Growth of larval and juvenile perch: the importance of diet and fish density

In August, growth rate of young–of–the–year (YOY) Perch In lake enclosures could be explained by both YOY density and mean cladoceran biomass, suggesting that in a lake where YOY perch are dominant, growth may be density dependent in late summer and mediated through top–down control on daphnid biomass. In June, growth rate of YOY perch could not be fully explained by YOY density or by mean cladoceran biomass, suggesting that growth and survival during the first part of the summer is negatively affected by a diet of Bosmina and cyclopoid copepods only. The experiments also suggest why YOY perch have a slow growth and a low abundance in eutrophic lakes where small zooplankton dominate. The June experiment also indicated that growth of late larval or early juvenile perch improved when a larger cladoceran became available and was included in the diet.     

Dispersion of vendace eggs and larvae around potential nursery areas reveals their reproductive strategy

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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.     

The influence of pre‐settlement and early post‐settlement processes on the adult distribution and relative dominance of two invasive mussel species

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Population regulation of vendace (Coregonus albula) in Lake Pyhäjärvi, southwest Finland

Previous time series analysis on vendace population dynamics in Lake Pyhäjärvi, 1971–1990, revealed a 2-year cycle in year-class strength, implying powerful density-dependent regulation. Here we have extended this analysis by using multiple regression models to test whether the recruitment series is influenced by density-independent factors. We chose population size with a lag of 1 year as the density-dependent factor; the density-independent factors were the summer water temperature with a lag of 2 years (temperature sum for June, July and August, indicating the year-class strength of predators) and the temperature-derived length of the larval period of vendace. For the years 1972–1990 the coefficient of determination ( r ²) of this regression model was 0·77. We suggest that the basic mechanism producing a persistent 2-year cycle of vendace in Lake Pyhäjärvi is the asymmetrical food competition between age groups. The abundance of predators in the lake and the warming of the water after the hatching of larvae in spring together determine the final year-class strength of vendace.     

Monthly variability of hydroacoustic fish stock estimates in a deep lake and its correlation to gillnet catches

Monthly (April to November) hydroacoustic surveys and parallel gillnet catches were used to determine vendace Coregonus albula abundance, biomass and population dynamics in a deep oligotrophic lake. By hydroacoustic surveys, recruitment of 0+ year vendace could clearly be detected. In contrast, gillnet catches resulted in low numbers of 0+ year fish, and similarly under-represented proportions of the oldest vendace. Consequently, the correlation between hydroacoustics and gillnets with respect to fish numbers and geometric mean fish total length was high only for the age groups 1 + to 4+ years. Annual variability in hydroacoustic estimates offish abundance and biomass was high (CV=26–29%) which reflects the seasonal population dynamics of vendace.     

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.     

Minnow predation on vendace larvae: intersection of alternative prey phenologies and size-based vulnerability

Time-intensive sampling was used to study minnow Phoxinus phoxinus density and foraging activity in the littoral area of Lake Lentua at a time of high vendace Coregonus albula larval abundance. Minnow activity and foraging during the late spring-early summer period at low temperatures was found to be mainly nocturnal and quite consistent with features reported in the literature, with the exception of feeding on vendace larvae. The absence of the latter finding from previous studies may be due to previous seasonally limited daytime samplings. The temporal or spatial scale of sampling is decisive when studying foraging on food animals with a brief period of vulnerability and seasonal phenology. However, minnows in Lake Lentua seemed to prefer Bosmina longispina whenever available and low B. longispina density during the first weeks after the break-up of ice directed minnow predation towards the vendace larvae. Estimated gross predation values showed that the minnow has the potential to become a remarkable predator on fish larvae. Slight variations in the vernal timing of the vendace hatching and larval development with respect to minnow activity, both of which are obviously temperature related, may be critical to vendace larval survival in the nearshore zone. However, the predation on the larvae is probably a strong factor only for a short period and the survival of the vendace larvae may be more related to the availability of suitably sized food resources than to predation by the minnow.     

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.     

Correlations between type‐indicator fish species and lake productivity in German lowland lakes

Morphotypes for 67 lakes in the German lowlands were derived, based on maximum depth and mixis type. A threshold of 11 m maximum depth was identified to be the best level to discriminate shallow from deep lake morphotypes. The fish communities in these two morphotypes were significantly different. Indicator species analyses based on fish biomasses found vendace Coregonus albula in deep lakes and ruffe Gymnocephalus cernuus , bream Abramis brama , white bream Abramis bjoerkna , roach Rutilus rutilus , pikeperch Sander lucioperca and small perch Perca fluviatilis in shallow lakes to be the most representative species of their communities. Lake productivity was closely related to biomass and in part abundance of the type‐indicator species, with vendace declining with increasing chlorophyll a concentration in the deep lakes, whereas biomass of pikeperch, bream, white bream and ruffe increased and biomass of small perch decreased with increasing chlorophyll a . These results indicate that assessment of ecological integrity of lakes by their fish fauna is generally possible, if lakes are initially separated according to a depth‐related morphotype before the assessment, and if eutrophication is considered to be the main anthropogenic degradation.     

Temperature- and light-mediated predation by perch on vendace larvae

The perch foraging pattern in a Finnish lake (64) N) was crepuscular and they moved into littoral areas at dusk and returned to deeper waters by dawn. Predation on vendace larvae was guided by environmental cues, mainly temperature and light periodicity, which affected the density and foraging activity of perch in the littoral area. The availability and the vulnerability of different food categories were both responsible for the perch diet. The hypothesis that perch have a generally decisive role in the regulation of vendace larval abundance through predation at the time of high vendace larvae concentration in the littoral area was not supported. We emphasize the need to evaluate biological phenomena, such as predator–prey relationships, in the light of the relevant physical variables, and to do so within appropriate time and space scales, instead of relying on indirect estimates of the predator and prey population abundance and using their reciprocity as evidence of the interaction.     

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.     

Effect of light stress from phytoplankton on the relationship between aquatic vegetation and the propagule bank in shallow lakes

1. The way light stress controls the recruitment of aquatic plants (phanerogams and charophytes) is a key process controlling plant biodiversity, although still poorly understood. Our aim was to investigate how light stress induced by phytoplankton, that is, independent from the aquatic plants themselves, determines the recruitment and establishment of plant species from the propagule bank. The hypotheses were that an increase in light stress (i) decreases abundance and species richness both of established aquatic plants and of propagules in the bank and (ii) decreases the recruitment success of plants from this bank. 2. These hypotheses were tested in 25 shallow lakes representing a light stress gradient, by sampling propagule banks before the recruitment phase and when the lakes are devoid of actively growing plants (i.e. at the end of winter), established vegetation at the beginning of the summer and phytoplankton biomass (chlorophyll a) during the recruitment and establishment phase. 3. The phytoplankton biomass was negatively correlated with the richness and abundance of established vegetation but was not correlated with the propagule bank (neither species richness nor propagule abundance). The similarity between the propagule bank and established vegetation decreased significantly with increasing phytoplankton biomass. 4. The contrast in species composition between the vegetation and the propagule bank at the highest light stress suggests poor recruitment from the propagule bank but prompts questions about its origin. It could result from dispersal of propagules from neighbouring systems. Propagules could also originate from a persistent propagule bank formerly produced in the lake, suggesting strong year‐to‐year variation in light stress and, as a consequence, in recruitment and reproductive success of plants.     

Projected shifts in fish species dominance in Wisconsin lakes under climate change

Temperate lakes may contain both coolwater fish species such as walleye (Sander vitreus) and warmwater fish species such as largemouth bass (Micropterus salmoides). Recent declining walleye and increasing largemouth bass populations have raised questions regarding the future trajectories and management actions for these species. We developed a thermodynamic model of water temperatures driven by downscaled climate data and lake‐specific characteristics to estimate daily water temperature profiles for 2148 lakes in Wisconsin, US, under contemporary (1989–2014) and future (2040–2064 and 2065–2089) conditions. We correlated contemporary walleye recruitment and largemouth bass relative abundance to modeled water temperature, lake morphometry, and lake productivity, and projected lake‐specific changes in each species under future climate conditions. Walleye recruitment success was negatively related and largemouth bass abundance was positively related to water temperature degree days. Both species exhibited a threshold response at the same degree day value, albeit in opposite directions. Degree days were predicted to increase in the future, although the magnitude of increase varied among lakes, time periods, and global circulation models (GCMs). Under future conditions, we predicted a loss of walleye recruitment in 33–75% of lakes where recruitment is currently supported and a 27–60% increase in the number of lakes suitable for high largemouth bass abundance. The percentage of lakes capable of supporting abundant largemouth bass but failed walleye recruitment was predicted to increase from 58% in contemporary conditions to 86% by mid‐century and to 91% of lakes by late century, based on median projections across GCMs. Conversely, the percentage of lakes with successful walleye recruitment and low largemouth bass abundance was predicted to decline from 9% of lakes in contemporary conditions to only 1% of lakes in both future periods. Importantly, we identify up to 85 resilient lakes predicted to continue to support natural walleye recruitment. Management resources could target preserving these resilient walleye populations.     

The surf zone: a semi-permeable barrier to onshore recruitment of invertebrate larvae?

The supply of larvae to the shore is important for population replenishment and intertidal community dynamics but its variability at most scales is not well understood. We tested the relationship between nearshore mussel larval abundance and intertidal settlement rates over several years at multiple spatiotemporal scales in Oregon and New Zealand. Abundance of competent larvae nearshore and intertidal recruitment rates were simultaneously quantified using collectors mounted at different depths on moorings 50-1100 m from shore, and at adjacent rocky intertidal sites. Total mussel larval abundance and oceanographic conditions were also measured in some locations. At all scales, abundance of nearshore mussel larvae was unrelated to intertidal recruitment rates. In the intertidal, patterns of mussel recruitment were persistent in space, with sites of consistently high or low recruitment. In contrast, nearshore competent larval abundance showed generally similar abundances among sites except for a high, and spatially-inconsistent, variability in Oregon during 1998 only. On moorings, recruitment tended to be greater on midwater collectors than shallower or deeper. Finally, on moorings larval abundance in traps and recruitment on collectors was unrelated. These results suggest that (1) among sites, the size of the nearshore larval pool is relatively uniform while onshore recruitment varies and is unrelated to larval abundance, (2) temporal variability in nearshore larval availability is not strongly expressed onshore, (3) vertical stratification of competent larvae nearshore is strong and may influence transport and recruitment, and (4) within-coast variability in onshore recruitment is strongly driven by processes occurring locally within the surf zone that need to be studied to understand coastal recruitment dynamics.     

Reproduction and early life history of an invasive ascidian Styela clava Herdman in Prince Edward Island, Canada

In the past decade, many aquatic invasive species have been introduced into Prince Edward Island, Canada, coastal waters, and they have had adverse effects on the aquaculture and fisheries industry. For instance, the mussel aquaculture industry has been overwhelmed by extremely high abundances of the invasive ascidian Styela clava in some areas. To understand and minimize the risk of spread of S. clava to other shellfish aquaculture sites, basic information on the reproductive biology of the ascidian is needed. This study investigated Styela gonad development, larval abundance and recruitment. Gonad development, using histological methods, indicated ovaries were ripe from late June through late October, 2004. Weekly water samples indicated larvae were present from 24 June to 29 October, with a distinct abundance peak in mid-August. Recruitment occurred from 24 June to 21 October. Timing of larval sampling was judged to be a crucial part of evaluating recruitment potential for monitoring purposes and a field experiment was carried out to document spatio-temporal larval concentrations. Data were gathered over 30 h sampling periods on 4 August and 14 September 2004. Water samples were taken simultaneously on an hourly basis at three depths: surface, 2 m and 4 m below the surface. Results indicated peaks in larval abundance were, in most cases, limited to a 3 h time interval in the early afternoon. Larval concentrations increased with water depth.     

REGULAR PAPERS Habitat distribution and size structure in freshwater fish communities: effects of vendace on interactions between perch and roach

In a study of habitat selection and size structure in three fish species in 115 Swedish lakes, roach Rutilus rutilus used the pelagic habitat to a lesser extent and were lower in pelagic biomass in the presence of vendace Coregonus albula. Size distributions of roach were skewed toward larger sizes in lakes with vendace, although a similar pattern of size-dependent habitat use was found regardless of lake group. In disagreement with predictions, vendace did not affect relative perch Perca fluviatilis biomass, size structure, or the proportion of large perch. However, a higher growth for large perch indicated higher per capita energy intake in lakes with vendace.     

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.     

Quality and status of fish stocks in lakes: gillnetting,seining, trawling and hydroacoustics as sampling methods

All sampling methods give selective or biased estimates of fish species abundance, distribution and size structure. This creates problems, e.g. in regard to the Water Framework Directive of the European Union, which demands evaluation of the quality and status of fish stocks in lakes. We compared fish sampling by means of Nordic multimesh gillnets, seining, trawling and hydroacoustics in two Finnish lakes in summer 2007 and 2008. Sampling methods were used ‘as such’, i.e. no special design was implemented for method comparison. In the shallow eutrophicated lake the species’ composition of gillnet sampling and seining were very different. The biomass-% of percids dropped from gillnet (61%) to seining (9%) and that of cyprinids grew from 39 to 90%, respectively. In the deep pelagic area of the oligotrophic lake, vendace and smelt predominated in trawl catches. The number of fish caught by gillnetting in that area was too small to make any conclusions about the species composition. In the eutrophicated lake, the combined length distribution for all fish species differed significantly between gillnetting and seining. In the oligotrophic lake, the gillnet catches were too small for any comparison of fish size. The difference in the length distribution of fish between trawl and echosounding was significant in most analysed depth layers. In upper depth layers acoustics sampled larger fish than trawling, and in deeper layers smaller fish. Using a combined acoustic-trawl method, the pelagic fish biomass was estimated to be approx. 17 kg ha⁻¹ in the deep and oligotrophic lake. We conclude that in large and deep-water areas, the use of active gear is enough in fish sampling to evaluate the quality and status of fish stocks. Gillnetting together with seining is an appropriate method to work out the quality and status of fish stocks in shallow and littoral areas of large lakes. Variation in the catch selectivity of fish sampling gear requires a discrete ecological classification for each type of gear.     

The food preference of vendace (Coregonus albula) in South Swedish forest lakes including the predation effect on zooplankton populations

Vendace (Coregonus albula, L.) is in the southern part of its distribution area during the summer period restricted to hypolimnion. Food is dominated by Bosmina coregoni and Daphnia sp. The selectivity index is highest for large cladocerans and large copepods.The pelagic crustacean fauna in vendace lakes is dominated by small species like Bosmina coregoni, Daphnia cristata, Thermocyclops and Mesocyclops sp. and Eudiaptomus sp. During periods of maximum vendace abundance only Bosmina coregoni is left of the cladocerans, while the abundance of cyclopoid copepods increases.