Cannibalism in whitefish (Coregonus lavaretus)

Summary Cannibalism in whitefish (Coregonus lavaretus) is studied on the basis of an evolutionary model. The model predicts that individuals may be cannibalistic if they gain sufficiently in terms of fitness relative to the reduction in survival caused by this behaviour. If cannibalistic forms mainly increase survival it is predicted that cannibalism is particularly likely to be an evolutionarily optimal strategy if only a few age classes are being cannibalized and if, simultaneously, many age classes are cannibalistic. The same pattern, although less apparent, emerges on the basis of a model assuming that cannibalistic forms mainly increase fecundity.The whitefish in River Sokna spawn during late October to late November. Males dominate the spawning run numerically. Growth rate in mature stages is low, 1.0 cm·y^-1 for males and 1.3 cm·y^-1 for females. Yearly survival rate in mature stages is 0.574, and survival from egg to mature is 0.000078%. Larger whitefish on the average lay more and larger eggs than do smaller. Females were more cannibalistic than males; in total cannibalism contributed to an egg mortality of at least 8.8%. This is consistent with the derived theoretical predictions.Our theoretical and empirical analyses on whitefish are supported by the observed lack of sexual differences in the cannibalistic behaviour of brown trout (Salmo trutta) and Arctic char (Salvelinus alpinus).     

Biochemisch-genetische Untersuchungen zur Frage der genetischen Differenzierung von Blaufelchen (Coregonus lavaretus wartmanni, Bloch) und Gangfisch (Coregonus lavaretus macrophthalmus, Nüsslin) aus dem Bodensee

A contribution to the differentiation between the blue whitefisb (Coregonus lavaretus wartmanni) and the densly rahered whitefish (Coregonus lavaretus macrophthalmus) by biochemical genetic analyses A sample of 86 fish of the Genus Coregonus were analysed by enzyme gel electrophoresis. 6 Loci out of 32 were polymorphic. Blue whitefish (Coregonus lavaretus wartmanni) and densly rahered whitefish (Coregonus lavaretus macrophthalmus) revealed different allele frequencies at the Hex-locus. Sub-population structure is further evidenced by a correlation between qhenotypes at the Hex-locus and the number of gill rahers, but there is no reason for ordering both groups in different species.     

'Bald spot' disease in powan Coregonus lavaretus (L.)

The stocks of powan Coregonus lavaretus (L.) in Loch Lomond (Scotland) were affected by an acute no-ulcerative inflammatory lesion in the centre of the head, at the same time as ulcerative dermal necrosis (U.D.N.) became apparent in salmon in the Loch. The numbers of powan were very great at the time, and fish were of poor quality and frequently fungus-infected. No pathogen was isolated from the lesions which rarely contained fungus, and were histologically very different from the lesions of U.D.N. in salmon. Its disappearance by the following year, coincided with a marked improvement in quality of the fish.     

Fecundity of whitefish (Coregonus lavaretus) during the eu-and oligotrophication of Lake Constance

About 20 ovaries of the whitefish 'Blaufelchen' ( Coregonus lavaretus L.) of Lake Constance (Bodensee) were collected annually from 1964-1991. Absolute and relative fecundity peaked in the early 1980s, lagging about 2 years behind the maximum of total-P in the water (during the circulation period). Out of the biological time-series of Lake Constance, whitefish fecundity is the only one known to follow the phosphorus curve, and this may be the first time-series documented case of parallel trend reversal in the trophic state of a lake and fish biology. It is concluded that fish fecundity serves, at best, as an unspecific monitor of the overall well-being of the fish.     

Digestive enzymes in larval Coregonus lavaretus L.

Using histochemical methods, morphofunctional aspects of the alimentary tract of larval coregonids were investigated. Larvae of Coregonus lavaretus were reared for 34 days with either zooplankton or one of two dry diets. Ontogeny, localization and diet-related modifications of the following enzymes were examined: trypsin (luminal digestion), aminopeptidase, maltase, alkaline phosphatase (brush border-bound digestion) and unspecific esterase (intracellular nutrient processing). All of the enzymes studied were present in 13-day-old larvae. Except for the intracellularly located unspecific esterase, there was an ontogenetic enhancement of enzyme staining intensities accompanied by a significant increase in the volume of the intestinal mucosa. Enzyme activities differed within and between intestinal regions. This finding suggests that a spatial gradient of nutrient breakdown and absorption already exists in the morphologically and physiologically incompletely developed digestive system of larval coregonids. Digestive enzyme activities were modified in response to the dietary regimen. There was no obvious correlation between enzymic response and growth performance of the larvae.     

Different growth trends of whitefish (Coregonus lavaretus) forms in the northern Baltic Sea

The sea growth of two whitefish forms, anadromous (Coregonus lavaretus lavaretus) and sea‐spawning (Coregonus lavaretus widegreni), was analysed using samples collected from the commercial sea catch in the Gulf of Bothnia (GoB) in the northern Baltic Sea during 1998–2014. In the GoB area, these two forms are possible to identify because the gill‐raker number and size at maturity vary between forms. The growth rate of the forms is linked to their feeding area. Sea‐spawning whitefish, which has a feeding migration near its home site, was shorter in the northern GoB (66°N–64°N) at the ages of 3–11 than those in the southern GoB (64°N–60°30′N). In the data, most whitefish were caught with gill nets in the GoB. The mesh sizes of gill nets capturing the anadromous form were mostly 35–45 mm, while those capturing the sea‐spawning form were <35 mm in the northern GoB. It is likely that the different growth trends for small and large whitefish were connected with differences in their recruitment for fishing. The length of anadromous females at the age of four sea years increased significantly, but the length of six‐year‐old anadromous female whitefish decreased over the catch years from 1998–2014. In contrast, the length of slow‐growing sea‐spawning whitefish of six years or older increased significantly in relation to the catch year in the gill‐net catch. The increase in the growth of young age groups in both forms was probably associated with the increasing temperature and the low fishing pressure on small fish. The decreasing age at capture for both forms and the depression of the mean size of old anadromous whitefish are signs of high fishing pressure with a high gill‐net effort that selectively removes the largest and oldest individuals of both forms.     

Ultrastructure of the retina of two subspecies of Coregonus lavaretus (Teleostei) from Lake Constance (Germany)

In the two studied subspecies of Coregonus lavaretus , the pollan ( C. l. wartmanni ) (which lives deep in the pelagial) and the gangfish ( C. l. macrophthalmus ) (which lives near the slope, closer to the bottom), duplex retinae containing rod and cone photoreceptors are found. Four morphologically different cone types were observed: unequal double cones, short single cones, long single cones and triple cones. The cones are arranged in a square pattern (four double cones around a central short single cone) in the ventral and ventrotemporal and in a row pattern in the nasal and dorsal areas of the retina. Moreover, intermediate patterns can be observed in several regions indicating that double cone twisting occurs, i.e. double cones twist about their longitudinal axis. The highest cone densities are found in the ventrotemporal area. Conversely, the rod photoreceptor density is the highest in the dorsal retina. While the basic morphology of the retina is the same in both subspecies, the distribution of long single and triple cones differs between the studied animals. While these cone types are very rare in the pollan, they are common in the gangfish, though not exhibiting a regular pattern. The findings are discussed with regard to the photic habitat conditions, the systematic position of coregonids and variation of retinal morphology in the two subspecies.     

Catalytic characteristics of monoamine oxidase of whitefish Coregonus lavaretus ludoga

Study of substrate-inhibitory specificity of liver mitochondrial monoamine oxidase (MAO) of sexually mature individuals of the whitefish Coregonus lavaretus ludoga P. from the Ladoga Lake has revealed distinguished peculiarities of catalytical properties of this enzyme. The studied MAO, on one hand, like the classical enzyme of homoiothermal animals, is able to deaminate tyramine, serotonin, benzylamine, tryptamine, and beta-phenylalanine, but, on the other hand, to deaminate histamine, the classic substrate of diamine oxidase. The found equal activity and sorptional ability of the enzyme toward six studied substrates including histamine, as well as results of the substrate-inhibitory analysis with use of specific inhibitors--deprenyl and chlorgilin--indicate homogeneity of the enzyme. The detected for the first time among the fish MAO wide substrate specificity and an unusually low sensitivity to both studied acetylene inhibitors does not allow ascribing unanimously the studied enzyme to the known MAO forms of organs and tissues of homoiothermal organisms. Apparently, the revealed enzyme form of poikilothermal organism is not the true MAO, but performs a large amine oxidase function.     

Histopathology, ultrastructure and immunohistochemistry of Coregonus lavaretus hearts naturally infected with Ichthyocotylurus erraticus (Trematoda)

Histopathological, ultrastructural and immunohistochemical investigations were conducted on 26 specimens of powan Coregonus lavaretus (L.) from Loch Lomond (Scotland). The hearts of all 26 powan (15 females and 11 males) investigated harboured metacercariae of the digenean trematode Ichthyocotylurus erraticus (Rudolphi, 1809). The vast majority of metacercariae were located either singly or as an aggregation of white cysts on the surface of the bulbus arteriosus. The intensity of infection ranged from 2 to 200 larvae heart(-1), although the number of metacercariae found on male powan did not exceed 13. Histochemically, the parasite cyst wall gave a strong positive reaction with periodic acid schiff (PAS) and a faint positive signal with Azan-Mallory stain. All the metacercariae cysts were embedded in a granulomatous proliferation of heart epicardium tissue, forming a reactive fibroconnective capsule around the parasite. The capsule enclosing the parasite (produced by the host's reaction to the parasite) measured 13.57 to 90.20 microm (37.43 +/- 3.56) in thickness. Within the capsule wall, eosinophilic granular cells (EGCs), granulocytes, melanocytes and, in some instances, partially degenerated or vacuolated epithelioid cells were observed in close proximity to the cyst wall. Pigment-bearing macrophages were scattered throughout the granulomatous host-tissue reaction and as macrophage aggregates (MAs) within the capsules surrounding parasites. Immunohistochemical tests were applied to infected heart sections using 12 different antisera. Nerve fibres immunoreactive to bombesin, substance P (SP), and atrial natriuretic peptide (ANP) antisera were observed in close proximity to the parasite larvae. The presence of a serotonin-like substance was also observed within host immune-cells surrounding trematode cysts. Large cells of the epicardium were found to be immunoreactive to met-enkephalin and vasoactive intestinal peptide (VIP) antisera but not immunoreactive to anti-protein gene-product 9.5 (PGP9.5) sera.     

Ontogenetic changes in habitat use by whitefish,Coregonus lavaretus

Synopsis The whitefish, Coregonus lavaretus, in the lake Mj?sa exhibited two niche changes during their life cycle. Juveniles (< 25 cm body length) were confined to the shallow (0–30 m) epibenthic zone. Medium sized whitefish (25–35 cm body length) expanded their habitat use to include the deep (30–90 m) epibenthic zone as well as the pelagic zone. From a body length of 35 cm, habitat use was restricted to the deep epibenthic zone. Small fish in the shallow epibenthic zone ate small and medium-sized prey (zooplankton, insect larvae and surface insects). Medium-sized fish in this zone were in addition feeding on the larger amphipod, Pallasea quadrispinosa. In the pelagic zone, the diet of medium-sized whitefish was dominated by zooplankton, although some larger prey like surface insects and age-0 smelt, Osmerus eperlanus, were also eaten. In the deep epibenthic zone, the diet of both medium-sized and large (< 35 cm) whitefish consisted mainly of the large prey P. quadrispinosa.     

Daily ration of juvenile Coregonus lavaretus (L.) fed on living zooplankton

Juvenile whitefish, Coregonus lavaretus (L.), wet weight 0.04 to 5.2 g, from Lake Constance were kept at 10, 12 and 16° C water temperature, respectively and fed with living zooplankton. The experimental duration lasted 72 to 120 h. Daily rations were defined as the amount of zooplankton remaining subtracted from the amount of zooplankton added after a 24 h interval. The mortality of the zooplankton was determined in parallel experiments without fish. Relative daily ration (zooplankton weight/fish weight) v. fish weight increased up to a fish dry weight of approximately 0.12 g and then decreased steadily. The maximum daily ration was about 270% of fish body wet weight (wet/wet) corresponding to 75% of body dry weight (dry/dry), respectively. In fishes of a dry weight higher than 0.12 g (wet weight 0.65 g) a significant difference in food intake was found between 12 and 16° C. The specific growth rate ranged from nearly 0 up to 33% per day. No correlation was found between daily ration and specific growth rate.     

A hermaphrodite specimen of Coregonus lavaretus (L.) (Salmoniformes, Salmonidae) from Loch Lomond, Scotland

A single hermaphrodite specimen of Coregonus lavaretus (L.) was caught mid-January in Loch Lomond, Scotland.     

Food selection of Coregonus lavaretus in a brackish water ecosystem

This study examined how variability in the abundance and biomass structure of benthic invertebrates affected the feeding choice of the whitefish Coregonus lavaretus on a hard bottom habitat of the brackish Baltic Sea. In general, crustaceans such as Idotea balthica and Gammarus spp. were preferred over molluscs. Although being the most numerous taxon in the invertebrate samples, Mytilus trossulus was the lowest ranking in C. lavaretus food preference. The availability of benthic invertebrate prey set the dietary range of fish but the selectivity largely described fish feeding within this range. There was no clear link between fish predation and the dominance structure of benthic invertebrate communities, suggesting that species composition, abundance and biomass of invertebrate species had no impact on the feeding selectivity of the fish. Thus, while fish predation may not affect the dominant species within a benthic community, due to strong selectivity fish may impose strong pressure on some rarer but highly preferred invertebrate prey species.     

Parallel and non‐parallel morphological divergence among foraging specialists in European whitefish (Coregonus lavaretus)

Parallel phenotypic evolution occurs when independent populations evolve similar traits in response to similar selective regimes. However, populations inhabiting similar environments also frequently show some phenotypic differences that result from non‐parallel evolution. In this study, we quantified the relative importance of parallel evolution to similar foraging regimes and non‐parallel lake‐specific effects on morphological variation in European whitefish (Coregonus lavaretus). We found evidence for both lake‐specific morphological characteristics and parallel morphological divergence between whitefish specializing in feeding on profundal and littoral resources in three separate lakes. Foraging specialists expressed similar phenotypes in different lakes in both overall body shape and selected measured morphological traits. The morphology of the two whitefish specialists resembled that predicted from other fish species, supporting the conclusion of an adaptive significance of the observed morphological characteristics. Our results indicate that divergent natural selection resulting from foraging specialization is driving and/or maintaining the observed parallel morphological divergence. Whitefish in this study may represent an early stage of divergence towards the evolution of specialized morphs.     

A proteomics approach reveals divergent molecular responses to salinity in populations of European whitefish (Coregonus lavaretus)

Osmoregulation is a vital physiological function for fish, as it helps maintain a stable intracellular concentration of ions in environments of variable salinities. We focused on a primarily freshwater species, the European whitefish (Coregonus lavaretus), to investigate the molecular mechanisms underlying salinity tolerance and examine whether these mechanisms differ between genetically similar populations that spawn in freshwater vs. brackishwater environments. A common garden experiment involving 27 families in two populations and five salinity treatments together with a large-scale, high-resolution mass spectrometry experiment that quantified 1500 proteins was conducted to assess phenotypic and proteomic responses during early development, from fertilization until hatching, in the studied populations. The populations displayed drastically different phenotypic and proteomic responses to salinity. Freshwater-spawning whitefish showed a significantly higher mortality rate in higher salinity treatments. Calcium, an ion involved in osmotic stress sensing, had a central role in the observed proteomic responses. Brackishwater-spawning fish were capable of viable osmoregulation, which was modulated by cortisol, an important seawater-adaptation hormone in teleost fish. Several proteins were identified to play key roles in osmoregulation, most importantly a highly conserved cytokine, tumour necrosis factor, whereas calcium receptor activities were associated with salinity adaptation. These results imply that individuals from these populations are most likely adapted to their local environments, even though the baseline level of genetic divergence between them is low (F(ST)=0.049). They also provide clues for choosing candidate loci for studying the molecular basis of salinity adaptation in other species. Further, our approach provides an example of how proteomic methods can be successfully used to obtain novel insights into the molecular mechanisms behind adaptation in non-model organism.     

Environmental effects of introducing whitefish,Coregonus lavaretus (L.), in Lake Ring

The impact of whitefish (Coregonus lavaretus (L.)) on the trophic structure of eutrophic lakes was studied in Lake Ring, a small eutrophic Danish lake (22.5 ha, mean depth 2.9 m) in which the natural fish fauna is dominated by pike (Esox lucius L.), perch (Perca fluviatilis L.), and eel (Anguilla anguilla (L.)), roach (Rutilus rutilus (L.)) and burbot (Lota lota (L.)) being the only other fish species present. A total of 10993 0+ whitefish were stocked in the lake from October 1989 to July 1990 and the structure of the fish, zooplankton and benthic invertebrate communities studied during the period 1989–1991. Stomach contents analysis revealed that the whitefish mainly ate Daphnia and copepods in 1990–1991, the proportion of copepods decreasing with increasing size of the fish and Daphnia being the overall most important food source. The density of Daphnia in the lake decreased from 72 ind. 1^-1 in 1989 to 9 ind. 1^-1 in 1991; concomitantly the large species Daphnia magna and D. pulex almost disappeared and the density of cyclopoid copepods increased from 72 to 101 ind. 1^-1, presumably because of improved food conditions, while that of calanoid copepods remained virtually unchanged. As a result chl-a increased from 19 to 47 µg 1^-1 and Secchi depth decreased from 2.4 m to 1.7 m, despite there being no change in total P and total N (0.6 mg P 1^-1 and 1.3 mg N 1^-1, respectively). Changes were also observed in the benthic invertebrates; Chaoborus, oligochaetes, and chironomids all decreased, whereas Pisidium increased. It is concluded that the stocking of whitefish in eutrophic lakes for commercial purposes may delay their recovery, or even lead to enhanced eutrophication.     

Human‐induced changes in the reproductive traits of Lake Constance common whitefish (Coregonus lavaretus)

Size‐selective fishery harvest leads to phenotypic changes in fish reproductive traits. When these changes represent an evolutionary response of a stock, they may have severe consequences for future stock dynamics and yields. In freshwater ecosystems, reproductive traits may also be affected by other human impacts such as changes in system productivity. The present study uses regression analysis to evaluate the impacts of changes in lake trophy and of an intensive size‐selective harvest over several decades on the reproductive traits of common whitefish in Lake Constance between 1963 and 1999. Fecundity was strongly linked to lake trophy but was also related to the calendar year, suggesting an evolutionary response to size‐selective harvest and to massive stocking of the lake with hatchery‐reared larvae. The present study is an example of how fish reproductive traits are influenced by the combined action of various human impacts: changes in system productivity, size‐selective harvest and massive stocking.     

Discrete foraging niches promote ecological, phenotypic, and genetic divergence in sympatric whitefish (Coregonus lavaretus)

Natural populations often vary in their degree of ecological, morphological and genetic divergence. This variation can be arranged along an ecological speciation continuum of increasingly discrete variation, with high inter-individual variation at one end and well defined species in the other. In postglacial fishes, evolutionary divergence has commonly resulted in the co-occurrence of a pelagic and a benthic specialist. We studied three replicate lakes supporting sympatric pelagic and benthic European whitefish (Coregonus lavaretus (L.)) morphs in search for early signs of possible further divergence into more specialized niches. Using stomach content data (recent diet) and stable isotope analyses (time-integrated measure of trophic niche use), we observed a split in the trophic niche within the benthic whitefish morph, with individuals specializing on either littoral or profundal resources. This divergence in resource use was accompanied by small but significant differences in an adaptive morphological trait (gill raker number) and significant genetic differences between fish exploiting littoral and profundal habitats and foraging resources. The same pattern of parallel divergence was found in all three lakes, suggesting similar natural selection pressures driving and/or maintaining the divergence. The two levels of divergence (a clear and robust benthic – pelagic and a more subtle littoral – profundal divergence) observed in this study apparently represent different stages in the process of ecological speciation.