Patterns in vendace recruitment in Lake Pyhäjärvi, south-west Finland

A vendace Coregonus albula population has long formed the basis of a flourishing fishery in Lake Pyhäjärvi, south-west Finland. Between 1971 and 1990, it exhibited variations in year-class strength generally characterized by a 2-year cycle of alternating strong and weak year classes, which implied density-dependent regulation probably due to asymmetrical food competition between age groups. The abundance of piscivorous predators in the lake and the wanning of water after the hatching of larvae in spring together determined the final year-class strength of vendace. Since 1990, year-class sizes have remained very low and the 2-year cycle has been disrupted. The continuously low levels of recruitment could not be explained by an increased larval mortality, although the decline started with 2 successive years with very high mortality of this life stage which reduced the whole spawning stock to a fraction of earlier levels, and consequently led to low larval numbers such that between 1988 and 1996 larval abundance was positively correlated with the spawning stock size. The importance of the spawning stock size to recruitment could also be seen in the whole 26-year time series as a significant correlation between year-class sizes with a 2-year lag. The observed recruitment patterns of the vendace stock in Lake Pyhäjärvi thus show the importance of spawning stock size, intraspecific competition, predator effects and abiotic environmental variation.     

Fish stock assessment in lakes based on mass removal

The efficiency of mass removal of fishes can potentially be assessed using catch statistics collected during intensive fishing periods. The calculations are based on the general assumption that a decline in population size will produce a decline in catch per unit effort (cpue). When the removal is efficient it is possible to estimate the population size both at the beginning of the fishing period and after the removal.
Two examples are based on the winter seine net fishing of vendace ( Coregonus albula ) in Lake Pyhäjärvi, SW Finland in (1) 1983–1984 and (2) 1989–1990. The effects of the error in catch composition samples and random changes in probabilities of capture during the removal period on the final estimates of initial stock sizes are also examined.
The precision of the population estimates shown here was greatly influenced by the violation of the underlying assumption that the probability of capture is equal for all members of the target age group. In any case, particular attention should be paid to optimizing the sampling programme so that it will reveal the best information on the exploited stock with the resources in hand.     

The size of a vendace, Coregonus albula L., stock in a deep lake basin monitored by hydroacoustic methods

An acoustic estimate of the number and biomass of a vendace stock (age >2 + years) in a deep basin of Lake Karjalan Pyhäjärvi was made in August 1985. The acoustic data were collected at night during the summer stagnation. The vendace were on the lake bottom during the day and rose by midnight to the hypolimnion; they did not rise to the thermocline or water layers above it. In the research area the mean number of vendace was 1900 fish ha ⁻¹ and the mean biomass was 76 kg ha ⁻¹. Total vendace biomass was 151 in the whole 200-ha research area.     

A validated bioenergetics model for ruffe Gymnocephalus cernuus and its application to a northern lake

A bioenergetics model was constructed for ruffe Gymnocephalus cernuus , and the model was used to estimate the food consumption of a native G. cernuus stock in Pyhäjärvi, a large mesotrophic lake in south-west Finland. The parameters for the bioenergetics model were determined through laboratory experiments conducted at several temperatures between 5 and 22° C for different sized G. cernuus (from 6 to 27 g). The model was verified and validated with independent laboratory growth experiments. The validated model was applied to field data from Pyhäjärvi to estimate the food consumption of G. cernuus and its effects on the zoobenthos community during the years 2002–2004. The total annual food consumption of the G. cernuus stock ranged from 11·6 to 15·7 g m⁻² (wet mass). Chironomidae was the most abundant prey, 81% of the total consumption. The annual total production of zoobenthos in Pyhäjärvi was 15·2 g m⁻² (wet mass). Therefore, it can be concluded that G. cernuus stock is likely to exert a significant influence on the zoobenthos community in this lake, indicating a potential role of G. cernuus as an influential invasive species in many freshwater ecosystems in Europe and North America.     

Year-class fluctuations in vendace,Coregonus albula (Linnaeus): Who's got the upper hand in intraspecific competition?

The vendace Coregonus albula (L.) populations in the lakes Mjøsa and Osensjøen exhibited fluctuating year-class strength. In Mjøsa, a strong year-class emerged every third year, except for the four year period between the strong year-classes 1969 and 1973. The difference between the strong and weak year-classes decreased from the 1960s, through the 1970s to the 1980s. The Mjøsa vendace matured sexually at age 2 +, and more than ten sexually mature age-groups were present in the population. Growth ceased at maturation, and asymptotic length was 23.6 cm. In Osensjøen, one strong year-class (1969) dominated the population during the period 1976–1987. The Osensjøen vendace matured sexually at age 3, and more than 15 sexually mature age-groups were present in the population. Growth ceased at maturation, and asymptotic length was 28.4 cm. In both lakes, vendace fed on crustacean zooplankton in the epilimnion throughout summer and autumn. Our data indicate that regular year-class oscillations occur as a result of the juvenile survival being negatively correlated to the number of adults.     

Seasonal changes in the numerical responses of predators to cyclic vole populations

Theoretical models predict that a delayed density-dependent mortality factor with a time lag of ca 9 months is able to drive 3–5-yr population cycles of northern voles. We studied numerical responses of predators in western Finland during 1986–92, in an area with 3-yr population cycles of voles. Abundances of small mammals were monitored in several farmland areas (each 3 km²) by snap-trapping in April, June, August, and October (only in 1986–90), and the abundances of avian, mammalian, and reptilian predators by visual censuses during trapping occasions. The 3-yr cycle studied was a cycle of Microtus voles (field vole M. agrestis and sibling vole M. rossiaemeridionalis ) and their small-sized predators (small mustelids and vole-eating birds of prey). The numerical responses of both migratory avian predators and small mustelids to changes in vole densities were more alike than different. In late summer (August), the time lag in the numerical response of all main predators was short (0–4 months), whereas longer time lags prevailed from spring to early summer. The length of the time lag in spring appeared to be related to the length of the winter, which indicates that strong seasonality may create longer time lags to the numerical response of predators at northern latitudes than at more southern latitudes. Our results suggest that, from spring to early summer, predation by migratory avian predators may act in concordance with mustelid predation to produce the long time lag necessary to drive the 3-yr cycle of voles, whereas almost direct density-dependent predation by all major predators in late summer may dampen spatial variation in prey densities.     

Influence of climate variability on whitefish (<Emphasis Type="Italic">Coregonus lavaretus</Emphasis>) year-class strength in a deep,warm monomictic lake

The year-class strength (YCS) of Blaufelchen (Coregonus lavaretus) in deep Upper Lake Constance was analysed for a 52-year period, from 1947 to 1998. Despite strong anthropogenic influences on the species' population dynamics due to cultural eutrophication and oligotrophication, intense fishing, and large-scale stocking, the influence of climate variability associated with the North Atlantic Oscillation (NAO) is apparent in the data set. This influence is significant although large-scale stocking of cold-bred larvae was performed to avoid a mismatch of larvae with their food. The importance of stocking on YCS, however, is unclear and was only detectable when analysing a subset of the data. In addition to climate variability a yet unidentified factor related to zooplankton suitability as food for fish larvae, and density-dependent mortality probably related to cannibalism do significantly influence YCS. The NAO seemed to influence YCS twofold, through temperature effects on egg development time and on larval growth rate. The first of these two mechanisms is related to the NAO via a time lag of 1 year due to the specific mixing dynamics of warm monomictic Lake Constance. Hence, a warm winter in the year before spawning results in earlier hatching of larvae, that is, hatching is decoupled from the actual meteorological conditions. This should make the larvae very prone to mismatch the dynamics of their food. However, we found no evidence for such a mismatch in this 52-year data set.     

Interactions between stochastic and deterministic processes in stream fish community assembly

Synopsis Numerous studies have attempted to determine whether stream fish communities are structured primarily by deterministic or stochastic processes. Previous work has assumed that stream fish communities will show either persistence about an equilibrium because of strong density-dependent processes or random variation in structure as a result of environmental stochasticity. In a 10-year study of a California stream, fish community structure changed under the influence of storm-induced high discharge events that impacted recruitment. Species' relative abundances were altered as pre-recruitment stream discharges differentially influenced year-class strength among species with contrasting life histories. Simulation of stream fish community assembly under flow-driven recruitment variation indicates that community structure will vary depending on how particular high-flow events affect species' relative abundances and ongoing density-dependent processes, including competition and predation. Results suggest that stream fish communities are likely to show alternate states rather than a single persistent equilibrium. However, community assembly will not be random but will depend on situation-specific interactions between density-independent and density-dependent processes.     

Egg quality and female condition in vendace (Coregonus albula L.) before and during spawning

The fat content of muscle, liver and eggs of vendace (Coregonus albula L.) were measured during the maturation of ovaries in summer. The fat content of eggs was at its highest in July and that of muscle in August during the optimal feeding season. Significant differences were not found in fat content between lakes Suvasvesi and Southern-Kallavesi in central Finland. There were no essential differences in fatty acid composition between muscle, liver and eggs of Lake Suvasvesi vendace. During the last month before spawning in October the gross energy and fat contents of eggs decreased in Lake Suvasvesi vendace. The fat and vitamin C contents of muscle as well as the liver index of females decreased during this time also. In males the corresponding decrease did not occur, indicating that spawning is a greater stress for females than for males. The egg quality and condition of Lake Inarijärvi vendace were about the same as those of Lake Suvasvesi vendace. The unfavourable conditions of Lake Inarijärvi, located in northern Finland, are reflected in the size of ovaries, which is known to be smaller (gonadosomatic index < 20%) than in Lake Suvasvesi (GSI 26–27%). Vendace, then, maintain the quality of eggs at the expense of their number.     

Two-year cyclicity in recruitment of a fish population is driven by an inter-stage effect

A 25-year time series for a vendace (Coregonus albula (L.)) population was analysed with mathematical models to reveal the factors that regulate its recruitment. The fitted recruitment model was an age-structured model incorporating the compensatory effects of the spawning population and the abundance of the previous year class. Wind forcing index was also added as an example of an external source of recruitment variability. The auto-correlation analysis revealed a tendency for 2-year generation cyclicity in recruitment. The compensatory effects of both spawning biomass and previous year class abundance on recruitment had to be incorporated into the model to remove cyclicity from residuals. Wind forcing during the larval and early juvenile periods negatively affected recruitment. Re-estimating the parameters of the Cushing recruitment function and the effect of previous year-class from simulated data revealed that the time series structure and measurement errors induce strong biases in parameters exaggerating the density independent population growth rate parameter and the amount of compensation. The negative effect of previous year class was also exaggerated but less severely. Simulations with the artificially perturbed deterministic model skeleton revealed a tendency for cyclicity in recruitment. The model typically generated dampening oscillations, but the dynamics appeared as limit cycles when assuming high mortality, a low level of compensation by spawning biomass and a considerable negative effect of the previous year-class. No single ultimate mechanism for inter-stage effects causing cyclicity can be presently designated despite the rather extensive studies on vendace population dynamics. Several of the suggested mechanisms may be operating in concert.     

Population regulation in adult, but not juvenile, resident trout (Salmo trutta) in a Lake District stream

1. Unlike a neighbouring sea-trout population that showed strong density-dependent survival, a resident trout population ( Salmo trutta L.) showed simple proportionate survival in the early life-stages. However, this persistent population fluctuated within narrow limits. Mature adults, especially during spawning, were the only possible life-stage left in which regulation might occur. 2. An October census, just prior to spawning, was made at five sites (total area 300 m ² ) from 1965 to 1983. Gravel nests (redds) associated with females of known size were excavated outside these sites to obtain a power-function relationship between egg density per redd and female length (range 181–280 mm, n = 26). This relationship and the census data for females (range 186–284 mm) were used to estimate egg densities in each year-class. 3. The census data for the early life-stages (0+, 1+, 2+ trout) confirmed proportionate survival with no evidence for density-dependent regulation. In contrast, the number of spawning females produced in each year-class was strongly density dependent on the initial number of females that laid eggs at the start of the year-class. Similarly, total egg production in each year-class was density dependent on initial egg density. 4 Both relationships were well described by the Ricker and Beverton-Holt stock-recruitment models (P < 0.001) and the goodness-of-fit was similar for both models. This study is probably the first to provide clear evidence for fish population regulation in the adult, rather than the juvenile, stage.     

Temporal changes in the strength of density-dependent mortality and growth in intertidal barnacles

1. In demographically open marine systems, the extent to which density-dependent processes in the benthic adult phase are required for population persistence is unclear. At one extreme, represented by the recruitment limitation hypothesis, larval supply may be insufficient for the total population size to reach a carrying capacity and density-independent mortality predominates. At the opposite extreme, populations are saturated and density-dependent mortality is sufficiently strong to reshape patterns established at settlement. 2. We examined temporal variation in the way density-independent and density-dependent mortality interact in a typical sessile marine benthic invertebrate, the acorn barnacle Semibalanus balanoides (L.), over a 2-year period. 3. Recruitment was manipulated at two high recruitment sites in north Wales, UK to produce recruit densities covering the range naturally found in this species. Following manipulation, fixed quadrats were monitored using digital photography and temporal changes in mortality and growth rate were examined. 4. Over a 2-year period there was a clear, spatially consistent, over-compensatory relationship between the density of recruits and adult abundance indicating strong density-dependent mortality. The strength of density dependence intensified with increasing recruitment. 5. Density-dependent mortality did not operate consistently over the study period. It only operated in the early part of the benthic phase, but the pattern of adult abundance generated was maintained throughout the whole 2-year period. Thus, early life-history processes dictated adult population abundance and dynamics. 6. Examination of the natural recruitment regime in the area of study indicated that both positive and negative effects of recruitment will occur over scales varying from kilometres to metres.     

Growth and food consumption of vendace (Coregonus albula (L.)) in Lake Pyhäjärvi,SW Finland: a bioenergetics modeling analysis

Hydrobiologia - We used a bioenergetics model to estimate daily food consumption of young-of-the-year (YOY) and adult vendace in the mesotrophic Lake Pyhäjärvi. Weekly samples of fish and...     

Multiple sources of evidence for density dependence in the endangered Hawaiian stilt (Himantopus mexicanus knudseni)

Hawaiian stilts (Himantopus mexicanus knudseni) are an endangered subspecies of the Black-necked stilt endemic to the Hawaiian Islands. Despite long-term study, the main drivers of Hawaiian stilt population dynamics are poorly understood. We tested for density dependence using two sources of evidence: a 30-year time series of annual estimated range-wide abundance, and two 15+ year time series of reproductive success. Using separate methods with independent data, sources allowed us to make up for the potentially positive bias of one approach with the more conservative nature of the second. We compared nonlinear density-dependent and density-independent population model fits to our time-series data, using both frequentist and Bayesian state-space approaches. Across both approaches, density-dependent models best fit observed population dynamics, with lower AICc and cross-validation statistics compared to density-independent models. Among density-dependent models, a conditional model in which density-independent dynamics occur below a population size threshold (~850–1,000 birds), and then density-dependent dynamics occur above that threshold, performed best across Bayesian and frequentist model comparisons, with the Ricker model ranked next or equivalently. Our analysis of reproduction data revealed a strong negative effect of local adult density on nest success (proportion of nests hatching at least one chick) at Kealia National Wildlife Refuge on Maui, where few alternative breeding habitats are available, but no such effect at another site where many nearby alternative wetlands are available. These congruent results across independent datasets and analytical approaches support the hypothesis that Hawaiian stilts exhibit density dependence across their range.     

Mercury and chlorinated hydrocarbons in the food chain of Lake Päijänne, Finland

The sediments and various organisms in Lake Päijänne were examined for contaminants. The average mercury content of water plants was 9, of plankton 14, of sediment 114, of zoobenthic predators 83, of fish 332–1510 and of birds 240–13685 μg kg⁻¹ (wet weight). The average PCB content of plants was 3, of plankton 21, of the zoobenthos 44, of fish 36-117 and of birds 219–13490 μg kg⁻¹. The average ΔDDT content of plants was 0.5, of plankton 6, of the zoobenthos 14, of fish 7–42 and of birds 144-8262 μg kg⁻¹. Regional differences in mercury content were most pronounced in sediment and fish. PCB concentration was highest near a town. ΔDDT was quite evenly distributed. Water plant species did not differ from each other, nor did the plankton fractions. The zoobenthic predators contained more chlorinated hydrocarbons than did the herbivores. There were clear differences between most species of fish and the chlorinated hydrocarbon content was highest in vendace. In adult birds levels of all residues were significantly higher than in juveniles.
In most cases PCB content was positively correlated with ΔDDT and in birds PCB, ΔDDT and mercury levels were correlated. DDT residues occurred mostly as DDE, but in vendace the proportion of DDT was high. At most trophic levels, ΔDDT/PCB was 0.15-0.40 but in birds it reached 1–2.     

Predator-induced synchrony in population oscillations of coexisting small mammal species

Comprehensive analyses of long-term (1977-2003) small-mammal abundance data from western Finland showed that populations of Microtus voles (field voles M. agrestis and sibling voles M. rossiaemeridionalis) voles, bank (Clethrionomys glareolus) and common shrews (Sorex araneus) fluctuated synchronously in 3 year population cycles. Time-series analyses indicated that interspecific synchrony is influenced strongly by density-dependent processes. Synchrony among Microtus and bank voles appeared additionally to be influenced by density-independent processes. To test whether interspecific synchronization through density-dependent processes is caused by predation, we experimentally reduced the densities of the main predators of small mammals in four large agricultural areas, and compared small mammal abundances in these to those in four control areas (2.5-3 km(2)) through a 3 year small-mammal population cycle. Predator reduction increased densities of the main prey species, Microtus voles, in all phases of the population cycle, while bank voles, the most important alternative prey of predators, responded positively only in the low and the increase phase. Manipulation also increased the autumn densities of water voles (Arvicola terrestris) in the increase phase of the cycle. No treatment effects were detected for common shrews or mice. Our results are in accordance with the alternative prey hypothesis, by which predators successively reduce the densities of both main and alternative prey species after the peak phase of small-mammal population cycles, thus inducing a synchronous low phase.     

On the origin of Solovetskaya vendace <Emphasis Type="Italic">Coregonus albula</Emphasis> and of the syamozero smelt <Emphasis Type="Italic">Osmerus eperlanus</Emphasis>

The genetic analysis is made on the population of the European smelt Osmerus eperlanus occasionally introduced to Lake Syamozero (Karelia) and of the vendace Coregous albula supposedly acclimatized in Solovetskie Islands as a result of fish cultural activities of the Solovetskii Monastery. They are compared with several natural populations—possible donors for both introductions. Genetic variation in a sample of smelt was estimated by means of restrictase analysis of mtDNA (fragment ND1/ND2) in samples of vendace—by means of allozyme analysis of six isoenzyme systems. The probable population for the Syamozero smelt is the population of Onega Lake in spite of the previously noted greater morphological similarity of colonizers with the smelt of Ladoga Lake. A high level of genetic variation in the Syamozero smelt in comparison with native populations indicates that, beside the introduction from Onega Lake, there were repeated introductions from neighboring water bodies. The genetic analysis of the Solovetskaya vendace does not prove that the vendace appeared on islands due to acclimatization. Frequencies of alleles of allozyme loci in the Solovetskaya population significantly differ from frequencies in continental populations. Still, it is compared with some populations of the Arkhangelsk oblast. Estimations of genetic diversity of the Solovetskaya vendace turned out to be comparable with those in native populations.     

Unpredictability of fish recruitment: interannual variation in young–of–the–year abundance

In 1989–1998, vendace larvae Coregonus albula were sampled in Finnish lakes following a stratified random sampling design. The abundance of young–of–the–year vendace after the first growing season was estimated using catch–per–unit–effort statistics. The number or total area of nursery places hardly limited the recruitment of vendace in the study lakes. The major proportion of prerecruit mortality of vendace occurred in the larval phase and larval sampling produced significant information on young–of–the–year survival. Although larval abundances and recruitment were clearly associated only in Lake Onkamo, generally high larval abundance was needed to produce high number of recruits.     

Relative roles of density-dependent and density-independent factors in population dynamics of British deer

It has become increasingly clear that both density-dependent and density-independent factors may influence the dynamics of mammalian populations; it remains more difficult, however, to determine which factors may play the more significant role in influencing population number in any particular case. In this paper we review published and unpublished data in an analysis of the various factors affecting population size and trend in three European species of deer: Red Deer ( Cervus elaphus ), Fallow Deer ( Dama damd ) and Roe Deer ( Capreolus capreolus). We select these species deliberately because they span a range of body size and reproductive strategy - it seems that different demographic parameters might thus play different roles in the dynamics of the three-which may also be differentially sensitive to the effects of density-dependent and density-independent factors. For each species we examine the available evidence to determine the relative roles and effects of density-dependent feedback mechanisms and density-independent factors such as climate on recruitment and mortality.
Despite differences in bionomic strategy between Red Deer (as essentially a K -strategist) and the more r-selected Roe, few differences emerge between the three species in the relative roles of density-dependent and density-independent factors - or of the stage at the life cycle at which each factor may act. Overall, however, it is clear that variation in density-independent factors, such as climate, appears primarily to affect levels of mortality within a population, while effects of density are particularly marked in relation to changes in recruitment.     

Formation of year-class strength in the bream population in the shallow eutrophic Lake Tjeukemeer

Formation of year-class strength of bream Abramis brama , the most abundant fish in shallow eutrophic Lake Tjeukemeer, was studied during a 14-year period. Although the size of the spawning stock of bream was rather stable during this period, analysis of length—frequency distributions indicates that it comprised only a few year-classes. The strength of a year-class was determined at a fork length of about 11 cm, at the end of the second summer. Mortality rates of fish larger than 11 cm were low (<50% year⁻¹). As the number of fish at the end of the second summer was not correlated to the number of 0+ in the previous year, mortality rates during the second summer (ranging from 75.6 to 97.7% year⁻¹) determine the final year-class strength. These mortality rates were positively correlated to the year-class strength of 2- to 4-year-old pikeperch. The abundance of this predator, therefore, is likely to be the most important factor controlling the year-class strength of bream.