Accumulation of plerocercoids of Triaenophorus crassus in the second intermediate host Coregonus lavaretus and their effect on growth of the host

Patterns of accumulation of Triaenophorus crassus in its second intermediate host whitefish Coregonus lavaretus s.l. were studied between 1991 and 1996 from two host populations in two separate areas of Lake Saimaa, Finland. Whitefish were infected commonly with several T. crassus plerocercoids and the parasites were aggregated into the oldest hosts. In one host population the annual parasite accumulation was 0·9 parasites in all host age groups between 3 and 8 years. In the other host population the annual accumulation was 1·6 parasites in 3–5-year-old fish, but increased up to 3 to 4 parasites per year in fish over 5 years old. The increase did not coincide with the period of maturation or any increase in whitefish growth, both of which could alter the food intake of the fish. The sharp increase in the annual accumulation suggests a threshold intensity above which the probability of acquiring further parasites increases. In spite of a heavy aggregation of parasites there was no evidence of parasite-induced host mortality. The annual increase in mean abundance was not correlated with the mean annual weight increase in 2–4-year-old fish within cohorts. However, evidence of a negative effect of parasites on whitefish growth was revealed by back-calculation of lengths of uninfected and infected whitefish and correlations between length or weight of fish and intensity of infection with fish age. Both analyses suggested that larger young fish harboured more parasites than the smaller ones while in older fish the reverse was true, a pattern that has not been shown earlier for parasitized fish.     

The growth of whitefish in relation to water quality and fish species composition

Water quality in 16 Finnish lakes did not affect directly densely gill-rakered whitefish growth, except possibly in an acid (pH 4·9) lake, Iso Lehmaälampi, where acidity may have retarded the growth of whitefish. The density of roach affected whitefish growth in the second year of life: highest growth rates were in lakes without a roach population and lowest growth rates in lakes having strong roach populations. Competition by vendace also retarded the growth of young whitefish. The efficient mass removal of roach from a eutrophic lake was considered to have increased the growth rate of young whitefish. It is suggested that an examination of the fish species composition and relative abundance, as well as the growth of whitefish, can be used as an aid in predicting the success of stocking with whitefish.     

Energy utilization during swimming and cost of locomotion in larval and juvenile fish

Oxygen consumption and ammonia excretion rates increased in an accelerated manner in larvae and juveniles of whitefish (Coregonus sp.) as a function of swimming speed. The three-dimensional patterns of fish metabolic rates (expressed as energy consumed or nitrogen excreted) versus body weight and swimming speed show that the total standard metabolic rate (i. e. at extrapolated zero swimming speed) increased during early life of whitefish, followed by the expected decrease. This phenomenon might be due to the profound changes in oxidative and glycolytic enzyme activities during fish “metamorphosis”. Standard metabolic rate of ammonia excretion, as a principal product of protein catabolism in fish, decreased by one order of magnitude in early coregonid ontogenesis. This means that protein utilization as an energy source decreases as far as standard metabolism is concerned, but increases with swimming speed. This trend is opposite that in adult fish, where protein utilization in the overall energy supply is diminished at increasing swimming speed. The cost of locomotion offish larvae and juveniles demonstrates that the energy expenditure increases logarithmically with decreasing fish size but at an accelerated rate as compared to adult fish. This contradicts earlier estimates of lower cost of swimming in fish larvae than cost of paddle-propulsion swimming in small invertebrates or cost of flying in insects.     

Ascorbate concentration in fish ontogeny

The ontogenetic trend of ascorbate has been quantified in three freshwater fishes: roach ( Rutilus rutilus ), whitefish ( Coregonus lavaretus ) and Arctic charr ( Salvelinus alpinus ). Total ascorbate (reduced and oxidized) declined from 150 to μg g⁻¹ as newly hatched larvae grew to become several-months-old juveniles. Declining total ascorbate with increasing size of metamorphosing fish could not be reversed by feeding on brine shrimp, Artemia salina nauplii, zooplanktonic food containing > 74μg g⁻¹ total ascorbate. The proportion of reduced ascorbate in total ascorbate increases with fish size/age. The physiological mechanism of the changes in transferable ascorbate forms remains unknown, but high dehydroascorbate concentrations suggest high vulnerability of larval fish to oxidation stress. This is the first report on quantity of vitamin C retained in actively-feeding larval and juvenile fish. The efficiency of ascorbate transfer from zooplankters to larval fish amounted to 5–20%. The ecological significance of larval fish feeding on various zooplankters and/or phytoplankton may reflect a trend toward maximum transfer of this vitamin in freshwater food webs.     

Morphological changes during metamorphosis in cod (Gadus morhua L.), with particular reference to the development of the stomach and pyloric caeca

Metamorphosis in cod, Gadus morhua L., was investigated with respect to morphological changes in the alimentary tract (mouth, stomach and pyloric caeca) and external characters (finfold and fin development). Morphometric measurements and histological investigations were made on both larvae and juveniles and all fish lengths are given as standard length. Median unpaired fins started to develop when the fish were 9–10 mm in length. The larval median finfold was resorbed when fish reached a size of 11–15 mm. The vertebrae were formed in fish of 11–20 mm. The supraterminal mouth, seen in larvae <12 mm, changed gradually to a sub-terminal position in juveniles larger than 65 mm. The development of the stomach and pyloric caeca started at 15 mm and the juvenile form was seen in fish of 40 mm. The ability to store and grind food particles in the stomach developed gradually as fish increased in size from 20 to 40 mm. Compared to many other teleost species, cod juveniles are relatively large when the stomach and pyloric caeca become fully developed. These findings may have important implications for both start-feeding of cod larvae and the weaning of juveniles on to artificial diets.     

Seasonal distribution of juvenile Atlantic cod in the southern Gulf of St Lawrence

Seasonal changes in the geographic distribution of juvenile (ages 1–4) Atlantic cod Gadus morhua were determined from research surveys conducted in the southern Gulf of St Lawrence during 1986–87, 1990–91, and 1994. From late June to early October, juvenile cod were most abundant in shallow water (<50 m deep). By early November, all age groups (adults and juveniles) had migrated to deep (>100 m) offshore waters for the winter. For most juveniles, this represented a minimum migration of >225 km (each way). Age 3 and 4 cod migrated greater distances than age 1 and 2 fish. Some age 3 and 4 cod migrated as far as 650 km from their summer and early autumn areas. Although there was partial geographic segregation of juvenile and adult fish during summer and winter, all age groups were found together during the migration. No cod were present in the shallow waters (<80 m deep) of the south-western Gulf during April due, in part, to later than average ice-melt. Large numbers of all age-classes were present in most of the south-eastern Gulf by mid-May.     

Metechinorhynchus salmonis infection and diet in the river-spawning whitefish of the Bothnian Bay

A total of 206 river-spawning whitefish Coregonus lavaretus (L.) from the Bothnian Bay was investigated for food objects and acanthocephalans in the years 1975–78. The only acanthocephalan species found was Metechinorhynchus salmonis (Müller 1780), the total number of which was 1295.
The whitefish is shown to be highly susceptible to infection by M. salmonis , with as many as 88·9% of specimens infected in a sample in June 1978, although the mean intensity was only 8·9 parasites per host. The river-spawning whitefish does not favour the intermediate host, Pontoporeia affinis , in its summer diet in the Bothnian Bay preferring to feed on eggs and molluscs, so that the incidence of M. salmonis infection decreases to 33·3% by the middle of summer. This is further reduced in the late summer by the fast associated with the migration to their upriver spawning grounds. The fact that the other acanthocephalan inhabiting the river-spawning whitefish in the southern Gulf of Bothnia, Echinorhynchus gadi Müller, 1776 is not to be found in the Bothnian Bay population suggests that the fish captured in spring have remained in the Bothnian Bay throughout the winter.     

Diet, growth and competitive abilities of sympatric whitefish forms in a dense introduced population: results of a stocking experiment

The food of five whitefish stocks Coregonus lavaretus planted in Lake Vuokalanjärvi consisted mainly of zooplankton and insects taken from the water surface. The amount of benthic food in the diet of the two sparsely gill-rakered (23 and 28 rakers) stocks increased from the age of 2 years onwards. The growth of whitefish was slow in all stocks. The densely gill-rakered stock (50–51 rakers) originating from the same lake system as Lake Vuokalanjärvi grew faster than the other stocks, and the food consumption of 4- and 5-year-old fish belonging to this stock was nearly double compared with the other stocks. The habitat choice of the whitefish differed between the stocks even in the densely-rakered stocks classified as the same form. A stock with flexible habitat use and feeding habits is most likely to succeed. Introducing whitefish stocks from distant areas is not advisable.     

Studies on the growth of juveniles of Cynoscion striatus in the sea and in aquaria

This paper reports on the studies on growth of juveniles, 20–140 mm in length, of Cynoscion striatus in the sea and in aquaria. The monthly growth of these juveniles in the sea was determined by means of the analysis of length-frequency distributions of 5500 individuals throughout the year. Fish in aquaria were measured and weighed every two weeks, and were fed on known amounts of food. The relationship between the length and the weight of fish, and between the length, width and thickness of the otoliths and the length of fish were calculated and were expressed as exponential functions. On the basis of the analysis of length-frequency distributions and of back calculations it was determined that the formation of the first annulus in the otoliths of juveniles of this species occurs in specimens of 45–100 mm in length and of 1.16–10.0 g in weight. In the majority of the individuals the first annual ring was formed at a size of about 70–80 mm. In juveniles of 79–102 mm, at a temperature of 15–22° C, the maintenance requirement was 0.189 g, and the gross efficiency is 0.310 g of shrimp per gram fish per week. The efficiency of food conversion is high, its value being of 18.8%. The increment in length in these juveniles was 2.01 mm per week and the increment in weight was 0.896 g per week.     

The effects of body mass and feeding on metabolic rate in small juvenile Atlantic cod

Apparent specific dynamic action (SDA) amplitude in young juvenile Atlantic cod Gadus morhua (1 to 8 g wet mass), fed a standardized meal and then exercised in a circular swimming respirometer at a constant swimming speed of 0·5 ± 0·3 body lengths s^-1, occurred within l h after feeding in all juveniles. SDA amplitude were 1·4 to 1·8 times higher in fed fish compared to unfed fish, and rates of oxygen consumption decreased as body mass increased. SDA duration had a tendency to decrease with increasing body mass and was shortest, at 6 h, in the smallest fish (1–1·5 g), but increased to 10–11 h in the largest fish. Apparent SDA in fed fish ( R _r) scaled with a mass exponent of 0·89, while maximum metabolic rate ( R _max) determined by chasing fish to exhaustion and then measuring oxygen consumption for 12 h, and unfed routine metabolic rate (R_r) scaled with a mass exponent of 0·79 and 0·76 respectively. Relative aerobic scope ( R _max– unfed R _r) did not appear to vary over the 1 to 8 g increase in wet mass. These results show that as body mass increased in young juvenile Atlantic cod: (1) apparent SDA ( R _f) increased more rapidly than R _max, and (2) apparent SDA took up >98% of the relative aerobic scope and that young Atlantic cod allocated most of the energy to growth, and left little for other metabolic activities.     

Interactions between invading benthivorous fish and native whitefish in subarctic lakes

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Characterization of the ultrastructure of the gastrointestinal tract mucosa, stomach contents and liver enzyme activity of the pinfish during development

Carnivorous juveniles (<16 mm L_S) of pinfish Lagodon rhomboides apparently lacked gastric glands in the stomach while larger fish, intermediates (30-33 mm L_S) and herbivorous adults (>80 mm L_S) had numerous gastric glands. Two cell types were identified in the gastric glands of larger fish: mucous secreting cells and secretory cells which had ultrastructure features typical of digestive enzymes and acid secretion. Lipid absorption occurred throughout the caeca and intestine in all sizes of fish. Microvilli found on the rectal epithelial cells of the intermediate and adult pinfish occurred on stalks and were possibly associated with water reabsorption. Liver enzyme activities changed in small fish (26–39 mm L_S) compared to the adult fish. Alanine amino transferase (ALT) and fructose diphosphotase (FDP) activities declined while pyruvate kinase (PK) activities increased significantly. These changes were consistent with a change in diet from a carnivorous (high protein) diet in juveniles to an omnivorous (lower protein) diet determined by stomach content analyses.     

Some positive and negative effects of stocking whitefish on the ecosystem redevelopment of Hjarbæk Fjord,Denmark

In 1984 the County of Viborg introduced a large-scale programme to improve the water quality in Hjarbæk Fjord, a freshwater fjord cut off from the sea in 1964. Measures were taken to reduce the discharges of nutrients from various sources. To reduce the nuisance of chironomids a large release of whitefish (Coregonus lavaretus L.) was incorporated in the programme. The effects of these efforts on the fish, benthos and plankton communities were studied during a period of five years. The whitefish established a self-reproducing population with a very good growth rate, and may have created quantitative and qualitative changes in the plankton community. Observed changes in the benthos community may also be related to predation by whitefish. The study did not confirm that whitefish could actually control the population of chironomids and diminish, thus, the nuisance caused by them. The stocking of whitefish created an exploitable fish stock of a valuable fish species in the hypertrophic Hjarbæk Fjord.

     

Resource partitioning between pelagic coregonids in a subarctic watercourse following a biological invasion

If a pelagic ecosystem is invaded by an efficient planktivorous fish, competition theory predicts that habitat and/or diet segregation should occur if the invader and native planktivores are to co-exist. We compared the diet and pelagic habitat use between invading vendace (Coregonus albula) and two native morphs of whitefish (Coregonus lavaretus) in three subarctic lakes located within the same watercourse. No clear vertical habitat segregation was found as vendace prevailed throughout the entire depth range. Zooplankton generally dominated the diet of all three coregonids. No obvious resource partitioning was found in two of the lakes, while in the third lake the diets of whitefish and vendace differed significantly from each other. Here, the predominant pelagic whitefish morph fed extensively on water mites whereas vendace mainly consumed surface insects and to some extent small fish, apparently as a result of highly depleted zooplankton resources. The results suggest that various levels of resource depletion following the vendace invasion caused the observed differences between the lakes, indicating that different levels of resource partitioning were associated with dissimilar competition intensities. At present, coexistence of these congeneric species seems to be possible, although the vendace invasion has apparently resulted in a challenging and unstable situation for the planktivorous whitefish.     

Empirical evaluation of phenotype–environment correlation and trait utility with allopatric and sympatric whitefish, Coregonus lavaretus (L.), populations in subarctic lakes

Adaptive phenotypic divergence of sympatric morphs in a single species may have significant evolutionary consequences. In the present study, phenotypic impacts of predator on zooplankton prey populations were compared in two northern Finnish lakes; one with an allopatric whitefish, Coregonus lavaretus (L.), population and the other with three sympatric whitefish populations. First, we examined whether there were phenotypic associations with specific niches in allopatric and sympatric whitefish. Second, trait utility (i.e. number of gillrakers) of allopatric and sympatric whitefish in utilizing a pelagic resource was explored by comparing predator avoidance of prey, prey size in environment, and prey size in predator diet. The allopatric living large sparsely rakered (LSR) whitefish morph, was a generalist using both pelagic and benthic niches. In contrast, sympatric living whitefish morphs were specialized: LSR whitefish was a littoral benthivore, small sparsely rakered whitefish was a profundal benthivore and densely rakered (DR) whitefish was a pelagic planktivore. In the lake with allopatric whitefish, zooplankton prey did not migrate vertically to avoid predation whereas, in the lake with sympatric whitefish, all important prey taxa migrated significantly. Trait utility was observed as significantly smaller size of prey in environment and predator diet in the lake with DR whitefish than in the lake with only LSR whitefish.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 561–572.     

Direct Estimation of Food Consumption of Juvenile Fish in a Shallow Inlet of the Southern Baltic

The species composition and feeding behaviour of fish larvae and juveniles was investigated in a shallow inlet of the Southern Baltic. The aim of the study was to examine if there was a predatory impact of young fish on the observed annual collapse of the copepod population. Dominant fish species were herring (Clupea harengus), perch (Perca fluviatilis) and roach (Rutilus rutilus). Roach larvae preferred shallower areas, whereas young herring and perch were found in open water. Highest consumption values were always observed in the evening. By computing complete evacuation of the digestive tract in approximately 3.5 hours, daily consumption in percent of fish fresh weight was 1.3, 4.7 and 5.5% for young herring, roach and perch, respectively. The impact of young fish on the dynamics of the population of the copepod species Eurytemora affinis is negligible.     

Spatiotemporal variation in the population structure of the European hake in the NW Mediterranean

The bathymetric distribution of Merluccius merluccius was studied as a function of length, age and maturity of specimens caught by commercial trawl and longline in Different seasons. Males matured first at 28·8 cm (3 years old), and females at 38·0 cm (3·5 years old). Reproductive activity was noted practically throughout the year with its most pronounced spawning peak in the autumn. Most fish <37–40 cm were males and >46 cm were females. Specimens occurred between 50 and 750 m depth, although density was low at >400 m. Adults were found at all depth strata studied. Recruits and juveniles were limited to inshore waters <400 m, most were found between 100 and 200 m. Spring and summer were the preferred seasons for recruitment; although for both seasons there was some interannual variation. Adult distribution also varied, according to the season. Young adults were spread over the entire depth range, with the biggest ones concentrated at the edge of the shelf (150–350 m), especially in autumn and winter. The main spawning peak coincided with this concentration of adults suggesting that spawning occurred in autumn/winter at the edge of the shelf.     

The biology of pygmy whitefish, Prosopium coulterii, in a closed sub-boreal lake: spatial distribution and diel movements

Using gillnets and trap nets, we examined the spatial distribution, diel movements, and environmental tolerances of pygmy whitefish, Prosopium coulterii, in a small boreal lake in north-central British Columbia. Most gillnets were set below the thermocline but we also fished a shore net in the littoral zone. During the ice-free season (May to November) there was a strong diel onshore–offshore movement: during the day pygmy whitefish were offshore and below the thermocline (water temperatures of 4–6°C) but at night they were inshore and above the thermocline (water temperatures of 12–18°C). This onshore–offshore movement occurred close to the bottom and, regardless of where they were caught, most fish were <4 m off the bottom. Oxygen concentrations in most of the hypolimnion dropped to <5.0 mg l⁻¹ in June and by late August to <1.0 mg l⁻¹; indicating pygmy whitefish can tolerate low oxygen conditions. The catch of pygmy whitefish in gillnets set below the thermocline was highly skewed: 53% of the nets were empty, 37% caught 18 or less fish, and 10% caught 70% of the total catch (742 fish). Trap nets produced similarly skewed results: most trap net sets caught no pygmy whitefish but one set caught over 2,000 individuals. Our catch data suggest that in Dina Lake #1 some pygmy whitefish aggregate.     

Shifts in the microhabitat of larval and juvenile roach, Rutilus rutilus (L.), in a floodplain channel

The microhabitat of roach larvae (stages L1–L6) and 0+ juveniles was examined in a braided channel (Ilon) of the Upper Rhone River, France, from weekly samples (spring-autumn 1985) using Point Abundance Sampling by electrofishing. During early development, microhabitat exploitation was characterized by two transitions: the first at larval step L3, when morphological and physiological attributes permit the active choice of microhabitat; the second at the end of the larval and start of the juvenile period. At larval step L3, and up to L5, roach were strongly associated with lentic waters of medium depth (0.5–1.0m) and with either macrophytes or ligneous debris. The L6 and 0+ juveniles were found more often than expected in shallower waters (< 0.2m, 0.2–0.5 m) and were no longer associated with dense vegetation or ligneous debris. However, both larval (L3–L6) and 0+ juvenile roach preferred silty sand and/or silted gravel, and a lack of current. This shift, from moderately deep waters with vegetation and/or branches as larvae to open shallow waters as juveniles, demonstrates the young roach's perception and response to environmental change, with shallows exploited as a refuge from predation when protective structures become scarce or unavailable.