Long-term changes in strontium-90 concentrations within a freshwater predator-prey system

It proved possible to determine the levels of Sr-90 in the opercular bones of individual pike, Esox Iucius , and in pooled samples of bones from perch, Perca fluviatilis . Results from both species from Windermere demonstrated that Sr-90 levels rose from below the detection limits in the 1940s to a peak in the 1960s, followed by a decline in the subsequent two decades. This decline was slower than would have been expected from the decline in northern hemisphere Sr-90 fallout, indicating the likelihood of recycling within the environment. Sr-90 levels were consistently lower in pike than in perch, their main prey fish. Thus, there is no concentration of Sr-90 up this part of the aquatic food chain. Tracking Sr-90 in bones taken in successive years from ages 3 to 8 for a single cohort of pike showed that the quantity of Sr-90 was closely related to opercular bone (and hence fish) weight. No significant increase in Sr-90 concentration in the bone with increasing age was demonstrated.     

Determinants of the distribution of juvenile fish in the littoral area of a shallow lake

1. Research has often focused on the pelagic areas of lakes; the littoral zone has received less attention. The few studies concerning fish distribution in littoral habitats have concentrated on stands of submersed macrophytes, whereas other littoral habitat types have seldom been investigated. 2. This study aimed to predict the occurrence of juvenile fish in several littoral habitats of a shallow lake as a function of food availability, complexity of habitat structure, water depth and substrate. Habitats comprising reed, woody structures, and two open water areas differing in depth were sampled for fish and invertebrate biomasses on two shores, over 6 months and during both daylight and at night. 3. The juvenile fish community consisted almost exclusively of 0+ and 1+ roach and perch. There was a strong diel component in habitat use, with a predominant occurrence of fish in complex habitats (mainly woody structures) during the day, and a partial migration towards the open habitats at night, more strongly expressed in roach than in perch. 4. The diet of all fish groups was relatively constant over the seasonal cycle, and was independent of habitat. There was a higher degree of planktivory in roach than in perch, but both species fed on benthic macroinvertebrates to a substantial extent. 5. According to a logistic regression model, the biomass of potential food organisms in the different habitats had little predictive effect on the spatial distribution of the fish, whereas the structural complexity of the habitats combined with the diel cycle explained about 28% of the occurrence patterns in 0+ and 1+ perch and 1+ roach.     

Changes in the fish community and water quality during seven years of stocking piscivorous fish in a shallow lake

SUMMARY 1. Piscivores (annual stocking of 1000 individuals ha^?1 of 0+ pike and a single stocking of 30 kg ha^?1 of large 20–30 cm perch) were stocked in seven consecutive years in a shallow eutrophic lake in Denmark. The stocking programme aimed at changing food‐web structure by reducing zooplanktivorous and benthivorous fish, with resultant effects on lower trophic levels and ultimately water quality. 2. The fish community and water quality parameters (Secchi depth, concentrations of total phosphorus, chlorophyll a and suspended solids) were monitored between 1996 and 2000 and relationships were evaluated between predatory fish and potential prey and between zooplanktivorous or benthivorous fish and water quality parameters. In addition, potential consumption of piscivorous fishes was calculated. 3. The density of fish feeding on larger zooplankton or benthos (roach >15 cm, crucian carp >15 cm) declined distinctly during the study period. This effect was attributed to predation by large (>50 cm) pike. Based on scale readings, we cautiously suggest that the stocking of 0+ pike boosted the adult pike population to produce an unexpected impact in later years. Conversely, no direct impact of stocked 0+ pike was detected on 0+ roach. 4. A major decline in the recruitment strength of 0+ roach was observed in 2000. A combination of (i) the indirect effect of large pike preying on adult roach, with negative effect on roach reproduction and (ii) the direct predation effect of 0+ pike and/or 1+ and 2+ perch recruited in the lake, provides the most likely explanation of this phenomenon. 5. A marked increase in Secchi depth in 2000 and declining trends in suspended solids, chlorophyll‐a and total phosphorus concentrations were observed. These changes may also be attributable to changes in the fish community, although the relationships were not straightforward. 6. This 7‐year study indicates that piscivorous fish may be a significant structuring force in shallow eutrophic lakes, suggesting that stocking piscivores can increase predation pressure on cyprinids. However, the general lack of impact of 0+ pike points to the need of refining current stocking practices in several countries across Europe.     

Diet composition and prey preference of tench, Tinca tinca (L.), common bream, Abramis brama (L.), perch, Perca fluviatilis L. and roach, Rutilus rutilus (L.), in two contrasting gravel pit lakes: potential trophic overlap with wildfowl

The abundance of benthic macroinvertebrate taxa and the relative abundance of zooplankton taxa were compared with the diet of bream, tench. perch and roach from two gravel pit lakes during May-July 1986 and 1987. Significant food preferences were demonstrated between species and between lakes. Chironomid pupae dominated the macroinvertebrate diet of perch, roach, ate predominantly either Spirogyra sp. (St Peters Lake) or Daphniu hyalina (Main Lake).
The Main Lake, bream fed largely upon chironomid larvae and the bivalve Sphuerium but in St Peters Lake they positively selected a variety of less abundant benthic invertebrate taxa. Bream switched from benthos to zooplankton in the Main Lake in 1986. Tench ate large numbers of Aselhs and showed positive selection of various macroinvertebrate prey in St Peters but ate D. hjulim in Main Lake. Tropic overlap for chironomids and other macroinvertebrate prey was demonstrated between perch, bream and tench, and potentially with wildfowl which used the gravel pits for breeding and wintering.     

Exceptionally big individual perch (Perca fluviatilis L.) and their growth

In Lake Windermere (U.K.) where there was a dense, slow-growing population of perch ( Perca fluviatilis L.) which had an l _x of about 180 mm, there co-existed a few individuals with an l _x of 463 mm. Data from 137 of these big perch, mostly caught in gillnets fishing for Esox lucius L., were examined. Their early growth was identical to that of normal perch, but, at an age, varying between individuals, from 1 to 8 and averaging 4 years, their growth accelerated for about 4 years and then followed a von Bertalanffy model with an l _x of 463 mm. They thus had a 'double' growth-curve that is revealed only if individual growth is examined. Acceleration tended to occur in certain calendar years and was correlated with strong year-classes of young perch. Growth was greater in years with warm summer water temperatures. There were no changes in the growth of big perch, after they had achieved 290 mm, during a period of great changes in the population density and an increase in the growth of normal perch < 290 mm. No more big perch were caught in Windermere between 1967 and 1990. Net selection, length-weight relationships, seasonal cycle in gonad weight, and opercular-body length relationship were the same as those of normal perch. The few available data suggest that big perch were piscivorous, feeding largely on small perch. The distribution of sites in the lake where big perch were caught is contagious, catches follow the negative binomial; but within fishings that caught big perch at the same site and time, catches were more evenly distributed than random. It is postulated that most adults would have accelerated their growth if they had sustained piscivory. Similar big perch have been found in other lakes.     

Seasonal changes in growth and standard metabolic rate of juvenile perch, Perca fluviatilis L.

The maximum growth rate of juvenile perch, PercaJuviatilis L., at different constant temperatures and in naturally changing day-lengths was studied in the laboratory. Standard metabolic rate was studied in starvation experiments at constant temperatures under short- and long-day conditions. Growth occurred in temperatures above 8 to 10°C. In winter, from mid-October until mid-April, maximal growth was considerably reduced and was relatively slow but constant. The standard metabolic rate was reduced c . 50% under short-day conditions. The seasonal change in metabolic rates, presumably controlled by an endogenous rhythm, was considered to be an adaptation to low food availability during the short winter days.     

Exceptionally rapid growth and early maturation of perch in a freshwater lake recently converted from an estuary

In the recently freshened Volkerak, in the SW Netherlands, age 0-2 perch attained mean lengths of 12, 24 and 30 cm fork length. Males matured at age 0 and females at age 1. This exceptionally rapid growth and early maturation was due to abundant availability of zooplankton and Neomysis integer , at low fish biomass during change from a marine towards a freshwater fish community.     

An application of a bioenergetics model to Eurasian perch (Perca fluviatilis L.)

A bioenergetics model was developed for Eurasian perch ( Perca fluviatilis L.) by revising an existing model for yellow perch and walleye. Data were gathered from field studies and the literature. Besides adjusting the original parameters of the model, effects of season on consumption and metabolic rates were added. The predictive capability of the revised model was high both concerning the levels of growth and its seasonal development in the Baltic coastal waters to which the model was applied. Perch young-of-the-year attained almost maximum consumption and growth except in the highest temperatures experienced. In larger fish, the model estimated consumption to be about 50 per cent of the maximum possible rate.     

Delayed spawning of perch, Perca fluviatilis L., in acidified lakes

The timing of spawning of perch was examined in four acidified lakes (pH 4.4–4.8) and in one circum-neutral lake (pH 6.3) in southern Finland in spring 1987. In three of the lakes, perch started to spawn soon after the ice melt (4–14 degree days > 5° C) and had spawned by the end of May at about 100 degree days > 5° C. In the two most acidified lakes, fish started to spawn later, at 35 and 60 degree days > 5° C, and had spawned in early June, at about 200 degree days > 5° C. The maturing of gonad products was delayed in both males and females.     

The link between respiratory capacity and changing metabolic demands during growth of northern pike, Esox lucius L.

The relationship between metabolic rate of pike (Y, mgO2) and body weight (X, g) over the range 40–1291 gat 15° C is of the form: Y=aX^b. For resting metabolic rate (V_o2, rest), the scaling coefficient, b , is 0.80 and for maximum metabolic rate measured after exhaustive swimming (V₀₂, max), b is 0.99. Factorial metabolic scope (V₀₂, max/ V₀₂, rest) increases with body weight. Peak postprandial oxygen consumption (V₀₂, ASDA) is a constant multiple of V₀₂ rest for any discrete meal (expressed as % of body weight) up to 10% body weight. V₀₂ASDA after a single meal can utilize the entire metabolic scope (V₀₂, max—V₀₂, rest) of juvenile but not adult pike.