Suomenlahden tilan muutokset – vaikutukset avomerialueen kalakantoihin ja kalastukseen

Hankkeessa arvioitiin ympäristötekijöiden ja niissä tapahtuvien muutosten vaikutuksia Suomenlahden avomerialueen kalojen alueelliseen runsaudenvaihteluun, kalojen ravinnonkäyttöön ja kalojen kuntoon. Tutkimuksessa havaittiin, että GAM-mallinnuksen (yleistetty additiivinen malli) avulla kalatiheyden vaihtelut avomerialueella voitiin selittää suureksi osaksi ympäristötekijöistä johtuviksi. Tutkimusajanjakson aikana Suomenlahden kalakannoissa tapahtui voimakkaita muutoksia. Sekä silakoiden että kilohailien osoitettiin vuonna 2003 kärsineen ravintopulasta. Kalat olivat laihoja ja suuri osa oli selvästi nälkiintyneitä. Nuorten kalojen kasvun havaittiin olevan erittäin vähäistä vuonna 2003. Kalojen nälkiintymisestä seurannut silakka- ja kilohailikantojen huono tila yhdessä samanaikaisesti voimaan tulleiden pyydysrajoitusten kanssa aiheutti Suomenlahden silakka- ja kilohailisaaliiden ennennäkemättömän voimakkaan romahduksen vuonna 2003. Hankkeessa myös arvioitiin, miten mahdolliset kalastuksessa tulevaisuudessa tapahtuvat muutokset vaikuttaisivat dioksiinien ja furaanien kertymiseen Suomenlahden ja Itämeren pääaltaan silakkaan ja kilohailiin. Hankkeessa kehitettiin mallit, joiden avulla osoitettiin olevan mahdollista kuvata kilohailin ja silakan kasvun riippuvuutta kalatiheydestä ja ympäristötekijöistä. Malleja käytettiin ennustettaessa kalastuksen vaikutuksia kalakantojen tilaan ja dioksiinien kertymiseen silakkaan ja kilohailiin. Kehitetyn mallin avulla kalastuskuolevuuden osoitettiin potentiaalisesti vaikuttavan erityisesti kilohailin kasvuun ja dioksiinien kertymiseen kilohailiin. Silakan kasvunmuutoksiin ja dioksiinien kerääntymiseen todennäköisesti vaikuttavat kalatiheyden ohella Itämeren suolapitoisuuden muutokset

Responses of food web to hypolimnetic aeration in Lake Vesijärvi

We studied the responses of a food web, especially fish and zooplankton, to summertime aeration, pumping of oxygen-rich epilimnetic water to the hypolimnion in Lake Vesijärvi, southern Finland. The aim of hypolimnetic aeration was to reduce internal loading of phosphorus from sediment. The population of smelt (Osmerus eperlanus L.), the main planktivore of the pelagial area, collapsed during the two 1st years of aeration due to increased temperature and low oxygen concentrations in the hypolimnion. The population recovered after the 4th year of hypolimnetic aeration, when oxygen conditions were improved. Despite elevated hypolimnetic temperature, smelt reached exceptionally high abundance, which led to a significant reduction in cladoceran body size. The density of perch (Perca fluviatilis L.) increased at first, but then decreased when the proportion of smelt and cyprinids increased. Biomasses of Daphnia decreased probably as a result of the disappearance of dark, low-oxygen deep-water refuge against fish predation and low availability of nutritionally high-quality algae. Occasionally filamentous cyanobacteria, such as turbulence tolerant Planktothrix agardhii (Gomont), were abundant, suggesting deteriorated food resources for zooplankton. The responses of food web were controversial with respect to the aim of the management, which was to prevent the occurrence of harmful algal blooms.Peer reviewe

Troubled by the humics — effects of water colour and interspecific competition on the feeding efficiency of planktivorous perch

Peer reviewe

The effects of age-at-maturation on the parameters of a biphasic Lester growth model in Sander lucioperca

An accurate estimation of growth is crucial for any fish species that is a target in fishery. We applied a biphasic Lester model for pikeperch (Sander lucioperca) population that is a slow-growing one. In this model, age at maturity divide the growth into immature and matuire phases. Logistic regression models showed that both age and length were significant in males and females when using maturity as a dependent variable, and both of these variables differed between sexes. To estimate how the changes in used age at maturity affect the Lester model parameters, the effects of ages from 10% to 90% probability of maturity were analysed. The gonadosomatic index of males (max. 2%) and females (max. 8.6%) was used to select Lester models that also gave low estimates for the investments in reproduction (g). Low g values were found in the Lester models for ages from 60% to 90% probability of maturity in males, and from 30% to 70% in females.An accurate estimation of growth is crucial for any fish species that is a target in fishery. We applied a biphasic Lester model for pikeperch (Sander lucioperca) population that is a slow-growing one. In this model, age at maturity divide the growth into immature and matuire phases. Logistic regression models showed that both age and length were significant in males and females when using maturity as a dependent variable, and both of these variables differed between sexes. To estimate how the changes in used age at maturity affect the Lester model parameters, the effects of ages from 10% to 90% probability of maturity were analysed. The gonadosomatic index of males (max. 2%) and females (max. 8.6%) was used to select Lester models that also gave low estimates for the investments in reproduction (g). Low g values were found in the Lester models for ages from 60% to 90% probability of maturity in males, and from 30% to 70% in females.Peer reviewe

Effects of non-selective and size-selective fishing on perch populations in a small lake

Retaining large individuals is considered intrinsic to sustainable fishing. In this nine-year study, we explored the effects of simulated recreational fishing on life-history traits of two perch (Perca fluviatilis) populations in a lake divided for experimental purposes into two sections. In each section, one of the two following fishing methods was used: non-selective and negatively size-selective i.e. large individuals released. Non-selective fishing rapidly decreased the average size and age of the spawning perch stock thus reducing the average size of spawned eggs. Both fishing procedures increased the share of females in the spawning population due to decreased age at maturity. The average age at maturity decreased more in females than in males. The reductions in the density and biomass of the populations and increase in the growth rate of perch were temporary but the effects on size and age structure persisted throughout the study period. The retention of large individuals can delay the adverse effects of fishing on populations, and enable reproduction of large females, thereby sustaining high genetic variability and better quality of offspring.Peer reviewe

Roach (Rutilus rutilus) populations respond to varying environment by altering size structure and growth rate.

To better understand the environmental factors and biological interactions affecting roach abundance and population structure in oligo-mesotrophic lakes, we explored roach population parameters in 24 small forest lakes in southern Finland. In those lakes, roach abundance was only little affected by the abiotic and biotic variables such as lake characteristics, water transparency, nutrient concentrations, pH or related variables, predation and competition. This was probably due to the generalist characteristics of the roach as it is adaptive to a wide variety of environments. Instead, the population structure and age-specific growth of the roach were strongly affected by various environmental variables. The growth rate was negatively related to water colour, calcium and nutrient concentration and roach abundance, and positively related to perch abundance and lake size. The growth patterns and relatively stable abundance showed flexibility in adaptation to different environmental conditions. Our results indicate that the ongoing environmental changes, recovery from acidification and brownification may notably affect the structure and production of roach populations, and may alter the role of the roach in lake ecosystems.Peer reviewe

Fishing-induced changes in predation pressure by perch (Perca fluviatilis) regulate littoral benthic macroinvertebrate biomass, density, and community structure

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