal
data as well, which reveals that there is less organic material in a fish’s digestive tract at impacted sites.
From this, it is safe to assume that grazing is less efficient at these sites. Thus, sedimentation would favor
more aggressive behaviors as territory becomes a more valuable resource. If a fish has enough to eat, it is
less likely to expend energy defending additional food resources.
This argument may be somewhat contradicted by the actual feeding observations. Fish at reference sites
were clearly shown to have a higher grazing rate then fish at impacted sites. If T. brichardi at impacted
sites were truly in suboptimal feeding habitats, one would expect that they have to graze at a higher rate to
obtain the same nutrients as fish at pristine sites. There are several explanations for the observed
retardation of feeding rate at impacted sites. First, the assumption that every bite by every fish consists of a
fairly constant volume of food may be faulty. Thus, it would be incorrect to calculate feeding rate based on
bites per unit time. Secondly, sediments may have a satiating effect on the fish. That is, fish at impacted
sites may ingest more mass per bite at impacted sites. A third explanation is that fish at sedimented sites
feed at a higher rate at a different time of day than the times at which the observations were done.
Observations were done at similar times at each site. Furthermore, these times were selected to minimize
wave action and resulting turbidity, both of which impede observations. However, as suggested by the
increased turbidity, waves may temporarily suspend fine sediments, thus unveiling a more palatable
substrate for T. brichardi.
The feeding rate data generally supports the hypothesis that there is a tradeoff between resource defense
and resource consumption. The time spent swimming data and fighting were greater at sediment-disturbed
sites than reference sites. Tropheus at references sites spent more time feeding than conspecifics at
sedimented sites, and their fecal organic content indicated that their food was of higher quality. The
implications of this sediment-induced shift in the relative investment in territoriality and feeding deserve
further study. We found no evidence that body condition (mass per unit length) was worse among fish at
disturbed sites, so apparently Tropheus can compensate for the observed reductions in feeding
opportunities and food quality under degraded conditions.
Pilot Sediment Manipulation
The only clear data to emerge from this experiment is how to design a better experiment in the future.
Rocks appeared to revert back to their original sedimentation states, and feeding changed more on some of
the control rocks than on manipulated rocks. My chief recommendation is to increase the number of rocks
studied. Perhaps rocks with more upward surface area would also help the situation. Lastly, rocks should
be chosen carefully to avoid ambient resedimentation or desedimentation. Such an experiment, if done
correctly, would provide powerful data about the immediate effects of sedimentation on fish grazing
behavior. However, in this case, the pilot did not warrant attempting a longer study due to time constraints.
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Conclusions
T. brichardi were more affected by sedimentation than P. moshi based on fecal data.. The data obtained in
this study raise more interesting questions than they answer. Specifically, a tradeoff between resource
defense and grazing is suggested. The effect of environmental conditions on these two behaviors has not
been previously documented, and it deserves further investigation. The dominant behavior in the trade off
would be dictated by resource (algae) availability and this resource may be governed by fine sediments.
Sediment manipulations are theoretically possible and would be invaluable in our search for answers on the
effects of sedimentation on fish. Future studies should examine fish diets with respect to sedimentation and
should strive to take into account factors that may have confounded this study such as fish mass, time of
day, and variable sampling techniques.
References
Alin, S.A. et al. 1999. Effects of Landscape Disturbance on Animal Communities in Lake Tanganyika, East Africa.” Conservation
Biology 13: 1017-1033.
Cohen, A.C. et al. 1993. The Impact of Sediment Pollution on Biodiversity in Lake Tanganyika. Conservation Biology 7: 667-677.
Rivers, A. 2001. The Effect of Sediment Deposition on Gastropod Fecal Content. The Nyanza Project Annual Report 2001: 63-66.
Table 1: ANOVA results from analysis of behavioral patterns.
Behavior
Bite rate
Swimming
Aggressions
Victimizations
Factor
Sedimentation
Site (Sedimentation)
Sedimentation
Site (Sedimentation)
Sedimentation
Site (Sedimentation)
Sedimentation
Site (Sedimentation)
df
1, 56
2, 56
1, 56
2, 56
1, 56
2, 56
1, 56
2, 56
F
14.160
3.095
5.407
8.334
12.922
3.125
0.000
3.382
P
0.000
0.053
0.024
0.001
0.001
0.052
1.000
0.041
Figure 1: Excretion rates by Site
0.45
Reference
Sedimented
0.4
NH4-N (ug N/ g fish / min)
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Jakobsen's
Cobralangabo
Hilltop
Kalalangabo
Site
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Figure 2: Average fecal ratio by site
Organic mass : total mass of feces
0.6
Tropheus
Reference
0.5
Petrochromis
Sedimented
0.4
0.3
0.2
0.1
0
Jakobsens
Cobralangabo
Hilltop
Kalangabo
Site
Figure 3: Territorial Interactions
4.5
Sedimented Agressions
Reference
4
Victimizations
3.5
# Events per 5 min
3
2.5
2
1.5
1
0.5
0
Jakobsen's
Cobralangabo
Hilltop
Kalalangabo
Site
Figure 4: Bite rate
200
Reference
Figure 5: Swimming Time
60
Sedimented
Reference
120
80
40
40
30
20
10
0
0
's
bsen
Jako
Sedimented
50
seconds swimming in 2 min
# Bites per 5 min
160
lan
Cobra
gabo
Hillto
p
lan
Kala
gabo
b
Jako
sen's
Cob
gabo
ralan
p
Hillto
lan
Kala
gabo
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