The Kinneret fishery dynamics were previously discussed and presented widely. During the last 20 years, the Kinneret ecosystem structure has undergone significant modifications. Precipitation regimes declined significantly, and water inputs into the lake were, therefore, reduced accompanied by lake water level (WL) decline. Nitrogen to Phosphorus ratio was turned around from Phosphorus to Nitrogen limitation   . The dominance of the major food source for Sarotherodon galilaeus, the bloom forming Peridinium, was replaced by Cyanobacteria, Chlorophyta and Diatoms, and fish plankton food resources (Phyto and Zoo) were modified, respectively    . Independently, other constraints created additional pressures on the fish population: Population increase of the migratory fish predator, Great Cormorant (Phalacrocorax carbo), in the lake   -  reduction of stocked S. galilaeus fingerlings, short-time (1992-2000) usage of illegal fishing gill-nets mesh size, the elimination of Bleaks (Sardine: Mirograx terraesanctae, Acanthobrama lissneri) fishing   , enhanced piscivory of S. galilaeus by Clarias gariepinus  and outburst of viral diseases, which infected mostly Tilapias      (Photo 1) as well as global climatological events of ENSO cycles (EL-NIÑO/Southern Oscillation (ENSO)  .
A fishery crisis alert was publicized when landings of S. galilaeus in 2007-2008 declined to less than 10 tons in 2008 whereas the total number of fish (>90% Bleaks) (Eco-Surveys) was gradually increasing from 1987 to 2005  . An ad- hoc emergent meeting was assembled and a resolution was made for a recommendation of a three-year fishing prohibition in Lake Kinneret. This recommended decision was submitted to the Minister of Agriculture and was later submitted to become a government decision. A group of scientists strongly opposed this resolution and alternatively recommended the continuation of fishing under legal legislations as normally implemented earlier. The fishing ban decision was canceled and fishing continuation was confirmed formally. During 2010-2016, the population of S. galilaeus and consequently their landings were recovered and came to its normal level. In the present paper, I evaluate the dynamics of the S. galilaeus crisis case with the aim of considering the comprehensive cyclic ecological trait of the Kinneret ecosystem (Plankton and Nutrients), which includes fish population and the significant impact of stocking. The significant decline of S. galilaeus landing during 2007-2008 was documented previously. Nevertheless, the present paper is the first option for evaluating if earlier predicted indications were implemented. The realization of unknown early novelties is demonstrated here.
The Limnological long-term (1970-2013) data-set  of nutrients, plankton,
Photo 1. Two specimens of Sarotherodon galilaeus. Commercial size: TL: 19 cm (left) and 18 cm (right) collected from purse-seine catch (fisher M.Lev courtesy) in Lake Kinneret (2015) with ruined left eye resulted by the Virus of Blindness (see text) infection.
Secchi depth, DO, TSS, Lake Water Level (WL), and numerical fish (all size frequencies) densities (echo-surveys) was analyzed for an indication of fluctuation dynamics trait. The fishery landings and stocking regimes were evaluated by the Fishery Department  and Shapiro per comm. Data source of Rain gauge is The Israeli Meteorological Service.
Statistical analyses used in this study were taken from STATA 9.1, Statistics-Data Analysis and StatView 5.1, SAS Institute Inc. The analyses used were: Polynomial Predicted Regressions, Fractional Polynomial (FP), and Trend of Changes, LOWESS (0.8).
Since the mid-1980’s, precipitation decline regime in the northern part of Israel in general and particularly in the Kinneret drainage basin was indicated (Figure 1). The direct consequence of it was a decline of river discharges and Lake WL lowering. This temporal dryness trend continued onwards and affected Lake Kinneret’s nutrient regime, mostly those that are externally sourced and the results shown in Figure 2 & Figure 3 are the respective outcomes. The Nitrogen species which are mostly affected by external inputs through river inflows are Total―Kijeldhal, Particulate Organic Nitrogen and obviously Total Nitrogen. These forms of Nitrogen are dependents of external inputs. Others are the secondary species, Dissolved Kjeldhal, TIN (NO3 + NO2 + NH4) (or DON), and TDN. They are produced by the activities of Bacteria and Cyanobacteria: Nitrogen mineralization, de-nitrification and atmospheric Nitrogen fixation, as well as fish and zooplankters excretions.
The external dependants of Nitrogen species declined and the internally affected increased (Figure 2). The opposite trend is presented by Phosphorus dynamics: The primary P species, TP and Particulate, increased and the secondary―SRP and PTD―declined (Figure 3). The increase of phytoplankton (Cyanophyta, Chlorophyta, Diatoms) biomass concentration (Gophen 2017, a ,b) was also confirmed by the multi-annual increase of Total Suspended Solids (TSS) measured in the upper 20 meters during 1975-2001 (Figure 4). Two statistical methods confirm it: Fractional Polynomial and LOWESS. Nevertheless, two unexpected and partly contradicted ecological results were recorded: increase of water clarity (deeper Secchi depth, Figure 5) and decline of DO concentrations (Figure 6) during 1990-2001.
3.1.1. Annual Landings
Annual landings of Oreochromis aureus, Bleaks and consequently total catch (LOWESS) declined since the early 1980’s, but those of S. galilaeus (Figure 7, upper right panel, LOWESS) are slightly different with two ebbs and one peak
Figure 1. Upper Panel: Fractional Polynomial prediction of Annual Rain Gauge (mm/year) in the Kinneret Watershed (Dafna Station) (Dependant) and years (Independent) (1939-2016). Lower Panel: Fractional Polynomial Prediction of Annual average of Kinneret Water Level (WL) (MBSL) (Dependant) and Years (Independent) (1939-2016).
Figure 2. Schematic chart of nitrogen loads long term dynamics in Lake Kinneret (surface to 40 m).
Figure 3. Schematic chart of phosphorus species loads long term dynamics in Lake Kinneret (surface to 40 m).
Figure 4. Total Suspended Solids (TSS) (ppm) (monthly means) (1974-2005) averaged for the upper 20 m. Upper Panel: LOWESS Plot; Lower Panel: Fractional Polynomial; Prediction: Years―independent, TSS concentration―dependant.
Figure 5. Annual means of monthly averages of Secchi depths (m) in Lake Kinneret (1969-2001): Fractional Polynomial prediction: Secchi depth (dependant) vs years as (independent).
Figure 6. Annual averages of monthly means of oxygen concentrations (ppm) (DO) above 3.0 ppm in Lake Kinneret during 1969-2001: Upper Panel: Line scatter; Lower Panel Left: Fractional polynomial prediction of DO concentration (dependant) vs years (independent); Lower Panel Right: Trend of Changes (LOWESS) of DO concentration during 1969-2001.
Figure 7. LOWESS plots of annual fish landings (t) in Lake Kinneret (1959-2016) of (clockwise): Oreochromis aureus, Sarotherodon galilaeus, sardines and total catches.
Table 1. Periodical means (SD) of Sarotherodon galilaeus landings (t/year) and indication of trend of changes.
  .
Periodical evaluation of annual landings of S. galilaeus is given in Table 1.
Fish (fingerlings) Stocking in Lake Kinneret started in the late 1950’s  . Since then, several species were introduced but the planting of only 4 of them lasted continually until the present: Mugilids, Silver Carp, and S. galilaeus. As of today, commercial profit is achieved from the stocking of Mugilids, S. galilaeus and Silver Carp. The stocking of O. aureus was profitable until termination of its introduction in the late 1980’s. The number of annual stocked fingerlings of O. aureus and S. galilaeus accompanied by their annual landings are shown in Table 2.
Not surprising is the significant increase of the fish (>90% sardine) population size from the late 1980’s and onwards (Figure 9) (Walline 1987-2005) because the elimination of Sardine fishery caused by near-zero market demands.
4.1. Plankton and Nutrients Fluctuations
As part of the Kinneret ecosystem dynamics, the Phytoplankton and Zooplankton communities in Lake Kinneret have undergone significant modifications of taxonomic structure and biomass compositions that have been widely documented
Table 2. Periodical averages of stocked O. aureus and S. galilaeus fingerlings (106/y) and annual respective landings (t/y) in Lake Kinneret during 1960-2010.
*During 1985-2010 O. aureus stocking was eliminated totally.
Figure 8. LOWESS plot of annually stocked (106) Sarotherodon galilaeus fingerlings (1998-2009).
Figure 9. Fractional polynomial prediction of annual means of total fish number (dependant) (106), (all sizes), acoustically recorded in Lake Kinneret during 1987-2016 (independent).
        . Nevertheless, for the accomplishment of the entire system, the nutrient fluctuations require additional considerations.
4.1.2. Nitrogen Dynamics
The majority of Nitrogen inputs originate externally and partly internally. External inputs of all nutrients are definitely correlated with precipitation and river discharges. Following the enhanced regime of precipitation prior to the 1980’s, a trend of decline was documented afterwards, consequently lowering the lake’s Water Level (Figure 1). The majority of external input of Nitrogen is represented as TN, Total Kjeldahl and Particulate forms and its decline is attributed to the retreat of river discharges. It should be considered that external input which enhances “Particulate Nitrogen” (Figure 2) also includes fixation of atmospheric N2 after the mid-1990’s by Cyanobacteria. The increase of dissolved-N forms (TIN: NO3, NO2, NH4) (Figure 2) is attributed to the enhancement of microbial activity within the processes of nitrification and de-nitrification. Conclusively, the Nitrogen load was suppressed. Those modifications of nitrogen changes induced two response types: the decline of Peridinium and enhancement of Cyanobacteria. Since Peridinium is known to be a significant component of the food composition of S. galilaeus, its decline might be accounted as a factor which has an effect on the decline of this fish. Nevertheless, there was not a periodical overlap between Peridinium (1987) retreat and S. galilaeus landing decline (2007-2008). Moreover, it was recently documented  that S. galilaeus efficiently consumes Zooplankton as a food resource replacement.
4.1.3. Phosphorus Dynamics
Contrary to Nitrogen, Particulate and Total Phosphorus (TP) were enhanced whilst dissolved P fractions declined (Figure 3). External P sources are river discharges and dust deposition, and dissolved inputs are due to bacterial mineralization in the sediments and epilimnetic supply also attributed to Peridinium Cyst-mediated P. Consequently, the dissolved forms’ decline is attributed to the Peridinium disappearance whilst the enhancement of the particulate forms is due to the biomass increase of non-Peridinium algae (Chlorophyta, Cyanophyta, Diatoms) known as efficient incorporators of dissolved P. The data given as Trend of Change (LOWESS) and Predicted Regression (Fractional Polynomial) plots (Figure 4) indicate the enhancement of Particles in the Epilimnion of Lake Kinneret since the mid-1980’s. Surprisingly, Predicted Regression of Secchi Depth (Figure 5) indicates increased clarity (deeper Secchi Depth) from the 1980’s. These data probably reflect the change in Phytoplankton composition from Peridinium (turbid water) to non-Peridinium algae (clear water) (Figure 5). The data of temporal fluctuations of DO concentration (Figure 6) support this conclusion: the decline of Peridinium replaced by non-Peridinium algae initiates a higher efficiency of photosynthetic DO production as well as reduction of DO consumption caused by organic matter decomposition due to the Peridinium Bloom collapse. Conclusively, independent dynamics of N & P has probably no impact on the decline of S. galilaeus stock and landing reduction.
Overfishing is a case of overexploitation of fish (one or more species, or population size) where stocks are reduced to the level below renewal capability. The outcome is, therefore, the disordered sustainable existence of the aquatic ecosystem. Further development is resource depletion through low fish biomass growth rate and consequently reduction of their stocks. The outcome of overfishing succession is a critical situation where fish population is no longer capable of sustaining itself. When this occurs, it is relevant to a single or several species or even the entire ecosystem’s fish assemblages. Overfishing malfunction occurs when more fish biomass is removed than can be replaced by natural or induced (stocking) reproduction.
S. galilaeus is a native component of the Kinneret ecosystem. Therefore, overfishing is not only commercial interference but also a measure of damage to the ecosystem structure. Overfishing occurs not only by biomass removal but also by long- term use of illegal small mesh-size of fishing gill-nets and the over-exploitation of the small-size specimen. If harvested target specimens are of an illegally small size, the maximum yield per recruit is reduced because fewer individuals reach maturity. Overfishing is also due to excessive removal of spawners. Excessive depletion of spawner biomass results in reduction of replenishment of reproductive capacity. Harvest control by implementation of regulated fishing legislations is, therefore, ultimately required.
The focus of this paper is clarifying whether the extreme decline of S. galilaeus landings during 2007-2008 was affected by over-fishing constraints or not and, therefore, other exceptional ecosystem modifications were involved. The practical benefit of such a search might give the optimal management indication to the fishery managers and provide guidance for optimal recovery of the harvest decline   . The decision makers considered, therefore, two contradicting options. 1) The harvest decline was caused by classic overfishing and, therefore, 3 years of total fishing ban is recommended. 2) The harvest decline represents an exceptional natural and partly anthropogenic fluctuation of the ecosystem, and a fishing ban might be damageable and is thus not recommended; predator removal, enhancement of S. galilaeus stocking, enforcement of existed legislations and bleaks fishing renewal are recommended.
Landau    documented outstanding studies on the impact of several factors on the stock and catches of S. galilaeus in
4.2. Ecological Perspectives
The event which is discussed here is extremism of decline of S. galilaeus. On the other hand, the major contributor to the total fish catch in
Long-term fluctuations of ecological parameters are commonly a natural trait of the environmental feature. Presently, anthropogenic involvement becomes more intensive as a result of events of extremism and human population increase accompanied by greater environmental demands. The extremity of the environmental event is mostly due to global warming, water scarcity, and consequent desertification processes. As a result of the elevation of the human mode of life, the society requires the magnification of aquatic (lakes) food supply as well as recreational infrastructure. Consequently, the intensity of pressure constraints on the Kinneret ecosystem was operated. Such a background encouraged the need for appropriate management of the Kinneret Ecosystem where fishery is a priority. Fishery management is aimed at both water quality protection and fisher profit. One of the required conditions for the implementation of those two missions is ecosystem stability. Nevertheless, ecological parameter fluctuations do not necessarily indicate instability. Moreover, cyclic trends (partial or complete) represent the ecosystem’s capability for environmental swinging or resilience where extremes are not desirable. The decline of S. galilaeus was part of an ecological cycling “seesaw” (Pisanti et al. 1987; Pisanti 2005). Beside several known parameters, which supported the decline of S. galilaeus landing, the forwarded consequent increase without a total fishing ban was predicted. The long-term study of the biological dynamics of the population of this fish in
Ironically, the “Ecological-Disordered” management design was basically evaluated on the misleading hypothesis of over-fishing pressure on the S. galilaeus stock. The stock was reduced as part of the natural trait, and its negative amplitude was accelerated slightly by several supported factors, including the short period (1992-2000) of the use of illegal small-mesh gill-nets. In   -  the decline of S. galilaeus stock was documented, but overfishing was not accounted as a cause are documented. Conclusively, it was suggested that “Decline Factors” should be considered and reformed but the tri-annual total fishing ban is not an optimal management approach. This case and the administrative treatment it was given exemplify the need for comprehensive view where not only entire conditions and\or limnological parameters should be incorporated but also long term records and full board of experts involvement are ultimately required. Future perspectives for research include a comprehensive analysis of the Kinneret ecosystem.
Filming verification (Gophen, M. unpublished data) of S. galilaeus nest construction confirmed that spawning ground type is quite often are at open bare (no vegetation) area. The dimple type of the nest constructed by S. galilaeus is shallow and constructed by removal (not dig out as O. aureus do) of coarse items like small pebbles and empty mollusc conches. These shallow nests on bare bottom is exposed to wave action impact and therefore promptly demolished after egg laying and consequently less observed and documented. In contrast to wave action impact on bare bottom, vegetation produce physical protection and therefore dimple spawn better survived, observed and documented. Recent published information  documented S. galilaeus nest densities ranged around 20 - 30 per 100 m2 which briefly calculated as much lower than required to support fish stock recruitment to maintain the present annual landings. The reason is probably eliminating active spawning on bare bottom and absence of wave action protection given by vegetation.
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