Vertical distribution and light effect on zooplankton density in relationship to fish
catch
Student: Mikhail Blikshteyn
Mentor: Pierre-Denis Plisnier
Introduction
Lake Tanganiyka is one of the world’s largest and deepest lakes with a high number of littoral endemic
species known worldwide by aquarium hobbyists. It also maintains a vitally important pelagic fishery,
composed mainly of a native centropomid species, Lates stappersi, and two sardines, Stolothrissa
tanganicae and Limnothrissa miodon (Coulter 1991). Of the two sardine species, S. tanganicae is by far
the most abundant species in the catch (pers. obs. in July-August 2000 near Kigoma), as only the largest L.
miodon adults venture into the pelagic to feed on smaller sardines. The pelagic fishing is done at night, with
a fishing unit consisting of four people in two boats that are joined together into a catamaran just before the
net is lowered. Very bright lanterns are suspended above the water to attract fish.
In this study, I’ve looked at the zooplankton variability and distribution at night near the fishing canoes,
and tried to correlate these findings to the total fish catch and catch composition. Zooplankton was
collected at the surface and from 100 to 0m at two locations: right from the fishermen’s boat and about
100m from it. The samples were compared to detect possible movement by zooplankton in response to
light put out by the fishermen.
In addition, total fish catch after the first haul and weight percentage of each contributing species were
determined and subsamples were taken from each species for length-frequency analysis. The data for fish
catch and composition are also a part of a longer-term monitoring program by the Nyanza Project to look at
the variability in economically important pelagic fisheries in the northern basin of Lake Tanganyika.
Materials and Methods
The fish data were collected during four night fishing trips, on the 24th , 27th , and 31st of July and 3rd of
August 2000, while the zooplankton data were collected on the first three dates. The R/V Echo towed two
fishing canoes, with two fishermen in each, to a site of their choice, between about 4 and 15 km from
Kigoma Harbor. At the fishing site, R/V Echo stayed usually no less than 50 to 75 m from the canoes so as
not to disturb fish and the fishermen.
All four nights, the fishermen followed the same procedure. As soon as the canoes were detached from R/V
Echo (at around 20:00 to 21:00), the fishermen secured their lit lanterns above water to begin attracting
fish. At around 23:00, the canoes were joined with long poles in the front and the back, forming a twocanoe catamaran. After that, the lift net was lowered to about 70 to 120 m, and left there for about an hour,
hour and a half. After midnight, with the lanterns continuously burning, the net was pulled back up.
Usually 3 or more hauls were done per night.
The sampling protocol for zooplankton was adapted and modified from Bosma et. al. (1997). Two
zooplankton samples were collected per fishing trip. Using a portable echosounder (Lowrance X65) on
board of R/V Echo, I determined the depth range of most zooplankton in the upper 200 m as well as the
depth of their densest aggregation near the surface. Zooplankton was observed on the LCD display as tiny
pecks, forming a very dense bend between 2 and 10 meters, observable as a thick line on the screen.
The zooplankton samples were collected with a 100 µm net and preserved in 10% formalin. The first
sample (sample A) was collected at midnight from R/V Echo, at the time when the fishermen were about to
raise their net. During that time, R/V Echo stayed about 75 to 100 m from the fishermen’s boat and thus the
sample was taken from water not lit by their lanterns. The depth with the thickest zooplankton aggregation
was determined with the echosounder and the A sample was collected by vertically towing the net from the
lower boundary of the aggregation region to the surface (which was either 6 to 0 or 12 to 0 m).
50000
Cyclopoid Adults
Number / m3
0-6m
Calanoid Adults
40000
Nauplii
0 - 12 m
30000
0-6m
20000
10000
0
24-Jul A
24-Jul B
27-Jul A
27-Jul B
31-Jul A
31-Jul B
Figure 1: Variation of the abundance of cyclopoid and calanoid copepods and their nauplii over a
one week period and with distance from the fishermen's boat. The "A" stations were located
about 100 m from the light of the fishermen's lanterns, while samples from the "B" stations were
taken right from the fishermen's canoe.
140
Number / m3
Shrimp
120
Medusae
100
0-6m
0 - 12 m
Fish Larvae
80
60
0-6m
40
20
0
24-Jul A
24-Jul B
27-Jul A
27-Jul B
31-Jul A
31-Jul B
Figure 2: Variation of the abundance of shrimp, medusae and fish larvae over a one week period
and with distance from the fishermen's boat. The "A" stations were located about 100 m from the
light of the fishermen's lanterns, while samples from the "B" stations were taken right from the
fishermen's canoe.
20000
Number / m3
Calanoid Adults
Cyclopoid Adults
Nauplii
16000
12000
8000
4000
0
24-Jul A 24-Jul B
27-Jul A 27-Jul B
31-Jul A 31-Jul B
Figure 3: Variation of the abundance of cyclopoid and calanoid copepods and their nauplii over a
one week period. The "A" stations were located about 100 m from the light of the fishermen's
lanterns, while samples from the "B" stations were taken right from the fishermen's canoe. The
water depth sampled was 100 to 0 m. Modified from Frank Basima's data.
The second zooplankton sample (Sample B) was taken directly from the fishermen’s canoe after the net
was brought up. For comparison, the net was towed through the same depth range as for sample A.
However, the second sample was collected from the water lit by the fishermen’s lanterns.
Also, two zooplankton samples were taken at approximately the same time but from 100 to 0 m using the
same procedure, i.e., one from R/V Echo (Sample A) and one from the fishermen’s canoe (Sample B).
Exactly the same gear was used and the same preservation method. However, they were collected and
enumerated by another researcher coming on these trips, Frank Basima (in this document).
A large bucket was brought on the fishermen’s boat to collect their fish catch. If the catch was less than a
bucket-full, all the fish caught during the first haul were taken back to R/V Echo. If more fish were caught
than could fit in the bucket, a subsample was taken and total weight estimated through extrapolation.
All the fish brought back to R/V Echo were weighed together. If the bucket was less than approximately ¼
full, all fish were sorted by species into separate buckets and total weight contribution of each species was
determined. If there were more fish than we could sort, a subsample was taken and sorted as above. Fork
lengths were measured of between 30 and 60 sorted fish of each species and used for length-frequency
analysis.
Results
There was a remarkable change in fish catch composition over a two-week period. The total catch weight
increased steadily over the last 3 sampling dates by at least one order of magnitude (Fig 5), while there was
almost no change between 24 and 27 July. The weight of the catch increased from about 1 kg on the first
two trips to ~ 3 kg on 31 July to over 200 kg on 2 August.
Limnothrissa was the least abundant species caught and was completely absent during the last two fishing
nights. On 24 July, the catch was composed of Stolothrissa, with about a 30% contribution, by weight, by
juvenile Lates stappersi. Juvenile L. stappersi are refereed to as those less than 10 to 15 cm in length as it’s
at that point that they switch from zooplanktivorous to their adult piscivorous diet. On 27 July, most of the
fish caught were juvenile L. stappersi, while on 31 July, the catch was dominated, by weight, by adult
Lates, with Stolothrissa making up only 10% of the total catch. On the last date, 2 August, mostly juvenile
Lates were caught, with about 10% adults and a small proportion of Stolothrissa (Fig 6).
With regard to the zooplankton, there was almost a 2- to 3-fold difference in the mean number of cyclopoid
and calanoid adult copepods and their nauplii between samples taken at the surface and the 0 – 100 m
samples. In the greatest aggregation near the surface, the density of cyclopoid copepods always exceeded
that of calanoids (Fig 1). The mean density of the nauplii was the same or even greater than the density of
cyclopoid copepods.
In addition, temporal variation over the one-week period in mean number of copepods and their nauplii per
m3 was observed. The density of zooplankton peaked on 27 July in both the surface and 0 – 100 m samples
(Figs 1 - 4).
The density of shrimp, medusae, and fish larvae were comparable between the surface and the 0-100 m
samples. As with the copepods, there was a peak in density on 27 July in both depth ranges. In general,
while in the surface samples higher density of shrimp was observed ~ 100 m away from the fishermen’s
canoes, in the 0 – 100 m samples, the highest density was right under the canoe. However, the density of
medusae was approximately constant between the samples taken from R/V Echo and the fishermen’s boat.
140
976
Number / m3
Shrimp
Medusae
Fish Larvae
120
100
80
60
40
20
0
24-Jul A
24-Jul B
27-Jul A
27-Jul B
31-Jul A
31-Jul B
Figure 4: Variation of the abundance of shrimp, medusae, and fish larvae over a one week period.
The "A" stations were located about 100 m from the light of the fishermen's lanterns, while samples
from the "B" stations were taken right from the fishermen's canoe. The water depth sampled was
100 to 0 m. The bar corresponding to the abundance of shrimp in the 27-Jul B Station is off the
scale and the number above it shows its true value.Modified from Frank Basima's data.
1.1 kg
1.3 kg
13.3 kg
201.4 kg
Percent weight contribution of
each species to the total fish catch
100%
Limnothrissa
Stolothrissa
L. stappersi, adults
L. stappersi, juvenile
80%
60%
40%
20%
0%
24-Jul
27-Jul
31-Jul
2-Aug
Figure 5: Percentages of Limnothrissa, Stolothrissa, and adult and juvenile Lates stappersi , by
weight, in the catch of pelagic night fishermen over a period of two weeks. Numbers above the
bars show total fish catch weights.
18
Stolothrissa
9
6
15
10
5
3
0
< 6.5
24-Jul
(40)
20
12
# Fish
# Fish
15
Lates
25
24-Jul
(35)
0
7.0
A
7.5
8.0
Fish Length (cm)
8.5
<8
10
E
18
27-Jul
25
15
(28)
20
12
20
30
Fish Lengths (cm)
40
50
27-Jul
(30)
15
9
10
6
5
3
0
B < 6.5
0
7.0
7.5
8.0
8.5
<8
F
18
31-Jul
25
15
(60)
20
12
10
20
30
40
50
31-Jul
(32)
15
9
10
6
5
3
0
C < 6.4
0
< 6.5
7.0
7.5
18
8.0
02-Aug
10
20
30
40
25
(35)
15
<8
G
02-Aug
(35)
20
12
50
15
9
10
6
5
3
D
0
< 6.5
0
7
7.5
8
8.5
H
<8
10
20
30
40
50
Figure 6: Variations of length frequency distribution of Stolothrissa (A - D) and Lates stappersi (E H) over a two-week period, caught during night-time fishing in the pelagic. Numbers in parenthesis
show the number of fish in sample.
Discussion
The most interesting observation from the fish catches was the wave of the dominant species, i.e., the one
making the largest contribution, by weight, to the total fish catch. There was a steady increase in the
proportion of juvenile Lates stappersi from 24th to the 27th July, with a peak between the 27th July and 2nd
August (Fig 5). Concurrently, the weight contribution of Stolothrissa declined through the sampling
period. Although the data collected are anecdotal for such extrapolations, with further sampling it might be
possible to see a cycle in the major species caught at successive fishing nights. One possibility is a peak in
Stolothrissa, followed by juvenile L. stappersi, then their adults, then back to the juveniles, with a cycle
starting all other with another peak in Stolothrissa catches.
Comparing the surface with the 0 – 100 m samples taken just from the R/V Echo (Samples A), there is a
much higher density of cyclopoid and calanoid adults and their nauplii across three sampling dates (24 July
to 31 July) near the surface than from the 100-m water column (Figs. 1 & 2). These samples were taken at
the natural lack of light for that time of night and thus represent diel vertical migration of zooplankton to
the surface to feed when there are not many visual predators around. However, there is a homogeneous
distribution in the density of shrimp and medusae from Sample A taken at the surface and from the whole
water column, implying that, at least for these dates, there was no vertical migration by them to the surface
(Figs 3 and 4).
However, all the zooplankton samples taken from the fishermen’s canoe on the 24th and the 27th of July had
a similar density at the surface as well as from the 100 m water column. This strongly suggests that the
light from the fishermen’s lanterns made the zooplankton descend to deeper waters, probably to escape
predation by fish aggregating near the canoes. This brings up an interesting question: whether the fish are
attracted directly to light put out by the fishermen or by their the ability to better see the zooplankton that
aggregate around the lights.
Conclusions
This project has provided some very interesting insights into the pelagic fish – zooplankton interactions of
Lake Tanganyika. However, it has also set-up a solid base for further investigation. More repeated
sampling of commercially important pelagic fish, using the same protocol, is needed to clearly identify the
possible cycle of predominant fish in the catch, suggested in this paper. Being able to predict the most
abundant fish species that would be caught during a specific time will allow a much better management of
the fisheries.
Also, more field and lab work is needed to determine the patterns of diel vertical migration of shrimp and
medusae. As was found in this project, they exhibited a peculiar lack of movement to the surface at night.
However, because the data used for this analysis are anecdotal, much more sampling is required, again
following the same sampling protocol and with sufficient repetition.
References
Bosma, E.M., Kalangali, A., Muhoza, S., Kaoma, K., Zulu, I. 1997. Results of zooplankton sampling from July 1995 to July 1996 and
comparison with the results from July 1993 to June 1995. FAO/FINNIDA Research for the Management of the Fisheries of Lake
Tanganyika. GCP/RAF/271/FIN-TD/64 (En): 33p.
Coulter, G.W. (ed.). 1991. Lake Tanganyika and its Life. Oxford University Press. London, Oxford, and New York.