Fish-Zooplankton — A Predator-Prey Relations as a Key Factor for the Design of Zooplankton Distribution Sampling Program in Lake Kinneret , Israel

Long-term data record of Kinneret Epilimnetic Zooplankton biomass distribution and fish stock assessment was analyzed. The objective is aimed at defining the representativeness of the present sampling stations distribution for the measure of the entire Lake Kinneret zooplankton biomass. Previous studies documented the preeminence of temperature and fish predation impacts on zooplankton density: water temperature through growth rate trait and fish predation. Acoustic surveys indicated that fish shoals are mostly inhabiting the Peripheral region and much less the central part of the Kinneret pelagial. Due to fish zooplanktivory, densities of planktonic crustacean in the Pelagial periphery presented by 5 7 sampling stations are low. Nevertheless, large central lake area with higher densities of zooplankton biomass is presented by only one station. Therefore, stations average is incorrect as is total lake measure. The average result as indicated presently as total zooplankton biomass in lake Kinneret is, therefore, underestimated. Two optional corrective suggestions are concluded: 1) Additional sampling stations in the central part of the Pelagial region; 2) Usage of station value of aerial coefficient (promoter).


Introduction
The long-term Record of Zooplankton, as part of the scientific routine comprehensive limnological research of the Lake Kinneret ecosystem [

Material and Methods
The zooplankton sampling procedure is given in Gophen [11] and Gophen and Azoulay [12].The data in tables 1 & 2 illustrate the sampling capacity and statistical evaluation (mean) of zooplankton (  but only Epilimnetic data were filtered for the present study.Original zooplankton data presented here were converted into Biomass units (g(ww) per m 2 ) [12].
The usage of biomass parameters of fish and its food consumption in an ecosystem with respect to long-term impacts is essential.Nevertheless, due to technical difficulties (calibration), fish data is not given by biomass but numerically.The biomass approach to the ecosystem long-term analysis of zooplankton-fish interrelation is essential because fish food (zooplankton) consumption rate is more biomass than numerically dependent.The study was restricted just to the Epilimnion because neither zooplankton nor fish densities in the Hypolimnion are negligible.
Such a retrospective analysis of data, including numerous items, require suitability of statistical methods.Raw date was taped and mean values per station, per periodical intervals (monthly, annual), were computed using indicative parame- C.V. SD X = and: Comparative analysis between stations and periods was carried out by Test of significance known as "null hypothesis" which is assessing the strength of the evidence against it.The "null hypothesis" is a definition of "no effect" or "no difference".The test of significance calculates the probability (p) of having an outcome at least as far from expected if "no difference" (null hypothesis; H 0 ) was true, the computed value of p (probability) assuming H 0 is true.Therefore, the smaller the p-value is, the stronger is the evidence against null hypothesis (H 0 ) provided by the data.Practically, if p < 0.05, H 0 is rejected, indicating that differences truly existed, and if p > 0.05, "no difference" is accepted.

Results
A summary of acoustic surveys carried out in Lake Kinneret during 1987-2005 [2] are presented in Figure 2 and in Figure 3   species fingerlings and sub-commercial body length.Larger size frequencies are due to sub-commercial and commercial fishes of about larger than 15 cm (Total Length) (TL).Zooplankton predation pressure is mostly operated by the small sizes including larvae, fingerlings of all species and adult Sardines [13]- [24].
Presently, some of the Tilapias partly consume zooplankton.The data about stock assessment of fish in Lake Kinneret is given in fish number qualified by Transducer, the acceptance of which is related to numerical density but not   non-significant (r 2 = 0.0004) and significant (r 2 = 0.5325) relations for the earlier and later periods, respectively.Diet composition for Bleak fishes of all ages in Lake Kinneret comprised mostly of Zooplankton.Feeding competition does not exist if food availability is not limited as occurred in the first period and consequently prey-predator stocks are not statistically related.On the contrary, during  the 1995-2005 period, zooplankton biomass significantly declined and fish stock (mostly Bleaks) increased.It is likely that food source became limited and competition forcefully affected, making prey-predator relations significant in the second period.

Discussion
The inverse relation between Zooplankton Biomass and fish (mostly Bleaks) densities was widely documented in previous studies.An increase of fish densities in the lake started from 1998 (Figure 2) when zooplankton (mostly prey favoured Cladocerans) started a decline [30], Figure 3).Moreover, during 1970-1993 a significant high harvest (app.1000 tons per annum) of bleaks was recorded.Probably reflecting a productive stock biomass producing intensive pressure on zooplankton which continuously declined.Nevertheless optimal conditions for zooplankton growth dynamics was indicated except fish predation.
Three major factors affecting Zooplankton ecophysiology in Lake Kinneret were considered: 1) Food availability; 2) Thermal impact and 3) predatory pressure.
The study of herbivore food resources in Lake Kinneret confirmed conditions of no food (algae, protozoa, detritus, and Bacteria) limitation.On the other hand thermal impact was found to have an influence but on a seasonal level and long term fluctuations were too small to produce significan conditional effect.Therefore fish predation was concluded the major factor which is significantly affect fluctuations of Herbivore zooplankton biomass in Lake Kinneret.The research team of the Kinneret Limnological Laboratory started, for the first time in Israel, a comprehensive integrated limnological research of the Lake Kinneret ecosystem in 1968.The research of the zooplankton in the lake was given to the author of this paper.As a collaborative group of scientists, one of their first missions was to fix sampling stations to be followed routinely for the long-term achieve- ter) for the calculation of total lake biomass.

Summary and Conclusions
The grand total average of zooplankton biomass in all sampling stations (A, C, F, D, G, K, L) indicates its low level in the peripheral stations and its even lower levels in the southern region of the lake.Conclusively, the design of sampling program aimed at representing the entire Kinneret Pelagial Epilimnion should include at least one station in the southern part (stations D, K, L).If not, the lake mean value will be higher than reality.Furthermore, if sampling program eliminates the central zone (station A) the resulting value will be lower than real.
A future suggestion aimed at improvement of zooplankton sampling design is leaving 3 northern and 3 southern peripheral stations and an additional sampling station in the central region south of station A where the depth is about 30 meters.
ters of Coefficient of Variation (C.V).CV parameter expresses the Relative Standard Variation (RSD), i.e. the Ratio of the Standard Deviation (SD) to the Mean (X).C.V. shows the extent of variability in relation to the Mean of the population:

Figure 2 .
Figure 2. Acoustic surveys in Lake Kinneret during 1987-2005: Polynomial regression (p and r 2 are given) between All Annual recordings of Fish number and years.

Figure 3 .
Figure 3. Acoustic surveys in lake Kinneret during 1987-2015: Polynomial regression (Confidence interval is indicated) between annual maximun recorded fish number and year.
of a monitoring program implementation.This mission was accomplished and the monitoring program operated.The routine monitoring of limnological parameters was directed through those stations from 1969 and continues at present.A daily spatial, bathymetrical and diurnal distribution of zooplankton was previously documented ([1] [4] [6] [7] [8] [9][10]).Those studies were carried out in a short time range: days and hours.But none of them gave an insight into a multiannual significance of long-term frame.This paper is the first approach to the issue of zooplankton spatial distribution in Lake Kinneret as reflected in 30 years of documentation of routinely collected samples integrated with fish abundance.Ecological factors in Lake Kinneret which have an impact on the natural zooplankton densities were widely explored but the analysis of zooplankton distribution as an outcome from multiannual sample data was never done before.The sampling program is first of all the outcome of available budget and manpower.These two parameters are the limiting factors for the decision about number of stations and frequency of sampling.The requirement for innovated design-based follow-up of the sampling efficiency and relevance to the research objective is obvious and the longer the term of implementation the higher the quality of the decision.The design of the present study advanced through double statistical evaluations: 1) each sampling station annually and monthly; 2) annually and monthly, including all stations.Each one of the analyses was done independently.In other words, the likeness of each station solely as a representative of the whole lake.It was documented in previous studies that zooplankton food availability is optimal and the limiting factors for zooplankton development are temperature and fish predation ([25]-[32]).The natural multiannual (not monthly) fluctuations of the Epilimnetic temperatures are too low to have any significant impact on zooplankton growth rate as confirmed experimentally.Therefore, the abundance of fish in Lake Kinneret was taken as a major impact on the fluctuations of zooplankton density.The first step in attempting the evaluation of the relation of zooplankton distribution to fish stock data was done by the multiannual fluctuations of both fish and zooplankton densities (Figures2-4).The data support the suggestion that M. Gophen DOI: 10.4236/ojmh.2017.73012219 Open Journal of Modern Hydrology zooplankton decline was due to Fish stock enhancement.The relevance of the measured fish stock size comprises from >95% of the recorded targets to small and sub-commercial body sizes which are known as zooplanktivores.Figures 2-4 confirm the inverse relation between Zooplankton density and the documented fish stock.The second step of the study was an attempt aimed at spatial allocating of fish shoals and zooplankton population.The acoustic surveys clearly confirmed assembling of fish flocks in the peripheral parts of the Kinneret Pelagial.It is, therefore, suggested that zooplanktivory pressure is more intensive in the peripheral stations and lower in the central part of the lake.Consequently, zooplankton biomass is higher in the central parts of the Kinneret pelagial and lower in the peripheral zones.In all stations zooplkankton biomass in winter is higher than in summer.It is the result of the lower energy investment in winter by zooplankton and higher reproduction efficiency[12] [33][34] together with the lower feeding rate of the fish.The next step forward in the investigation was due to the dissimilarity of zooplankton density found between the northern and southern parts.WaterCurrent and water-mass moving directions in Lake Kinneret were widely studied and documented as well as the distribution of the Jordan River input waters in the lake[35] [36].The direction of the flow of the Jordan waters is mostly from the river mouth to the west interlocked in the dominant direction pattern which is anticlockwise.It is suggested that such a current pattern enhances higher densities of Jordan River fluxed suspended particles in the northern stations.Reduction of water clarity by the enhanced concentration of suspended matter also reduces prey (zooplankters) visibility and consequently suppress fish zooplanktivory[37].The suggested lower concentration of suspended matter in the southern stations (Figure1) probably enhances zooplankter vulnerability.The outcome is lower zooplankton biomass in the southern part of the Kinneret pelagial.The long-term data record of zooplankton biomass and fish distribution in Lake Kinneret clarified that the previous results are an underestimation.Multi-variable complicated modeling attempts that were carried out aimed at a quantitative chart of energy flow pattern in the Kinneret ecosystem [14] [38] revealed underestimated zooplankton biomass values [31].The previous data of zooplankton biomass density are based on sampled regions where fish density is high and, therefore, zooplankton predation is intensive.Weighting averages of 7 stations (or even less) where only one of them (Station A, central lake zone) represent most of the Kinneret Epilimnion is fairly unbalanced.Fish density in the central part of the Kinneret Epilimnion is low and the zooplankton biomass is, therefore, high.For a fairly justified representation of the entire lake value, additional sampling stations in the central zone are required.Conclusively, it is therefore suggested to distribute same number of sampling stations for zooplankton study in the peripherial and in the central parts of the Kinneret Pellagial.An alternative option is to give each station a value of aerial coefficient (promo- Table 1-annually; Table 2-monthly) in (Epilimnion) and are, therefore, feasibly comparative.The documented reports of the original zooplankton raw date were recorded and computerized on tapes Table 1.Annual means (all stations, all dates) of Zooplankton Biomass and annual means of C.V. (see text) and total number of sampled stations, in Lake Kinneret during 1969-1985.