Long-term dynamics of pelagic fish density and vendace (Coregonus albula (L.)) stocks in four zones of a lake differing in trawling intensity

– An 11-year time series of hydroacoustic fish density estimates and fisheries statistics of vendace (Coregonus albula (L.)) from four zones of a lake differing in trawling intensity was analyzed in order to test the hypothesis that intensive trawling has detrimental effects on pelagic fish stocks, especially vendace recruitment. The standardized fish density estimate in trawled zones showed no decrease in comparison to the non-trawled zone. No signs of recruitment failure associable with trawling intensity were found. The growth of vendace at the end of the study period was slower than that at the beginning, indicating a higher density, most clearly so in the zone with highest trawling int…

Genetic-based evaluation of management units for sustainable vendace (Coregonus albula) fisheries in a large lake system

Abstract The goal of the processing industry, trade and consumers is to get eco-labelled freshwater fish products from sustainable fisheries into the market as soon as possible. The fourth largest natural lake system in Europe, the Saimaa lake system supports a fishery for vendace (Coregonus albula). Certification of the fishery requires an understanding of population structure to help determine the number and spatial extent of management units. In this study, we analysed the genetic diversity of local vendace populations in the Saimaa lake system and aimed to identify the conservation and management units of vendace. Within the Saimaa, the genetic divergence between local populations of ve…

Mortality of pike-perch (Stizostedion lucioperca), brown trout (Salmo trutta) and landlocked salmon (Salmo salar m. sebago) caught as by-catch in pelagic trawling in a Finnish lake

Altogether 146 pike-perch, 49 brown trout and 19 landlocked salmon were caught in 23 exploratory trawl-hauls in lake Pyhaselka, Finland. Brown trout and salmon were caught only in surface trawling, whereas pike-perch were taken also in mid-water trawling. Most of the fish were smaller than the minimal legal size of 40 cm. To estimate their post-trawling mortality, 129 fish were kept in net cages for 1 week. Less than 15% of the pike-perch and brown trout caught by surface trawl died during 1 week caging, whereas every caged salmon died. Mid-water trawling was far more lethal to pike-perch than surface trawling. We conclude that pike-perch and brown trout can recover from surface trawling. S…

Estimating the effects of winter-fishing: mobile echosounding under ice

Abstract An echosounder with a tape recorder in a propeller-driven waterproof box (an echosounder shuttle) was used under the ice to monitor the vertical distribution and density of fish in a winter seining area of a Finnish lake in April 1997. The surveyed fish were at 3.5–8 m depth, and fish density was significantly lower after seining than before. The seine catch was about 10 kg ha−1, and the echosounding results showed a decrease of 7 kg ha−1 in the hauling area. There were some difficulties due to the low temperature, and minor problems with steering and leakage of the shuttle. We conclude that it is possible to make mobile echosurveys under the ice and estimate densities of wintering…

Growth of pikeperch in relation to lake characteristics: total phosphorus, water colour, lake area and depth

The growth of pikeperch Sander lucioperca was studied in 41 lakes in central Finland. The backcalculated average total length of 3 year-old pikeperch was used as an indicator of growth. The growth correlated positively with total phosphorus and water colour and negatively with lake area and depth. The reason for differences in growth may be differences in the amount of suitable food, foraging success or temperature dynamics in different lakes.

Indice de réflexion latérale du saumon Atlantique (Salmo salar), de la truite de mer (Salmo trutta), du corégone (Coregonus lavaretus) et du brochet (Esox lucius).

L'indice de reflexion acoustique laterale (TS) de 19 saumons Atlantique (Salmo salar), 16 truites de mer (Salmo trutta), 10 coregones (Coregonus lavaretus) et 7 brochets (Esox lucius) a ete mesure en utilisant un sondeur de 200 kHz, afin d'etudier la relation entre l'indice TS et la taille, le poids et la surface du poisson. Les effets de l'angle d'incidence sur l'indice TS ont aussi ete etudies. Des modeles lineaires entre TS et le logarithme de l'indice de taille du poisson ont ete ajustes et ont montre que la meilleure prevision provient de l'indice de longueur. Classiquement, l'erreur standard de l'estimation etait de 1,2 a 2,9 dB. Les resultats des mesures de TS sur des poissons de lon…

La remontée du saumon pour le frai dans le fleuve Tornionjoki, suivie au moyen d'un sonar à double faisceau horizontal.

Fixed location split-beam horizontal echosounding was used to assess the size and timing of the Atlantic salmon (Salmo salar) spawning run in the River Tornionjoki. Four transducers, two on each river bank, were mounted across the river at the study site 4 km upstream from the river mouth. Net weirs were used on both shores to direct the passage of fish through the acoustic beams. Hydroacoustic monitoring covered 40-50% of the river cross-sectional area. Also test fishing and yearly catch statistics of salmon were used as an indication of the size of the spawning run in the river. Altogether, 7 700, 5 300 and 4 300 salmon-sized targets (target strength, TS ≥ -29 dB) moving upstream were det…

Predation by signal crayfishPacifastacus leniusculuson fish eggs and its consequences for coregonid recruitment

The character and magnitude of predation by the invasive, ectothermic Pacifastacus leniusculus, a crayfish widely introduced to Europe and Japan from North America, on the eggs of coregonid fishes, vendace Coregonus albula and whitefish Coregonus lavaretus were examined by experimentation, modelling and field data. The present results showed that P. leniusculus has the potential to be very efficient predator of fish eggs under winter conditions, but the predation by P. leniusculus did not significantly decrease production of coregonid larvae during the years with a high P. leniusculus population in the study lake. Hence, the mortality caused by the novel invertebrate predator appeared to co…

Analysis of the spawning stock-recruitment relationship of vendace (Coregonus albula (L.)) with evaluation of alternative models, additional variables, biases and errors

–  Spawning biomass and recruitment data for vendace from a central Finnish lake were analysed by fitting various recruitment models and comparing the fits statistically. The compensatory models of Ricker, Cushing and Beverton & Holt fitted the data better than the H0 hypothesis of constant proportionality, but model and parameter uncertainties were high. Additional variables were included in an attempt to reduce uncertainties. Heavy wind forcing during the first month after hatching of larvae reduced the recruitment success. For the Ricker and the Cushing models, recruitment seemed to be also negatively associated with the density of the previous year-class. The r2 increased considerably w…

Population size estimation of larval coregonids in large lakes: Stratified sampling design with a simple prediction model for vertical distribution

Abstract The lake-specific vertical distribution of larval coregonids was assessed annually in Finnish lakes by bongo net sampling. The effect of inter-annual and inter-lake variation in vertical distributions on population estimates of larval coregonids were analysed in 1999–2008. The vertical distribution of newly hatched vendace ( Coregonus albula (L.)) and European whitefish ( Coregonus lavaretus L. s.l.) larvae was analysed and high inter-annual and lake-specific variation in the larval vertical distribution was found. Generally, in both littoral and pelagic areas, larvae were aggregated near the water surface, mostly in the top 30 cm layer. We compared observed and predicted density e…

Effect of glochidia infection on growth of fish : freshwater pearl mussel Margaritifera margaritifera and brown trout Salmo trutta

AbstractEffect of freshwater mussels’ (Unionoida) glochidia on the growth of fish host has remained poorly studied. We compared the specific growth rate of the juvenile, PIT-marked brown trout (Salmo trutta) between uninfected controls to those experimentally infected (average initial intensity of infection 8000 fish−1) with Margaritifera margaritifera glochidia, kept in high and low feeding. Growth and mortality of fish were monitored for 10 months. Our hypothesis was that glochidiosis would impair the growth of fish. According to our hypothesis, infected fish gained statistically significantly less weight than the control fish throughout the experiment. A proportional increase in weight o…

Vendace (Coregonus albula) Disperse Their Eggs Widely during Spawning

Depending on their reproductive strategy, different fish species either aggregate or disperse eggs and larvae in their reproductive habitat. Because yolk-sac larvae of vendace (Coregonus albula) disperse widely across the littoral and pelagic zones of boreal lakes, it is unclear where the exact spawning and egg incubation locations are. Vendace egg and larvae densities were studied in Lake Southern Konnevesi to clarify its spawning strategy. In autumn 2019, 1–2 weeks prior to spawning, 500 egg samplers were installed in five depth zones in 20 sampling plots. Fertilized eggs were found in 18 plots. The mean density of eggs was 74 eggs m–2 and the mean fertilization rate 85%. During spawning,…

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

Depending on their reproductive strategy, different fish species aim to aggregate or disperse eggs and larvae in their reproductive habitat. Many pelagic species disperse their eggs widely around the potential nursery areas. Larval dispersion or aggregation affects population sub‐structuring, which has important implications in fisheries management and conservation of the natural spatial diversity in populations. The dispersion of larval vendace (Coregonus albula) was quantified in two oligotrophic Finnish lakes, and effects of density and environmental variables on the inter‐annual variation in the larval distribution were examined by analysing spatial abundance data from the lakes from 19…

Phenotypic plasticity in growth and fecundity induced by strong population fluctuations affects reproductive traits of female fish

Fish are known for their high phenotypic plasticity in life-history traits in relation to environmental variability, and this is particularly pronounced among salmonids in the Northern Hemisphere. Resource limitation leads to trade-offs in phenotypic plasticity between life-history traits related to the reproduction, growth, and survival of individual fish, which have consequences for the age and size distributions of populations, as well as their dynamics and productivity. We studied the effect of plasticity in growth and fecundity of vendace females on their reproductive traits using a series of long-term incubation experiments. The wild parental fish originated from four separate populat…

Gastric evacuation rate of burbot fed single-fish meals at different temperatures

The gastric evacuation rates of burbot Lota lota, fed a single meal of vendace, Coregonus albula, were measured in the laboratory at five temperatures (1·3, 2·6, 4·8, 9·4 and 12·6° C). Gastric evacuation rate increased exponentially with increasing temperatrure, but the results suggest that gastric evacuation rates of burbot at low temperatures are lower than those of other freshwater fish species. Temperature and the ratio of meal weight to burbot weight were the most important factors affecting gastric evacuation rate. There was no significant difference in gastric evacuation rate between three different prey species: vendace, perch Perca fluviatilis, and smelt Osmerus eperlanus.

Cormorant (Phalacrocorax carbo) predation on a coastal perch (Perca fluviatilis) population: estimated effects based on PIT tag mark-recapture experiment

Abstract The number of cormorants has rapidly increased in the northernmost Baltic Sea. In 2018, 50 km × 50 km ICES catch rectangle 55H1 had 3140 breeding pairs. To estimate the predation effect of cormorants on perch populations, we Passive Integrated Tags tagged 1977 perch and 9.9% of tags were found. The median instantaneous cormorant-induced mortality during the breeding time, with consumption by non-breeding individuals, was estimated at 0.23 and at 0.35 during the whole residing period. We estimated with a yeild-per-recruit model that the long-term maximum loss of perch yield of tagged sub-population would be at 80% probability interval 32–67%, and when extended to the entire 55H1, 10…

Seasonal and ontogenetic variability in stomach size of Eurasian perch (Perca fluviatilis L.)

Eurasian perch (Perca fluviatilis) were sampled in April, from May to June and at the end of August to test whether the current season (i.e. feeding conditions) affects the fishes’ stomach size (i.e. volume and weight). A wide range of size data were analysed to reveal the relationship between fish size (length and weight) and stomach size. No significant differences in length-specific stomach volume or stomach weight were found in fish sampled at different times of the year. However, there were differences between seasons in the size of the stomach in relation to body weight, as length-specific body weight changes during the year because of the development of gametes and changes in nutriti…

Recruitment variability in vendace, Coregonus albula (L.), and its consequences for vendace harvesting

Timo Marjomäki tutkii väitöskirjassaan muikun rekryyttimäärän (=vuosiluokan runsaus ensimmäisen elinvuoden syksyllä tai talvella) vuosien välisen runsaudenvaihtelun ominaispiirteitä ja syitä sekä tarkasteli vaihtelun vaikutuksia kalastukseen ja sen säätelyyn ja toisaalta kalastuksen heijastumista vuosiluokkavaihteluun. This thesis analyses the characteristics and causes of interannual variability in vendace recruitment and its consequences for, and interactions with, vendace harvesting. The interannual variability of prerecruit mortality was high. The daily mortality during the stage 1-3 weeks after hatching was about 10% and in stage 3 weeks - recruitment about an order of magnitude lower.…

The success of crayfish stocking in a dredged river with and without artificial shelter increase

Abstract The success of crayfish ( Astacus astacus L.) stocking in a heavily dredged river in Finland was compared between sections with and without artificial shelter. The materials used for creating shelter were a) broken rock and, b) ceramic drain pipes and perforated bricks. Group-marked crayfish (mean carapace length 42.6 mm) were released into the sections and test trappings conducted one year later. The highest crayfish densities were found in the broken rock sections. The reasons for the observed crayfish densities are discussed. No significant differences in growth between groups were detected. Migrations in the range of 2.5 km in one year were recorded.

Differences in parasite community composition support ecological differentiation in a freshwater gadoid fish

Several northern freshwater fishes have gone through rapid adaptive radiation after the last glacial period, resulting in new species or intraspecific morphs with distinct life histories. Parasite infections can promote adaptive radiations and spatiotemporal differences in patterns of infections can potentially reveal incipient or ongoing speciation processes. We investigated intraspecific differentiation in a freshwater gadoid fish, burbot (Lota lota), by exploring differences in parasite infections between two potential life-history morphs in Lake Konnevesi, Central-Finland, one reproducing species characteristically in shallow littoral waters in February and the other possibly in deep pr…

Empirical Estimation of Accumulation-Induced Change in Gill Net Catchability: Mind the Observation Errors

We analyzed cumulative catches for 24 h gill net exposures divided into 4*6 h, 2*12 h and 1*24 h soak time treatments to estimate the reduction in its catchability due to accumulation of fish. The effects of loss of catch during net lifting, disturbance effect and fouling were eliminated as far as possible to reveal the true effect of accumulation. First we applied simple nonparametric and parametric tests in comparison of treatments. As expected, considerable reduction in catchability took place along with the increase in soak time, indicated by significantly lower total 24 h catches from longer soaks in comparison with shorter ones. The reduction was more pronounced for roach than for per…

The effect of meal size on the rate of gastric evacuation of burbot, Lota lota (L.)

– The effect of meal size on the gastric evacuation rate of burbot Lota lota (L.), was determined at a constant temperature of 1.5°C. Burbot were fed vendace Coregonus albula (L.), in varying amounts from 1.0% to 12.6% of burbot weight. Although gastric evacuation rate decreased exponentially with increasing meal size, the absolute amount of food evacuated per day (g. day−1 increased when meal size increased. The estimated maximum daily gastric evacuation rates were 0.8, 1.3 and 1.6 g/day with meal sizes 1%, 5% and 10% of burbot weight (200 g). No difference in gastric evacuation rate was observed between meals of the same size (8 g) but which consisted of either one or three vendace.

Influence of warming temperatures on coregonine embryogenesis within and among species

The greatest response of lakes to climate change has been the increase in water temperatures on a global scale. The responses of many lake fishes to warming water temperatures are projected to be inadequate to counter the speed and magnitude of climate change, leaving some species vulnerable to decline and extinction. We experimentally evaluated the responses of embryos from a group of cold, stenothermic fishes (Salmonidae Coregoninae) - within conspecifics across lake systems, between congeners within the same lake system, and among congeners across lake systems - to a thermal gradient using an incubation method that enabled global comparisons. Study groups included cisco (Coregonus artedi…

Kasvatetun siian (Coregonus lavaretus) ja kirjolohen (Oncorhynchus mykiss) maksa elintarvikkeena

Communal pair spawning behaviour of vendace (Coregonus albula ) in the dark

Mating in nature is rarely random, and most fish species have refined mating systems. The vendace (Coregonus albula) is a short-lived, small-sized, cold-water-adapted pelagic schooling species that is known to spawn in groups, but the actual mating system of this species, like many other group-spawning fishes, has not been described in detail. Vendace typically spawn in the littoral or sublittoral zones of lakes in late autumn, and the hatching of larvae occurs close to icebreak in the following spring. In our large study lake, vendace larvae were caught in 93% of 1,149 random sampling locations lake-wide. We examined the courtship and mating of vendace under experimental conditions by noni…

A comparison of two methods for surveying aquatic macrophyte communities in boreal lakes: Implications for bioassessment

To compare the performance of two common methods for surveying boreal lake aquatic macrophyte communities (the transect survey and the phytolittoral inventory) in lake bioassessment, we studied five small humic lakes with both methods. In addition to this pairwise comparison, we contrasted available independent sets of reference lake data of either methodological origin. Specifically, we compared the observed species richness and a number of ecological quality (community) metrics and their variability both between the methods and within them in relation to sampling effort and operator. With the phytolittoral inventory, more taxa (mean 38.8 ± s.d. 12.1) were observed than with transect surve…

Indications of polymorphism in the behaviour and morphology of burbot (Lota lota) in a European lake

AbstractIn this study, polymorphism in behaviour, morphology, and stable isotope signatures of burbot from Lake Southern Konnevesi, Finland, Europe, was examined. First, local knowledge was collected on exceptional polymorphism of the spawning behaviour and morphology of burbot. These phenomena were then studied based on catch samples. Interviews and catch sample analyses suggested two morphs of burbot: one morph spawning in late February in the littoral zone and other in late March, in deep profundal, depths of about 30 m. Fish caught from the profundal zone had higher average proportional somatic body weights and wider heads than those caught from the littoral spawning sites in February. …

Sisävesien talouslajien saalispotentiaali Suomessa

Densité et mobilité des poissons sous la glace, sur une zone de pêche à la senne de deux lacs finlandais, par écho-intégration.

The density and movements of fish under ice were studied with single-beam mobile surveys and fixed location split-beam surveys, as well as exploratory fishing in winter-seining areas of two shallow Finnish lakes during winter 1999. Fish schooled near the bottom during the day but the schools dispersed and fish ascended at night. Single and split-beam target strength distributions corresponded fairly closely with the length distribution of seine catch samples. Estimated fish densities were greater at night than in daylight. The swimming speed of smelt (Osmerus eperlanus) was 0.18 m·s -1 in daylight and 0.36 m·s -1 at night. The corresponding figures for vendace (Coregonus albula) were 0.11-0…

Night, day, sunrise, sunset: do fish under snow and ice recognize the difference?

1. Although boreal lakes are ice-covered for several months annually, little is known about the behaviour of fish under ice. To consider the reasons for diel vertical migrations (DVM) it is important to compare periods under ice as opposed to under open water. Echosounding provides a tool for non-intrusive continuous monitoring of fish, even in winter. 2. Changes in the vertical distribution of fish through six 48-h periods were monitored using a stationary, mounted echosounder that beamed vertically either from the bottom up or from the surface down from February to April, 2003. The up-beaming and down-beaming transducers were run alternately for 24 h each over the 48-h period. Standard ec…

Effect of low dissolved oxygen on the viability of juvenile Margaritifera margaritifera : Hypoxia tolerance ex situ

The decline of endangered freshwater pearl mussel (FPM, Margaritifera margaritifera) has been attributed to juvenile mortality caused by low concentrations of dissolved oxygen in the stream substrate resulting from fine sediments (siltation) that impede water exchange in the interstitial microhabitat of juveniles. If low oxygen concentration causes recruitment failure of FPMs, knowledge on the oxygen tolerance of juvenile FPMs is essential for the conservation of the species, as it will justify conservation efforts improving water exchange in the bottom gravel. However, the tolerance of low oxygen of FPM juveniles has not been directly studied. Juvenile FPMs (9–11 months old) were exposed i…

An alien fish threatens an endangered parasitic bivalve: the relationship between brook trout (Salvelinus fontinalis) and freshwater pearl mussel (Margaritifera margaritifera) in northern Europe

Host–parasite interactions may play a significant role in biological invasions: for example, an invader may benefit from lower infectivity by native parasites in competition against the native hosts (‘enemy release hypothesis’). The invasive North American brook trout (Salvelinus fontinalis) is replacing the native salmonid hosts of the freshwater pearl mussel (FPM, Margaritifera margaritifera) in northern Europe, but the suitability of brook trout as a host for FPM is poorly known. In this study its suitability was investigated using an array of laboratory and field experiments, and several FPM populations from a catchment in northern Finland. The occurrence of brook trout in FPM rivers in…

Age is not just a number—Mathematical model suggests senescence affects how fish populations respond to different fishing regimes

Abstract Senescence is often described as an age‐dependent increase in natural mortality (known as actuarial senescence) and an age‐dependent decrease in fecundity (known as reproductive senescence), and its role in nature is still poorly understood. Based on empirical estimates of reproductive and actuarial senescence, we used mathematical simulations to explore how senescence affects the population dynamics of Coregonus albula, a small, schooling salmonid fish. Using an empirically based eco‐evolutionary model, we investigated how the presence or absence of senescence affects the eco‐evolutionary dynamics of a fish population during pristine, intensive harvest, and recovery phases. Our si…

Kalastuslain kolmastoista – onnen vai epäonnen pykälä?

Rehabilitation of two northern river lamprey (Lampetra fluviatilis) populations impacted by various anthropogenic pressures : lessons learnt in the past three decades

The pioneering work done during the past three decades in the regulated Rivers Perhonjoki and Kalajoki, Finland, to study and rehabilitate river lamprey populations is presented. The effects of various anthropogenic activities and rehabilitation measures are evaluated based on habitat surveys and long-term monitoring of larval densities, numbers of adults migrating upstream and of transformers migrating downstream. Telemetric tracking and tagging experiments were used to determine the efficacy of fishways. Lamprey populations in both rivers decreased in the 1980s and 1990s. This was linked to obstructed upstream migration of adults and deterioration of habitats for different life stages due…

Age is not just a number : Mathematical model suggests senescence affects how fish populations respond to different fishing regimes

Senescence is often described as an age-dependent increase in natural mortality (known as actuarial senescence) and an age-dependent decrease in fecundity (known as reproductive senescence), and its role in nature is still poorly understood. Based on empirical estimates of reproductive and actuarial senescence, we used mathematical simulations to explore how senescence affects the population dynamics of Coregonus albula, a small, schooling salmonid fish. Using an empirically based eco-evolutionary model, we investigated how the presence or absence of senescence affects the eco-evolutionary dynamics of a fish population during pristine, intensive harvest, and recovery phases. Our simulation …

An Empirical Evaluation of the Utility of Convex Hull and Standard Ellipse Areas for Assessing Population Niche Widths from Stable Isotope Data

Stable isotope analyses are increasingly employed to characterise population niche widths. The convex hull area (TA) in a δ¹³C–δ¹⁵N biplot has been used as a measure of isotopic niche width, but concerns exist over its dependence on sample size and associated difficulties in among-population comparisons. Recently a more robust method was proposed for estimating and comparing isotopic niche widths using standard ellipse areas (SEA), but this approach has yet to be tested with empirical stable isotope data. The two methods measure different kind of isotopic niche areas, but both are now widely used to characterise isotopic niche widths of populations. We used simulated data and an extensive e…

Features of intercohort cannibalism of Vendace (Coregonus albula(L.)) under laboratory conditions

Cannibalism in Vendace (Coregonus albula (L.)) was studied under laboratory conditions by exposing Vendace eggs (100) or newly hatched larvae (20) to an older individual in aquaria. Predation on the eggs was not detected whereas both biting and ingesting of newly hatched larvae were observed in 23% of older Vendace individuals. Small individuals (<100 mm in total length) were significantly more likely to attack larvae than larger individuals. Differences were also found in attack rates between individuals. The rate ranged from no attacks to attacks towards every larva exposed to the cannibalistic individual. These results confirm that intercohort cannibalism is neither an exceptional nor a …

Miljardeja mätimunia, satoja miljoonia poikasia, kymmenen miljoonaa kutukalaa, miljoonien kalojen saalis – kuinka paljon kaloja on järvessä?

Lifting the Vendace, Coregonus albula, on the Life Table : Survival, Growth and Reproduction in Different Life-Stages during Very High and Low Abundance Regimes

We analyzed the variability in vendace population parameters in the context of a life table. Parameters related to growth, fecundity and survival were estimated for post-recruitment (age > 1 growing season) life stages of Lake Southern Konnevesi vendace in both very high and low population abundance regimes. Pre-recruitment survival producing population stability was then determined. We found a very strong compensatory density dependence in growth, fecundity and survival: during low abundance, a lifetime reproductive output of a female was almost 20 times that of the abundant regime. To maintain the low abundance regime, pre-recruitment survival must counterbalance it by decreasing to a ver…

Occurrence of Two-Year Cyclicity, “Saw-Blade Fluctuation”, in Vendace Populations in Finland

The tendency towards two-year cyclicity is considered typical of many Fennoscandian vendace populations, especially in fluctuation of recruitment, based on time series of individual lakes. We used two robust indicators to identify and quantify two-year cycles in vendace population proxy time series at different life-stages — spawning stock biomass (SB), density of newly hatched larvae (LD) and recruitment (REC) — from 22 Finnish lakes. Then we applied Fisher’s meta-analytical test to assess the adequacy of the evidence to support the hypothesis that vendace population dynam-ics include two-year cyclicity. The results supported this hypothesis for RECbut not for SB or LD. Yet, the indicators…

Density and biomass of smelt (Osmerus eperlanus) in five Finnish lakes

Abstract Hydro-acoustic stock assessment and exploratory sampling with small mesh-sized trawls and seines have sometimes suggested that the importance of smelt (Osmerus eperlanus) in pelagic fish communities might be greater than sampling from commercial fishery and gillnetting indicate. We studied the proportion of smelt in the total fish density (fish ha−1) and biomass (kg ha−1) of pelagic fish with echo sounding and trawling in five southern boreal lakes. Fish density varied between 460 and 2000 fish ha−1 in the study lakes. Smelt and vendace (Coregonus albula) accounted for more than 95% of the exploratory trawl-catches. The total fish biomass in the study areas varied between 3 and 13 …

Diet and prey size spectrum of pikeperch in lakes in central Finland

The stomach content of pikeperch Sander lucioperca was studied in 24 lakes in central Finland. The most important prey species was smelt Osmerus eperlanus then perch Perca fluviatilis. The proportion of cyprinids as prey increased with pikeperch total length (LT) and lake phosphorus content. Prey L T and pikeperch L T correlated positively, but the prey-to-predator size ratio correlated negatively with pikeperch L T .

Age is not just a number – senescence affects how fish populations respond to different fishing regimes

Abstract The presence of senescence in natural populations remains an unsolved problem in biology. Described as an age-dependent increase in natural mortality (known as actuarial senescence) and an age-dependent decrease in fecundity (known as reproductive senescence), the role of senescence in nature is still poorly understood. Based on empirical estimates of reproductive and actuarial senescence, we explored how senescence affects the population dynamics of Coregonus albula, a small, schooling salmonid fish. Using an empirically-based eco-evolutionary model, we investigated how the presence or absence of senescence affects how the fish population responds to pristine, intensive harvest, a…

Echo-sounding can discriminate between fish and macroinvertebrates in fresh water

SUMMARY 1. Acoustic scattering from fish and macroinvertebrates was studied in a boreal Finnish lake at three echosounder frequencies (38, 120 and 200 kHz). Split-beam transducers with partly overlapping 7� beams were employed. Acoustic, fish and invertebrate sampling were undertaken simultaneously. Vertical gradients of temperature and oxygen concentration were measured during the exercise. 2. At all frequencies, a narrow scattering layer coincided with the thermocline. At 38 kHz, fish were detected well with practically no reverberation from invertebrates while 200 kHz detected both fish and invertebrates. 3. Minor differences in the magnitude of acoustic scattering from fish were found b…

Genetic-based evaluation of management units for sustainable vendace (Coregonus albula) fisheries in a large lake system

The goal of the processing industry, trade and consumers is to get eco-labelled freshwater fish products from sustainable fisheries into the market as soon as possible. The fourth largest natural lake system in Europe, the Saimaa lake system supports a fishery for vendace (Coregonus albula). Certification of the fishery requires an understanding of population structure to help determine the number and spatial extent of management units. In this study, we analysed the genetic diversity of local vendace populations in the Saimaa lake system and aimed to identify the conservation and management units of vendace. Within the Saimaa, the genetic divergence between local populations of vendace was…

Upstream migration activity of cyprinids and percids in a channel, monitored by a horizontal split-beam echosounder

A 200 kHz digital echosounder (HTI) with two split-beam transducers was aimed horizontally to monitor the upstream migration activity of fish, from 24 April to 28 June, in Aijalansalmi channel (mean width 35 m, length 700 m, and maximum depth 5 m) from large mesotrophic Lake Paijanne to small eutrophic Lake Jyvasjarvi. This study was part of a larger project which aims to analyse the movement of commercially unimportant fish species and reduce the abundance of these fish in L. Jyvasjarvi. Catch samples were collected with a trap net located immediately upstream from the acoustic beams. The most common species in the catch were roach (Rutilus rutilus), perch (Perca fluviatilis), bream (Abram…