Intraspecific competition drives multiple species resource polymorphism in fish communities

It has been hypothesized that inter-specific competition will reduce species niche utilization and drive morphological evolution in character displacement. In the absence of a competitor, intra-specific competition may favor an expansion of the species niche and drive morphological evolution in character release. Despite of this theoretical framework, we sometimes find potential competitor species using the same niche range without any partitioning in niche. We used a database on test fishing in Sweden to evaluate the factors (inter- and intraspecific competition, predation, and abiotic factors) that could influence habitat choice of two competitor species. The pattern from the database shows that the occurrence of perch and roach occupying both littoral and pelagic habitats of lakes in Sweden is a general phenomenon. Furthermore, the results from the database suggest that this pattern is due to intra-specific competition rather than inter-specific competition or predation. In a field study, we estimated the morphological variation in perch and roach and found that, individuals of both species caught in the littoral zone were more deeper bodied compared to individuals caught in the pelagic zone. Pelagic perch fed more on zooplankton compared to littoral perch, independent of size, whereas the littoral perch had more macroinvertebrates and fish in their diet. Pelagic roach fed more on zooplankton compared to littoral roach, whereas littoral individuals fed more on plant material. Furthermore, we sampled littoral and pelagic fish from another lake to evaluate the generality of our first results and found the same habitat associated morphology in both perch and roach. The results show a consistent multi-species morphological separation in the littoral and pelagic habitats. This study suggests that intra-specific competition is possibly more important than inter-specific competition for the morphological pattern in the perch-roach system.     

Diet-dependent body morphology and ontogenetic reaction norms in Eurasian perch

Individual morphology and performance are directly or indirectly under the influence of variation in resource levels. To study the effects of different resource conditions and their effects on morphology and ontogenetic reaction norms in young‐of‐the‐year (YOY) perch (Perca fluviatilis), we used three different approaches. First, we examined the morphological trajectories over early ontogeny in relation to lake‐specific resources in a field study. Second, one lake that lacked perch recruitment was stocked with perch eggs from a control lake in a whole‐lake experiment to study ontogenetic reaction norms. Third, we compared the development of YOY perch in the three lakes that mainly inhabited the littoral zone with YOY perch experimentally confined to enclosures in the pelagic zone of the lakes.
Overall body morphology of the YOY perch changed both as a function of size and as a function of diet. As perch increased in size they developed a deeper body morphology corresponding to an increased proportion of benthic macroinvertebrates in their diet. In pelagic enclosures where perch were constrained to feed mainly on zooplankton they had a more fusiform body morphology than perch in the lakes that fed on a mixture of zooplankton and macroinvertebrates. Similarly, the ontogenetic reaction norm of perch was related to the diet and lake‐specific zooplankton levels in the whole‐lake experiment.
In the pelagic enclosures, perch with high growth rates had a more fusiform body morphology than slow‐growing perch, whereas the opposite was found in the lakes, where perch included more macroinvertebrates in their diets. Perch in lakes with a higher proportion of macroinvertebrates in the diet also had deeper body morphology. The opposite morphology – growth rate relationship found between perch in the pelagic versus those using the whole lake suggest a morphological trade‐off between foraging on zooplankton and foraging on macroinvertebrates. Our results suggest that YOY perch show different ontogenetic reaction norms as a function of lake‐specific resource levels, which may allow YOY recruitment to later stages. Our results further suggest that diet‐related changes in morphology are a rapid process.     

Diet,stable isotopes and morphology of Eurasian perch (Perca fluviatilis) in littoral and pelagic habitats in the northern Baltic Proper

We present morphology, stable isotope signals and stomach contents of Eurasian perch (Perca fluviatilis) from littoral and pelagic habitats in a brackish water embayment in the northern Baltic Proper. Studies conducted in freshwater habitats repeatedly state that littoral perch have deeper bodies than the pelagic ones. In this study we observed the opposite; the perch from the pelagic study site had deeper bodies than the littoral ones, indicating that more factors than habitat structure affect the perch morphology. A possible explanation to this discrepancy is the diet choice; the pelagic perch in this study were more benthivorous than freshwater pelagic perch. Our results on stable isotope signals combined with the stomach contents also shed new light on the dietary preferences of perch. Perch is known to be a generalist predatory fish, but our results indicate that perch have individual diet preferences. Based on our results, it seems that at some point in their lives the perch in brackish water choose between the littoral and pelagic habitats and also specialise in a certain diet. This study shows that the perch morphology and diet in the Baltic Sea coast differ among habitats, but the patterns are not similar to those observed in freshwater studies.     

Morphology dependent foraging efficiency in perch: a trade‐off for ecological specialization?

Trade-offs in foraging efficiency leading to divergent natural selection between and within populations exploiting different resources are thought to be a primary cause of trophic polymorphism. In this study we focused on the trade-offs in foraging efficiency and growth in a polymorphic perch population. Specifically, we related habitat-specific growth and diet of perch to perch morphology. In a subsequent laboratory study we experimentally tested the trade-off by testing the efficiency of perch with different morphology feeding on pelagic ( Daphnia sp., Chaoborus sp.) and littoral (mayfly larvae) food resources. The feeding performance was tested in different physical environments to see if we could predict growth patterns in the field based on foraging rate and behavior of perch.
In the field study, we found that the perch from the littoral and the pelagic zones differed in both morphology and diet. Within the littoral zone the deeper-bodied individuals grew faster compared to the more streamlined individuals, whereas the opposite pattern was found in the pelagic zone. In the aquarium experiments, perch from the littoral zone had higher capture rates on the pelagic prey types in vegetation trials and on mayfly larvae in both open water and vegetation trials. The pelagic perch had higher capture rates on the pelagic prey types in open water trials. The littoral perch had lower search velocity than the pelagic perch in open water trials whereas the opposite pattern was found in vegetation trials. The attack velocity of the pelagic perch was also higher than that of the littoral perch independent of vegetation structure. Our results suggest that there is a functional trade-off between performance in alternate habitats and general body form in perch. Such trade-offs may promote divergent natural selection and could be the mechanism that give rise to and upholds the pattern in the field.     

Ecological stoichiometry of Eurasian perch – intraspecific variation due to size,habitat and diet

The turnover and distribution of energy and nutrients in food webs is influenced by consumer stoichiometry. Although the stoichiometry of heterotrophs is generally considered to vary only little, there may be intraspecific variation due to factors such as habitat, resources, ontogeny and size. We examined intraspecific variation in Eurasian perch Perca fluviatilis stoichiometry, a common species that exhibits habitat and resource specialization, ontogenetic niche shifts and a large size range. This study investigated the elemental stoichiometry of a wide size range of perch from littoral and pelagic habitats. The mean C:N:P stoichiometry of whole perch was 37:9:1 (molar ratios). However, %C, %P, C:N, C:P and N:P varied with size, morphology, habitat and diet category. These factors together explained 24–40% of the variation in C:N:P stoichiometry. In contrast, perch stoichiometry was not related to diet stoichiometry, suggesting that the former is homeostatically regulated. The results suggest that the high P content of perch may result in stoichiometric constraints on the growth of non‐piscivorous perch, and that piscivory is an efficient strategy for acquiring P. Resource polymorphism, individual diet specialization and intraspecific size variation are widespread among animals. Thus changes in stoichiometry with size, habitat, morphology and resource use, and therefore also stoichiometric demands, are probably common.     

Morphological and life-history differentiation between littoral and pelagic forms of pumpkinseed

Trophically dimorphic pumpkinseed populations were investigated in five lakes in Ontario, Canada to determine: (1) whether the morphological traits that distinguish littoral from pelagic forms are consistent among populations; (2) whether the pharyngeal jaw apparatus is diminished in pelagic pumpkinseeds because of a lower proportion of hard-bodied prey in their diets; (3) whether there is life-history differentiation between littoral and pelagic forms. Pumpkinseeds captured from the littoral and pelagic zones differed significantly in morphology in four of the five lakes, but the number of external measures that differed significantly within the differentiated populations ranged from zero to six. Littoral pumpkinseeds generally had longer heads, more rearward placement of dorsal and pectoral fins, longer pectoral fins and deeper bodies than pelagic pumpkinseeds. Littoral and pelagic pumpkinseeds were more readily differentiated by internal morphometric measures, with littoral individuals having larger molars and wider spacing between gill rakers than pelagic individuals. Littoral and pelagic differences in age at maturity, size at maturity and gonado-somatic index were present only in one of three populations assessed for these traits, suggesting that morphological divergence is not necessarily accompanied by life-history differentiation.     

The effect of core compression on the measurement of zinc concentrations and anthropogenic burdens in lake sediments

The whitefish (Coregonus lavaretus) in Lake Tyrifjorden, southeastern Norway, is monomorphic with regard to gill raker number: (x=32), and exhibits a bimodal length distribution. The small whitefish (<27 cm) were distributed mainly in the littoral zone and the deeper layers of the pelagic zone. The larger whitefish (>27 cm) were mainly distributed in the upper layers of the pelagic zone. Both small and large whitefish fed on zooplankton in the pelagic zone. In the littoral zone the diet of the two size groups was significantly different; the small whitefish fed mainly on zooplankton whereas the large whitefish fed mainly on zoobenthos.     

Growth,morphological variation and ontogenetic niche shifts in perch (<Emphasis Type="Italic">Perca fluviatilis</Emphasis>) in relation to resource availability

Despite the common occurrence of ontogenetic niche shifts, their consequences for morphological adaptations have been little studied. To address this question, we studied morphological adaptations related to ontogenetic niche shifts in Eurasian perch (Perca fluviatilis) in eight lakes that varied in density of benthic resources and planktivorous fish biomass. Perch start to feed on pelagic zooplankton, then shift to benthic resources at intermediate sizes, and finally, when large enough, mainly feed on fish. These three functional niches over ontogeny are expected to set constraints on the morphology and size-specific growth of perch. The growth of perch was negatively related to planktivorous fish biomass in the zooplanktivorous niche, but positively related to planktivorous fish biomass in the piscivorous niche. The number of gill rakers of perch was negatively related to the biomass of planktivorous fish, providing evidence for the occurrence of character displacement as a consequence of competition in the zooplanktivorous niche. Perch in lakes with low densities of predator-sensitive macroinvertebrates had greater body height measurements and a larger mouth early during ontogeny. This pattern is suggested to be a result of a selection for increased efficiency in the benthic niche when the availability of benthic resources is low. Perch in lakes with a high biomass of planktivorous fishes had fusiform body morphology, a thicker tail and a larger mouth then the average piscivorous perch. The different responses of perch morphology to variation in the availability of benthic resources compared to variation in planktivore biomass are suggested to be partly because the availability of the former resource to a larger extent is set by abiotic conditions (humic content). We suggest that the key factors affecting size-specific growth and body morphology of perch in the system studied are the availability of resources in the benthivorous and piscivorous niches. We also provide evidence for morphological trade-offs, especially between the benthivorous and the piscivorous ontogenetic niches. Received: 6 July 1999 / Accepted: 8 September 1999     

Two size classes of 0+ year perch: is phenotypic plasticity based on food resources?

Perch Perca fluviatilis of age 0+ years were caught in a gravel pit lake in June (unimodal size distribution) and in July (bimodal size distribution) to analyse morphological differences between the two growth cohorts. Independent of size, 0+ year perch developed a deeper body and perch of the large size cohort had an even deeper body than perch of the small size cohort. This might have been adaptations to either piscivory or planktivory, but might also be a hint that 0+ year perch of the small size cohort were undernourished and that they developed on a different trajectory than individuals of the large size cohort. In a second step the study was extended with a mesocosm experiment. This part of the study was designed to provide preliminary evidence for the extent to which morphological variations may be due to the consumption of different food resources when other factors such as habitat use could be neglected. Two groups of 0+ year perch in four mesocosms were fed for 40 days with the same biomass of either plankton or cyprinids. Although the experimental groups at the end of the experiment did not differ in size, they differed in morphology. The mouth of the piscivorous 0+ year perch became larger, the pectoral fins and the centre of mass of the posterior abdomen were shifted backwards. These results provide further evidence that the type of food is important and might lead to further functional adaptations in morphology.     

Indirect trophic interactions with an invasive species affect phenotypic divergence in a top consumer

While phenotypic responses to direct species interactions are well studied, we know little about the consequences of indirect interactions for phenotypic divergence. In this study we used lakes with and without the zebra mussel to investigate effects of indirect trophic interactions on phenotypic divergence between littoral and pelagic perch. We found a greater phenotypic divergence between littoral and pelagic individuals in lakes with zebra mussels and propose a mussel-mediated increase in pelagic and benthic resource availability as a major factor underlying this divergence. Lakes with zebra mussels contained higher densities of large plankton taxa and large invertebrates. We suggest that this augmented resource availability improved perch foraging opportunities in both the littoral and pelagic zones. Perch in both habitats could hence express a more specialized foraging morphology, leading to an increased divergence of perch forms in lakes with zebra mussels. As perch do not prey on mussels directly, we conclude that the increased divergence results from indirect interactions with the mussels. Our results hence suggest that species at lower food web levels can indirectly affect phenotypic divergence in species at the top of the food chain.     

Lipid‐rich zooplankton subsidise the winter diet of benthivorous Arctic charr (Salvelinus alpinus) in a subarctic lake

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Seasonal changes in the diets and relative abundances of perch and roach in the littoral and pelagic zones of a large lake

In the Enonselkä and Laitialanselkä basins of Lake Vesijärvi, perch Perca fluviatilis and roach Rutilus rutilus were abundant in the littoral and in the pelagic zones throughout the summer. In the littoral zone, roach was always more numerous than perch, while perch dominated in the open water. Intraspecific diet overlap values were higher than interspecific values. In the pelagic zone, perch <155 mm fed mainly on the cladoceran Leptodora kindtii , while small bosminids were most important food items for roach. Large perch were piscivorous, feeding mainly on smelt Osmerus eperlanus . In the littoral zone small perch foraged on zooplankton and chironomid larvae and large perch on chironomids and fish (small perch). Small roach fed mainly on bosminids and detritus, while for roach <185 mm macrophytes ( Elodea Canadensis, Lemna trisulca ) were also of importance. Detritus was more common in the food of roach in Laitialanselkä than in Enonselkä. The slower growth rate of roach in Laitialanselkä compared with Enonselkä was probably connected with this. However, considering the latitude of the lake, the growth rate of both roach and perch was relatively fast in both basins. The results indicated that in a large lake both perch and roach are able to utilize effectively the different habitats and diverse food resources. By segregation in food resource utilization they are able to co-exist in large quantities, at the same time maintaining a relatively fast growth rate.     

Distribution of perch (Perca fluviatilis L.) during their first year of life in Lake Constance

The distribution and behaviour of larval and juvenile perch (Perca fluviatilis L.) were studied for two years in large, deep Lake Constance. After hatching larvae were transported by water currents to the open water. The majority of larvae remained in the pelagic zone for about one month. In both years, their return to the littoral zone coincided with the decline of pelagic zooplankton abundance. After returning to the littoral zone, juveniles stayed among submerged macrophytes within 5 m depth and lived apart from larger perch which lived at depths of 6–20 m. By late summer, juveniles changed their distribution pattern: during the day they stayed intensively close to piers and ports, but increased their swimming activity at dusk, cruising among shallow and deep waters and feeding on zooplankton, and rested on the bottom at night. This behaviour appears to be related to the decrease of inshore food resources and to the presence of predators in deeper water. 0⁺ perch left the littoral zone and moved into deep waters when autumnal mixing began in late October. They overwintered near the bottom at depths of more than 30 m. During most of the year, juvenile and adult perch were separated from each other. But as soon as they occupied the same habitat, the occurrence of cannibalism increased.     

Visual conditions and habitat shape the coloration of the Eurasian perch (Perca fluviatilis L.): a trade‐off between camouflage and communication?

In theory, selection for effective camouflage (i.e. dull coloration) in fish should be strongest when the conditions for visual predation are most favourable, such as in structurally simple pelagic habitats. By contrast, in more sheltered (e.g. littoral) habitats, selection may favour effective intra‐specific communication (i.e. bright coloration) (at the expense of crypsis). Poor transparency, as in highly humic waters, should constrain colour adaptations. We investigated phenotypic variation in body coloration of Eurasian perch (Perca fluviatilis L.) in littoral and pelagic habitats of four humic boreal lakes. Perch from the most transparent lake had the lightest and less coloured belly and perch were more colourful in the littoral habitats than in the pelagic areas, with the pattern being clearest in the most transparent lake. In addition, perch in the most transparent lake exhibited sexual dichromatism, with males having a more colourful belly than the females, whereas no indications of sexual dichromatism were found in more humic lakes. Moreover, in the most transparent lake, the condition of fish correlated with bright belly coloration in the littoral, but with dull belly coloration in the pelagic habitat. The results obtained in the present study suggest that selection on perch coloration may differ between lakes as a result of visual properties of the water, and within lakes as a result of divergent selection for camouflage and communication in pelagic and littoral habitats. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 47–59.     

Niche segregation of coexisting Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) constrains food web coupling in subarctic lakes

1. Generalist fish species are recognised as important couplers of benthic and pelagic food‐web compartments in lakes. However, interspecific niche segregation and individual specialisation may limit the potential for generalistic feeding behaviour. 2. We studied summer habitat use, stomach contents and stable isotopic compositions of the generalist feeder Arctic charr coexisting with its common resource competitor brown trout in five subarctic lakes in northern Norway to reveal population‐level and individual‐level niche plasticity. 3. Charr and trout showed partial niche segregation in all five lakes. Charr used all habitat types and a wide variety of invertebrate prey including zooplankton, whereas trout fed mainly on insects in the littoral zone. Hence, charr showed a higher potential to promote habitat and food‐web coupling compared to littoral‐dwelling trout. 4. The level of niche segregation between charr and trout and between pelagic‐caught and littoral‐caught charr depended on the prevailing patterns of interspecific and intraspecific resource competition. The two fish species had partially overlapping trophic niches in one lake where charr numerically dominated the fish community, whereas the most segregated niches occurred in lakes where trout were more abundant. 5. In general, pelagic‐caught charr had substantially narrower dietary and isotopic niches and relied less on littoral carbon sources compared to littoral‐caught conspecifics that included generalist as well as specialised benthivorous and planktivorous individuals. Despite the partially specialised planktivorous niche and thus reduced potential of pelagic‐dwelling charr to promote benthic–pelagic coupling, the isotopic compositions of both charr subpopulations suggested a significant reliance on both littoral and pelagic carbon sources in all five study lakes. 6. Our study demonstrates that both interspecific niche segregation between and individual trophic specialisation within generalist fish species can constrain food‐web coupling and alter energy mobilisation to top consumers in subarctic lakes. Nevertheless, pelagic and littoral habitats and food‐web compartments may still be highly integrated due to the potentially plastic foraging behaviour of top consumers.     

Water Transparency Drives Intra-Population Divergence in Eurasian Perch (Perca fluviatilis)

Trait combinations that lead to a higher efficiency in resource utilization are important drivers of divergent natural selection and adaptive radiation. However, variation in environmental features might constrain foraging in complex ways and therefore impede the exploitation of critical resources. We tested the effect of water transparency on intra-population divergence in morphology of Eurasian perch (Perca fluviatilis) across seven lakes in central Sweden. Morphological divergence between near-shore littoral and open-water pelagic perch substantially increased with increasing water transparency. Reliance on littoral resources increased strongly with increasing water transparency in littoral populations, whereas littoral reliance was not affected by water transparency in pelagic populations. Despite the similar reliance on pelagic resources in pelagic populations along the water transparency gradient, the utilization of particular pelagic prey items differed with variation in water transparency in pelagic populations. Pelagic perch utilized cladocerans in lakes with high water transparency and copepods in lakes with low water transparency. We suggest that under impaired visual conditions low utilization of littoral resources by littoral perch and utilization of evasive copepods by pelagic perch may lead to changes in morphology. Our findings indicate that visual conditions can affect population divergence in predator populations through their effects on resource utilization.     

A new approach to quantifying morphological variation in bluegill Lepomis macrochirus

Bluegill Lepomis macrochirus showed intraspecific morphological and behavioural differences dependent on the environment. Pelagic L. macrochirus had more fusiform bodies, a higher pectoral fin aspect ratio, a larger spiny dorsal fin area and pectoral fins located farther from the centre of mass than littoral L. macrochirus (P < 0·05). The shape of the body and pectoral fins, in particular, were suggestive of adaptation for sustained high-speed and economical labriform swimming. Littoral L. macrochirus had a deeper and wider body, deeper caudal fins and wider mouths than pelagic L. macrochirus (P < 0·05). Additionally, the soft dorsal, pelvic, anal and caudal fins of littoral L. macrochirus were positioned farther from the centre of mass (P < 0·05). The size and placement of these fins suggested that they will be effective in creating turning moments to facilitate manoeuvring in the macrophyte-dense littoral habitat.     

Ontogenetic shifts in phenotype–environment associations in Nile perch,Lates niloticus (Perciformes: Latidae) from Lake Nabugabo,Uganda

Habitat‐associated trait divergence may vary across ontogeny if there are strong size‐related shifts in selection pressures. We quantified patterns of phenotypic divergence in Nile perch (Lates niloticus) from ecologically distinct wetland edge and forest edge habitats in Lake Nabugabo, Uganda, and we compared patterns of divergence across three size classes to determine whether trends are consistent through Nile perch ontogeny. We predicted that inter‐habitat variation in biotic (e.g. vegetation structure) and abiotic (e.g. dissolved oxygen concentration) variables may create divergent selective regimes. We compared body morphology using geometric morphometrics and found substantial differences between habitats, although not all trends were consistent across size classes. The most striking aspects of divergence in small Nile perch were in mouth orientation, head size, and development of the caudal region. Medium‐sized Nile perch also showed differences in mouth orientation. Differences in large individuals were related to eye size and orientation, as well as caudal length. The observed patterns of divergence are consistent with functional morphological predictions for fish across divergent trophic regimes, high and low predation environments, and complex and simple habitats. Although this suggests adaptive divergence, the source of phenotypic variation is unknown and may reflect phenotypic plasticity and/or genetic differences. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 449–465.     

Asymmetrical habitat coupling of an aquatic predator—The importance of individual specialization

Predators should stabilize food webs because they can move between spatially separate habitats. However, predators adapted to forage on local resources may have a reduced ability to couple habitats. Here, we show clear asymmetry in the ability to couple habitats by Eurasian perch—a common polymorphic predator in European lakes. We sampled perch from two spatially separate habitats—pelagic and littoral zones—in Lake Erken, Sweden. Littoral perch showed stronger individual specialization, but they also used resources from the pelagic zone, indicating their ability to couple habitats. In contrast, pelagic perch showed weaker individual specialization but near complete reliance on pelagic resources, indicating their preference to one habitat. This asymmetry in the habitat coupling ability of perch challenges the expectation that, in general, predators should stabilize spatially separated food webs. Our results suggest that habitat coupling might be constrained by morphological adaptations, which in this case were not related to genetic differentiation but were more likely related to differences in individual specialization.     

Rapid changes in fish community structure and habitat distribution following the precipitation of lake phosphorus with aluminium

1. Fish community structure and habitat distribution of the abundant species roach, perch and ruffe were studied in Lake Nordborg (Denmark) before (August 2006) and after (August 2007) aluminium treatment to reduce internal phosphorus loading. 2. Rapid changes in fish community structure, abundance and habitat distribution occurred following a decline in in‐lake phosphorus concentrations from 280 to 37 μg P L^?1 and an increase in Secchi depth transparency from 1.1 to 1.9 m (August). The proportion of perch in overnight gill net catches increased, whilst roach decreased, and the average weight of all key species increased. 3. The habitat distribution of perch and roach changed from a high proportion in the upper pelagic and littoral zones in 2006, towards enhanced proportions in the deeper pelagic and profundal zone in 2007. The abundance of large‐bodied zooplankton increased and the abundance of benthic invertebrates decreased in the same period, suggesting that the habitat shift was not induced by food limitation. 4. Ruffe shifted from the littoral and upper profundal zones towards the deep profundal zone, likely reflecting an increased predation risk in the littoral zone and better oxygen conditions in the deep profundal. 5. Our results indicate that enhanced risk of predation in the upper pelagic and the littoral zones and perhaps improved oxygen concentrations in the deeper profundal zone at decreasing turbidity are responsible for the observed habitat shift. The results indicate that fish respond rapidly to changes in nutrient state, both in terms of community structure and habitat use.