Environmental Factors Influencing the Zonation And

ELSEVIER Journal of Experimental Marine Biology and Ecology

190 (1995) 135-149

JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY

Environmental factors influencing the zonation and activity patterns of a population of Periophthalmus

sobrinus Eggert in a Kenyan mangrove

I. Colombini”‘*, R. Berti”, A. Ercolini”, A. Nocita”, L. Chelazzib

“Dipartimento di Biologia Animale e Genetica “Leo Pardi’: Universitri degli Studi di Firenze, Via Romana 17, 50125 Firenze, Italy

hCentro di Studio per la Faunistica ed Ecologia Tropicali de1 C.N.R., Via Ronana 17, 50125 Firenze,

Italy

Received 14 June 1994; revision received 25 January 1995; accepted 31 January 1995

Abstract

The activity rhythms and zonation of a population of Periophthalmus sobrinus were studied along the muddy mangrove banks of the Tana river delta (Kenya). A transect, perpendicular to the water channel, was subdivided into sections 1 m in width parallel to the river bank. The number of surface-active mudskippers and their zonation was recorded over several tidal cycles. Fishes found resting on their nests were also recorded. The observations were carried out over two periods of 48 consecutive tidal hours corresponding to a spring and a neap tide. Environmental variables such as tidal level, air temperature and relative humidity were also recorded. Periophthalmus sobrinus was active both diurnally and nocturnally and during spring tide all the observed mudskippers were active on the mud surface. At neap tide more than half of the total number of animals sighted were found resting on their nests. During this synodic phase, daytime sightings decreased drastically because the mudskippers took refuge inside the nests. Since P. sobrinus

normally tends to avoid water, its mean hourly zonation at spring tide was more variable than at neap tide. Differences in mean hourly zonation were also found between night and day of the spring tide. Activity patterns and zonation of the mudskippers were directly influenced by the synodic and tidal cycles and depended more on environmental factors such as air temperature and relative humidity than on the die1 light cycle.

Keywords: Activity pattern; Environmental factor; Mangrove system; Mudskippers; Periophthalmus sobrinus; Zonation

* Corresponding author.

0022-0981/95/$09.50 @ 1995 Elsevier Science B.V. All rights reserved SSDZ 0022-0981(95)00020-S

136 I. Colombini et al. I J. Exp. Mar. Biol. Ecol. 190 (1995) 13.5-149

1. Introduction

The biology of the small amphibious fishes known as mudskippers has received much attention and there are many papers concerned with their mor- phophysiological (Sponder & Lauder, 1981; Low et al., 1988; Yadav et al., 1990), physiological (Teal & Carey, 1967; Tamura et al., 1976; Hillman & Withers, 1987; Lee et al., 1987; Chew & Ip, 1990, Chew et al., 1990; Ip et al.,1990 a,b,c, 1991) and ecophysiological adaptations (Gordon et al., 1969, 1978, 1985; Bridges, 1993). Other works consider behavioural aspects of these fishes (Stebbins & Kalk, 1961; Brillet, 1969, 1975, 1976, 1980a,b, 1984, 1986; Sarker et al., 1980) or analyse their use of space and time (Ishibashi, 1973; Nishikawa & Ishibashi, 1975; Gibson, 1978, 1986, 1987, 1992; Hartnoll, 1987). More recently, an orientational capacity was demonstrated in some populations of Periophthafmus sobrinus Eggert 1935 (synonymous with P. argentilineatus according to Murdy, 1989) inhabiting a Kenyan mangrove system. The ability to orient in one direction is probably based on a solar compass mechanism (Berti et al., 1992, 1994).

During three expeditions to Kenya in 1990, 1991 and 1993 ecoethological researches were conducted on P. sobrinus whose cyclical aspects of feeding behaviour (Colombini et al., 1992) and use of space and time were studied. This work reports the results obtained from a population of this species living along the mangrove banks of the Tana river delta. The ways in which the mudskippers use and maintain position in their intertidal habitat, which is continuously subjected to periodic change on a diel, tidal and synodical basis, are analysed.

2. Study area

A population of mudskippers inhabiting a mangrove system of the Tana river delta was studied. The mangrove is located at Shekiko (02” 35’S: 40” 21’E) along the muddy banks of one of the many channels that have been cut off from the main watercourse of the delta. This mangrove is principally composed of Avicennia marina (Forsk.) Vierh. (Fig. la) and is about 50 m wide when measured perpendicular to the channel’s bank. Additionally, nearer the channel, there is a band of small plants of Avicennia marina about 4 m wide followed by another about 5 m in width, where only the aerial roots of the mangrove trees can be found. The substratum is composed exclusively of mud and it is slightly sloped towards the water banks where the inclination greatly increases. The mud contains an enormous quantity of both inhabited or abandoned burrows, principally belonging to the conspicuous population of decapods represented by Uca urvillei H. Milne Edwards, Uca inversa (Hoffmann), Uca lactea annulipes H. Milne Edwards, Sesarma meinerti De Man. and Sesarma smithii H. Milne Edwards. According to the tidal and synodic phase the mangrove is more or less invaded by water from the channel. An unvegetated area about 10 m in width

I. Colombini et al. I J. Exp. Mar. Biol. Ecol. 190 (1995) 135-149

a

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Fig. 1. (a) Profile of the mangrove transect on the Tana river delta. Mean hourly zonation of P. sobrinus during the night (b) and the day (c) of spring (0,O) and neap (0,m) tide. Confidence limits

(93%) are also shown. The dashed line refers to the tidal levels of spring tide, the dotted one to the

tidal levels of neap tide. In (b) and (c) the time of the nocturnal and diurnal HT, respectively

(astronomical time of Shekiko) is shown for spring and neap tides. The vertical axis is in tidal hours.

138 1. Colombini et al. i J. Exp. Mar. Biol. Ecol. 190 (1995) 135-149

occurs to the landward edge of the mangrove and is separated from the salt marsh by an erosion step about 0.50 m in height (Fig. la). The salt marsh is covered by Sporobolus sp. with a few bushes of Suaedu monoica Forssk. ex J.F. Gmel, Salvadora persica L. and small plants of Avicennia marina. The marsh is covered by water during the extreme high water of spring tide (EHWS) and during our study period this occurred once (Fig. la).

The transect considered is perpendicular to the channel’s bank and its length, starting from the erosion step, varies according to the tidal level: from a minimum of 0 m during EHWS to a maximum of about 90 m during the extreme low water of spring tide (ELWS). The maximum vertical range between EHWS and ELWS was 4 m. During neap tide the vertical range between the mean water levels of the high tides (MHWN) and that of the low tides (MLWN) was about 1 m. The great uniformity in morphology, vegetation cover and climate of the bands parallel to the channel’s banks is one of the major characteristics of this mangrove.

3. Materials and methods

During November 1993, the zonation and surface activity of P. sobrinus were studied in a 6-m wide transect. Observations were carried out every tidal hour for two periods of 48 tidal hours, one around spring tide (14-16/11/93) and the other the following neap tide (20-22/11/93). The time between two successive high tides was subdivided into 12 equal segments (tidal hours). Equivalent segments of time (relative to the tidal cycles) were then combined and a total tidal cycle constructed that was independent of the actual length of the HT-HT time, which varies from day to day throughout the synodic phase. Two diurnal (day = 0600- 1800) and two nocturnal (night = 1800-0600) tidal cycles were obtained for each considered period by appropriately rearranging the different segments. Tide tables (Admirality Tide Tables and Tidal Stream Tables Vol. 2, 1993 published by the Hydrographer of the Navy, England, 1992) provided the exact time of the tides but local measurements allowed corrections to be made so that the extent of the tidal excursions could be calculated.

Ten days before the observations began, a path of mangrove tree trunks was placed down the centre of the transect to decrease the disturbance made by walking on the mud during the observations. The transect was then subdivided into l-m wide bands parallel to the river banks and marked with labels indicating the distance in metres from the erosion step (0 m). Starting from 0 m, observations along the transect were made every tidal hour and a torch was used to spot the animals. It had been previously observed that low light intensity and human presence temporarily inhibited the fishes’ surface activity making counts easier. The number of mudskippers sighted (frequency of sightings), either active on the surface or resting on their nests, was recorded together with their zonation. Tidal levels, measured horizontally along the transect, air temperatures and relative humidities were also recorded with tidal hour frequency. The latter two variables

I. Colombini et al. I J. Exp. Mar. Biol. Ecol. 190 (1995) 135-149 139

were recorded at 1 m from the ground surface and at the mangrove’s centre. This was done so as to be independent from the tidal levels and to obtain mean values of temperature and relative humidity given the great uniformity of the mangrove. The data here reported are always referred to in tidal hours.

The statistical programme “Statgraphic Version 2.6” was used to analyse the data. Tidal levels, temperatures and relative humidities were analysed with the one-sample analysis method. To test if the numerical differences were significant the X*-test was used. The one-sample analysis method was also employed to analyse the differences in zonation (n 2 5, confidence limits 95%). The summary statistic for contingency (p < 0.05) was used to compare the distributions. Multiple regression analysis was employed to test the different aspects of the surface activity of the fish and their presence on the nests in relation to the different environmental factors. Circular statistics were used (Rayleigh Test, V-Test; Batschelet, 1981) to test the uniformity of the distributions, the mean distribution in time of the surface activity and of the presence of fish on the nests.

4. Results

4.1. Environmental factors

Mean air temperatures of 27.31”C and of 27.64”C were recorded during spring and neap tides, respectively. In the latter case the maximum thermal ranges were recorded (minimum = 24°C maximum = 31°C).

Mean relative humidities of the air recorded during spring and neap tides were 87.02 and 85.81%, respectively. The variations of the relative humidity were from 75 to 100% during spring tide and from 68 to 100% during neap tide.

The mean tidal level measured horizontally along the transect during spring tide varied from 0 m at HT (high tide) to 87 m at LT (low tide) (Fig. lb,c dashed line) with a mean of 49.58 + 9.19 m (95% confidence limits). During neap tide the tidal level varied from 56 m at HT to 76 at LT (Fig. lb,c dotted line) with a mean of 65.18 + 1.36 m.

During the entire period of observations it did not rain.

4.2. Total frequency of sightings

A total number of 629 observations on P. sobrinus were made in the transect during the two periods of study (spring tide n = 314, neap tide n = 315 x2 = 0.0015 N.S.). Of the 314 sightings during spring tide all fish were seen active on the mud surface, whereas during neap tide a total of 153 mudskippers were observed active on the surface and 162 resting on nest turrets (x2 = 0.26 N.S.). During spring tide there were 152 day sightings of mudskippers and 162 during the night (x2 = 0.32 N.S.). At neap tide during daytime hours 121 fish were observed whereas during the night 194 mudskippers were seen (,$ = 16.92 p < 0.001). Of the 121 fish, 34 were active on the mud surface and 87 were resting on the nest (,$ = 23.21

140 I. Colombini et al. I J. Exp. Mar. Biol. Ecol. 190 (1995) 135-149

p -=c 0.001) whereas during the night 119 fishes were seen active and 75 resting on nests (x2 = 9.97 p < 0.005). During the two synodic periods the total number of animals observed active on the mud surface and resting on the turrets were 273 during the day and 356 during the night (x2 = 10.95 p < 0.001).

4.3. Zonation of surface activity

4.3.1. Spring tide During spring tide all the mudskippers observed were active on the surface and

none was resting on nest turrets. P. sobrinus mean zonation was 24.31 m in this synodic phase. During the nocturnal tidal cycles the mean zonation of the surface active animals was 26.12 5 4.25 m and this did not differ from the diurnal one that was 22.38 2 2.45 m.

During the night on the spring tide (Fig. lb) at HT and Hour 1 the mudskippers were all concentrated at the base of the erosion step in the vegetation. As the tide ebbed, at Hours 2 and 3, the mean zonation of the fishes followed the water line:, respectively, at 18.1 m and at 45.44 m. The mean zonation did not vary during the following hours, including LT (57.4 m) and Hour 9 (47.66 m). At Hour 10, as the tide advanced, the mudskippers were found in a significantly landward zone at 24.41 m. The fishes were again concentrated at the base of the erosion step at Hour 11.

During daytime hours of spring tide (Fig. lc), when the tidal excursion is smaller than the one recorded during the night, P. sobrinus was found at 1.35 m from the erosion step at HT. At Hour 1 the mudskippers were active at 9.38 m and more concentrated than at Hour 2 when the fish (n = 5) had a mean position at 8.4 m. During the following seven tidal hours, from Hour 3 to Hour 9 and especially at Hours 3, 4 and LT, the animals were active in a more advanced zone as compared to the previous hours, At Hour 11 the fishes were again active in a landward zone: 18.19 m. Note that active mudskippers during the nocturnal and diurnal LT were found in significantly different zones: at night at 57.4 m and during the day at 33 m.

4.3.2. Neap tide During neap tide P. sobrinus has a mean zonation at 61.6 + 0.98 m and this is

significantly different from the mean zonation of 24.31 + 2.49 m found during spring tide. As on spring tide, during neap tide the nocturnal mean zonation of 62.27 2 0.85 m did not differ from the diurnal one of 59.26 + 3.26 m. During the nocturnal ebbing neap tide (Fig. lb), from HT to Hour 4, the mudskippers were active at different positions from the erosion step and these zonations differed from one another. At Hour 5, LT and Hour 7, P. sobrinus was active in a much wider zone of the mangrove when compared with the other hours of the night-time neap tide (see confidence limits and means in Fig. lb). In the following hours, as the tide rises, the fishes were again more concentrated and the zonations


differed from one another. No animals were seen active at the surface three hours after LT.

During the day on the neap tide (Fig. lc) the mudskippers (n = 34) were found active on the surface only at HT and throughout Hours 9-11 and Hours 1-3. Significantly, active animals were found only at Hour 9 (nine animals all at 67 m) and they had a different zonation from those active during the other tidal hours (for example the hour with highest upper limit is at HT: mean 58.87 -+ 1.3 m).

4.4, Surface activity

At night a total of 281 P. sobrinus were observed active at the surface (spring tide II = 162; neap tide n = 119; x2 = 6.58 p < 0.025) whereas 186 mudskippers were seen active during the day (spring tide II = 152; neap tide n = 34; x2 = 74.86 p < 0.001). The difference between night and day was significant (x2 = 9.66 p < 0.005).

The plots of the total surface activity calculated in tidal hours (Fig. 2a) show that the mudskippers were surface active during the entire tidal cycle both at spring and at neap tide. The comparison of the two distributions of spring and neap tide is significantly different (x2 = 19.27 df 11). With circular statistical analysis during spring tide the distribution is bimodal with a peak of activity at 0020 (about HT) and one at 0620 tidal hour (about LT) (V-Test u = 5.37 p < 0.0001). In contrast, during neap tide the total activity was unimodal with mean activity at 0010 tidal hour (about HT) (V-Test u = 5.23 p < 0.0001).

If daytime activity is considered separately from the nocturnal one at spring tide (Fig. 2b) the distributions are significantly different (x2 = 34.83 df 11) and in both cases are bimodal. At night the peaks of activity occurred at 0042 and at 0642 (u = 2.59 p < 0.005) and during the day at 0003 and at 0603 (U = 5.05 p < 0.0001).

Also, (Fig. 2c) the distribution of the surface activity observed during the night-time neap tides is different from the one recorded during daytime hours (,$ = 8.39 df 3). In both cases the activity was unimodal with a mean nocturnal activity at 0030 (U = 4.01 p < 0.0001) and a mean diurnal activity at 1144 (u = 3.60 p < 0.0005).

4.5. Presence on nests

At spring tide no mudskippers were observed resting on the turrets that are characteristic of the nests of this species even considering the areas outside the transect.

It was only at neap tide that many fishes were seen on the nests with turrets. In the transect (Fig. la) there were eight inhabited nests located in three different zones at increasing distances from the erosion step. Two of these nests, located at about 21 m from the erosion step, were old constructions and had been abandoned the previous spring tide. Of the other six nests, all recently constructed, two were found at about 47 m and four at about 62 m. Only one fish was


.z”

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Fig. 2. The total surface activity of P. sobrinus at spring and neap tide (a), during the day and night of

spring tide (b) and of neap tide (c) are shown. The frequency of active fish as percentage are calculated

separately on the total number of sightings during spring and neap tides.


observed on each nest except for one nest at 62 m in which sometimes two animals were contemporaneously present.

On a total of 315 sightings at neap tide, 153 mudskippers were active on the surface whereas 162 were resting on the nest. Comparing for each tidal hour the number of active fish with the number of those found resting on the nest (Fig. 3a) only twice the former was significantly greater than the latter. This occurred at HT when there were 22 active and six resting mudskippers (x2 = 9.14 p < 0.005) and at Hour 1 where there were 27 surface-active fishes and seven resting ones (x2 = 11.76 p < 0.001). The opposite occurred twice: the number of mudskippers resting on the nest was significantly greater than those active on the surface at Hour 5 (5 active and 15 resting specimens x2 = 5 p < 0.05) and Hour 8 (5 active and 18 resting specimens x2 = 7.34 p < 0.01). If the 6 h around HT (from Hour 10

q activity l-i=153

19 nests n= 162

m night

0 day

a

b

HT I z 3 4 5 LT 7 8 9 IO I I HT tidal hours

Fig. 3. (a) Frequency of mudskippers sighted active on the mud surface and resting on the nests during neap tide. X2-values are indicated only in significant cases. (b) Comparison of sighting frequency of P. sobrinus at night and day. Both horizontal axes are in tidal hours.

144 I. Colombini et al. I .I. Exp. Mar. Biol. Ecol. 190 (199.5) 1.35-149

to Hour 3) are summed there were 105 surface-active mudskippers whereas there were 67 resting on the nest (x2 = 8.39 p < 0.005). If the other 6 h around LT (from Hour 4 to Hour 9) are combined, the number of active fishes (n = 48) is less than those found resting on the nest (n = 95; x2 = 15.45 p < 0.001). Comparing the distribution of the surface active mudskippers in the different tidal hours with that of the fishes resting on the nest (Fig. 3a) the results are significantly different (x2 = 47.12 df 11). The mean hour of activity is at 0010 about HT (u = 5.23 p < 0.0001) whereas the mean hour of presence on the nest is 0545 tidal hours after HT, i.e. about LT (u = 2.92 p < 0.0025).

A total of 75 mudskippers resting on the nest was recorded during night observations whereas there were 87 daytime sightings (,$ = 0.89 N.S.) (Fig. 3b). The comparison of the two distributions during the entire tidal cycle was not significantly different (x2 = 3.91 df 9). At night the mean hour of presence on the nest was 0503 tidal hours after HT (u = 2.11 p < 0.025), during the day 0031 tidal hours after LT (u = 2.024 p < 0.025).

4.6. Environmental factors and relationship with surface activity

Using multiple regression analysis four dependent variables, (the number of surface active mudskippers, the number of fishes found resting on the nest, the zonation of the active and of the resting mudskippers on the nest) were tested separately with the three environmental factors i.e. air temperature, its relative humidity and tidal levels as independent variables.

At spring tide the number of surface active animals was correlated negatively with the tidal level (regression coefficient (b) = - 0.080, p < 0.0001; coefficient of multiple determination R* = 0.27). Note that the horizontal distance of the water from the erosion step (0 m) is maximum at LT. Again at spring tide the zonation of the active mudskippers was correlated positively with tidal level and negatively with temperature (respectively b = 0.554 p < 0.0001 and b = - 2.973 p < 0.003; R= = 0.7889).

At neap tide the number of surface active animals was again correlated negatively with tidal level (b = - 0.324 p < 0.0019 R2 = 0.172). The zonations were correlated positively only with the relative humidity (b = 0.701 p < 0.0001 R2 = 0.9863).

The number of fish seen resting on nests, the number resting on the nests closer to the erosion step, those resting on the intermediate zones and those resting on the nests in a zone closer to the river bank were analysed separately. Of these, the first group of fish was correlated negatively with the relative humidity (b = - 0.033 p < 0.009 R2 = 0.1199). The presence of mudskippers on nests with intermediate zonation was correlated negatively with the air temperature and relative humidity (respectively: b = - 0.227 p < 0.002 and b = - 0.057 p < 0.0005; R* = 0.2051). The fish resting on the nests closer to the channel were correlated positively with the distance of the tidal level from the erosion step (b = 0.204 p < 0.0001 R2 = 0.4974).

I. Colombini et al. I .I. Exp. Mar. Biol. Ecol. 190 (1995) 135-149 145

5. Discussion

The same number of surface active mudskippers observed during the night and the day on the spring tide suggests that surface activity is independent of the die1 light cycle. Moreover, the fact that at spring tide the number of surface-active mudskippers is greater than during neap tide indicates that the fish are influenced more by the synodic cycle than by the light-dark cycle. With regard to F’. sobrinus of Madagascar, Brillet (1975) asserts that this species is mainly active during the day and that during the night activity is almost completely interrupted. The data reported here show that during both synodic periods the nocturnal surface activity includes foraging during the LT period. This observation is also supported by the analysis of the stomach contents of P. sobrinus that revealed a great number of prey even during the nocturnal hours (pers. unpubl. data). In other species there is a partial or total interruption of surface activity at night (Nishikawa & Ishibashi, 1975; Gordon et al., 1978; El-Ziady et al., 1979; Eissa et al., 1978). In the case of P. chrysospilos of Kuwait, during the winter the fish are only active again after the nocturnal interruption when the air temperature increases to 14°C from the low nocturnal values (Eissa et al., 1978). This indicates that in this species surface activity is restrained by low temperature. On the other hand, at neap tide the number of Z? sobrinus observed active during the night was greater than the number active during daytime. Also, considering the two synodic phases together, the total number of animals observed during the day was smaller than the total number of those observed during the night. This indicates that during the day P. sobrinus can take refuge in occasional burrows especially at neap tide when the tidal excursions are smaller and impose minor movements on the fish.

Moreover, the difference in the number of surface-active mudskippers between spring and neap tides indicates a different behaviour at different phases of the synodic cycle. At neap tide, when the tidal excursion is smaller, mudskippers are found resting on the typical turrets of the recently built nests, or on old reoccupied constructions. Contrary to the observations of Brillet (1975) on P. sobrinus and of Low et al. (1988) on Boleophthalmus boddaerti, in the P. sobrinus population studied here the mudskippers always abandoned the occasional burrows on the incoming spring tide or the nest turrets on the incoming neap tide and never remain submerged during high tide.

The influence of the tidal and synodic cycles on the population of P. sobrinus studied is also confirmed by the mean hourly zonation recorded at spring and neap tides. The data show that at spring tide the mudskippers escape from the water but tend to remain under cover of the vegetation. The velocity of the tidal rise and fall is maximum immediately before and after HT because of the slight inclination of the substratum. During these tidal hours the fish move on the mud closely following the tidal line. In the hours around LT the mudskippers remain active in a zone of the mangrove (more or less wide) in which the air temperatures are adequate for surface activity. Furthermore, the thick vegetation canopy probably offers protection from bird predation (Ardeidae) that was commonly


observed during LT along the channel bank and out of the mangrove vegetation. This suggestion may be supported by the observation that the fish are only active in a more advanced and wider zone with reference to the erosion step during the nocturnal low tides, when they leave the cover of the vegetation and enter the zone where only the aerial roots of the mangroves are found. Evidence for bird predation on mudskippers has also been reported by Clayton (1993).

At neap tide, when the tidal excursion is minimum and the mangrove consequently dries up, the mudskippers closely follow the tidal levels during the night. During the day around LT of neap tide no fish were ever seen active on the mud surface, although many were observed resting on nest turrets. The differences in zonation of P. sobrinus registered at spring and neap tide and the different diurnal and nocturnal zonations of spring tide probably depend not only on the climatic conditions and on predation pressure but also in changes of the fish’s trophic niche (pers. unpubl. data).

Using multiple regression analysis it was demonstrated that the rhythms of surface activity and the zonation of P. sobrinus are strictly correlated with the tidal levels and also with air temperature and relative humidity. During spring tide the zonation is closer to the channel as the temperature decreases and during neap tide as the relative humidity increases and this occurred during the nocturnal hours. Note that the minimum temperatures and the higher relative humidities were recorded during the night. The fact that during spring tide the zonation was correlated negatively with the air temperature and during neap tide positively with the relative humidity indicates that these independent variables become alternatively important and limiting according to the synodic phase. In different species of mudskippers many authors (Stebbins & Kalk, 1961; Gordon et al., 1969; Gordon et al., 1978; El-Ziady et al., 1979; Tytler & Vaughan, 1983; Gordon et al.. 1985) have pointed out the importance of these two environmental factors both in the field and under laboratory conditions.

It is interesting that different types of activity were recorded in the two phases of the synodic cycle: bimodal activity on spring tides and unimodal activity on neap tides. The bimodality is due to a zonal recovery immediately before and immediately after HT and to foraging activity during LT (Colombini et al., 1992). On neap tides, since the tidal excursions are less extensive, the distances covered by the mudskippers at HT are smaller and foraging activity occurs at the same time. At LT, especially during the day, some fish take refuge in occasional burrows, whereas others rest on the turrets of their nests because they are in their reproductive phase. Nest building and the presence of P. sobrinus on the nests are related to the synodic cycle (neap tide) and to the tidal cycle (LT), respectively. The latter case is more evident for nests closer to the channel. The presence of mudskippers on the more landward nests is more likely to be influenced by the temperature and relative humidity. During the day the mudskippers take refuge in the nests so as to avoid desiccation whereas during the night the fish leave their nests to forage at the edge of the tide.

As a whole the surface activity of the population of P. sobrinus that was studied is independent of the die1 light cycle, but depends directly on air temperature and


relative humidity that are related to it. These environmental factors influence the zonation of the activity itself and the fish’s presence on and inside the nests. Moreover, the zonation of the surface activity and the quantity and quality of the activity itself both directly depend on the tidal and synodic cycles.

Acknowledgements

We thank Mr. R. Retief for all their help in solving logistic matters and Mr. L. Kazungu for his unstinting help on the field. We also would like to thank the entire staff of the Shekiko camp whose assistance has made this work possible.

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Environmental Factors Influencing the Zonation And

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