Behav. Ecol. Sociobiol. 4, 357-368 (1979) Behavioral Ecology and Sociobiology �9 by Springer-Verlag 1979

Notes on Ethology and Ecology of the Swashi River Mormyrids (Lake Kainji, Nigeria)* **

Peter Moller 1, Jacques Serrier z, Pierre Belbenoit 2, and Stephen Push 3

1 Hunter College of the City University of New York and the American Museum of Natural History, New York, USA 2 Laboratoire de Physiologic Nerveuse, Dbpartement de Neurophysiologie Sensorielle, C.N.R.S. F-91190 Gif-sur-Yvette, France 3 City College of the City University of New York, New York, USA

Received December 28, 1977

Summary. 1. A survey of abundance, distribution, and electric organ dis- charge (EOD) activity of mormyrid fish as well as associated aquatic condi- tions was conducted in a selected study area, 8 km upstream in the Swashi River, a tributary of Lake Kainji, Nigeria (Fig. 1).

2. Mormyrid locomotor activity and EOD repetition rate exhibited a daily activity rhythm (Fig. 2) with changes in light intensity and possibly temperature (Fig. 3b) affecting the onset and offset of the fish's nocturnal activity period.

3. Eight sympatric mormyrid species were collected: Petrocephalus bovei, Marcusenius cyprinoides, M. senegalensis, Mormyrops deliciosus, Mormyrus rume, M. hasselquisti, Hyperopisus bebe, and Pollimyrus isidori (Table 1).

4. The distribution of mormyrids with regard to pulse type, time of day, and microhabitats (inlet, opening of inlet, and river) was analysed. During the night, mormyrids were most abundant at the inlet's opening, while during their (daytime) inactive period, most EODs were monitored inside the inlet (Fig. 5: T). At the inlet's opening, a temporal separation of species (related to pulse duration, Fig. 5 :$1 and $2) suggested the exis- tence of a temporal resource partitioning mechanism.

Introduction

The African freshwater mormyr i form fishes and the neotropical gymnotoids produce weak electric organ discharges (EODs) which are part of an electrosen-

* Supported by grants from the National Geographic Society, the City University of New York (PSC-BHE 11478E), and the Explorers Club to P.M.; the C.N.R.S. Recherche Coop6rative sur Programme No. 080350 to P.B. and J.S. Facilities and assistance were provided by the Kainji Lake Research Institute, New Bussa, Nigeria ** This paper was presented at the Symposium 'Electroreception and Behaviour' organized by Dr. T. Szabo in Gif-sur-Yvette, France

0340-5443/79/0004/0357/$02.40

358 P. Moller et al.

sory system involved in electrolocation and e lec t rocommunica t ion (cf. reviews by Scheich and Bullock, 1974; Heil igenberg, 1977).

Most of the behavioura l data on weakly electric fish were ob ta ined from labora tory studies. Field invest igat ions of these fish and their behaviour are scarce. Ethological data on gymnoto id fish were provided by L i s smann (1961), L i s smann and Schwassmann (1965), Steinbach (1970), and Hopk ins (1974a, b, and c); field observat ions on mormyr i fo rms were made by L i s smann (1958), Heymer and Harder (1975), and Moller et al. (1976).

The a im of this invest igat ion was to conduct a field survey on mormyr id electric organ discharge behaviour as associated with species a b u n d a n c e and accompany ing ecological, aquat ic condit ions.

Materials and Methods

Study Site. The survey was conducted in the Swashi River, a tributary of Lake Kainji, a man-made lake in Northwest Nigeria. The effects of the formation of the lake on the indigenous fish populations have been carefully monitored (Imevbore and Bakare, 1970; Henderson, 1973; Lelek, 1973; Lewis, 1974; Blake, 1977). We selected a study area in the lower Swashi River (Fig. la) approx. 8 km upstream, consisting of a cross-section of the Swashi River (approx. 150 m wide) and an inlet extending 80 m perpendicularly in a southerly direction from the river.

Recording Procedures. Twelve pairs of detecting electrodes were distributed over the study area with four pairs across the Swashi River, four pairs close to the 1eft and right banks of the inlet's opening to the Swashi, and four pairs of electrodes (average depth: 2 m) inside the inlet (Fig. 1 b). The interelectrode distance of each pair was 45 cm. The EODs from electric fish were pre-amplified, monitored with a loudspeaker and a portable oscilloscope (Tektronix 455), and recorded on magnetic tape (portable stereo cassete recorder: Sony SD 152). Systematic data on mormyrid activity were collected over four consecutive days from 13 to i6 November 1976, covering four 24-h periods. Every hour, in a consecutive order, a 50-s recording sample was taken from each of the 12 pairs of electrodes (10-rain total per hour).

Mormyrid activity, including individual EODs (without pre-amplification) were also pho- tographed directly from the screen of the oscilloscope. Ecological data on aquatic conditions (conductivity, temperature, pH, turbidity, dissolved 02, and light intensities at various depths) were obtained with portable, battery-powered equipment.

Analysis. The tape-recorded data were analysed by (a) determining the presence (passages) of mormy- rid EOD activity (regardless of the number of fish passing together through the detection range of the electrodes), (b) identifying the EOD repetition rate (EOD frequency) from inspection of recordings of single fish, and (c) estimating the approximate shape and duration of single EODs.

A comparison between mormyrid activity during the day and the night was made using standard statistical procedures (t-test,)~2 test).

Results

L Daily Activity Rhythm

1. Locomotor Activity. M o r m y r i d fish cons tant ly emit EODs with variable repeti- t ion rate (EOD frequency). Thus, the detect ion of EODs f rom fish passing through the detect ion range of a pair of electrodes can be used as cri terion for mormyr id locomotor behaviour . The total n u m b e r of mormyr id passages from all 12 pairs of electrodes (for a consecutive four-day sampling period) is shown as a funct ion of the t ime of day in Fig. 2a. The average hour ly

Ethology and Ecology of Morrayrid Fish 359

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1 ~ 12 8o

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Fig. 1. a Electric organ discharges were monitored from a study area (thick arrow) in the Swashi River, a tributary of the man-made Lake Kainji, Nigeria. b Twelve pairs of recording electrodes were distributed in an inlet (electrodes: 8, 9, 10 and 11) at the right and left banks of the inlet's opening (1, 2, 7 and 12) and in the river (3, 4, 5, and 6). R denotes site of a daytime 'resident' mormyrid fish. Small numbers adjacent to $ indicate water depth. Shaded area represents patches of the grass Echinoehloa sp. The location of dead trees in the Swashi shows the original flow of the river before impoundment of the River Niger

rate o f passages f rom 0800 h th rough 1700 h (3.5) was significantly smaller (t-test; P < 0 . 0 0 1 ) than that of passages obtained f rom 1800 h th rough 0700 h (12.4).

A n analysis o f the fish's locomotor activity with regard to local habi tat differences within the study area (cf. Fig. 1: inlet, electrodes No. 8, 9, 10, and 11; opening of the inlet, electrodes No. 1, 2, 7, and 12; and river, electrodes No. 3, 4, 5, and 6) pointed to local daily migrat ions o f mormyr ids into and out of the inlet. Dur ing the day, f rom 0700 h to 1800 h, mos t of the fish were

360 P. Moller et al.

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Fig. 2. a Locomotor activity. Length of each bar indicates the number of mormyrid passages through the detection range of 12 pairs of electrodes during four 10-min sampling periods recorded at four consecutive days. The daytime passage rate is significantly smaller (P < 0.001) than the night-time one. b EOD frequency (repetition rate). Each dot represents a mean EOD frequency value (except for 0700 h, 0800 h, and 0500 h: only one value obtained) determined from tape recordings of single fish. Bars indicate corresponding standard deviations. The compound mean discharge fre- quency as determined between 0700h and 1600h is significantly lower (P<0.05) than that determined between 1700 h and 0600 h. Note: EOD frequency increased about 1 h prior to locomotor activity

found inside the inlet (37 passages); 12 passages were moni tored at the inlet 's opening and two in the river. Dur ing the night, f rom 1900 h th rough 0600 h, mormyr ids were most often recorded f rom the inlet's opening (42 passages), fol lowed by 31 passages inside the inlet. Only five passages were moni tored f rom the river electrodes.

The statistically significant reversal (X z test; P < 0 . 0 1 ) of the distr ibution ratio o f fish (opening of inlet vs. inside inlet) suggests that during their inactive period during the day, the fish stay inside the inlet, while at night, during their active period, a large number swim out o f the inlet to feed along the opening.

2. E O D Frequency. The E O D frequency o f mormyr id fish is subject to changes in light intensity as observed under labora tory condit ions (e.g., Harder et al., 1964; K e m m e r et al., 1970; Moller, 1970). To investigate such changes under natural conditions, we determined the E O D frequencies o f single fish of un- identified species passing th rough the detection range o f the electrodes. Figure 2 b illustrates the hour ly means and s tandard deviations f rom a total o f 123 record- ings. The c o m p o u n d mean discharge rate for the samples taken f rom 0700 h

Ethology and Ecology of Mormyrid Fish 361

through 1600 h (15.4+8.2 Hz; n=34) is significantly different (t-test; P<0.05) from that for the samples taken from 1700 h through 0600 h (26.6_+ 12.7 Hz; n = 89). Similar to the locomotor activity (Fig. 2a), the EOD frequency of mor- myrids exhibits a daily rhythm. The decrease in the EOD frequency is correlated with a change in light intensity, i.e. sunrise at about 0620 h; the EOD frequency increase, however, occurs at least 1 h prior to sunset.

IL Aquatic Conditions

To assess the role of environmental variables such as light, temperature, and conductivity as possible 'zeitgebers' for the observed daily activity rhythms, we monitored daily changes, both in the river and in a representative location in the Swashi inlet. The average morning (0655 h to 0745 h) and midday readings (1230 h to 1320 tl) for temperature and conductivity in the river (in the vicinity of electrode No. 5, cf. Fig. 1 b) are shown in Fig. 3 a.

Maximum daily temperature variations were recorded at the surface (At= 4.1 ~ C; range 26.2 to 30.3 ~ C). Below 2.5 m the daily temperature difference was at maximum 0.3 ~ C. Temperatures remained constant between 2.5 and 6 m at 28.1-27.9 ~ C, followed by a thermocline in the original riverbed down to the bottom at 12 m (26.5 ~ C) depth.

The morning and midday conductivity measurements did not differ signifi- cantly from each other but depended on the water depth. From the surface to about 6 m depth, i.e. in the layer of inundation, the conductivity increased from 65 to 75 gS.cm-1 (formerly gmho.cm- t ) . In the original riverbed the conductivity stayed constant close to 90 ~tS. cm- 1 down to 11 m with the bottom sediments affecting a further conductivity increase to up to 110 ~tS.cm -1 at 12 m depth.

The daily readings for dissolved oxygen ranged from 3.7 ppm (surface) to 1 ppm (at 4 m depth); the turbidity values were from 20 to 40 FTU (formazin turbidity units); and the pH varied between 6.2 and 6.9. TotM hardness was determined as 26 ppm (Ca ++ : 15.8 ppm).

Daily changes in light intensity were monitored in the inlet (Fig. 3b) at 0.5 m depth, at a site marked ' R ' in Fig. 1 b. Rapid changes in light intensity were observed between 0600 h and 0700 h, and 1800 h and 1900 h, respectively. Between 0530 h and 1830 h, concurrent water temperature and conductivity readings were taken (Fig. 3b). The temperature gradually increased from a morning (0630 h) low at 27.2 ~ C to 30 ~ C at 1800 h. The conductivity ranged from 65 to 70 laS.cm -1

IlL Species Abundance

After completion of the mormyrid EOD survey in the Swashi inlet, the mormyrid population was sampled with gill-nets (17 and 19 November), and with rotenone, a respiratory blocking agent (23/24 November 1976). The specimens caught were determined according to Reed et al. (1967), and Taverne (1971). We found

362 P. Moller et al.

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Fig. 3. a Temperature (circles) and conductivity (triangles) data from the Swashi River. Morning (black symbols) and midday readings (open symbols) indicate daily temperature variations of At= 4.1 ~ C at the surface and At=0.3 ~ C below 2.5 m; no daily variations in conductivity were recorded; however, conductivity increased with water depth, b Changes in light intensity (circles, dotted line), temperature (triangles), and conductivity (squares) at 0.5 m depth at a representative recording site in the Swashi inlet

eight sympatr ic m o r m y r i d species coexisting with two other electric fish, Gymnar- chus niloticus and the electric catfish Malapterurus electricus (Belbenoit et al., 1979). As shown in Table 1, catches obta ined by poisoning the inlet were much more representat ive of the mormyr id popu la t ion than the gill-net catches. The gill-net catches suggest, however, that Marcusenius cyprinoides is underrep-

Ethology and Ecology of Mormyrid Fish 363

Table 1. Sympatric mormyr id species sampled in the Swashi inlet (17, 19 and 23, 24 November 1976)

Species" Number of individuals Standard length (mm) caught with

Gill-nets Rotenone Range Median M a x i m u m values (Durand, 1970)

Petrocephalus bovei - 24 41-97 73 95 Marcusenius cyprinoides 18 3 119-181 166 330 Mormyrops deliciosus - 18 66-222 166 890 Pollimyrus isidori - 17 38-71 61 90 Morrnyrus fume 2 8 90-507 125 870 Marcusenius senegalensis - 10 78-119 108 208 Hyperopisus bebe 1 5 147-333 198 508 Mormyrus hasselquisti - 4 145-170 155 480

a Determined according to Reed et al. (1967), and Taverne (1971)

resented in the rotenone estimate. Standard lengths (ranges and median values) in Table 1 give an estimate of the population structure and are compared with results by Durand (1970). The population sampled was composed of adults of Petrocephalus bovei and Pollimyrus isidori and young specimens of Marcusenius cyprinoides, M. senegalensis, Mormyrops deliciosus, Mormyrus rume, M. hassel- quisti, and Hyperopisus bebe. Large size variations in three of these species were due to the presence of several very young specimens of M. deliciosus as well as a few adult individuals of M. rume and 11. bebe.

IV. Temporal and Spatial Distribution of EOD Types

Several characteristics (such as time course and wave-form) of mormyrid EODs are genus and/or species specific. The duration and wave-form of a single EOD, though stable under constant recording conditions, vary within limits as a func- tion of the fish's position and distance from the recording electrodes. To estimate the possible origin (species or group of species) of the tape-recorded EODs, we determined the range of the duration of single EODs from specimens of a known species. The duration of an EOD, emitted by a fish kept in a gauze cage at 1 m depth in its natural habitat, was determined with the aid of an oscilloscope by monitoring directly from the fish (Fig. 4), and from a replay of the tape. From the four-day sample we selected at random 105 EODs which were recorded in the three microhabitats: inlet, opening, and river. Based on our 'reference ranges' and the data on species abundance, we distinguished two short pulse types $1 (P. isidori/P, bovei) and Sz (M. cyprinoides/H, bebe and possibly M. senegalensis/M, rume). Longer lasting pulses (> 1.5 ms) could have been emitted by M. hasselquisti, M. deliciosus, and also M. rume (L). While the distinction of the $1 type from the S 2 type was unambiguous, there could have been overlap between the $2 and L type species.

364 P. Moller et al.

Pollimyrus isidori Marcusenius cyprinoides

Hyperopisus bebe Mormyrus rume Fig. 4, Electric organ discharges from four Swashi River mormyrids photographed in the field from the screen of a portable oscilloscope. Specimens were kept in a gauze cage at 1 m depth in their natural habitat (conductivity: 65 gS.cm 1). White bars indicate 0.1 ms. (The EOD for P. isidori was redrawn from several photographs)

During the night, mormyrids were most active at the inlet's opening (cf. I/l). The occurrence of different pulse types at different times of the night suggested a further analysis of the fish's abundance with respect to pulse type, time of day, and microhabitat. The temporal and spatial distribution of EODs (total: 105 ; $1:29; $2:42) and their respective theoretical 95% limits of confi- dence (obtained from binomial distribution tables) were compared with an assumed hypothetical homogeneous distribution in all three microhabitats (33.3% level).

When all EODs are pooled (upper graph in Fig. 5) the reversal of activity from the inlet during the day to its opening during the night becomes obvious. The daytime abundance in the inlet significantly exceeds the 33.3% level (P<0.01). The abundance of mormyrids in the inlet during the night and at its opening during the day does not differ from the 33.3% homogeneous distribu- tion level. During the night, at the opening, we observed an increase of EOD activity during the first 6 h after sunset, exceeding the 33.3% level. The number of EODs detected at the river electrodes is significantly smaller (0700 h 1 2 0 0 h: P<0 .01 ; 1300 h-2400 h: P<0.01) than the 33.3% level (with the exception at 0100 h-0600 h).

A comparison of the spatial and temporal distribution of the two most abundant EOD types $1 (P. bovei/P, isidori) and $2 (M. cyprinoides/H, bebe, etc.) shows that in the inlet during the day, $2 exceeds the 33.3% level while $1 does not (Fig. 5). During the night, $1 and $2 occur at the 33.3% distribution

Ethology and Ecology of Mormyrid Fish 365

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Fig. 5. Temporal and spatial distribution of EOD types. The temporal (four time periods: 0700 h - 1200 h, 1300 ~1800 h, 1900 h 2400 h, and 0100 h-0600 11) and spatial distributions (inlet, opening, and river) of EODs (short horizontal bars), (T, total n = 105; $1, P. isidori/P, bovei type: n=29 ; S2, M. cyprinoides/H, bebe type: n=42) and their respective theoretical 95% limits of confidence (vertical dashed lines) are compared with an assumed homogeneous distribution of 33.3% (horizontal dashed lines). Arrows indicate a significant deviation from this distribution (P values, cf. text). Asterisks: only one value available. With all EODs pooled (7), note the reversal from daytime abundance in the inlet to night-time abundance at the inlet's opening. A comparison of the night-time distribution of the S~ and $2 types at the opening illustrates a significant temporal separation of species related to their pulse type: $1 is more abundant during the second part of the night than $3, and vice versa, $2 is more abundant during the first part of the night than $1

level. While at the opening, during the day, the occurrence of St and S 2 again does not differ from the homogeneous distribution, an interesting difference emerges when the night-time distributions of $1 and $2 are compared. During the first part of the night (1900 h to 2400 h), St does not exceed the 33.3% level while Sz does (P< 0.01); conversely, during the second part of the night (0700 h to 0600 h), $2 does not exceed the homogeneous distribution level whereas $1 does (P< 0.05). The abundance of mormyrids at the inlet's opening during the night demonstrates a temporal separation of species (or groups of species) related to their pulse duration.

Regardless of the time of day, the occurrence of St and $2 types at the river electrodes is not significantly different from the 33.3% level, with the exception of a significant drop during 1900h to 2400h ( S t : P < 0 . 0 5 ; S2:P< 0.01).

366 P. Moller et al.

Discussion

The adaptive significance of nocturnal or diurnal activity patterns is, in most instances, well understood in terms of competition for limited resources, and/or maintaining reproductive isolation (Mayr, 1963). For the Swashi mormyrids, we have identified two daily activity cycles concerning their locomotor behaviour and EOD frequency. As has been demonstrated under laboratory conditions, changes in light intensity seem to be the most powerful exogenous zeitgeber (Harder et al,, 1964: Gnathonernus petersii; Lissmann and Schwassmann, 1965: G ymnorhamphichthys hypostornus ; Moller, 1970: [ Gnathonemus] Brienornyrus ni- ger). While most of the laboratory studies on EOD activity rhythms with light as the controlled independent variable show a direct and immediate effect of light changes on EOD frequency, field recordings from weak electric gymno- toids and mormyrids reflect an interaction of several factors. The times of local sunrise and sunset in the Swashi area were close to 0620 h and 1820 h with 15 min of twilight (Fig. 3 b). While at dawn, the decrease in morning EOD frequency was directly related with a light intensity increase, and the decline of general locomotor activity was slightly delayed by about half an hour, at dusk, with comparable low light intensities, the EOD frequency had risen since 1600 h and reached its maximum value (about 30 Hz) at least 1 h prior to sunset (1700 h). Maximum locomotor activity occurred 7 h after sunset at 0100 h.

Lissmann and Schwassmann (1965) recorded EODs and related locomotor activity from the South American nocturnal sand-fish Gymnorhamphichthys hy- postomus. The onset of the daily EOD frequency increase took place before the fish emerged from the sand and - similar to the Swashi mormyrids - before a noticeable decrease in light intensity occurred. The return to the sand and a low frequency rate - different from the mormyrids - took place well before sunrise. During our survey of the inlet, we detected, during daytime, several well spaced-out individual mormyrids (interindividual distance: 5 10 m) that were hiding under protective shelters such as rocks and submerged tree trunks. A direct correlation was found between such a 'resident 's ' daily migra- tory behaviour and changes in light intensity. Intensities of around 10 lux were correlated with the fish's return to the hiding place at dawn (0623 h) and its departure from it at dusk (1800 h). It seems likely that most of these site-attached fish leave the inlet at about 1800 h which would explain the sudden increase in mormyrid passages at 1800 h (cf. Fig. 2a). Hopkins (1974c) observed similar migratory behaviour in the gymnotid Sternopygus rnacrurus.

Aside from light, water temperature in up to 2.5 m depth showed a daily cycle with At > 0.3 ~ C. Laboratory studies on the effect of temperature on EOD frequency report Qlo values as high as 2.08, 2.46 for G. petersii, and 2.81 for G. moori (Gallon et al., 1967). With the water depth in the inlet not exceeding 2.8 m, the afternoon temperature increase (Fig. 3b) could explain the initial EOD frequency increase at 1600 h.

During the night, at the inlet's opening, we found a significant separation of mormyrids with respect to their EOD duration. While the P. isidori/P. bovei ($1) type was more active during the second part of the night, the M.

Ethology and Ecology of Mormyrid Fish 367

cyprinoides/H, bebe (82) type was more active during the first 6 h after sunset. Though additional data are needed, we suggest that the time-sharing strategy of the Swashi mormyrids may be part of a temporal resource partitioning mechanism which in turn could be related to the role of the EODs in electrocom- munication and electrolocation.

Acknowledgements. The research was carried out from the Kainji Lake Research Institute, New Bussa, Nigeria, and its field station at Shagunu. We are grateful to the Director, Mr. V.O. Sagua, and his staff, who generously provided facilities and invaluable assistance. Special thanks are to Mr. E.O. Ita and his team who helped us with many catches and never forgot us in the bush. S. Push acknowledges a stipend from Sigma Xi.

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