Miroslav Nikčević

Accumulation of 20 elements in great cormorant (Phalacrocorax carbo) and its main prey, common carp (Cyprinus carpio) and Prussian carp (Carassius gibelio).

In this study, 20 heavy metals and trace elements (Al, As, B, Ba, Cd, Co, Cr, Cu, Fe, Hg, Li, Mg, Mn, Mo, Ni, Pb, Si, Se, Sr and Zn) were analyzed in different tissues (muscle, liver, intestine, feather and bone) of the great cormorant (Phalacrocorax carbo) and in different tissues (muscle, liver, gills, kidney and gonad) of their main prey (common carp-Cyprinus carpio and Prussian carp-Carassius gibelio) during the nesting season. Cormorant and fish specimens were collected at the Ečka Fishing Farm (Serbia) and from the nearby river. Principal components analysis (PCA) showed that cormorant livers were differentiated from other four tissues by higher concentrations of Hg, Fe, Cu and Mo, feathers by higher concentrations of Al and Si, and bones by higher concentrations of Sr and Mg. Differentiation among the three age classes of cormorants was observed only with regard to elemental concentrations in their feathers: subadult cormorants were differentiated by higher Hg and Zn concentrations, while the adults were separated by higher concentrations of Fe. In comparison with their prey (common carp and Prussian carp) cormorants were differentiated by higher concentrations of Fe, Hg and Cu in the muscle. Toxic mercury concentrations above 4μgg(-1)dw were determined in the liver of juvenile cormorants, as well as in the liver and feathers of subadults. Hg concentrations were significantly correlated among all studied tissues. Cormorant had significantly higher concentrations of Fe and Hg in muscle and liver than two studied fish species.

Spectral sensitivity of the dogfish shark (Scyliorhinus canicula)

The action spectrum of the electroretinographic (ERG) b-wave and the unmasked late receptor potential (LRP) were measured under a variety of conditions (isolated eyecup, detached retina, b-wave recording in fresh preparations, LRP measurements after low-temperature aging, dark and light adaptation). It was shown that in the dogfish, Scyliorhinus canicula (L.), eyecup spectral sensitivity matched closely the pigment 502 spectral curve like other rhodopsin-possessing marine species. The ERG b-wave is as good an indicator of spectral sensitivity as the unmasked LRP that directly reflects the responsiveness of photoreceptors. Differences in spectral sensitivity were not revealed between dogfish preparations studied under a variety of conditions (isolated eyecup vs. detached retina; b-wave recording in fresh preparations vs. LRP measurements after low-temperature aging; dark vs. light adaptation). We developed a new model for spectral sensitivity data.

Feeding habits of huchen Hucho hucho (Salmonidae) fry in the River Tresnjica, Yugoslavia

Abstract About 10000 huchen larvae, obtained by artificial spawning, were introduced on 28 May 1995 into the River Tresnjica, a tributary to the River Drina, using procedures applied routinely by local fishing associations. During next two months, only 33 fry were caught in the river. Nevertheless, valuable information has been obtained for the first time on young huchens in the Drina basin. The fry apparently selected slower or almost stagnant water habitats outside the main river course. The variability of prey organisms found in fry stomachs decreased with fry length. Larger and particularly motile prey, especially mayfly larvae, gradually dominated the fry diet. Also for the first time, we document homing instinct in huchens with mark‐recapture information: one out of the nine adults, tagged after spawning, was recaptured the following year at the same spawning site.

Effects of acute cooling on fish electroretinogram: a comparative study.

Temperature dependence of electroretinogram (ERG) was investigated in 3 fish species occupying different habitats--dogfish shark (Scyliorhinus canicula), Prussian carp (Carassius gibelio) and European eel (Anguilla anguilla). Acute cooling of the shark isolated eyecup from 23°C down to 6°C induced suppression of the electroretinographic b-wave--a complete degradation of this component was observed at 6°C. On the other hand, photoreceptor component of the ERG, the negative late receptor potential was not affected by cooling. The fact that the suppression of the dogfish shark b-wave at low temperatures was as a rule irreversible testifies about breakdown of neural retinal function at cold temperature extremes. Although in vivo experiments on immobilized Prussian carps have never resulted in complete deterioration of the b-wave at low temperatures, significant suppression of this ERG component by cooling was detected. Suppressing the effect of low temperatures on Prussian carp ERG might be due to the fact that C. gibelio, as well as other cyprinids, can be characterized as a warmwater species preferring temperatures well above cold extremes. The ERG of the eel, the third examined species, exhibited the strongest resistance to extremely low temperatures. During acute cooling of in situ eyecup preparations of migrating silver eels from 30°C down to 2°C the form of ERG became wider, but the amplitude of the b-wave only slightly decreased. High tolerance of eel b-wave to cold extremes shown in our study complies with ecological data confirming eurythermia in migrating silver eels remarkably adapted to cold-water environment as well.

Spectral sensitivity of the electroretinogram b-wave in dark-adapted Prussian carp ( Carassius gibelio Bloch, 1782)

One of the purposes of this study was to examine whether b-wave measurements can be used in the evaluation of scotopic spectral sensitivity in Prussian carp measurements when the eyes were surgically deprived of cornea, lens, and most of the vitreous. Another goal was testing the new fitting procedure for A2-based photopigments. Using fitted amplitude-log intensity functions for threshold calculation, and two models for computer-assisted fitting of spectral sensitivity curves, no significant differences in λmax were found between rod photopigments and b-wave-based spectral sensitivity.

The geometric morphometrics and condition of Pontic shad, Alosa immaculata (Pisces: Clupeidae) migrants to the Danube River

Anadromous fish could have intraspecific groups with different life histories; identification of these is very important for management and conservation. Geometric morphometrics represents a quick and cost-effective method to identify such intraspecific groups. In this study, geometric morphometrics of Pontic shad (Alosa immaculata) migrants from the Danube River were investigated, as was Fultons condition factor (K), hepatosomatic index (HSI) and gonadosomatic index (GSI). Pontic shad specimens were collected during April and May of 2004 and 2006. The sample comprised four groups (65 males and 56 females in 2004; 110 males and 78 females in 2006). Geometric morphometrics revealed differences in shape that were statistically significant for both year (nested analysis of variance: F = 13.84; p < 0.001) and sex (nested analysis of variance: sex nested in year, F = 1.71; p < 0.01). The major differences between years were in head shape and position of fins. The most significant difference between females and males was in the dorsolateral expansion of the mid-body section with emphasis on the higher dorsal part of the body. Values for K in this work (0.65–0.70) were the lowest recorded for Pontic shad in the Danube River. Additional investigations involving both geometric morphometrics and molecular genetics are necessary to confirm the presence of races. This could be important information for conservation purposes as Pontic shad is listed as Vulnerable on the International Union for Conservation of Nature list.

Modeling of the migration of the European glass eel

Abstract: The characteristics of glass eel migration at the mouth of river Bojana have been investigated by seasonal field studies from February to April 1998. Samples were collected by two fyke nets, one on each riverside, for 12 hours each night. A total of about 3,300 individuals were caught. In this work we present the results obtained from three successive migration waves in 1998. The data obtained on the number of eels caught during one migration wave, as a function of time, had two components: a bell‐shaped curve, lasting 7–14 days, over which an impulse (sudden burst in the number of caught eels, during 1–2 days) was observed. We propose a mathematical model for total number of captured eels within one migration wave. Impulse components were observed during the first and second migration waves. After their removal, the remaining experimental data of the three migration waves were fitted with normal distribution functions. A decrease in the values of fitting parameters as a function of time (migration wave number) was obtained. Our data indicate multiple causes for the observed migration waves.