Ecological Notes of the Fishes and the Interspecific Relations among Them in

Taizo MIURA

Introduction

Lake Biwa has been much interested by biologists, specially by ichthiologists because of the richness and variety of the lake fish fauna and of their diverse origin. Therefore, a good number of papers on taxonomic researches of the fishes have been published. However ecological studies of the fishes have been poorly made. Freshwater environments as well as terrestrial ones have been practically reformed, accompanied with development of human society and specially of industries and agriculture. Lake Biwa is also not an exception. Recently the ministry of construction is planning to construct a dam between the north basin and the south basin of the lake, and to pump up water from the north basin to the south one for the purpose of holding a continuous water supply to the outlet river. By such programme the two basins will nearly be disconnected and the water level of the north basin is often expected to be lowered. It can be easily expected that production of the organisms in the lake will be affected by the reformation, so that it 'is important to predict how the organic production will be affected, and to propose how the programme should be made in order to benefit the local people, especially the fishermen. Thus, about sixty scientists including biologists, physicists, chemists and agricultural economists had formed a co-operative research team and have investigated the biotic re- sources of the lake from 1962 to 1965. Through this investigation ecological studies of some of the fish species have been intensively made. This paper presents, in the first place, descriptions of ecological elements of the fishes including spawning, habitat, feeding and growth etc., and introductions of some other ecological works. Secondly, by comparison of their ecologies,

Contribution from the Animal Ecological Research Group, Department of Zoology, Kyoto University. Contribution from the Otsu Hydrobiological Station, Foreign Language Series, No. 182. Contribution from the B. S. T. (Research Group for Biotic Resources in Lake Biwa), No. 64. 50 interspecific relationships are considered. The materials presented here are inadequate for discussing the fish community of the lake, however, the author believes that this article is usable as a step of further investigation of the fishes in this lake.

History and environmental descriptions of the lake

Lake Biwa (35•‹15'N., 136•‹05'E.), the largest lake in Japan, is situated just on the isthmus which seems to be the distored portion of arcs building the Honshu

Island (HORIEE, 1961). Through analysis of Post-Pliocene sediments around Lake

Biwa and of fossils from the deposits, TAKAYA (1963) has established the follow- ing three paleolimnological stages of of Paleo-Biwa Lake ; "Old Closed Lake",

"Open Lake" "Young Closed Lake" . According to TAKAYA (1963) the first stage

of late Pliocene age has provided the immigration of the fauna and flora of

continental element to the lake under a warm climate. At the second stage of

the earliest Pleistocene, the lake, "Open Lake", had a broad channel leading to

the sea at the southwestern end of the lake without any distinct topographicar

barriers, though it was thoroughly lacustrine. The channel is supposed to have

prepared a quite different condition for marine dwellers and to have given them

the opportunity of invasion into the lake. When the channel was disappeared

by a very strong crustal movement at Early Pleistocene the third stage started

and has continued till the present time. Perhaps, almost all the species which

had invaded before and during the channel stage were compelled to become

perfect land-locked species. Passing through several occasions of water levelling

which might be responsible for terrace formations during the "Young Closed

Lake" period, the present lake basin had been formed.

The lake is about 68 km long and 22.6 km in width. The surface area is

approximately 674.4 km2, the length of shore line 188 km, the maximum depth

104 m, the volume 22.6 km3. The lake is devided into two basins; the north

basin and south one. The former occupies approximately 92% in area and 99%

in volume. The latter is small and shallow, the maximum depth 7.5 m and

mean depth 2.8 m.

The lake has numerous inlet streams which is occasionally dried up in dry

periods. The main tributaries are located around the north basin. There is

only one outlet, the Uji River, which has a high control dam at the southwestern

end of the south basin. 51

According to MORIKAWA and OKAMOTO (1963) and MORIKAWA (1964), the surface

temperature of offshore water seldom rises 30•Ž in summer. The lake begins to

show thermal stratification in May and thermocline gradually becomes remarkable

toward August (Fig. 1). It develops between the depths of 10 m and 30 m in

summer. After then it is gradually reduced by cooling of the surface water and disappears in the end of November. In winter and spring thermal stratification entirely disappears and the water temperature is nearly the same from the surface to the lake bottom (Fig.2). The lowest temperature of offshore water seldom shows less than 6•Ž in the northern basin. FOJINAOAet al. (1966) indicates that even during the summer stagnation a fair amount of dissolved oxygen can be observed in the hypolimnion, and oxygen deplession never observed in the entire area of the south basin and of the north basin. The same authors describe that the chemical nature of the north basin is intermediate between oligotrophic and mesotrophic and of the south basin is eutrophic, as far as contents of nutrients are considered. Such differences of chemical nature as well as lake topography more or less reflect upon organisms. Density of phyto-and zooplankters are observed greater in the south basin than in the north basin, with the exception of winter. Typical example can be seen in larger aquatic plants. IKUSHIMAet al. (1962) state that the small bottom area of the south basin which is only 8.5 % of the total area sustains 40% of the total yield of aquatic vegetation.

Fish fauna of the lake

Lake Biwa and its tributaries have been recorded to contain 62 fish species and 2 subspecies. However, during the intensive investigation of the lake by the Research Group for Biotic Resources in Lake Biwa, 49 species and 2 subspecies observed as the inhabitants of the lake today (Table 1). Among these species are several that include endemic ones, namely, nigorobuna, gengorobuna, hon- moroko, hasu, iwatokonamazu, biwako-onamazu and isaza. Hasu and isaza have been considered as a relict (UENO, 1943, HORIE 1961, TOMODA1963 a, b), while the rests may be due to autochthonous intra-lacustrine speciation (TOMODA1962,1963a, b, 1965). 52

Table 1. Species list of the fishes in Lake Biwa. 53 54

Ecological notes of the fishes in the lake

Ecological notes of 9 species, which have been intensively investigated in recent years because of their ecological as well as economic importance will be described below. Besides these, brief notes of spawning period, spawning place, growth, habitat and food of fairly abundant species are listed in Table 2. Biwa-masu (Oncorhynchus rhodurus) Biwa-masu appear to . spawn in inlet streams. Spawning occurs from the middle of October until late November. The main spawning streams are the River Ado, R. Ishida, R. Chinai, R. Shiozu-okawa, R. Ane and R. Inukami. The majority of spawning migrants are caught by weirs and traps before they arrive at the spawning place. Eggs of the spawners caught are artificially fertilized before marketing and the eggs are sent to the hatchery. The youngs reared in the hatchery are stoecked into the lake in spring of the succeeding year. It is likely, therefore, that the majority of recruits are hatchery fish. The main part of the male is covered by 2+ and 3+ year fish and of the female by 3+ and 4+ ones. The 5+ spawners are extremely rare (MIURA,unpublished M.S.) In warm season, both the youngs and adults inhabit in the hypolimnion and in cold season they expand the habitat into the epilimnion. The result of growth estimation by the scale method indicates that mean body length attains 24.0 cm by the second year., 36. 5 cm by the third and 42.0 cm by the fourth (MIURA,unpublished M. S.). The types of food organisms vary with the size of the fish. The youngs eat planktonic crustaceans and gammarids, while the adults take small fishes such as ayu and isaza (Shiga Pref. Fish. Res. Lab. 1942). Ayu (Plecoglossus altivelis) AZUMA(1964) summarized many features of the ecology of ayu in Lake Biwa. He reveals that there exist two groups of ayu which differ in their mode of life as well in morphological characters. Either of them spawn in the inlet streams in Autumn (September-October) and die there. The larvae hatched out drift with the current toward the lake at night. After then they appear on the coastal bottom near the streams. In October the larvae (40 mm BL) can be collected at night in the surface water of the off-shore area. Further developed larvae (50 mm EL) seem to have a habit of aggregation around the bottom layer of shelf during the daytime and migrate toward the inshore area during the 55 nighttime. Such distributional and migratory patterns are continued untill late winter. Food organisms observed in the alimentary tract of the fry and juvenile are water fleas and copepods. The fish of these stages likely feed on these zoopla- nkters during the daytime and unlikely feed during the nighttime, because the majority of alimentary tracts of the fish caught during the daytime are filled with the zooplankters but of the fish caught at night at the inshore area are entirely empty. A remarkable change occurs in diurnal migration in spring. They migrate toward the shore during the daytime and return to the deep part of the lake at night. They still feed on zooplankters in considerable amount at night. In the same period they diverge into two different directions, one is the" upstreaming population which migrates into inlet streams from early spring to early summer and stays there until the spawning period, while the other, the lake remained population, occupies the surface and mid-layer of the pelagic area throughout warm season and feeds on zooplankters. Through comparison of relative abundance of each two successive life history stages of ayu in Lake Biwa, MIURA (1965) points out that up to the late fry stage density dependent effect may not be important and abundance of each stage is proportional primarily to the size of biomass of spawner population, although spawning success plainly depends on presence or absence of heavy rain in the spawning period. While from the fry stage to the juvenile one the rate of mortality and/or of growth seem to fluctuate in compensatory way which is supposed to be related to a shortage of food during the cold season. MIURA (1966) gives further suggestion on population fluctuation of ayu in the lake. The results of comparison of ecology of the two species, ayu and isaza, suggests that interaction of these possibly occurs in cold season when there is no other competitor. The significance of interspecific competition between them can be evaluated as a factor controlling the rate of recruitment of juvenile through analysis of the Prewar catch statistics. However, presence of the compensation between juvenile stage and the adult one relieves the ayu popula- tion of influence of interspecific competition which they will suffer in the stage of juvenile. Ayu is ecologically important as a prey of the predatious species such as hasu and biwamasu. This will be discussed later. 56

Honmoroko (Gnathopogon elongatus caerulescens)

Matured fish which stayed in the bottom layer of the offshore area (60 to 80m

deep) in winter start to migrate into the inshore area in late March and spawn in the bays and lagoons from April to late June (MAKI 1964, 1966a). Habitat of the larval fish has been left unknown although they seldom be observed in

the submarged plant zone where they are spawned. The youngs of 20 to 40mm

in body length occupy the 'bottom layer of the inshore area (3 to 5m deep) during the first summer, and in late autumn they migrate into the bottom layer

of the central part of the north basin (60•`80m) and stay there throughout winter. The adult fish can be commonly found in the bottom layer (5 to 15 m deep) of the inshore area in summer, while in winter they stay in the same water

that the youngs inhabit (MAKI 1964, 1966 a).

Although the fish are primarily zooplankton feeder, when they migrate into

the shallow inshore area in spring, they feed also on chironomid larvae and terrestrial insects as well as on zooplankters (MAKI 1964). SIINAGA (1964) ob- served noticeable food preference of the fish. Several species of minnow selectively feed on Daphnia.longispina where the plankter is abundant,. but even in the water where the plankter is scarce, honmoroko still concentrates on the plankter although the rest show entirely different food habits.

MAKI (1966 a, b) observed that one of the critical life-cycle stages for the yearly fluctuation of this fish population lies in the first winter, when the fish inhabit

in the bottom layer of the deepest part of the lake. By comparison of the length frequency curves and of the weight ones, he indicates that small sized individuals disappear during the cold season, probably die out. The poorly grown individuals are, he suggests, more disadvantageous in winter life than the well grown ones, because the small sized individuals may be less resistant for severe winter, presumably due to poor accumulation of fat which may be used by the fish for energy source in winter because their feeding activity is extremely lowered during

the period.

Hasu (Opsariichthys uncirostris)

The fish spawn over an extended period, from late May to early August.

Spawning occurs over sandy gravel bottoms in the inlet streams and at the lakeside. Most part of males mature at 3 year of age (16cm, mean B. L.), and approximately a half of female spawners are at 2 year of age (13cm mean B. L.) and another half at 3 year (16 cm, mean B. L.). A few individuals of 4 and 5 57 year of age can be observed among the spawning population (TANAKA 1964).

The fry appear in the open lakeshore around the spawning place, where there is a tendency to form a summer aggregation. In late summer the fish that reach to the juvenile stage with the body length of 4 to 5cm leave the lakeshore, and disperse along the inshore area of the whole lake. They move down into deeper water (30 to 60m deep) during winter. In spring of the successive year they appear in the inshore area without forming a dense aggregation as they show in the first summer. The adult fish also occupy the inshore area through out the year with the exception of the spawning period.

A tagging experiment suggests that movement of the species is remarkable.

Some of the tagged fish released in a place were caught from the entire area of the north basin in a week.

TANAKA (1964) made a study on the relation between food habit and growth of the fish. The mean body length of the 0-year fish reaches 6cm by early spring, and the 1-year ones 16 cm (•Š) and 18 cm (•‰) in the same period. Stomach analyses show that their diet changes with increasing in size. Until reaching

7cm their principal diet is zooplankton and the fish of 7cm in body length start to eat fish though they still feed partly on zooplankton. Big individuals of longer than 18cm in body length do not eat zooplankton but mainly take fish.

The main prey fish species is ayu.

MIURA (unpublished M. S.) suggests that the ayu population has an important role for production of the hasu population through analysis of three variates; yearly abundance of the juvenile population of ayu, that of the adult population of ayu and that of the adult population of hasu.

Wataka (Ischikauia steenackeri)

The fish spawn from late June to early July. Eggs deposited on floating ma- terials in the marsh-reed zones. Hatched larvae are not found near the spawned place, but in the muddy bottom zone of 2 to 3 meter deep. The juvenile and adult inhabit in the reed and submerged plant zone for a whole year except the spawning period of gengorobuna and of nigorobuna (MART, 1964).

The fish reach a total length of 7 to 8cm in their second summer and that of

15 to 20cm in the third summer and mature. Growth in length is much reduced in higher aged fish. The oldest male recorded is 5 years (total length of 20cm) and the female is 8 years (total length of 30cm) (NAKAMURA 1950).

The O year fish are a typical plankton feeder, and the 1 year fish are omni- 58 vorous and feed on chironomid larvae, shrimps, small clams in addition to zooplankters. Principal diet of the fish older than 2 year is submerged plants

(MAKI 1964).

Nigorobuna ( carassius grandculis)

The fish spawn over an extended period, from mid-April to early July. When water level of the lake goes up after heavy rain, they frequently spawn and deposit eggs amongst submerged plants and floating seston in the bays and

lagoons. The catch statistics of the spawning population indicates that the south

basin is a principal area.of spawning of the species.

The fry stay in the submerged plant zone until midsummer when they reach

a length of 4 to 5 cm. In late summer they leave the place toward a deeper

water (MAKI 1964). The juvenile and adult inhabit in the bottom layer of the

depth of 20 to 40m a whole year around except in the spawning period, although

they occupy a deeper water in winter.

Mean body length of each year class is estimated by scale method as 7.4•}

0.22 cm in the 1 year class, 12. 9 •} 0.34 cm in the second, 17.1 •} 0.37 cm in the

third and 19.5•}0.66cm in the-fourth (MAKI, unpublished MS).

Stomach analysis shows their dietal change with progress in the life history

stages. The fry not exceeding 20mm in body length feed mainly Chydorus, a

small cladoceran, and partly Bosmina, , and chironomid larvae

(HIRAI, unpublished MS). The fish exceeding 20mm in body length continue to

feed on algae attaching on the submerged plants until they leave there (MAKI

1964). The juvenile and adult eat mainly zooplankters and partly chironomids

and other aquatic insects.

Gengorobuna (Carassius carassius cuvieri)

The spawning period of the species is very long, extending from early April

to early June. Spawning places are fairly restricted on some areas of the north

basin, and the most important one is the Hayasaki Lagoon which is situated at

the north-eastern coast of the north basin (NAGOSHI 1965). Some spawn in the

south basin. NAGOSHI (1965) reports that the majority of constituents of the

male population are three to five years old, and those of the female population

are four to six years old. Both the constituents occupy 90 per cent of the

spawning population.

According to Nagoshi (personal communication) the body length of the 0-year

fish reaches 9 to 11cm, of the 1-year ones 15cm, of the 2 year ones 23 to 25cm 59

and of the 4 to 6 year ones 30cm. There are some differences between growth

patterns of male and female. The growth rate of female is higher than that

of male among the individuals greater than 16 cm in body length, and the ultimate length, in Walford's definition, of female is calculated as 33.7cm and of

male as 30.4cm.

First summer life of the species is similar to that of nigorobuna, as far as their habitat and feeding habit' are concerned. While the juvenile and adult show

quite different life from nigorobuna. They occupy the surface water of the offshore area of the north basin and form a large school. They are known as a phytoplankton feeder.

NAGOSHI and MIURA (1964) estimated size of the spawner population at the

Hayasaki Lagoon by tagging method. The estimated number of the spawners in the lagoon from May 11 till May 23, 1963 is 248, 000•`649, 000, of the total immigrants 151,000•`555,000, and of the total emigrants 132,000•`541,000.

NAGOSnI (1965 and unpublished MS), studied the fishing rate on the same popu- lation in 1964, and then he estimated the total biomass of the adult in the lake

as 1,469 tons.

Yoshinobori (Rhinogobio brunneus)

The fish spawn over an extended period, from April to October. Spawning occurs over the bottom from the shore line to the depth of 12m. Eggs are deposited on gravels, shells and deed plants such as reeds. The male protects fertilized eggs until they hatch out.

The fry are distributed in the whole lake, but they are much fewer in aquatic plant zones. When the larval fish reach a length 20mm, their life form changes from pelagic one to bottom, and some migrate into the inlet streams. The juvenile and adult tend to inhabit in the inshore water shallower than 10 m in warm season. In cold season they migrate towards deeper water (MIYADI et al.

1965).

The length of hatched fry is approximately 4mm. Individuals hatched in the early part of the season reach a length of 15 to 25 mm in late summer and of

30 to 40mm in the second summer. They mature in the second warm season and very few live till the third summer (MIYADI et al., 1965). At the stage of pelagic larvae they feed on zooplankters, specially on cladocerans, and the juvenile and adult on chironomids in addition to the zooplankters.

Yoshinobori is one of the important preys of predatious fish species, especially 60

Table 2. Ecological notes of the fairly abundant fishes in Lake Biwa. 61 62

for the juvenile of hasu in warm season (TANAKA1964). KOBAYASHI(1952) pointed presence of negative correlation between yearly abundances of yoshinobori and of isaza although actual processes are still unknown. Isaza (Chaenogobius isaza) This species, preferring cold water, is well known as an endemic one of this lake. NAGOSFII(1966) describes that the spawning season covers the period from April to June and eggs are shed under the bottom pebbles at the inshore area ranging from the shore line to the depth of 7m. Newly hatched larvae which are 3.0mm in length migrate from the spawning bed to a depth of approxima- tely 30m. Both juvenile and adult of isaza show diurnal vertical migration (NACosru 1966). They stay in the bottom layer deeper than 30 m during the daytime and come up to the middle layer but never into the epilimnion. In cold season, however, when the temperature of the surface layer goes down, the upward migration reaches to the surface layer. The fish of age 0 feed mainly on zooplankters such as cladocerans and cope- pods. Though the fish of age 1 also feed on- zooplankters, there-is a certain difference from the age 0 group in their diet. The 1+ fish consume a large gammerid, Anithogammarus annandalei, in addition to zooplankters (NACOSHI, 1966), and specially in winter their stomachs are filled with the gammarid (KOBAYASHIand YAMANAKA1950). NAGOSHI(1966) studied the effect of population density on growth of isaza of the lake. He indicates that a considerable yearly variation occurring in growth is in close connection with the fluctuation of population density, and suggests that the influence affected by the density of age 1 group is greater than that by age 0 group. Actually, the body length of the 0 year fish widely varies from year to year;that of the year of minimum size was 3.5cm and of maximum size was 4.3cm, and those of the 1 year fish were 4.5 cm in the year of minimum size and 6.3cm in the year of maximum one.

Fish association in the lake

As described in the previous chapter, habitat requirements of the fishes signi- ficantly differ from one species to another and are changed with progress of developmental stages and also with changes of season. One major division may be made by their preferences to temperature. The 63

fishes could be devided into three groups ; the first group includes warm water species such as most cyprinids, the second includes cold water species such as biwamasu and isaza, and the third includes ayu, gengorobuna and adult hasu which adapt to a wide range of temperature. Thermal stratification in warm season prepares three different layers for the fish inhabitants. The warm water species and the third ones occupy the epilimnion of the lake and form the shallow water association, while the cold water species form the deepwater association (Fig. 1). In cold season when the thermal stratification disappears, the cold water species and the third ones form the shallow water association and the warm water ones which pass the extreme period in inactive condition form the deepwater association (Fig. 2). It is interesting that the fish species belonging to the third group occupy the euphotic zone and continue an active life throughout the year. The summer shallow water association may be devided into several ones by habitat preferences of the fishes. Vertical stratification of the fish inhabitants

Fig.1. Associations and food relations of the fishes and temperature distribution in Lake Biwa in summer. 64 can be seen. Ayu, gengorobuna and hasu occupy the surface layer, and zezera, gigi, namazu and yoshinobori live closely attached on the bottom. Among the mid-layer dwellers honmoroko, sugomoroko, dememoroko, nigorobuna and higai are included. Horizontal stratification is also significant. In weedy lagoons and bays bitterlings, fry of nigorobuna and of gengorobuna and kamuruchi can be observed, and along the open lakeside fry of hasu and oikawa occupy the shallow area while honmoroko, nigorobuna, zezera and yoshinobori occupy somewhat deeper water area. In addition to this, among the demersal fish species further habitat segregation can be seen, probably depending on their preferences to bottom types.

Fig.2. Associations and food relations of the fishes and temperature distribution in Lake Biwa in winter.

The food relations of the fishes of the lake

LARKIN(1956) suggests that by comparison with the complex type of population interspersion for a forest community, the spatial organization of plants and animals in freshwater habitats is extremely simple. However, MIURA (1962) 65 states that Larkin's conclusion is fully acceptable for a stream community, while in large lakes a rather complex distribution of organisms does exist and is probably related to the physico-chemical factors. In Lake Biwa there exist well developed stratifications of environmental conditions which are very significant for the lives of the fishes if the inshore area of the lake is carefully observed. Such stratifications reflect on the diversity of food habits of the fishes in the lake.

Fig. 1 shows the food relation of the abundant fishes in the lake in warm season. Diversity of their food habits is fairly significant. Although some interspecific overlapping of stomach contents can be seen, specially among zoo- plankton feeders, interspecific relations could be minimized if spatial segregations between them exist. Interspecific relation of ayu, a surface dweller, and isaza, a cold water species, can be neglected here because there is no other zooplankton feeder in their habitats. Among the mid-layer dwellers, honmoroko may not come into contact with the other species since the fish occupy somewhat shal- lower areas. While the rest of the mid-layer dwellers ; sugomoroko, dememoroko and nigorobuna which are considered to occupy very similar waters, may have a close food relation with one another. The predator-prey relations among the fishes in warm season occur independ- ently in each temperature zone ; between hasu and ayu, and partly hasu and yoshinobori in the epilimnion, and between biwamasu and isaza in the hypo- limnion. In cold season food relations of the fishes are entirely different (Fig.2). As described in the previous section, majority of the fish species migrate into the bottom layer of the central part of the north basin and stay there without feeding actively, so that these fishes may be out of consideration here. Occurence of interspecific food relations have to be checked among hasu, biwamasu, ayu, isaza and gengorobuna. Gengorobuna which keeps the same mode of life as in warm season, is also negatively related with others. As briefly mentioned already, MIURA (1966) points out that interaction of ayu and isaza possibly occurs in cold season. In this season not only the horizontal distribution of both species is overlapped, but also the vertical distribution is very similar with each other during daytime when ayu feeds on zooplankters. Although the food of 1+ isaza significantly different from that of ayu in winter, the composition of the diet of ayu and the 0+ isaza is largely overlapped, which 66 indicates that their requirements of food organisms are very similar. Density of the zooplankters is extremely low in winter. The scarce planktonic crustaceans may give for ayu and the 0+ isaza the opportunity of severe competition for food between them, if the plankters are virtually eliminated by vigorous of both. Even if the plankters are not eliminated, it might be expected that they compete indirectly for food as a result of competition for other resources such as space. In addition to the prey-predator relation between hasu and ayu, the extension of spacial distribution of biwamasu towards the surface water induces the new relation between biwamasu and ayu. Since isaza also extends its habitat into the surface water in cold season, the relation between biwamasu and isaza exists. It is unknown whether interrelation between these predatious species does exist or not because biology of biwamasu in the lake has been only little known.

Interspecific relations and coexistence among the fishes in the lake

One of the most frequently argued ecological principles may be an interspecific competition. GRINNEL(1909), as recently quoted by UDVARDY(1959); said: "Two- species of approximately the same food habits are not likely to remain long enough evenly balanced in numbers in the same region. One will crowd out the other. If each species occupied a different ecological niche, then competition for the resources of the environment would be eliminated and two species would be expected to coexist." FRYER (1959 a, b) demonstrated this principle the fishes of a tropical lake. MiuaA (1962) indicates through series of observation of fry of five inshore species in a Canadian lake that interspecific divergence in their mode of life including habitat preference and food habit appears in fairly early stages of their life span. Actually, he states, the fry of the five species require the same environmental resources for first two weeks of their first summer. MIURA (1962) gives a further suggestion that interspecific competition is the reason why there is a good positive correlation between the number of species of fish in a lake and the size of the lake which is chosen from the lakes in the Fraser River system. This may indicate that competition is less in large lakes because they usually show more varieties of habitats to choose from and they have a greater area for regional isolation to develop. Since Lake Biwa is very large and markedly rich in varieties of habitats, specialized feature of the fishes in mode of life, which is probably one of the 67 most important elements for their coexistence, is highly expected. Divergence seems to be distinct among phylogenetically related species. Three species of piscivorous catfish in the lake occupy entirely different habitats from each other (TOMODA1962). Namazu occupy aquatic plant zones of the shallow inshore area while the other inshore dweller, iwatakonamazu, occupies rocky or gravel bottoms. Biwako-onamazu dwells in the deep bottom layer of the offshore area. Similar spatial and food segregation by fishes of a phylogenetically related group can be seen among gengorobuna, nigorobuna and hiwara, and between honmoroko and tamoroko. It is interesting that either of the groups mentioned above includes endemic ones which are assumed to be a fish specialized in this lake. This might indicate that stratifications of the environments and the diversity of habitats as seen today had existed for a long paleolimnological age. There exists spawning segregation besides spatial or food one. HIRAI (1964) shows an example of this among the four species of bitterlings in the lake. Ichimonjitanago and kanehira lay the eggs into the bodies of mussles distributed in a shallow bay, however the former spawns from May to August and the latter is an autumn spawner. The eggs of yaritanago and tabira are spawned into the bodies of mussles distributed in an open inshore area in the same season, however their hosts are largely different although partly overlapped (see Table 2). All of the fishes in Lake Biwa seem to have species specific mode of life which may be differentiated into several developmental stages at which they require more or less different resources of the environments. This feature does not mean absence of interspecific competition. They still have opportunities of competition in some occasions. Aforementioned interspecific relation between isaza and larval ayu in winter can be a typical example. Although competitive influence of isaza on juvenile ayu seems to be great, they coexist for many hundreds years. One of reasons why isaza does not crowd out ayu may be that presence of the compensation between the juvenile stage and the adult one of ayu relieve the ayu population of influence of interspecific competition which they will suffer in the stage of juvenile (MIURA 1966). Another mechanism by which two or more species share the resources they require has been discussed. It is known that populations of closely related, allopatrically living species can have closely situated ecological optima, and that in this case intraspecific competition alone can compel each of the species to 68 utilize all its ecological potency. If, on the other hand, the populations live sympatrically, and under interspecific compeition, the optima are removed from each other, while at the same time the amplitudes around these now optima become narrower (SVARDSON1949, NILSSON1955, 1956 cited by KALLEBERG1958). If such displacement of the optima occured only temporarily or seasonally, it would be possible for the fish to recover from these unfavourable periods. In such cases, if the fish is able to survive, there may be few detrimental effects. Temporal or seasonal interspecific competition may be expected to exist among some other fishes of Lake Biwa, for instance between sugomoroko and dememo- roko and among fry of gengorobuna, of nigorobuna and of several species of bitterlings in weedy bays and lagoons. The mechanisms of avoiding severe competition and of recovering from competitive influence, as mentioned above, may be at work for minimizing these competitive effects, and may make the unfavourable situation possible to coexist. Most of the reports of fish control programs in ponds and lakes emphasize that overpopulation and stunting are a result of too 'small a population of natural predators "(LAGLER1944, BENNETT1944, 1947, SWINGLEand- SMITH1941). LAGLER (1944), while basing his discussion in part on the research done on other animals, suggests that the end effect following predation may be little different from that which would have resulted had predators been absent. MIURA (unpublished MS) observed a similar phenomenon between hasu and ayu in Lake Biwa. Through analysis of three variates;yearly abundance of the juvenile population of ayu, that of the adult population of ayu and that of the adult population of hasu, he concludes that the ayu population has an important role for production of the hasu population, while there seems to be no-effect of hasu upon the production of the ayu population because the increasing rate of biomass from the juvenile stage to the adult one is not correlated with the abundance of hasu population but with negatively correlated with abundance of the juvenile population of ayu. Recently from series of observations of relation between rainbow trout, a predator, and young sockeye in a Canadian lake, WARDand LARKIN(1964) suggest that the trout may be more abundant as a result of food provided by the dominant year sockeye population, but that the ultimate size of the trout popu- lation is limited by the other three years of small sockeye populations which influence trout survival, growth and fecundity. At least predation should not 69

be overestimated as a controlling factor of prey population although it should not be neglected as a controlling factor of predator populations. Thus, coexis- tence of predatious fish and prey fish may be warranted as far as the prey fish exist. It is also possible that some other factors associated with the environmental stratifications or size of the lake may give opportunities of coexistence of many fish species in Lake Biwa. For further consideration on this problem it is nece- ssary to accumulate new findings of ecologies of the fishes. The author eagerly wants to rewrite this article in near future.

Summary

This paper presents, in the first place, descriptions of ecological elements of the fishes in Lake Biwa, including spawning, habitat, feeding and growth and introductions of some other ecological works done in the lake. Secondly, by comparison of their ecologies, fish association and the food relations of them to other members of the lake are considered. Finally, these materials are discu- ssed in relation to the current controversy concerning interspecific relations and coexistence of the fishes.

Acknowledgments

The author wishes to express his gratitude to Dr. S. Mori of Kyoto University for valuable advices and critical reading of the manuscript. Thanks are also due to the members of the Fish Biology Research Group of the B. S. T. (the Research Group for Biotic Resources in Lake Biwa) who offered advices and unpublished materials. The author is grateful to the staffs of the Otsu Hydrobiological Station of Kyoto University for giving him the financial support for publishing this paper.

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(Author: Taizo MIURA,Department of Zoology, Kyoto University, Kyoto)