Effects of different diets on juvenile tench, Tinca tinca (L.) reared under controlled conditions

In a laboratory experiment at 28 °C, four commercial starters (Carp Starter, Uni Starter, Perla Plus, Asta) were fed intensively to 11-month-old juvenile tench Tinca tinca of initially 41 mm TL and 0.70 g BW, over a period of 120 days. Frozen chironomid larvae were fed to fish below satiation as a reference diet for comparison of the fish body chemical composition. Final survival rates of 99–100% and satisfactory growth were attained with all dry diets, ranging from 73.3 (Carp Starter) to 84.8 mm TL (Asta) and from 5.39 (Carp Starter) to 8.09 g BW (Perla Plus). External body deformities of up to 96.4% specimens appeared in those fed Uni Starter, Carp Starter and Perla Plus, whereas 0% were recorded for Asta and chironomids. Fish in groups with deformities showed significantly elevated lipid content in their bodies, lower ash content and higher condition coefficient values. Juveniles in groups without deformities did not substantially change lipid content, but the ash content increased. Thus, among four dry diets studied, only Asta starter can be fed effectively and safely to juvenile tench reared intensively under controlled conditions.     

Diet and water temperature affect growth and body deformities in juvenile tench Tinca tinca (L.) reared under controlled conditions

The experiment was designed to determine the combined effect of fish diet and water temperature on juvenile tench Tinca tinca (L.). Three diets were used: commercial dry diet for fish Aller Futura (diet F); frozen Chironomidae larvae (diet C); and Aller Futura substituted with Chironomidae at a ratio of 3:2 (dry weight; diet FC). Daily food rations and duration of the experiment were adjusted to temperatures of 20°C, 23°C and 26°C based on a correction factor q equal to 1.000, 0.779 and 0.609 respectively. The experiment lasted 92, 72 and 56 days for the respective temperatures. No mortality occurred. The highest relative growth rates were found at 26°C in diets F and FC. The lowest food conversion ratio of 1.12–1.22 (recalculated for dry weight of feed diet) was determined in fish fed diet F at 23°C and 26°C. A high value of condition index appeared to be the early warning of decreasing biological quality of fish and deformities. The lowest incidence of fish with deformities (IDef) was found in groups fed diet C (<1%), while the highest (90%) in fish fed diet F at 26°C. A lower share of dry diet in the fish food and a lower water temperature led to a lower IDef. Thus, amount of dry feed in fish diet was the major factor inducing body deformities, while water temperature only modified the effects of the dietary factor. Diet C at 23–26°C was the most cost‐effective of the tested combinations of diet and temperature.     

Dynamics of ammonia excretion in juvenile common tench, Tinca tinca (L.), during intensive rearing under controlled conditions

This paper presents the first data on ammonia excretion by fed and starved juvenile common tench. The aim of the study was to determine the amount of ammonia excretion by juvenile common tench Tinca tinca (L.), fed with a commercial feed under intensive rearing conditions. Rearing was conducted for a period of 4 weeks at a water temperature of 27 °C. On the test day, the amounts of ammonia excreted by starved fish (over 12 h: control group) and fish fed with a morning feed dose (3 % of biomass: experimental group) were determined at 1, 3, 5, 7, 9 and 11 h after feeding. Ammonia excretion measurements were taken in 7-day intervals. Despite the increased growth rate in successive weeks of experiment, the amount of ammonia excreted by the fish was always at a similar level within the control (maximum 0.26 mg g^?1) and the experimental group (maximum 0.39 mg g^?1). This paper presents for the first time reliable amount of ammonia excreted by fed and starved juvenile common tench. The obtained results may also increase the effectiveness of intensive rearing procedures (taking into account stocking density and feeding regime) and allow to design the most effective biofiltration capacity of recirculating aquaculture systems for the commercial production of this species.     

Survival and growth of larval and juvenile tench, Tinca tinca L., fed different diets under controlled conditions

Abstract. Tench, Tinca tinca L., larvae were reared under controlled conditions at water temperatures of 28°C and 31°C during a 15-day period. Feeding with exclusively live Artemia nauplii gave best results, irrespective of rearing temperature (total length 13·6mm; average weight 31 mg; survival rate 83–85%). High survival rates of 81–88% were found also in all groups fed mixed diets, whereas growth rate of tench larvae was significantly influenced by the frequency of supplemental feeding with Artemia. When applied alone, the tested carp starter feed turned out to be insufficient for rearing the larval tench.     

Does overfeeding affect tench Tinca tinca (L.) juveniles?

Duplicate groups of tench (Tinca tinca) juveniles (mean weight 0.69 g) were fed at 28 °C for 70 days either a commercial formulated diet, Futura, or frozen Chironomidae larvae at four levels: below satiation, at about satiation, above satiation and well above satiation. These are represented by the feeding groups F1–F4 and C1–C4, respectively. No mortality was observed throughout the experiment. In groups F2–F4 a considerable incidence of uneaten food (>40% observations), deformities of the caudal penduncle, retarded growth, elevated condition coefficient (>1.2), reduced amounts of minerals in the tissue (<10% dry matter), high C/N ratio (>5) and high caloric value of the tissue (>28 J mg⁻¹ dry matter) (the latter two suggesting excessive fat deposits), were found. The combination of these indices was indicative of overfeeding in fish fed the Futura diet at the daily doses ≥2.7% fish biomass. First symptoms of overfeeding: a considerable incidence of uneaten food, retarded growth, and elevated C/N ratio were observed in the group fed Chironomidae larvae at the highest level, (C4, wet Chironomidae 20.7% of the fish biomass daily, that is, 3.9% of dry chironomids per fish biomass and day). Thus, restricted daily doses of formulated diet not exceeding 2.5% fish biomass are recommended for tench juveniles aged 130–200 days post hatch. Safe daily doses of frozen Chironomidae (in terms of larvae dry weight) remain below 3.5% of fish biomass.     

Induced meiotic gynogenesis in tench, Tinca tinca (L.) using irradiated heterogenic sperm

Effects of the starting time and duration of cold shock, as well as the source of heterogenic sperm on the percentage of viable gynogenic larvae (PVGL) in tench were studied. The DNA in sperm of red common carp (Cyprinus carpio var singuonensis) was inactivated by ultraviolet radiation prior to use to induce meiotic gynogenetic development in tench. In experiment 1, tench eggs were cold-shocked for 30 min starting at 1, 3, 5, 7, 10 and 15 min post activation. In experiment 2, cold shock began 5 min after activation and lasted for 10, 20, 30, 40, 50 min, respectively. Each experiment was run in triplicate using 3 tench females, and one group not treated with cold shock was included in each experiment to serve as a control group. In experiment 3, sperm of bigmouth buffalo (Ictiobus cyprinellus), red common carp or grass carp (Ctenopharyngodon idella) was used to induce gynogenesis in tench with a 20-min cold shock starting 5 min post activation. The results showed that PVGL from the control group was very low (0.11–0.47%). In experiment 1, the highest average PVGL (9.60%) was observed when cold shock treatment was applied 5 min post activation. When cold shock treatment was started 5 min post activation, duration of cold shock affected PVGL. Cold shock lasting 20 min resulted in the highest average PVGL (12. 57%) among the selected duration of cold shock studied in experiment 2. The average PVGL was 2.3, 8.6, and 9.3%, respectively, for eggs induced by sperm of bigmouth buffalo, red common carp and grass carp. Average PVGL was significantly lower for eggs induced by sperm of bigmouth buffalo, compared with that for eggs induced by sperm of the other two species. However, average PVGL were similar for eggs induced by sperm of red common carp and grass carp. In summary, the optimal conditions for gynogenesis in tench include the use of irradiated sperm of grass carp to activate the eggs and cold shock of 20 min starting 5 min post activation. Since female tench grow much faster to a larger size than male tench, gynogenesis of tench holds great potential for production enhancement.     

Insemination, fertilization and gamete management in tench, Tinca tinca (L.)

Various procedures for artificial insemination in tench, Tinca tinca (L.) were re-examined with evaluation of fecundity of males and females among different tench strains. The objectives of this study were to enhance fertilization and hatching rates through optimization of the activation solution, the insemination process, the activation of gametes, and the elimination of eggs stickiness. Sperm for all experiments was collected directly into immobilization solution of modified Kurokura solution containing 180 mM of NaCl and stored at 2 °C for 2.5–5 h prior to the experiment. When dechlorinated tap water was used for activation a gamete ratio of 1150 spermatozoa per egg showed the best significant fertilisation and hatching rates. Optimal ratio between eggs (weight in g) and activation solution (in cm³) was 1:1. Different concentrations of activation solutions such as NaCl from 0 to 68 mM (0–136 mOsmol kg⁻¹) without buffer statistically decreased fertilization and hatching rates. The activation solution containing 17 mM of NaCl, 10 mM Tris–HCl, pH 8 and 9 significantly increased fertilization and hatching rates compared to dechlorinated tap water of pH 7 or activation solution containing 17 mM of NaCl, 10 mM Tris–HCl, pH 6 and 7. Adhesiveness of the eggs was successfully removed by incubation in Alcalase and activity: 3.16 Anson units per cm³.     

Studies on sperm of diploid and triploid tench, Tinca tinca (L.)

The tench Tinca tinca is an interesting fish from the viewpoint of polyploidy and related atypical reproduction aspects. Triploid tench were produced artificially. Studies of spermiation as well as of sperm motility and structure were performed on several triploid and diploid males simultaneously with individual experimental crosses with diploid females to define their reproductive capacities. The testes of triploids visually looked less developed in the most of cases with lower sperm production (0.05 cm³ sperm per male), GSI and weight of testes compared to diploids (0.58 cm³ sperm per male). Analysis of variance showed significant influence of ploidy level on the percentage of motile spermatozoa. Triploidy did not change percentage of live spermatozoa and velocity of spermatozoa at the first time of sperm movement. The study of sperm structure by scanning electron microscopy revealed that most sperm cells were of normal structure with some anomalies. Sperm heads of triploid and diploid males were mostly round-shaped, 1.86±0.2 and 1.6±0.18 μm in diameter. The midpiece of triploid spermatozoa was slightly narrower than that of diploid ones with typical cylindrical shape. Flow cytometry revealed sperm cells of triploids to be largely aneuploid (1.47 n) with high mosaic DNA, oscillating from haploid DNA content (1.0 n) to diploid DNA content (1.9 n). Experimental crosses between triploid males and diploid females revealed that these males were capable to stimulate effective development with relatively high level of fertilization and hatching rates from 0 to 70%. In conclusion, triploidization does not seem to guarantee sterility of tench.     

Morphological changes of digestive structures in starved tench Tinca tinca (L.) juveniles

The aim of the study was to evaluate the effect of short-term starvation on the morphology of the digestive system of juvenile tench Tinca tinca (L.). The morphological changes in digestive structures of juveniles, fed 70 days starter or Chironomidae larvae, were examined after 1, 12, 24 and 48 h of starvation at 28 °C. The morphological changes in the organisation of the digestive structures were as follows: (1) Intestine – proteolysis of the intestinal mucosa, especially of enterocytes in apical part of mucosal folds; (2) Liver – progressive reduction of lipid vacuoles and glycogen granules, and, in effect, volume reduction of hepatocytes cytoplasm; (3) Pancreas – progressive degeneration of exocrine pancreatic cells, parallel with decrease of zymogen activity. The fish fed natural food prior to starvation seem to be more resistant to starvation than those fed starter.     

Respiratory and hematological response of tench, Tinca tinca (L.) to a short-term cadmium exposure

The effects of 3 h exposure to 96hLC50 of cadmium (4.5 mg dm⁻³) on oxygen consumption rate, and hematological parameters (RBC, WBC, erythrocyte and leukocyte pattern) of juvenile tench were evaluated. Oxygen consumption significantly decreased beginning from 24 h postexposure, and remained reduced until the end of the experiment (96 h postexposure). RBC gradually increased, together with the percentage of juvenile cells in circulation. On the other hand, cadmium induced damage to the red cells – the share of cellular anomalies significantly increased with time postexposure. They included abnormal cell shape, vacuolization, swelling, chromatin disintegration in the nucleus, and nucleus indentation. The exposed fish showed a gradual and significant decrease in WBC without a shift of lymphocyte/neutrophil proportion. No significant changes in thrombocyte count occurred. The results show that short-term exposure to cadmium reduced fish energetic metabolism, and suppressed immune abilities. The symptoms gradually developed after the end of exposure, and no recovery took place until␣96 h.     

Evaluation of pea protein concentrate as partial replacement of fish meal in practical diets for juvenile tench (Tinca tinca L.)

With the aim to evaluate different replacement levels of fish meal (FM) by pea protein concentrate (PPC) on survival, growth performance and body composition of juvenile tench (Tinca tinca), a 90‐day experiment was conducted with 6‐month‐old juveniles. Four practical diets (50% crude protein) differing in the level of replacement of FM protein by PPC protein were tested: 0% (control), 25%, 35% or 45%, corresponding to 0, 207.5, 290.4 or 373.3 g PPC kg^?1 diet respectively. Survival rates ranged from 96.4% to 98.5%. The 25% and 35% replacement diets resulted in similar growth values (P > 0.05) to those obtained with the control diet (average of the three feeding treatments: 57.57 mm total length, 2.48 g weight and 1.87% day^?1 specific growth rate). The 45% replacement diet had the lowest growth (P < 0.05). Fish with externally visible deformities ranged from 0% to 1.5%. The relation among amino acid profiles of the diets, body composition, growth performance of juveniles and amino acid requirements of other fish species is discussed. An amount of 290.4 g PPC kg^?1 diet (35% replacement of FM protein) can be included in juvenile tench diets without impairing growth performance.     

Decapsulated Artemia cysts of different quality (high or low hatch‐rate) as direct food for tench (Tinca tinca L.) larvae

Three 30‐day experiments were conducted to evaluate decapsulated Artemia cysts with different quality (high or low hatch‐rate) as food for tench (Tinca tinca L.) larvae from the onset of exogenous feeding. In experiment 1, three diets were tested: Artemia nauplii‐only or cysts‐only for 30 days, and nauplii for the first 7 days and cysts thereafter. The cysts used had 86% hatching rate (high hatch‐rate cysts). The same feeding treatments were replicated in experiment 2 but with low hatch‐rate cysts (10% hatching rate). In experiment 3, five diets were tested: high hatch‐rate cysts only or low hatch‐rate cysts only for 30 days, and nauplii for the first 7, 4 or 2 days and low hatch‐rate cysts thereafter. In overall, survival was high, except with the low hatch‐rate cysts only diet. Feeding tench larvae with cysts resulted in higher growth and lower FCR compared to feeding with live nauplii only. High hatch‐rate Artemia cysts are a suitable food from the onset of exogenous feeding and low hatch‐rate cysts can be successfully used after 2–7 days feeding on nauplii.     

Morphological sex change upon treatment by endocrine modulators in meiogynogenetic tench (Tinca tinca L.)

Exogenous steroids alter sex differentiation in fish substantially. In the present study we have evaluated the effects of 17α-methyltestosterone (MT) and oestrogen receptor antagonist Tamoxifen (TA) on gonadal development and skewness of the sex ratio in all-female tench juveniles. In the first two experiments, sexually undifferentiated juveniles were orally treated with three doses of MT and TA (50, 100 and 150 mg kg⁻¹). Both the treatments resulted in a moderate dose-dependent masculinization, with neomale production ranging from 17% (50 mg kg⁻¹) to 26% (150 mg kg⁻¹) for the MT only treatment, and from 0% (50 mg kg⁻¹) to 27% (100 mg kg⁻¹) only for the TA treatment respectively. In the third experiment treatment of sexually differentiated tench females with single steroid treatments or combinations of the two resulted in populations composed of females and intersex individuals. The significantly highest occurrence of intersex individuals (45.5%) was found in the group subjected to combine treatment of MT+TA (150+200 mg kg⁻¹). No masculinization effect of the single or the combined treatment occurred. It can be concluded that oral treatment with MT or TA only slightly modifies the normal process of sex differentiation in gynogenetic tench juvenile, but treatment with the above-mentioned combinations has a highly significant potential to skew the sex ratio in sexually differentiated tench females. However, from an applied point of view, the treatment procedure will need optimization before use at a commercial level.     

Evaluation of poultry by‐product meal as partial replacement of fish meal in practical diets for juvenile tench (Tinca tinca L.)

The aim of this study was to evaluate different replacement levels of fish meal (FM) by poultry by‐product meal (PBM) on survival, growth performance and body composition of juvenile tench (Tinca tinca). A 90‐day experiment was conducted with 5 month‐old juveniles (31.95 mm total length, 0.396 g weight). Eight practical diets (50% crude protein) differing in the level of replacement of FM protein by PMB protein were tested: 0% (control), 25%, 31%, 37%, 43%, 49%, 55% and 61% corresponding to 0, 184.8, 229.2, 273.5, 317.8, 362.1, 406.5 or 450.8 g PBM kg^?1 diet respectively. Significant differences were not found (P > 0.05) between 25% replacement of FM protein by PBM protein (184.8 g kg^?1 PBM in diet) and control diet. At higher replacement levels, fish had significantly lower growth, higher feed conversion ratio and lower protein productive value (P < 0.05). Fish with externally visible deformities ranged from 1.1% to 3.3%. The relation among amino acid profiles of the diets, body composition, growth performance of juveniles and amino acid requirements of other fish species is discussed. Up to 184.8 g PBM kg^?1 diet can be included in diets for juvenile tench without impairing growth performance.     

Comparative growth and diet of tench Tinca tinca (L.) larvae and juveniles from river floodplain biotopes in France and England

The growth and diet of larvae and 0 + juveniles of riverine tench Tinca tinca (L.) were compared in two stagnant floodplain biotopes from the upper River Rhône, France, and the River Great Ouse, England. The diet of tench was generally similar at the French and English sites, with Cladocera being the predominant prey of young larvae (English site only). Cladocera and Copepoda were the principal prey of older larvae and 0 + juveniles, though the latter took a variety of other, larger prey items. Ontogenetic changes in diet at both sites probably reflect prey availability. Protracted spawning at the French site contrasted a single spawning effort in the English backwater. Fluctuations in growth rate and condition were observed to coincide with shifts in diet, suggesting that tench developing in riverine biotopes may be vulnerable to competition at particular transitional intervals in their early ontogeny.     

The effect of feeding on oxygen consumption and ammonia excretion of juvenile tench Tinca tinca (L.) reared in a water recirculating system

The effect of feeding frequency (one, three, and continuous feeding), feed ration (0.2, 0.5, 0.8% of total fish biomass), and feeding per se on the oxygen consumption (OC, mg O₂ kg⁻¹ h⁻¹) and ammonia excretion (AE, mg TAN kg⁻¹ h⁻¹) of juvenile tench (body weight 15–19 g) and variations in these parameters in daily cycles were examined. Fish metabolism was studied in a recirculating system (rearing tanks of 0.2 m³, water temperature 23 °C). It was found that oxygen consumption and ammonia excretion depended significantly on feed ration. An increase of feed ration from 0.2 to 0.8% of fish biomass caused an increase of OC and AE from 126.80 mg O₂ kg⁻¹ h⁻¹ and 1.95 mg TAN kg⁻¹ h⁻¹ to 187.35 mg O₂ kg⁻¹ h⁻¹ and 8.80 mg TAN kg⁻¹ h⁻¹ (p<0.05). There was no dependence between feeding frequency and the mean rate of oxygen consumption. However, the relationship between feeding frequency and ammonia excretion by juvenile tench was statistically significant (p<0.05). Feeding frequency significantly affected daily fluctuations of AE and OC. It was found that diurnal variations in metabolic rates were strictly related to tench feeding, and the daily variations of AE were significantly higher than OC.     

Production, release and olfactory detection of sex steroids by the tench (Tinca tinca L.)

The present study is concerned with pheromone communication in tench (Tinca tinca L.), establishing firstly whether males have a high olfactory sensitivity to some typical teleost sex steroids and prostaglandins; and secondly whether males and females might be able to synthesise and release some of these steroids into the water. The C₂₁ steroid, 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) was found to give large electro-olfactogram responses with an estimated threshold of detection of 10⁻¹² M. The male tench were equally sensitive to glucuronidated 17,20β-P (10^−11.6 M) but 100 times less sensitive to sulphated 17,20β-P (11^−9.7 M). Preliminary data from cross-adaptation studies suggest that both the free and conjugated forms are detected by the same olfactory receptor(s). Male tench also had high olfactory sensitivity to prostaglandin F_2α (PGF_2α) and 15-keto PGF_2α (11^−11.5 and 10^−11.4 M). They were relatively insensitive, however, to testosterone (T), androstenedione (AD), 11-ketotestosterone (11-KT), 17β-oestradiol (E₂), 17,20β,21-trihydroxy-4-pregnen-3-one (17,20β,21-P) and 17,20α-dihydroxy-4-pregnen-3-one (17,20α-P). Radioimmunoassays were used to measure the steroids in plasma and water and all samples were processed for the measurement of free, sulphated and glucuronidated fractions. In females, free 17,20β-P, 17,20α-P, free and glucuronidated T, and AD in plasma showed the largest increases in response to injection with mammalian gonadotropin-releasing hormone analogue (GnRHa) or Ovaprim (a mixture of GnRHa and a dopamine inhibitor). Free 17,20β-P was released into the water at the greatest rate. Plasma concentrations of the two conjugated forms of 17,20β-P were also elevated 18 h after the administration of GnRHa, but not by as much as the free steroid. In males, AD and 11-KT showed the greatest increase in response to GnRHa and were moreover released into the water at a higher rate in the treated group than in the control. The data support a possible pheromonal role for free and glucuronidated 17,20β-P. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selective behaviour of a tench, Tinca tinca (L.), stock in different light–substrate combined conditions

Two physical factors light or substrate (fibre glass, artificial vegetation, gravel and mud) were determined in a 24 m long artificial channel to have importance on the distribution of 33 tench 2+, mean size 11.8±0.9 cm S.L. For this purpose in five trials tench could select between the half of the channel occupied by their preferred light intensity (inferior to 10 lux) combined with the less preferred substrate (fibre glass) or the half with the combination of one of the substrates considered and higher light conditions (40 lux). Fish preferred, with a 100% frequency, the optimal light-less preferred substrate when compared with the combinations fibre glass, gravel and mud. When artificial vegetation was the substrate considered, tench selected the combination preferred substrate – non-optimal light intensity with a 98.26±2.68% frequency. This result changed when increasing light intensity up to 150 lux, avoiding the shading effect. We demonstrated that light has a bigger influence than substrate in juvenile tench habitat selection under controlled conditions.     

Decapsulated Artemia Cysts: A Suitable Dietary Supplement for Juvenile Tench (Tinca tinca L.)

There are no specifically formulated dry foods for tench (Tinca tinca L.), which forces farmers to use diets formulated for other fish species. This has major drawbacks, such as high mortality, slow growth, and body deformities. A 120 day experiment was performed with five-month-old juvenile tench (initial mean weight: 0.388 g; total length: 31.78 mm) to evaluate decapsulated Artemia cysts as a supplement to a dry diet for other fish species. Three treatments, differing in the daily supplement, were tested: 1,800 freshly hatched nauplii, 1,800 cysts, and 300 cysts per g of tench biomass. Final survival ranged between 95.3% and 97.9%. Juvenile tench that received the supplement of 1,800 decapsulated Artemia cysts had a specific growth rate (1.28), weight (1.83 g), and total length (52.30 mm) significantly higher than those with the same amount of nauplii. The lowest supplement (300 cysts/g of fish biomass) allowed significantly lower growth and higher condition coefficient (1.40) than the rest. Animals with body deformities (1.06%) were only recorded in the groups that received the lowest cyst supplement. Results showed Artemia cysts are a suitable dietary supplement for juvenile tench, being an advantageous alternative to live nauplii.     

Changes in commercially exploited populations of tench, Tinca tinca (L.) in lakes of Northeastern Poland

Commercially exploited tench populations in 755 lakes in the Northeastern part of Poland were observed on the basis of the fishing data from 1951 to 1994. The observations were analyzed in five size categories of water reservoirs, whose total area reached over 123,900 ha. The analyses were performed according to the parameter of a commercial fish catch per area unit, taking into consideration the number of months per year when the lakes were exploited. Statistically significant differences were determined in the mean value of the tench catch parameter between the different size categories of lakes. Similar time-related differences were observed. In the lakes >100 ha the relative tench catch gradually declined in the successive time intervals. Tench was the rarest in catches from lakes <50 ha. The complete disappearance of tench from commercial catches occurred first as early as the late 1960s in lakes <200 ha. The largest quantities of tench stocking material were introduced to the lakes from 1951 to 1972. The amount of stocking material per 1 ha of stocked lakes decreased for larger lakes. The average frequency of tench stocking in the analyzed lakes was higher in larger lake size categories. On the other hand, in each lake size group the frequency of stocking was found to have declined along with the average volume of the stocking material in the subsequent 11-year-long periods.