The feeding habits of the cichlid Tilapia zillii (Gervais) in Lake Kinneret (Israel) were experimentally studied in indoor glass containers (2.5 hrs) and two trials in outdoor 5 m 3 tanks (20 - 25 days). The trait of food particle collection by adult fishes was measured. A survey was carried out in the littoral (0 - 1.0 m deep) zone and fingerlings were sampled by electro-chocker. The gut content of the fingerlings was analyzed. The feeding habits of T. zillii were indicated as planktivorous filtration with more enhancements of small zooplankters (Nauplius, Brachionid rotifers) and fewer of Cladocerans suppressions.
Fish feeding habits and food particle selectivity are known to be correlated with their actual habitat or their geo-ecological origin. Nevertheless, natural ichthyofaunal diversity initiates food resources partitioning, and the ecological structure of Lake Victoria is a well known versatile system. The original fish community of Lake Kinneret includes 19 species from three primary and four secondary freshwater originated families [
The Glass container experiments design was as follows. The experiment was carried out in five 120 L glass indoor containers (Aquarium), each containing 112 L of filtered (63 µ mesh size net) lake water under ambient indoor conditions: diffused light; 23˚C - 26˚C stable room temperature. T. zillii specimen were placed (
Fishes were introduced into the containers with filtered lake water two days before experiments started for acclimatization. Fresh plankton was collected in the lake containing 63 µ mesh size plankton net and re-suspended in lake water. The same aliquot of fresh plankton suspension was given to the fishless and each other container. Samples were collected in the containers immediately after the insert of the plankton suspension (Initial time) and 2.5 hours later. The containers were not aerated and re-suspension of dead organisms was prevented. Experimental sampling was done with a plastic cylinder open on both sides and a rubber stopper. The stopper was gently placed on the bottom and the plastic pipe was vertically lowered onto the stopper closing
Total Length (cm) | Weight (g) | Container No. |
---|---|---|
13 | 50.5 | 1 |
19 | 126.4 | 2 |
16.3 | 80.0 | 3 |
21 17 | 81 153 | 4 |
19.3 16.6 | 147.8 88.5 | 5 |
sampled water within the pipe. Then the cylinder was picked with its bottom blocked by the stopper. The volume of the sample was measured before filtering through 63 µ mesh size net. All collected organisms were flushed from the net collector into a small beaker and preserved by 0.5 cc (1/10 of the sample volume) of 10% formalin. Counting was carried out under Wild Binocular through wheel-counting-chamber. Each sampling included 3 repetitions and result was averaged. The plankton was divided into the following four categories: 1) Copepod nauplii, 2) 1 - 4 copepodite stages, 3) 5th copepodite stage and adult copepods, and 4) all cladocerans (Bosmina spp., Diaphanosoma sp., Ceriodaphnia spp). All concentrations were expressed as number per litre. Number of consumed (eliminated) organisms was considered as those resulting from the subtraction of 2.5 hrs concentration from initial concentration after the elimination of mortality as resulting from concentration measured in fishless containers. In containers with two fishes, results were calculated as number of consumed organisms per individual fish.
The study of the feeding habits of the fish include 4 steps: 1) gut content analysis of lake sampled fishes; 2) Glass containers with individual body size measured specimen (1 - 2 per container) for the investigation of single fish fed by known food items for the study of Index Of Electivity; 3) The 5 m3 Outdoor tanks contained the background of natural un-treated food resources removed from the lake and the addition of zooplankton, fish and both combination for the preferential habits of the fish. The gut content study was presented earlier and the two steps forward are given here.
Two trials (20, 25 days each) were run in 5 m3 outdoor tanks filled with lake water to examine the impacts of fish and zooplankton on lake plankton. Tanks were filled at the beginning of each experiment with water pumped from approximately 30 m offshore at a depth of 1.5 m. T. zillii were placed into 4 of 8 tanks: 2-with supplemented fresh zooplankton collected in the lake by 300 µ mesh size net and equal portions of the composite added to tanks; 2-control, and 2-with both fish and zooplankton. Treatment combinations were in a 2 × 2 factorial design. Several parameters were measured weekly. Results were analyzed by ANOVA for the significance of main effect and interactions by Duncan grouping for fish and zooplankton treatment. Fish (T. zillii) were collected from the lake and acclimated in the tanks filled with lake water several days prior to experimental periods. During experimental time, tanks were mixed for 2 hours a day by an air-lift mixer system. Mixers moved a water volume equivalent to tank volume in about 1 hour destratifying and aerating the tanks. Tanks were weekly sampled for zooplankton counts and Chlorophyll analysis (Wetzel and Likens 2000); by mixing 5 replicate samples collected with 2.5 m-long 1.5 cm-diameter plastic pipe lowered to few cm above tank bottom. Chlorophyll analysis was carried out on pipe samples [
Results in terms of consumed organisms per fish per 2.5 hrs in 5 containers are given in
The highest number of consumed organisms is due to the smallest-sized Nauplius and the highest consumed biomass to the largest body-sized caladocerans. Nevertheless, the highest index of electivity is that of Nauplius and the lowest that of Cladocerans. The efficiency of small organisms (Nauplius and small copepodites) ingestion (E = 0.73 and 0.22, respectively) is much higher than those of large body organisms (4 - 5 copepodites, adult cyclopoids and cladocerans) (E = −0.04 and −0.11, respectively). T. zillii is conclusively considered as filter feeding fish and partly visual attacker.
where:
r1 = % of Consumed Food Component;
p1 = % of Food Component in the Control at Initial Time.
Analyses of three effects were done: Zooplankton Effect, Fish Effect and Main Effect based on data sums across sampling dates in the tanks: 2-control, no fish no
Container No. | Nauplius | 1 - 3 Stage Copepodite | Copepodi 5 + Adult copepods | Cladocerans |
---|---|---|---|---|
1 | 83 | 8 | 14 | 9 |
2 | 121 | 57 | 2 | 24 |
3 | 105 | 18 | 6 | 9 |
4 | 78 | 9 | 3 | 3 |
5 | 39 | 6 | 1 | 9 |
Averaged Consumed (SD): No./L | 85 (31) | 20 (21) | 5 (5) | 11 (8) |
Organism | Biomass Consumed (µg/L) | Biomass Consumed (%) | Control Initial (No./L) | Control Initial (µg/L) (%) |
---|---|---|---|---|
Nauplius | 77 | 13 | 44 | 39.6 (2%) |
1 - 3 Copepodite | 85 | 14 | 49 | 207.3 (9%) |
4 - 5 Copepodite & Adults | 64 | 11 | 20 | 254 (11%) |
Cladocerans | 383 | 62 | 50 | 1740 (78%) |
Zooplankton addition; 2-Zooplnkton addition, 2-Fishe addition and 2-addition of Zooplankton and Fish. There was a significant main effect of Fish on Chlorophyll and on Cladoicera in the 1st experiment: p = 0.048 and 0.012, respectively; Zooplankton addition significantly affected copepod concentration in the 1st experiment and Cladocera concentration in the 2nd experiment. No significant interaction between zooplankton and fish additions was indicated. In the tanks with fish, chlorophyll increased in the two experiments accompanied by a decline of Cladocera. Evaluation of Fish and Zopoplankton main effects were achieved by comparing the mean values for treatments containing fish and zooplankton with those from which it is absent (Control). Probability values are given in
As part of the ecological study of the Kinneret littoral-shallow waters, fingerlings are captured by Electro-Shocker. The sampling program included shallow water (0 - 1.0 m depth) stations along total shoreline length. Among other species, fingerlings of T. zillii were fished mostly in the West-Southern and Northern regions. The bottom in those sites was varieties of muddy-sandy-pebble stony compositions. The body size (TL, cm) range of the captured fingerlings was 4 - 8 cm. Samplings were carried out monthly and 5 specimens were sub-sampled. The sub-sampled fingerlings were measured and placed immediately into 10% formalin solution and were later on dissected for the analysis of the gut contents under dissecting and inverted microscopes.
Parameter | Zooplankton Main Effect | Fish Main Effect | Interaction |
---|---|---|---|
1st Experiment | |||
Chlorophyll | 0.364 | 0.406 | 0.048 S |
Copepods | 0.021 S | 0.462 | 0.156 |
Nauplius | 0.751 | 0.590 | 0.305 |
Copepodite | 0.092 S | 0.961 | 0.294 |
Cladocera | 0.240 | 0.619 | 0.012 S |
Rotifera | 0.310 | 0.737 | 0.914 |
Turbidity | 0.874 | 0.895 | 0.461 |
2nd Experiment | |||
Turbidity | 0.463 | 0.303 | 1.000 |
Chlorophyll | 0.808 | 0.704 | 0.048 S |
Nauplius | 0.669 | 0.622 | 0.114 |
Copepodite | 0.532 | 0.820 | 0.760 |
Cladocera | 0.050 S | 0.790 | 0.725 |
Rotifera | 0.976 | 0.740 | 0.092 S |
The most common items in the fingerling intestines were Intact and broken Foraminifer shells, Intact and broken shells of the Gastropod Melanoides sp., sand grains, Frustulae of Centrales and Pennales Diatoms, short (broken) filaments of Melosira sp. body parts, (fragments) of Chironomid larvae, small size chlorophytes (Scenedesmus spp., Cosmarium spp., Pediastrum spp.,), high plant debris, small rotifers (Brachionids), and Spiculae of Porifera. This type of food composition is typical to bottom burrowers or dweller freshwater fishes.
The adults are omnivores which collect food by different techniques [
The very high range of water salinity (0.4 - 43 ppt Chloride concentration) populated by T. zillii is indicates the high saline tolerance of T. zillii. Nevertheless, the optimal condition for a natural complete lifecycle is probably not marine. It is suggested that the origin might be marine conditions but after long-term adaptation the fish’s natural habitat was established in freshwater. The high saline tolerance of T. zillii is probably an evolutionary relict of marine origin. The food composition of T. zillii in marine condition is not known but the documented absence of fingerlings and reproductive behavior confirmed freshwater conditions as preferential habitats. The intensive consumption of the lake and running water food sources indicate an adaptive evolutionary process in freshwater. Moreover, the aggressive reproductive behavior, the diversity of the granulometric composition of the nest structure and substrate composition [
To indicate the level of freshwater adaptation performed by T. zillii, four major features are considered: reproduction, feeding and food relation, tolerance of salinity changes [
The parameters of feeding and food relations were studied in this paper. It is suggested that adult T. zillii preferentially select small zooplankters. Nevertheless, this preferential selection is a result, not of visual collection of food particle, but of swimming and the escapeability trait of the species preyed on. Visible adult cyclopoid copepods and 1 - 4 copepodite stages are better escapers then the non-visible nauplius, young copepodites and small rotifers (Brachionids). Therefore, these small and poorer escaper organisms are consumed through pumping activity of the fish at a higher rate than adults and older copepodites. The ingestion of small organisms (Nauplius, young copepodites and Brachiond rotifers) is maintained by filtration of pumped water, and large animals (matured copepodite, adult cyclopoid copepods and cladocerans) are captured by a visual attack. Significant probabilities were indicated for interaction effect on Chlorophyll enhancement in the two tank experiments, as well as on Cladocera in the first experiment and on Rotifera in the second experiment (
The strong adaptation of reproductive behavior, nest construction and taking care of the fertilized eggs and newborn larvae by both male and female are also correlated between the available food sources for the YOY in the shallows of the Kinneret littoral zone. It is suggested that the information collected in the present study and those performed earlier enable lake managers to establish evidence for rules of fishery management legislations aimed at water quality protection and fishery regulations.
The ecological success of the evolutionary adaptation process of T. zillii in Lake Kinneret is expressed by the fish’s passivity to suitable food required for the adult and the young life cycle stages, a suitable substrate for nest construction and suitable temperatures. This paper documented the food resources suitability for the optimal existence of T. zillii in Lake Kinneret. Experiments carried out in glass containers confirmed T. zillii’s feeding habits of planktivor filtration and the 5 m3 tank trials indicated more enhancements of small zooplankters and less cladocerans suppression.
I wish to express my thanks to Dr. O. Sonin, Z. Snovski and J. Shapiro for supporting of facilities and assistance of fingerling sampling in the littoral zone (Project No. 596- 0527-12). Design, operation and statistical analysis of the Outdoor tank experiments (NSF Research Grant No. BSR-8416519) were carried out by the late Dr. G.L. Vinyard, an outstanding scientist, friend and collaborator.
Gophen, M. (2017) Experimental Study of the Feeding Habits of Tilapia zillii (Gervais) in Lake Kinneret. Open Journal of Modern Hydrology, 7, 1-10. http://dx.doi.org/10.4236/ojmh.2017.71001