Tapio Keskinen

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

Abstract 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 ( Abramis brama ), ruffe ( Gymnocephalus cernuus ), and white bream ( Abramis bjoerkna ). The upstream migration of fish was correlated with water temperature ( r = 0.40) with time lag of 1 d. In spring, L. Jyvasjarvi warmed faster than L. Paijanne, causing spawning migration from L. Paijanne to L. Jyvasjarvi. Clear diurnal rhythm in activity was observed. The migration rate through the channel peaked around dawn and dusk. Catch per unit effort of the trap net suggested that the peak of the spawning migration of different species was separate. Upstream migration was induced by the temperature difference between two lakes, and the activity of the migration was regulated by temperature changes and light rhythm.

Altered energy flow pathways in a lake ecosystem following manipulation of fish community structure

We used carbon and nitrogen stable isotope analyses to assess the relative contributions from pelagic and littoral energy sources to higher trophic levels in a lake ecosystem before and after a major food web perturbation. The food web structure of the lake was altered when the population sizes of the most abundant fish species (small perch, roach and bream) were reduced during an attempt to improve water quality by biomanipulation. Fish removal was followed by dense year classes of young fish, which subsequently increased the utilisation of pelagic resources. This was reflected as a decrease in relative energy contribution from littoral sources and also led to more distinct pelagic and littoral food chains after fish removal. Community metrics calculated from stable isotope data indicated increased trophic diversity and occupied niche area, and reduced trophic redundancy in the food web. However, only minor changes were observed in fish trophic positions, although roach and pike occupied slightly lower trophic positions after fish removal. Despite the Jyväsjärvi ecosystem becoming more dependent on pelagic energy after fish removals, the littoral energy contribution was still substantial, particularly to certain fish species. Hence, our results support recent arguments for the importance of benthic production in lake ecosystems. More generally, our results illustrate how large-scale perturbations of food web structure can alter energy flow patterns through an entire ecosystem.

Recovery of Lake Jyväsjärvi after pollution by municipal and industrial waste waters

Water bodies near cities are generally recipients of municipal and industria! waste waters, which leads to their deterioration. Following the diversion or treatment of waste waters, the situation generally improves but often remains unsatisfactory (KETo 1982, VAN LIERE & ÜULATI 1992). Consequently, interest has developed to find ways ( e.g. biomanipulation; MEIJER & HosPER, 1997) to support the recovery and hasten the improvement of water qua1ity for recreational and economic purposes. In this paper we summarise the pollution history of Lake Jyvãsjãrvi and present possible methods for continued improvement.

A Bioenergetics Model for Zander: Construction, Validation, and Evaluation of Uncertainty Caused by Multiple Input Parameters

Abstract A bioenergetics model for zander Sander lucioperca was constructed using data from respirometer and feeding experiments to estimate temperature and mass-specific functions of metabolic and food consumption (C) rates. Other parameters were taken from the literature. The model, together with a previously published model for walleyes S. vitreus (Kitchell et al. 1977) and its modification for zander (Salonen et al. 1996), was validated with independent feeding experiments in the laboratory and was field evaluated in two lakes by using mercury (Hg) as a marker. Both validation procedures gave high modeling efficiency, indicating that the model was able to predict zander C under field conditions. The effect of uncertainty in 29 input parameters on Hg concentration and C was estimated using the Latin hybercube sampling technique (a modified Monte Carlo approach that uses a form of stratified sampling). The simulation results showed that our model was rather insensitive to given uncertainties in input pa...

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

Abstract 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 populations with markedly different densities, and hence naturally induced differences in their growth and fecundity. The energy allocation to somatic tissues and eggs prior to spawning served as a proxy for total resource availability to individual females, and its effects on offspring survival and growth were analyzed. Vendace females allocated a rather constant proportion of available energy to eggs (per body mass) despite different growth patterns depending on the total resources in the different lakes; investment into eggs thus dictated the share remaining for growth. The energy allocation to eggs per mass was higher in young than in old spawners and the egg size and the relative fecundity differed between them: Young females produced more and smaller eggs and larvae than old spawners. In contrast to earlier observations of salmonids, a shortage of maternal food resources did not increase offspring size and survival. Vendace females in sparse populations with ample resources and high growth produced larger eggs and larvae. Vendace accommodate strong population fluctuations by their high plasticity in growth and fecundity, which affect their offspring size and consequently their recruitment and productivity, and account for their persistence and resilience in the face of high fishing mortality.

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 perch. Further, we compared a functional relationship regression (FRR), admitting correctly observation error variance also in the x-axis variable, with ordinary least squares regression (OLS) in modelling the relationship between cumulative 24 h catches for different treatments. We estimated the between-replicates proportional observation error variance within a treatment and found it to be similar in different treatments. Therefore the variance ratio could be assumed to be close to 1 enabling the use of major axis solution FRR. In this particular case the incorrect use of OLS obviously gives a seriously biased result, exacerbating the negative effect of accumulation for high x-axis values in comparison with FRR. We recommend the use of FRR for any analysis comparing different notoriously low precision fish abundance proxies.

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 universal phenomenon when freely swimming Vendace larvae are exposed to older conspecifics. Field observations and bioenergetic modelling are recommended to assess whether cannibalism plays a role in recruitment variation, especially since Vendace show a biennial cycle in the strength of their year classes.