ISSN 1067-4136, Russian Journal of Ecology, 2007, Vol. 38, No. 6, pp. 430–435. © Pleiades Publishing, Ltd., 2007. Original Russian Text © Yu.V. Gerasimov, 2007, published in Ekologiya, 2007, Vol. 38, No. 6, pp. 461–466.

Intrapopulation Spatial Segregation in Bream (Abramis brama) and Roach (Rutilus rutilus) in the Rybinsk Reservoir Yu. V. Gerasimov Papanin Institute of the Biology of Inland Waters, Russian Academy of Sciences, Borok, Nekouzskii raion, Yaroslavl oblast, 152742 Russia e-mail: [email protected] Received February 12, 2007

Abstract—In the course of studies on mass fish species (bream and roach) in water bodies of the Upper Volga basin, intraspecific groups of individuals have been distinguished. Conditions and possible mechanisms of spatial differentiation of these groups are considered. The intrapopulation divergence of individuals by some adaptive characters, primarily behavioral, enables them to utilize alternative resources, which provides for more efficient use of the environment by populations. DOI: 10.1134/S1067413607060094 Key words: bream, roach, intraspecific groups, food resources, spatial segregation.

The presence in populations of intraspecific groups of organisms differing in the level of adaptation to certain environmental conditions allows the population to use the environment with higher efficiency, more completely colonizing the set of spatial and temporal subniches with alternative resources available within the species range. Such an intraspecific structure is adaptive and conforms to I.I. Schmalhausen’s (1968) concept of adaptive norms, according to which a population contains groups of organisms that have been formed as a result of implementation of different discrete genetic programs. Savvaitova and Mednikov (2000) consider that the role of numerous intraspecific groups of chars and ciscoes in aquatic ecosystems of high latitudes is no less important than the role of fish complexes comprising tens or even hundreds of species in temperate and low latitudes. Studies on water bodies of the Upper Volga basin revealed such intraspecific groups in the populations of mass fish species such as bream, roach, and perch (Kas’yanov et al., 1982; Kas’yanov and Izyumov, 1995; Gerasimov and Poddubnyi, 1999; Gerasimov et al., 2005; Stolbunov, 2005a, 2005b; Gerasimov, 2006; Stolbunov and Pavlov, 2006). The purpose of this study was to analyze conditions and probable mechanisms of spatial differentiation within populations of bream and roach in the Rybinsk Reservoir. Bream (Abramis brama). Studies on the bream population of the Rybinsk Reservoir (Slyn’ko, 1992) revealed its polymorphism by the peroxidase locus represented by alleles Po79 and Po100. Homozygous individuals of these genotypic groups were obtained by directed crossing, and special tests revealed differences

in trophic, exploratory, and defensive behavior between them (Gerasomov and Slyn’ko, 1990, 1991; Gerasimov, 2006). In studies on breams of these two groups in the natural environment, differences in the choice of feeding grounds and food spectrum were observed (Gerasimov and Poddubnyi, 1999). The bream is not a schooling fish. Aggregations of bream in natural water bodies are a result of fish concentration in habitats with more favorable feeding conditions (Gerasimov and Linnik, 1993). According to P’yanov (1992), such aggregations have a certain structure based on the hierarchical type of relationships between individuals under conditions of differences in microbiotope quality: some individuals occupy the best feeding sites, while other fish are displaced to the periphery. There are also data that the bream is territorial and aggressive in the spawning period (Poncin et al., 1996). According to the model of population regulation based on the polymorphism of behavior (Ehrman and Parsons, 1984), animals in a population differ in their tolerance to crowding, with the varying population density acting on these behavioral types as a factor of selection. Accordingly, population growth leads to closer interactions of individuals and, therefore, increasing selection for aggressive behavior. As shown in studies on rodents of different inbred lines (Vale et al., 1972), individual aggressiveness shows a statistically significant correlation with population density, with the strength of this correlation depending on genotype; i.e., changes in aggressiveness upon an increase in density are not equally manifested in all lines.

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INTRAPOPULATION SPATIAL SEGREGATION IN BREAM (a) PerchWhite bream 4% Blue bream 1% 21% Bream 53%

Roach 18% Pikeperch Pike 2% 1% (b) Blue bream 17%

Bream 17%

Roach 66% Fig. 1. Qualitative composition of trawl catches in floodplain areas with depths less than 8 m at the western shore of the Rybinsk Reservoir (a) with zero wind and (b) with a moderate northern wind.

As shown in our previous studies (Gerasimov and Slyn’ko, 1991; Gerasimov and Poddubnyi, 1999; Gerasimov, 2006), aggressive and low-reactive breams with allele Po79 are more tolerant to overpopulation than low-aggressive and highly reactive breams with allele Po100. An increase in the density of fish aggregations on natural feeding grounds should lead to higher frequency of contacts between individuals and, consequently, to higher aggressiveness of breams with allele Po79. Therefore, in the course of interaction with breams carrying this allele, individuals with allele Po100 may be displaced to the periphery of the aggregation and even leave overpopulated habitats in search of alternative feeding grounds. Tests of this hypothesis under natural conditions (Gerasimov and Poddubnyi, 1999; Gerasimov, 2006) showed that the Po79 breams had a narrower feeding spectrum than the Po100 breams. The qualitative composition of gut contents indicated that the former fed in the riverbeds flooded by the reservoir, whereas the latter actively foraged not only in these riverbeds but also in shallow areas of the inundated floodplain. The composition of gut contents in both groups agreed well with data on the qualitative and quantitative composition of invertebrates in the corresponding biotopes (Shcherbina, 1993). The difference in habitat use was confirmed by biotelemetric observations on fish tagged with ultrasound transmitters (Malinin et al., 1990). The breams with a narrow food spectrum remained within in the riverbed area with depths of 13–17 m virtually throughout the period of observations, traveling from the place of their RUSSIAN JOURNAL OF ECOLOGY

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release for no more than 0.5–1 km. In some cases, they moved to shallower areas (8–10 m in depth) but went no farther than 0.2–0.5 km from the riverbed. The time spent in the floodplain accounted for only 10–15% of the total period of observations. The breams with a wide food spectrum moved for more than 10 km from the place of their release during the same period, and some of them permanently stayed in the floodplain, at distances of up to 4 km from the riverbed (Malinin et al., 1990). The observed differences in the range of fish movements in the water body agree with the results of laboratory experiments demonstrating that the swimming capacity of Po100 individuals was significantly higher than that of Po79 individuals among both yearlings (Gerasimov and Poddubnyi, 1999) and fish at age 5+ (Gerasimov and Lapshin, 2005). In addition, the Po100 breams are characterized by a higher level of functional development of the visual analyzer than the Po79 breams, with the olfactory system functioning similarly in both groups (Gerasimov, 2006). This feature of breams with allele Po100 apparently provides for more efficient foraging on epibenthic invertebrates under conditions of high illumination level in shallow floodplain habitats. Studies on the spatial distribution of bream (Malinin et al., 1990; Gerasimov and Poddubnyi, 1999) show that feeding migrations of Po100 fish to the floodplain have a periodic pattern. The stability of floodplain aggregations depends on hydrodynamic activity (wave action and wind currents) in shallow-water areas. When winds grow stronger, breams leave their floodplain habitats and migrate to riverbed areas with depths exceeding 10 m (Fig. 1). During long windless periods they stay in the floodplain for several days and do not return to the riverbed areas. Comparison of gut contents in breams of different genotypic groups in calm and stormy weather showed that their food spectra differed only in calm weather. After long storms, the guts of fish in both groups contained mainly invertebrates characteristic of riverbed areas (Fig. 2). However, the situation with the distribution of these groups of bream has not always been so. At the first stages of formation of the Rybinsk Reservoir, its shallow-water areas were still covered with the remains of forest inundated upon damming. Habitats formed under protection of these remains were not exposed to adverse hydrodynamic influences (Gordeev, 1971). Studies performed in that period (Klyuchereva, 1960) showed that part of the bream stock lived in such habitats together with Carassius and Tinca. The impact of waves and ice movements gradually destroyed these habitats, and stable feeding fish aggregations in floodplain areas disappeared. Roach (Rutiluis rutilus). The population of roach in the Rybinsk Reservoir comprises two ecological groups differing in some morphological characters (Izyumov et al., 1982): the coastal group (having a mixed food spectrum) and the floodplain-bottom, or 2007

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migratory group (feeding mainly on mollusks). Due to extensive migrations and gene exchange, the roach of the floodplain-bottom group forms an integrated population. The coastal morph of roach, being more conservative with respect to spawning and feeding grounds, is subdivided into several small local populations confined to river mouths, large bays, and areas behind islands (Kas’yanov et al., 1982). There is evidence (Poddubnyi, 1966, 1971; Kas’yanov et al., 1982) that juveniles of the coastal and floodplain-bottom groups feed jointly and separate only at the age of 4–6 years, when the latter moves to depths 4–8 m and starts feeding on Dreissena mollusks (D. polymorpha and D. bugensis) while the former remains in the same habitats and continues to feed on animals living in aquatic vegetation. However, the results of recent morphological studies on juvenile roach caught in different habitats (Gerasimov et al. 2005; Stolbunov, 2005a, 2005b, 2006) show that differentiation of roach into two morphotypes occurs already at early stages of development. Their spatial distribution coincides with the main coastal biotopes of the reservoir: the open littoral zone exposed to wind action (areas with sandy or sandy– stony grounds and sparse vegetation) and the protected coastal zone (areas with silted sandy ground and welldeveloped higher aquatic vegetation in bays, river mouths, or behind). In juveniles of roach from the open part of the reservoir, the relative size of the mouth is significantly larger and the body is significantly more slender than in juveniles from other habitats (Stolbunov, 2006) (Fig. 3). Hydrodynamic activity in the open littoral zone of the Rybinsk Reservoir is high, and there are no suitable spawning grounds for roach. Therefore, irrespective of spatial segregation of the coastal and floodplain-bottom morphs in the feeding period, they spawn on the same grounds in the protected littoral zone (Kas’yanov et al., 1982). Nevertheless, the results of studies on the species composition of early juvenile aggregations formed in biotopes of the open littoral zone in June and July show that the proportion of roach in them exceeds 70% (Ekologicheskie problemy…, 2001). The question arises as to in what way mass quantities of juvenile roach appear in areas of the open littoral? An analysis of the migratory behavior of roach larvae (stages C1–D1) revealed two different phenotypic groups among them, migrants and residents (Pavlov et al., 2001). Differences in the responses of larvae to currents determine their spatial segregation at night: individuals with a positive rheotaxis reaction migrate with currents, whereas individuals with negative rheotaxis avoid currents and remain in place (Pavlov et al., 2001). Such a picture is observed in the riverbed flow. However, the rates of various currents may also be relatively high in shallow areas of the Rybinsk Reservoir, at the boundary of the overgrown littoral zone: there are discharge currents of 0.04–0.2 m/s, shore currents of 0.2–1.0 m/s, and wind currents of 0.1–0.2 m/s (Gerasimov and Poddubnyi, 1999). These currents, being com-

Euclidean distance 3.99 3.19

(a)

2.39 1.60 0.80 0

9 1 5 8 3 7 2 10 6 15 4 112017161219181413

16.52 13.21

(b)

9.91 6.61 3.30 0

1 9 131014 3 11 2 15 5 6 7 8 1612 4 Individual number

Fig. 2. Dendrograms of differences in the food spectra of breams with alleles Po100 and Po79 (a) at zero wind and (b) after a storm: nos. 1–10 (a) and 1–8 (b), breams with allele Po100; nos. 11–20 (a) and 9–16 (b), breams with allele Po79.

mensurate with flow rates in lowland rivers, may aid in realization of different strategies of migratory behavior by bringing migrant larvae from protected to open areas of the littoral zone. The morphological phenotype of roach is formed either due to selection in favor of large-mouthed individuals in the open littoral zone or due to linked inheritance of positive rheotaxis together with the large mouth and slender body. This provides for a high survival of migrants in the fairly aggressive environment of open waters characterized by active hydrodynamics, the absence of shelters, dominance of pursuing predators, and abundance of large active pelagic forms of plankton (food for juveniles). The possibility of linked inheritance of characters in the migrants is confirmed by the fact that the coefficient of variation in morphological characters remains unchanged after their appearance in the open littoral zone (Stolbunov, 2005a). Laboratory studies on juvenile roach obtained by directionally crossing small-mouthed spawners caught in nature (Stolbunov, 2005c) showed that the range of variation in the size of their mouths coincided with that in juveniles of small-mouthed roach from the natural environment but significantly differed from the corre-

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INTRAPOPULATION SPATIAL SEGREGATION IN BREAM D, cm2 0.35 0.30

433

Y, rel. units. 0.55 (a)

(b)

0.50

0.25 0.20

0.45

0.15 0.40 0.10 0.05

0.35 I

II

I

II

Fig. 3. (a) The size of mouth opening D and (b) index of body form Y in juvenile roach from different habitats of the Rybinsk Reservoir: (I) littoral zone protected with vegetation and (II) open littoral zone without vegetation (vertical bars show standard deviation) (Stolbunov, 2005b).

sponding values in the large-mouthed morph. This is evidence that characteristic features of the mouth structure are inherited. Thus, there are reasons to consider that spatial differentiation of the roach population takes place prior to the transition to feeding on mollusks of the genus Dreissena. Small-mouthed roach remains in its habitats confined to river mouths, large bays, and areas behind islands, forming the coastal morph. Large-mouthed juveniles of the open littoral zone give rise to the bottom-floodplain morph feeding on mollusks. With age, this morph moves to areas with numerous Dreissenidae colonies between isobaths 3 and 8 m, where feeding on these mollusks becomes obligatory. This follows from the fact that the bottom-floodplain and coastal groups of roach retain the same differences in the size of the mouth opening that are observed in juveniles. In fish of all ages occurring in catches (up to 8+), this parameter is significantly higher in the first group (Stolbunov, 2005b) (Fig. 4). A somewhat different situation is observed in the case of body form: the bottom-floodplain roach at the age of 3–4 years becomes less slender than the coastal roach. This is due to its transition to feeding on Dreissena mollusks, which leads to higher fatness (Poddubnyi, 1966) and the corresponding change in body proportions. The mouth size still has adaptive significance at this age, as mollusks of the genus Dreissena reach a considerable size. Large roach can eat mollusks up to 20 mm in size, thereby winning the competition for this resource with mass benthos-eating species such as common and silver bream (Blicca bjoerkna) whose diets include mollusks up to 10 and 14 mm in size RUSSIAN JOURNAL OF ECOLOGY

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(Shcherbina and Buckler, 2006). Under these conditions, the slender body form does not give obvious advantages to the large roach, since it has already escaped the pressure of mass predators and passed over to obligatory feeding on immovable mollusks. In addition, comparatively low rates of discharge, shore, and wind currents are not critical for the large roach (Gerasimov and Poddubnyi, 1999). Thus, both bream and roach have mechanisms providing for spatial segregation of intraspecific forms under certain conditions. This allows the bream population to efficiently utilize food resources of varying accessibility (macrobenthos of the littoral and sublittoral zones) and the roach population to utilize largeD, cm2 3 1 2 2 1 0

3+

4+

5+

6+

7+ 8+ Age, years

Fig. 4. Age-related changes in the size of mouth opening D in roach from different biotopes: (1) protected littoral zone (roach from seine catches) and (2) open water (roach from trawl catches) (Stolbunov, 2005b). 2007

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sized plankton of the open littoral zone at early developmental stages and Dreissena mollusks at later stages. The main difference between these fish species is that spatial differentiation into ecological groups in bream is periodical, depending on hydrodynamic activity in shallow-water areas, while this differentiation in roach is more stable. Due to spatial differentiation, the populations of bream and roach can more completely colonize the assortment of spatial and temporal subniches with alternative resources within their ranges. Thus, the intraspecific diversity of ichthyofauna in water bodies of temperate latitudes is important for utilization of available habitats, which contributes to the stable functioning of aquatic communities. ACKNOWLEDGMENTS The author is sincerely grateful to V.N. Yakovlev, E.I. Izvekov, and Yu.G. Izyumov for their critical comments on the manuscript. REFERENCES Ekologicheskie problemy Verkhnei Volgi (Ecological Problems of the Upper Volga), Yaroslavl: Yaroslav. Gos. Univ., 2001. Ehrman, L. and Parsons, P., Behavior Genetics and Evolution, New York: McGraw-Hill, 1981. Translated under the title Genetika povedeniya i evolyutsiya, Moscow: Mir, 1984. Gerasimov, Yu.V., The Role of Behavioral Polymorphism in the Process of Intrapopulation Segregation of Ecological Niches in Bream, Abramis brama (Cyprinidae), Ichthyology, 2006, vol. 46, no. 2 (Suppl.), pp. S204–S212. Gerasimov, Yu.V. and Lapshin, O.M., Specific behavioral Features of Bream (Abramis brama) with Different Genotypes in the Peroxidase Locus in the Zone of Intensity Fishing, Povedenie ryb: Mat-ly dokl. mezhdun. konf. (Proc. Int. Conf. on Fish Behavior), Moscow, 2005, pp. 97–103. Gerasimov, Yu.V. and Linnik, V.D., Condition of Establishment of Individual Fattening Areas in Fish, Ekologicheskie faktory prostranstvennogo raspredeleniya i peremeshcheniya gidrobiontov (Ecological Factors Determining the Spatial Distribution and Movements of Hydrobionts), St. Petersburg: Gidrometeoizdat, 1993, pp. 211–259. Gerasimov, Yu.V. and Poddubnyi, S.A., Rol’ gidrologicheskogo rezhima v formirovanii skoplenii ryb na melkovod’yakh ravninnykh vodokhranilishch (Role of Hydrologic Conditions in the Formation of Fish Aggregations in Shoals of Lowland Reservoirs), Yaroslavl, 1999. Gerasimov, Yu.V. and Slyn’ko, Yu.V., Feeding and Defensive Behaviors of Fishes on Experimental Substrates Differing in the Degree of Complexity: Ecological and Genetic Aspects, in Iskusstvennye rify dlya rybnogo khozyaistva: Sb. nauch. tr. VNIRO (Artificial Reefs in Fishery: Collected Works of the All-Russia Research Institute of Fisheries and Oceanography), Moscow: VNIRO, 1990, pp. 177–193. Gerasimov, Yu.V. and Slyn’ko, Yu.V., Differences in Elements of Defensive and Social Behaviors in Juvenile Bream Genotyped by the Peroxidase Locus, Trudy Vsesoyuznogo soveshchaniya po voprosam povedeniya ryb (Proc. All-

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