The role of fish in Lake Kinneret ecology
by R. Landau, 351/14 Yefeh Nof St., Safad 13403, Israel. E-mail: ruthlandau@hotmail.com
Sections:
Introduction
Since the 1960's, Lake Kinneret has supplied ~1/4 of Israel's water, including water for domestic purposes. However, by the mid-1970's limitations in its usage were apparent. Serruya & Pollingher (1977) described the deleterious effect on water quality of a drop in lake level to -211.8 in 1973 due to poor rains (the lake is 209 m below sea level when full).
On the basis of theory as well as observation the 'red line' was set at -212 m, i.e. the level below which water quality would be noticeably affected. Nevertheless, lake level was allowed to drop close to -213 m in l990 and again in 1991. One of the resulting ecological changes was a sudden expansion of the dominant fish population, the cyprinid Mirogrex terraesanctae, known as the lavnun or Kinneret sardine.
Lowering of lake level has drastic ecological consequences for L. Kinneret because of its inner contours: from a narrow littoral zone, the lake bottom descends steeply to the depths (maximum depth < 40 m). Therefore, removal of water has greater impact on volume than on surface area, so that heat transfer is increased, stimulating processes that release phosphates from the sediments (ibid). Usually, the dominant alga of L. Kinneret is Peridinium, which thrives in phosphorus-poor environments and is not a pollution problem. The more troublesome organisms, small algae and toxic blue-greens, increase in biomass and duration when lake level falls to -212 m or lower.
Concurrently, there is bound to be an increase of organic matter and its breakdown products in the water column (ibid; Hambright et al, 1997). This is a public-health problem because some of the breakdown products are carcinogenic. Also, without adequate filtration to remove organic matter, chlorination must be increased, again raising carcinogens in the domestic water supply.
In lakes and rivers throughout the world there are cyprinids that form dense populations when conditions are favorable, and like Mirogrex terraesanctae these species tend to be difficult subjects for growth studies. Since age and growth data are basic to the assessment of fish populations and their impact on the environment, problems of age determination are stressed in this presentation. In this respect, the population explosion of the Kinneret sardine was providential, as it occurred when older, larger specimens were scarce, so that growth of the young could be traced by year-to-year changes in modal size. Growth studies were also facilitated by the 'dilul' or 'thinning' project that removed 'under-sized' specimens.
After lowering of the water level, the commercially valuable catch of Tilapia (Sarotherodon) galilaea, amnun hagalil or musht, fell from a peak of 500 T/a to 110 T in 2001; less than 100 T is expected in 2003. Analysis of trends in T. galilaea stock size since the 1940's indicate that eutrophicaton is a primary factor. The impacts of changes in Mirogrex biomass, stocking of tilapia fingerlings, and extant of the littoral are examined.
Kinneret research is largely sponsored by the same government bodies that manage the lake; nevertheless, until the mid-1980's, work was objective and on a high standard, as recognized internationally. Difficulties arose when the authorities decided to enlarge storage capacity and usage of L. Kinneret by lowering the 'red line' from -212 m. to -213 m, requiring a new definition of the 'red line'. Drought and a complex of economic and political factors have brought water level down to -214m, dubbed 'the new new red line'. Att the time of writing (September 2001) it is clear that this 'red line' will soon be breached; a decision was made to set the 'red line' at -215.5 m. Update, 2003: Lowest level in 2001 was -214.87 m; lowest level in 2002, -214.42 m.
I do not presume to comment on this policy. My intention is to reveal suppressed data and counter misinformation arising from the tendency of research results to support government policy.
Presented herein are unpublished data on the expansion of the Mirogrex population in the 1990's These and other data on biomass changes in the Mirogrex stock illustrate how cyprinids indicate organic pollution and also serve as alleviating factors.
About the author
Born and educated in Canada (B.Sc. McGill Univ., 1951; M.A. Univ. of Toronto, 1953), my first research was on speciation of blackflies, by mapping their salivary gland chromosomes. Employed by the Israel Dept. of Fisheries from 1962 to 1972 to study tuna and sardine species, I delineated sub-stocks of Sardinella aurita by means of morphometric and growth parameters as well as spacial and seasonal distribution. One of these races disappeared from Israel’s coast after Egypt’s Aswan High Dam went into operation (see http://www.oocities.org/sardinella2002). A low assessment of the ‘palamida’ stock (little tuna, Euthynnus alleteratus) dissuaded investment in this fishery.
From 1972 to 1988 I was employed by Israel Oceanographic & Limnological Research (IOLR) to assess Lake Kinneret fish stocks and delineate their role in the ecosystem. Publications in Hebrew and English deal mainly with Mirogrex terraesanctae (lavnun or Kinneret sardine) and Sarotherodon (Tilapia) galilaeus (see T. galilaea webpage and references).
Misinformation and suppressed data on Lake Kinneret ecology
Listed below are some
examples of distorted information on phytoplankton and on the Kinneret
sardine. Reported changes in these populations obscure their connection
to the increased eutrophy resulting from lowering of lake level.
Not all Kinneret research
is obscurant. Parparov
(1994) found that oxygen depletion rates in the period 1969 - 1991 tended
to increase with lowering of Kinneret water level. He attributed this
trend to a combination of factors including a 'sharp increase of internal load
by autochthonous suspended organic matter with high specific oxygen
demand'.
Comparing Lake Kinneret to
Lake Sevan in Armenia Parparov &
Hambright (1996) noted that increased fish production is a predictable
outcome of lowering of lake level, as well as deterioration in water quality.
Berman et al (1998) describe
the stable pattern of seasonal succession of phytoplankton populations that
lasted many years, and the drastic changes since 1994. The characteristic
Peridinium (dinoflagellate) blooms of late winter- early spring intensified in
1994 and 1995, but disappeared in later years. Cyanobacteria (blue-green
algae), generally unimportant in earlier years, became prominent. Massive blooms of the toxic blue-green alga Aphanizomenon were
first observed in 1994 and have appeared sporadically in late summers
since then.
However, like most authors dealing
with Kinneret ecology, Berman et al (1998) suppress the
lake-level-connection. They attribute phytoplankton changes mainly to inflow of
nutrients and/or growth factors, but no evidence of increased input since
1994 is presented, and changes in water level are not mentioned.
Dealing with the impact of
increased nutrient load in L. Kinneret, Gophen (Israel Oceanographic &
Limnological Research internal report T8/2003) was apparently unimpressed by
the mininum water level in 2002, -214.42 m (the legal 'red line' was
-215.5 m). He advised 'lake load reduction by pumping water for
supply'.
Hambright et al
(1997) noted that lowering of water level increased availability of
phosphorus, while Hadas
et al, 1999 and others reported that phosphorus is critically important for
the development of blue-green algae. The obvious connection of
water level to toxic algae was not mentioned by these authors.
In a publication by N.
Abulafia & R. Amir intended for high school biology teachers (Alon l'morei
biologiah 151, Hebrew University, 1997) the Aphanizomenon blooms of 1994 and
1995 are attributed mainly to the ability of this organism to utilize
atmospheric nitrogen. Increased availability of phosphorus is also
recognized as an essential factor; decomposing Peridinium is considered by
these authors to be its source. This article, based on Gophen &
Nishri's paper 'Blooms of blue-green algae in Kinneret' (Ecologia v'sviva 4:1,
1997), does not mention the lowering of lake level or its effect on phosphorus
availability.
On the basis of other
scientific authorities, the Water Commissioner's Office informed the public
that toxic algae in the lake are mainly due to influxed nutrients from cowsheds
and fertilizers (The Jerusalem Post, Feb. ll, 2003 and other newspaper
reports). This implies a rise in pollution from agricultural sources
since the 1990's; there is no evidence of such a change.
Nearly every aspect of the
life history and population dynamics of the Kinneret sardine has been 'revised'
since 1990.
The lavnun population is
regarded as detrimental to water quality due to its predation on zooplankton (Gophen, 1995 &
others). Well-known stabilizing effects of predation on the ecosystem are not
mentioned, nor the possibility that eutrophic conditions may alter food
preferences.
The lavnun is familiar to
fishermen for ages as a substrate spawner in both deep and shallow water, but Gafny et al, 1992
and others recognize only inshore spawning. A reduction in the sardine
population was expected to follow the receding shoreline.
Ostrovsky & Walline (1999) suppress data indicating slow
growth of the lavnun (see Mirogrex growth).
Fisheries data have been
widely accepted as evidence of biomass fluctuation in the sardine stock (Gophen
& Landau, 1977; Reich, 1978; Landau, Walline & Gophen, 1988; Gophen
& Threlkeld, 1989; Landau, 1991 and others). As an expansion in the order of
ten times between the 1950's and the 1980's is indicated (Landau, 1991),
there can be little doubt that changes in natural abundance far outweigh error
due to methodology.
However, when Walline et al,
1992 published their acoustic assessment of pelagic fish in L.
Kinneret, they dismissed the time-honored use of fisheries data in stock
assessment as complicated by "…market effects on catch and difficulties in
estimating effort…". Henceforth, all estimates of the lavnun stock (other
than Landau estimates) vary between 1000 and 5000 T (Walline et al, 1992;
Pisanti & Shapiro, 1997; Ostrovsky & Walline, 1999). In contrast,
a peak biomass of ~20,000 T in the mid-1980's was estimated by
conventional methods of stock assessement which include age & mortality
estimates as well as catch & effort data (Landau, 1991; WebPage Mirogrex biomass).
Acoustic monitoring of
pelagic fish in L. Kinneret is based on counts of fish, divided into size
categories; the most important category, 10 - 20 cm, encompasses the Mirogrex
commercial stock. Biomass is estimated by multiplying number by mean weight in
each category. Instead of estimating mean weight from observations
Walline et al (1992) assigned the same value to all years
from 1981 to 1990. In effect, they assigned stability to the Mirogrex
population, in contradiction to the wide fluctuations in size distribution that
were observed from 1981 to 1990 (Davidoff, 1982; Landau, 1991).
In 1993, two years after
lake level was lowered to ~ -213m for the first time, massive quantities of 8 -
10 cm lavnun began to appear, and the 'dilul' (thinning) project
was initiated as a remedy for the damage supposedly inflicted on water quality
by the fish (Gophen, 1995; Pisanti & Shapiro, 1997; Ostrovsky &
Walline, 1999 and others). However, acoustic estimates of lavnun, a
few thousand tonnes, are so low that the fish would hardly have an impact on
zooplankton, presumed to be the main food of the lavnun. The 'dilul' project continues,
year after year, to the time of this update, 2003.
The Kinneret Authority has
kept accurate records of 'dilul' catch and effort by the 3 units
operating. These data indicate a fish biomass much higher than in the
mid-1980's when peak catches were taken. In any stable fish stock,
juveniles constitute a small part of total biomass; therefore, the massive
increase in small sardines in the 1990's was indeed a population
explosion.
According to Berman et al
(1998) there were no reliable estimates of Kinneret fish populations until 1988
when routine acoustic surveyal began; other estimates are considered unreliable
due to 'highly dubious effort records'. On the contrary, acoustic estimates are
virtually meaningless due to the data manipulation described above, and to high
sample variance. Partial detection of the stock in each sampling trip can
account for this variance and also for the low numbers observed.
Some researchers regard the study of fish populations more as a political
excercise than as scientific work. Replacement of conventional stock
assessment by acoustic surveyal is politically expedient since the higher (and
more reliable) estimates indicate a great increase in organic matter in the
water column. By suppressing this information researchers have obscured
the consequences of management policy.
The potential impact of
organic pollution on lavnun stock size was demonstrated in the mid-1980's by
studies on food and distribution of fish larvae (Landau et al, 1988 ,
Mirogrex biomass).
Earlier, in dealing with the effect of organic matter on water quality, Serruya and Pollingher, (1977) made no mention of fish
populations, probably because fish were not regarded as ecologically important
in the 1970's.
However, even after the
lavnun population explosion, Hambright et al, 1997 failed to mention fish
stocks, while confirming the earlier hypothesis (of Serruya & Pollingher)
on effects of low lake level on water quality.
While the lavnun population
is either ignored or grossly underestimated, it is also blamed for
deterioration of water quality due to its consumption of zooplankton which is
regarded as the main cleaning agent. The webpage Mirogrex biomass changes
presents data showing sardine biomass in the 1990's to be so far in excess of
zooplankton that other food sources must be postulated, - smaller organisms and
lake snow. Thus, in eutrophic and hypertrophic conditions, fish are more
important than zooplankton as cleaning agents.
The Israeli public is well
aware of L. Kinneret's receding shoreline and the increased danger of drowning
due to deep water and strong currents close to shore. But the term 'water
quality' is usually assumed to refer to taste or salt content. The media never
use the 'C-word', as far as I know, in respect to Kinneret water. Oft-used
phrases such as 'water quality' or 'irreversible damage to the lake's
ecosystem' have little meaning because essential information is missing. This
explains, at least in part, the reluctance to accept water saving measures in
spite of a barrage of pronouncements on the ongoing water crisis.
Tilapia galilaea biomass
changes