Background

Salmo trutta is a native freshwater fish species for almost all the Europe belonging to the family Salmonidae. It is widespread and overall Least Concern. Its preferred habitat are cold streams, rivers and lakes. Spawns in rivers and streams with swift water. The phylogeographic structure is almost destroyed by stocking. Spawning sites are usually characterized by downward movement of water into gravel. Spawns between late October and March, usually in November-December (Freyhof, 2011). In small streams, brown trout are important predators of macroinvertebrates, and declining brown trout populations in these specific areas affect the entire aquatic food web (http://www.climate-and-freshwater.info/rivers-temperate-ecoregions/climate-change-species/). The brown trout has been popular subject to angling for centuries in Europe, and in Bulgaria, it is one of the preferred quarry for the anglers. Even though S. trutta occurs in high-mountain Rivers its populations suffer from severe anthropogenic influence such as pollution, habitat destruction, angling and poacher activities, and etc. That is why it is very important the status of the brown trout’s populations to be periodically checked. Determination of the growth rate is very important in inchthyological investigations as fish growth is one of the main factors that determine stock conditions (Mikhailov and Prodanov, 1983). Growth is perhaps the most studied of all parameters used to describe the life history of exploited fish (Khan and Khan, 2014). The growth rate of fish populations is related to the environment conditions and is indicative for the status of the populations. The good water quality determines the good growth rate of the fish. Contemporary studies on the biology and growth rate of S. trutta in Bulgaria are missing. The Valley of the studied Iliyna River is part of the National Park “Rila” and Natura 2000 site “Rilski monastery”in Bulgaria.

The aim of the study was to establish the growth rate of the brown trout from Iliyna River in Bulgaria and to compare it with the growth rates of other populations from the area of distribution of the species.

1 Materials and Methods

The material was collected in the autumn 2011 by electrofishing with 700 A/50-70 Hz straight, pulsating current. Two sampling sites along the river, with the following coordinates were studied: 1. N 42.09592° E 023.40512°; 2. N 42.10099° E 023.34944°. During the sampling the water temperature, oxygen saturation, conductivity and pH were measured (Table 1). Altogether 38 brown trout specimens from Iliyna River were analyzed. Each specimen was measured the length to the end of the scale cover (L) to the nearest mm and the total weight (W) to the nearest g. After the measurements on site, the fish were returned back alive in the river of their catchment.

The age of the fish was determined on their scales. The diagonal radius was measured by the use of Dokumator, Lasergeret (Carl Zeiss, Jena) at magnification 17.5х.

Length and weight at age were back calculated and the received values were used to calculate von Bertalanffy’s growth parameters (Bertalanffy, 1938): L_t = L_∞ [1-e^-k(t-to)] and W_t = W_∞ [1-e^-k(t-to)]ⁿ, where L_∞ (W_∞) = the asymptotic length (weight); k = relative growth rate; t_o = prenatal time (the hypothetic age at which the fish would have 0 length/weight).

To compare the linear and weight growth rate between the different populations 3 approaches were applied: 1. The populations were arranged in ascending order of the length of the highest age group. Comparing two populations of different ages, the length of the highest age group of the youngest population was compared (Zivkov, 1972; Zivkov et al., 1999); 2. Through ω = parameter (Gallucci and Quin, 1979): ω_L = L_∞ k; ω_W = W_∞ k, where k, L_∞, и, W_∞ are parameters from the von Bertalanffy’s equation; 3. Through the index of length/weight growth performance (Pauly, 1979; Munro and Pauly, 1983; Pauly and Munro, 1984): φ’=lgk+2lgL_∞ and φ’=lgk+2/3lgW_∞, where L_∞ (W_∞) and k are parameters from the von Bertalanffy’s equation.

2 Results

2.1 Linear growth

The relation between the fish length (L) and scale radius (R) is described by the equation L=12.067R+1.4829; r=0.94.

The back calculated average length and length increments are presented in Table 2. Increasing the fish age, the average annual increments (t) are changing irregularly. The highest length increment occurs during the second year (6.71 cm) and the smallest during the fifth year (1.96 cm). The increase of the length (L) with the age is well described by von Bertalanffy’s linear growth equation (Figure 1): L_t=23.16 [1-e^{-0.4666 (t-0.38)}], r=0.995, and SD=6.8.

2.2 Weight growth

The relation between the length (L) and the weight (W) is described from the equation W=0.0162L^2.9444, r=0.999. Based on the regression between L and W, the weights and the weight increments were back calculated (Table 3). The highest was the weight increment during the fourth year = 37.91 g. The increase of the weight (W) with the age (t) is well described by the equation of von Bertalanffy for the weight growth (Figure 2): W_t=160.692[1-e^{-0.4978 (t-0.4164)}]^2.94, r=0.998 and SD=7.15.

2.3 Comparative analysis of the growth rate

The studied population of S. trutta from Iliyna River has relatively fast temp of growth. Arranging the 20 populations of the species in ascending order of the fish lengths at the same age, the population from Iliyna River takes 7^th position (Table 4). Table 5 and Table 6 present comparison between the parameters from the von Bertalanffy’s linear and weight growth equations, φ’ and ω parameters for the brown trout from different rivers in Bulgaria. The smallest asymptotic length (L_∞, cm) and weight (W_∞, g), and respectively the highest growth constant (k) are established for S. trutta from Iliyna River. The changes in the parameters φ’ and ω doesn’t show clear tendency.

3 Discussion

Growth is a bio-energetic process and is defined as a change in the length and weight over a period of time. It indicates the health of the population and has been extensively studied for various species of fishes (Khan and Khan, 2014).

The length and weight increments of the brown trout from Iliyna River change irregularly during the years. This indicates the presence of compensation growth (Rozdina and Raikova-Petrova, 2014). The compensation growth is a process when initially bigger fish slow down their growth and the smallest individuals increase it. This mechanism ensures the best possible utilization of the environment resources. Density-dependent compensation growth in brown trout has been reported from Sundström et al. (2013).

The growth constant (k) determines how fast the fish approaches its L_∞. Some species, most of them short-lived, almost reach their L_∞ in a year or two and have a high value of k. Other species have a flat growth curve with a low k-value and need many years to reach anything like their L_∞ (http://www.fao.org/docrep/W5449e/w5449e05.htm). The highest values of the growth constant (k) for S. trutta from Iliyna River indicate the high growth rate of the studied population. The fast growth rate is due to the prevalence of two and three year old individuals and determines the relatively low asymptotic length (23.16 mm) and weight (160.69 g) (Raikova-Petrova and Živkov, 1987; Hamwi et al., 2007). The good growth rate is determined from the good physicochemical conditions in the river. The measured physicochemical parameters of the water exclude anthropogenic pollution or other fluctuations in the river in the period of sampling. The conditions are good and ensure the normal development of the brown trout in the river according to its biological requirements. To ensure the wellbeing of the species it is recommended to keep monitoring the growth rate and the status of the population in the studied river as well as in other rivers from the area of distribution of the species in Bulgaria.

The changes in the parameters φ’ and ω doesn’t show clear tendency. Other studies have shown that these two parameters are not appropriate to be used for comparison of the growth rate of freshwater fish species (Rozdina and Raikova-Petrova, 2014).

Authors’ contributions

GR participated in the sampling, data processing, drafting the manuscript and have given final approval of the manuscript to be published; DR have been involved in the manuscript preparation, analysis and interpretation of data; RV took part in the data procession. All authors read and approved the final manuscript.

Acknowledgements

Sampling of the material has been done for the project “Mapping and defining the conservation status of natural habitats and fish-Phase 1”, funded by the Operational Program “Environment”. Special thanks to Martin Iliev for the active participation during the sampling process.

References

Bertalanffy L., 1938, A quantitative theory of organic growth, (Inquiries on growth laws. II), Human Biology, 10(2): 182-213

http://www.jstor.org/stable/41447359

Freyhof J., 2011, Salmo trutta, The IUCN Red List of Threatened Species 2011: e.T19861A9050312

http://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T19861A9050312.en

Gallucci V., and Quinn T., 1979, Reparameterizing, fitting and testing a simple growth model, Transactions of the American Fisheries Society, 108: 14-25

https://doi.org/10.1577/1548-8659(1979)108<14:RFATAS>2.0.CO;2

Hamwi N., Raikova-Petrova G., and Petrov I., 2007, Growth rate, condition and mortality of Chub (Leuciscus cephalus) from the middle stream of the Iskar River (Bulgaria) and a comparison with populations from another water bodies, Acta Zool. Bulg, 59(3): 325-335

http://www.acta-zoologica-bulgarica.eu/downloads/acta-zoologica-bulgarica/2007/59-3-325-336.pdf

Khan S., and Khan M.A., 2014, Importance of age and growth studies in fisheries management, Proceedings of the National Seminar "Next Generation Sciences: Vision 2020 and Beyond', 194-201, Maharshi Dayanand University, Rohtak 124001 Haryana (India)

Mikhailov K., and Prodanov K., 1983, Approximate assessment of the natural mortality rate of the anchovy in the Bulgarian Black sea coast, Pross. IIR, Varna 20: 173-182

Munro J., and Pauly D., 1983, A simple method for comparing the growth of fishes and invertebrates, Fishbyte, 1(1): 5-6

http://www.worldfishcenter.org/Naga/FB_1976.pdf

Pauly D., 1979, Gill size and temperature as governing factors in fish growth: a generalization of von Bertalanffy's growth formula, Ber. Inst. Meereskd. Christian-Albrechts Univ. Kiel, 63: 156

http://oceanrep.geomar.de/14774/1/IFM-BER_63.pdf

Pauly D., and Munro J., 1984, Once more on the comparison of growth in fish and invertebrates. Fishbyte, 2(1): 21

https://core.ac.uk/download/pdf/6389328.pdf

Raikova-Petrova G., and Živkov M., 1987, Biological significance and practical use of Von Bertalanffy's equation in fishes]. pp. 103-106, In: Naidenow W., Golemansky V., Beshovski V., Russev B., Kehayov I., Lavchiev V., and Gerassimov S., (Eds.), Contemporaгry Achievements of Bulgarian Zoology, Sofia, Bulgarian Academy of Sciences Press, pp. 400 (in Bulgarian)

Rozdina D., and Raikova-Petrova G., 2014, Growth Rate of Barbus cyclolepis (Cyprinidae) in the Middle Stream of the Maritsa River, Bulgaria, Acta zool. bulg., 66(2): 265-270

http://acta-zoologica-bulgarica.eu/downloads/acta-zoologica-bulgarica/2014/66-2-265-270.pdf

Sundström L.F., Kaspersson R., Näslund J., and Johnsson J.I., 2013, Density-Dependent Compensatory Growth in Brown Trout (Salmo trutta) in Nature, PLoS ONE 8(5): e63287

https://doi.org/10.1371/journal.pone.0063287

PMid:23658820

PMCid:PMC3643939

Zivkov M., 1972, Critical analysis of some relative indices of the intensity of the fish growth, Bulletin de L'Institut de Zoologie et Musée, 36: 81-101 (In Russian, English summary)

Zivkov M., Trichkova T., and Raikova-Petrova G., 1999, Biological reasons for the unsuitability of growth parameters and indices for comparing fish growth, Environmental Biology of Fishes, Kluwer Academic Publisher, Dordrecht-Boston-London, 54(1): 67-76

https://link.springer.com/article/10.1023/A:1007425005491