Zeynep Pekcan-Hekim

Effects of clay turbidity and density of pikeperch (Sander lucioperca) larvae on predation by perch (Perca fluviatilis)

Increased turbidity reduces visibility in the water column, which can negatively affect vision-oriented fish and their ability to detect prey. Young fish could consequently benefit from high turbidity levels that can provide a protective cover, reducing predation pressure. Perch (Perca fluviatilis) are commonly found in littoral zones of temperate lakes and coastal areas of the Baltic Sea. Pikeperch (Sander lucioperca) spawn in these areas, so perch is a potential predator for pikeperch larvae. We conducted laboratory experiments to test the predation of perch on pikeperch larvae at different turbidity levels (5–85 nephelometric turbidity units), densities of pikeperch larvae (2–21 individuals l−1) and volumes of water (10–45l). The logistic regression showed that the probability of larvae eaten depended significantly on turbidity and volume of water in the bags, while density of larvae was not significant. However, because container size is known to affect predation, the data was divided into two groups based on water volume (10–20 and 25–45l) to reduce the effects of container size. In either group, probability of predation did not significantly depend on volume, whereas turbidity was significant in both groups, while density was significant in larger water volumes. Thus, high turbidity impaired perch predation and protected pikeperch larvae from perch predation. Because density of larvae was also a significant factor affecting predation of perch, the dispersal of pikeperch larvae from spawning areas should also increase the survival of larvae.

Feeding efficiency of planktivorous perch Perca fluviatilis and roach Rutilus rutilus in varying turbidity: an individual-based approach.

The feeding rate of perch Perca fluviatilis showed high individual variation at low and moderate turbidities, when one individual had consumed more Daphnia pulex than any other fish, whereas no such variation in feeding efficiency was observed with roach Rutilus rutilus. There was a significant decrease in total consumption of D. pulex by P. fluviatilis with increasing turbidity, but no correlation was observed in the case of R. rutilus. The results suggest that the difference in the ontogeny of P. fluviatilis and R. rutilus may be detectable as behaviour-related species-specific trait differences in the early planktivorous feeding stage of the two common species.

Climate Warming and Pikeperch Year-Class Catches in the Baltic Sea

Climate change scenarios concerning the Baltic Sea predict increase in surface water temperatures. Pikeperch (Sander lucioperca (L.)) inhabits the coastal areas of the northern Baltic Sea and is an important fish species for the Finnish fisheries. The year-class strength of pikeperch varies strongly between years and significantly depends on water temperature. We aimed to study the effects of changing temperature conditions on pikeperch fisheries and distribution based on commercial catch data from the period 1980–2008 in the Finnish coastal areas of the Baltic Sea. The results indicated that warmer summers will produce stronger pikeperch year-classes that consequently contribute significantly to the future catches. The average temperature in June–July explained 40% of the variation in the year-class catches in the Gulf of Finland and 73% in July–August in the Archipelago Sea. During the study period, the distribution of pikeperch catches expanded toward north along the coasts of the Bothnian Sea.

Effect of prey type and inorganic turbidity on littoral predator-prey interactions in a shallow lake: an experimental approach

Predation often represents the prevailing process shaping aquatic ecosystems. As foraging and antipredatory behaviour frequently relate to vision, turbidity may often impair the interactions between the predator and its prey, depending on prey type and source and level of turbidity. We studied the effect of inorganic turbidity (0–30 NTU) on the effectiveness of fish feeding on two types of prey in different habitats: free-swimming cladoceran (Daphnia pulex) in open water and plant-associated cladoceran (Sida crystallina) attached to Nuphar lutea leaves. For the planktivore, we used vision-oriented perch (Perca fluviatilis) common in the littoral zone of temperate lakes. In our study, increasing inorganic turbidity did not appear to initiate any significant change in the feeding efficiency of perch on free-swimming Daphnia pulex. However, we saw a markedly different feeding efficiency when perch targeted plant-attached Sida crystallina. Our results substantiate that floating-leaved macrophytes in turbid lakes may provide a favourable habitat for plant-attached cladocerans.

Combined Effects of Turbulence and Different Predation Regimes on Zooplankton in Highly Colored Water—Implications for Environmental Change in Lakes

In aquatic ecosystems, predation is affected both by turbulence and visibility, but the combined effects are poorly known. Both factors are changing in lakes in the Northern Hemisphere; the average levels of turbulence are predicted to increase due to increasing wind activities, while water transparency is decreasing, e.g., due to variations in precipitation, and sediment resuspension. We explored experimentally how turbulence influenced the effects of planktivorous fish and invertebrate predators on zooplankton when it was combined with low visibility caused by high levels of water color. The study was conducted as a factorial design in 24 outdoor ponds, using the natural zooplankton community as a prey population. Perch and roach were used as vertebrate predators and Chaoborus flavicans larvae as invertebrate predators. In addition to calm conditions, the turbulent dissipation rate used in the experiments was 10−6 m2 s−3, and the water color was 140 mg Pt L−1. The results demonstrated that in a system dominated by invertebrates, predation pressure on cladocerans increased considerably under intermediate turbulence. Under calm conditions, chaoborids caused only a minor reduction in the crustacean biomass. The effect of fish predation on cladocerans was slightly reduced by turbulence, while predation on cyclopoids was strongly enhanced. Surprisingly, under turbulent conditions fish reduced cyclopoid biomass, whereas in calm water it increased in the presence of fish. We thus concluded that turbulence affects fish selectivity. The results suggested that in dystrophic invertebrate-dominated lakes, turbulence may severely affect the abundance of cladocerans. In fish-dominated dystrophic lakes, on the other hand, turbulence-induced changes in planktivory may considerably affect copepods instead of cladocerans. In lakes inhabited by both invertebrates and fish, the response of top-down regulation to turbulence resembles that in fish-dominated systems, due to intraguild predation. The changes in planktivorous predation induced by abiotic factors may possibly cascade to primary producers.

Reversed Diel Horizontal Migration of Fish: Turbidity Versus Plant Structural Complexity as Refuge

ABSTRACT Diel horizontal migration (DHM) by small perch (Perca fluviatilis) between the littoral vegetation zone and open water was studied in the eutrophic Kirkkojärvi basin (southern Finland). Small perch showed reversed DHM by occupying the open water area during daylight and moving into the littoral zone at dusk. They avoided the vegetation zone during daylight because water clarity among the plants was much higher than in the open water, which was attributed to the reductive effect of macrophytes on sediment resuspension. The migration of perch into the littoral zone at dusk was explained by the higher availability of zooplankton among the plants. The results indicated that the importance of structural complexity of vegetation as a refuge decreases with increasing turbidity of the water. The effects of macrophytes on water turbidity may create circumstances where predation threat for small planktivorous fish during daylight is highest among the vegetation, and such circumstances may thus induce reversed DHM.

Bridge under troubled water: Turbulence and niche partitioning in fish foraging

Abstract The coexistence of competing species relies on niche partitioning. Competitive exclusion is likely inevitable at high niche overlap, but such divide between competitors may be bridged if environmental circumstances displace competitor niches to enhance partitioning. Foraging‐niche dimension can be influenced by environmental characteristics, and if competitors react differently to such conditions, coexistence can be facilitated. We here experimentally approach the partitioning effects of environmental conditions by evaluating the influence of water turbulence on foraging‐niche responses in two competing fish species, Eurasian perch Perca fluviatilis and roach Rutilus rutilus, selecting from planktonic and benthic prey. In the absence of turbulence, both fish species showed high selectivity for benthic chironomid larvae. R. rutilus fed almost exclusively on zoobenthos, whereas P. fluviatilis complemented the benthic diet with zooplankton (mainly copepods). In turbulent water, on the other hand, the foraging‐niche widths of both R. rutilus and P. fluviatilis increased, while their diet overlap simultaneously decreased, caused by 20% of the R. rutilus individuals turning to planktonic (mainly bosminids) prey, and by P. fluviatilis increasing foraging on littoral/benthic food sources. We show that moderate physical disturbance of environments, such as turbulence, can enhance niche partitioning and thereby coexistence of competing foragers. Turbulence affects prey but not fish swimming capacities, with consequences for prey‐specific distributions and encounter rates with fish of different foraging strategies (pause‐travel P. fluviatilis and cruise R. rutilus). Water turbulence and prey community structure should hereby affect competitive interaction strengths among fish species, with consequences for coexistence probability as well as community and system compositions.

Essential fish habitats (EFH)

Many fish species in the Baltic Sea are dependent on shallow and sheltered near-shore habitats for their spawning, nursery, feeding and migration. Still, the role of these essential fish habitats, ...