'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.     

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.     

Notes on the biology of the schelly Coregonus lavaretus (L.) in Haweswater and Ullswater

Some aspects of the biology of the schelly Coregonus lavaretus (L.) were investigated by examining 188 that had passed down an aqueduct from Haweswater and 437 that were caught in gill nets set in Ullswater. In both lakes the main breeding season is from mid January to mid February, and in Ullswater spawning takes place in shallow water [less than 7.6 m (25 feet)] on a sandy bottom with stones and weed, at the entrance to a bay. The males in Haweswater first spawn when 3 years old and in Ullswater when 2; and the females first spawn in Haweswater when 4 years old, and in Ullswater when 3. Scales were kept from every fish and were found suitable for age determination. Some Haweswater schelly live for 9 years, but from Ullswater the oldest was 8. The rate of growth of male Haweswater schelly was very well described by the equation: (where l_t(=the length at time t) but the curve had to be transposed a further year to describe adequately the Ullswater male schelly growth. It is clear that the rates of growth are similar in the two lakes after the first year, during which it is much faster in Ullswater. A study of the length-weight relation showed that the weight is proportional to the length cubed, and no difference in condition was found between males and females. The schelly egg was found to be slightly sticky, spherical and 2.98 mm in diameter. Some fertilized eggs were kept at various constant temperatures. At 4°C and 6°C the death rate was low, but was higher at 8°C and total mortality occured at 10°C. This low upper lethal temperature probably restricts the spawning season to mid-winter and also limits the species' geographical range, as has also been suggested for the char Salvelitms alpinus (L.) whose eggs have similar temperature requirements. Some eggs were reared to adults, and these provided corroboration of the validity of the scales for age determination. Two other points mentioned in this paper were the occasional winter strandings of schelly, and the incidence of a supernumerary pelvic fin.     

Limits for the detection of daily otolith increments in whitefish (Coregonus lavaretus L.) larvae

The formation of daily otolith increments in European lake white-fish was evaluated in the laboratory during 40 days at 4, 6, and 8 °C under limited and ad libitum food supply. Daily increments were easily identified in larvae reared at 8 °C. At 6 °C and, more, at 4 °C, an unstructured perinuclear zone was formed, and daily increments could only be recognized in the light microscope starting from 10 to 35 days after hatching. In some larvae from the 4 °C groups, no increments at all were found. Scanning electron microscopy (SEM) could not improve increment resolution. Only those increments could be viewed by SEM which were visible in the light microscope as well. We conclude that whitefish larvae which experience low (4–6 °C) temperatures during their first weeks of life, hence those which hatch in the lake, cannot be aged by the currently employed preparation techniques.     

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.     

Morphological differentiation and resource polymorphism in three sympatric whitefish Coregonus lavaretus(L.) forms in a subarctic lake

Three field‐identified whitefish Coregonus lavaretus forms in Lake Muddusjärvi, Finland, were compared in morphology, diet and prey size. First, these forms were studied with univariate and multivariate analysis to assess morphological divergence at a higher resolution level than in the field. Second, stomach contents were analysed to estimate diet‐overlap among forms. Finally, the relationship between prey size and morphology was examined. The whitefish were assigned to the initial field‐classification with 99·2% and 98·8% accuracy for morphologic and meristic traits, respectively. The small sparsely‐rakered form (SSR) had the shortest rakers and largest gillraker space, followed by the large sparsely‐rakered form (LSR) with intermediate gillraker length and gillraker space, while densely‐rakered whitefish (DR) had the longest rakers and smallest gillraker space. The two sparsely‐rakered whitefish forms (LSR and SSR), consumed mainly benthic macroinvertebrates, while densely‐rakered whitefish (DR), utilized pelagic food items. Average diet‐overlaps between whitefish forms were low in June‐September (Schoener's α = 0·02 − 0·23). Gillraker number and length were negatively correlated to prey length in the diet ( r  = −0·73, and r  = −0·60), while gillraker space was positively correlated with prey length ( r  = 0·81). The fact that these whitefish forms were morphologically and ecologically segregated, and that gillraker traits probably have a functional value in food selection, further suggests that natural selection has been important in structuring life‐history trajectories into divergent niche use.     

Induction of fever in a teleost fish (Lepomis gibbosus, L.)

Although fishes are ectotherms they are nevertheless able to thermoregulate behaviorally by selecting appropriate water temperatures (1). In a temperature gradient fish will congregate to a species-specific range of preferred temperature (“final thermal referendum”) which is unaffected by previous thermal history of the individual (2,3). Several aquatic (and terrestrial) ectothermic vertebrates have been found to exhibit “behavioral fever” which is manifested as an increase in preferred temperature above the final thermal preferendum (4). Fever can be elicited by pyrogens: whole bacteria (alive or killed), components of bacterial cellwall (endotoxins), endogenous pyrogens, prostaglandins or from several other sources (5). Since the results with fever induction in fish using whole bacteria or endotoxins are very scarce the aim of the present work was to compare possible thermoregulatory effects of endotoxins and prostaglandins in the same species (Lepomis gibbosus, L.) by means of identical methods.     

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).     

Evolutionary history of the European whitefish Coregonus lavaretus (L.) species complex as inferred from mtDNA phylogeography and gill-raker numbers

We compared mitochondrial DNA and gill-raker number variation in populations of the European whitefish Coregonus lavaretus (L.) species complex to illuminate their evolutionary history, and discuss mechanisms behind diversification. Using single-strand conformation polymorphism (SSCP) and sequencing 528 bp of combined parts of the cytochrome oxidase b (cyt b) and NADH dehydrogenase subunit 3 (ND3) mithochondrial DNA (mtDNA) regions, we documented phylogeographic relationships among populations and phylogeny of mtDNA haplotypes. Demographic events behind geographical distribution of haplotypes were inferred using nested clade analysis (NCA) and mismatch distribution. Concordance between operational taxonomical groups, based on gill-raker numbers, and mtDNA patterns was tested. Three major mtDNA clades were resolved in Europe: a North European clade from northwest Russia to Denmark, a Siberian clade from the Arctic Sea to southwest Norway, and a South European clade from Denmark to the European Alps, reflecting occupation in different glacial refugia. Demographic events inferred from NCA were isolation by distance, range expansion, and fragmentation. Mismatch analysis suggested that clades which colonized Fennoscandia and the Alps expanded in population size 24 500-5800 years before present, with minute female effective population sizes, implying small founder populations during colonization. Gill-raker counts did not commensurate with hierarchical mtDNA clades, and poorly with haplotypes, suggesting recent origin of gill-raker variation. Whitefish designations based on gill-raker numbers were not associated with ancient clades. Lack of congruence in morphology and evolutionary lineages implies that the taxonomy of this species complex should be reconsidered.     

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.     

The effect of dietary phosphorus deficiency on the immune responses of European whitefish (Coregonus lavaretus L.)

Low phosphorous (P) feeds in aquaculture are recommended to reduce eutrophication of water systems. However, the feed should be adequate for normal growth and intact immune defence. Influence of low dietary P supply on non-specific and specific immune defence of European whitefish (Coregonus lavaretus) was studied in two trials. For Trial 1, a semi-purified, low-P diet was formulated and supplied with 0, 2.5, 5.0, 7.5, or 10.0 g P kg(-1)to obtain a P gradient of 4.4-14.9 g kg(-1)diet. Diets were fed to four replicate groups of fingerling whitefish for 42 days in a flow-through, freshwater system maintained at 15 degrees C. Fish fed with the P-unsupplemented diet had significantly lower plasma immunoglobulin M (IgM) levels than fish fed with the P-fortified diets (means 2.53 vs. 3.19 mg ml(-1); P=0.047). Plasma lysozyme activity did not differ between fish fed with the P-unsupplemented diet and fish fed with the P-fortified diets (means 21.8 vs. 25.5 U ml(-1); P=0.107). For Trial 2, fish were acclimated for 49 days by feeding the lowest or highest dietary P contents, and thereafter immunised with a single intraperitoneal injection of a novel antigen, bovine gamma globulin (BGG). A clear antibody response against BGG was elicited but the antibody levels did not differ between fish fed low-P or high-P contents. Similar to Trial 1, plasma IgM was lower in fish fed low-P diets, and low-P diet did not influence plasma lysozyme activity. In both trials the growth of fish, having received low-P diet, was significantly lowered. It is concluded that P deficiency has only minor effects on the immune parameters in whitefish, and the practical aquafeed with P contents sufficient for normal growth does not compromise immune functions of this species.     

Parallel evolution of ecomorphological traits in the European whitefish Coregonus lavaretus (L.) species complex during postglacial times

The extensive phenotypic polymorphism in the European whitefish has triggered evolutionary research in order to disentangle mechanisms underlying diversification. To illuminate the ecological distinctiveness in polymorphic whitefish, and evaluate taxonomic designations, we studied nine Norwegian lakes in three watercourses, which each harboured pairs of divergent whitefish morphs. We compared the morphology and life history of these morphs, documented the extent of genetic differentiation between them, and contrasted the niche use of sympatric morphs along both the habitat and resource axes. In all cases, sympatric morphs differed in the number of gill rakers, a highly heritable trait related to trophic utilization. Individual growth rate, age and size at maturity, diet and habitat use also differed between morphs within lakes, but were remarkably similar across lakes within the same morph. Microsatellite analyses confirmed for all but one pair that sympatric morphs were significantly genetically different, and that similar morphs from different lakes likely have a polyphyletic origin. These results are most compatible with the process of parallel evolution through recurrent postglacial divergence into pelagic and benthic niches in each of these lakes. We propose that sparsely and densely rakered whitefish sympatric pairs may be a likely case of ecological speciation, mediated in oligotrophic lakes with few trophic competitors.     

Morphological divergence and origin of sympatric populations of European whitefish (Coregonus lavaretus L.) in Lake Femund, Norway

Combining morphological and genetic analysis, we compared patterns of diversification within and between morphs among sympatric European whitefish (Coregonus lavaretus L.) populations in Lake Femund, Norway. Seven external populations, from potential colonization routes into Lake Femund were included. We found that deep-, shallow-, river- and bay spawning populations are distinct morphs in Lake Femund. Within morphs, populations range from being similar genetically (Fst=0-0.005) among deep-spawning populations to being highly differentiated (Fst=0.153) between bay-spawning populations. Between morphs, genetic differences ranged from a low (Fst=0.008-0.022) between deep- and shallow-spawning populations to high difference (Fst=0.125-0.143) between shallow- and bay-spawning populations. A higher proportion of molecular variance was seen among (3.9%) than within morphs (2.8%). The adaptive gene combinations behind the four morphs seem to have originated within the lake, although the lake could have been colonized from more than one source population.     

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.     

Post-stocking perturbations in a population of subarctic whitefish,Coregonus lavaretus (L.): effects on growth,condition and cestode infection

Lake Kilpisjärvi was stocked with a total number of 440 000 migratory and benthic whitefish fry during the period of 1959–1964. Thereafter, the Diphyllobothrium ditremum plerocercoid infection became a problem decreasing the feasibility and commercial exploitation of the whitefish stock. Since dense fish populations may cause stunting, all stockings in the study area were stopped in an attempt to enhance the fisheries. The aim of the present study was to explore the effect of the cessation of the stocking on the growth, condition, catches and D. ditremum infection of the whitefish population. A total of 1594 whitefish were sampled with benthic gill nets. Growth was estimated from length-at-age and mass–length relationship was estimated with a non-linear regression in 1982–1983, 1992–1993 and 1997. Parasite infection was studied in 1992–1993 and 1997. Growth and condition were poor in the beginning of the 1980s. In 1992–1993, the growth and condition of the fish improved as the numerical catch per unit effort decreased from 1982 to 1992. The mean catches of other fish species were low. There were no significant changes in whitefish growth or condition from 1992–1993 to 1997. From the 1970s to 1982–1983, during slow growth and poor condition of the fish, the median gillraker count was decreasing to 19 (min–max 14–26). In 1992–1993, the gillraker count had increased significantly to 23 (min–max 19–26). However, the improved growth and condition did not cause decrease in the D. ditremum burden. On the contrary, based on our own results and literature notes, the parasite abundance seems to increase during the observation period of 1964–1997.     

Utilization of body stores in embryonated ova and larvae of two coregonid species (Coregonus lavaretus L. and C. albula L.)

• 1.1. Developing eggs of whitefish (Coregonus lavaretus L.) and vendace (Coregonus albula L.) were kept at 1–2°C and some eggs taken gradually up to 8°C to provoke mass hatching of embryos.
• 2.2. Wet weight, dry matter and the contents of lipid, protein and ash were measured in fish during the course of experiment.
• 3.3. Dry matter content decreased gradually in whitefish eggs from 15.64 to 11.95% during 1 month at 1–2°C, whereas vendace eggs showed only a slight decrease from 16.27 to 15.53%.
• 4.4. In both species protein content decreased but lipid increased when approaching the natural time of hatching.
• 5.5. During delayed hatching at low water temperatures protein contributes to catabolism, whereas lipid content decreased only in the later phase of the experiment.
• 6.6. Larvae starved for 10 days after hatching lost increasing amounts of dry matter (from 26.1 to 50.3% of body weight) and protein (from 18.7 to 45.9% of body weight) as they remained longer in cold water as embryos.
• 7.7. A correspondence was found between assessment of metabolic utilization of body stores based on chemical analysis of fish body and previous work on oxygen consumption and nitrogen excretion.

     

Life histories of the powan,Coregonus lavaretus (L.) (Salmonidae,Coregoninae) of Loch Lomond and Loch Eck

Four aspects of the life histories of the two populations of powan Coregonus lavaretus (L.) in Scotland are described: growth (Eck powan are shorter and with greater year to year variance than Lomond); sexual maturation (Eck powan mature younger, but at similar weight to Lomond); spawning (timing in Eck varies, but is consistent in Lomond); and recruitment/mortality (fecundity, sex ratios, and mortality also vary in the short term). Short term differences between the physiological ecology of the populations can be ascribed to the size and topography of the lochs. Long term differences are more difficult to account for, and are more important in that they may signal changes in sustainability. Conservation of powan must be considered in terms of their synecological relationships, not in isolation.