IntroductionCurrently, there is no production of sturgeon fish on an industrial scale, only a quota is allocated for the catch of producers to ensure the operation of sturgeon fish breeding plants and scientific purposes. An alternative to catching sturgeon is commercial sturgeon farming. One of the ways to increase the efficiency of growing sturgeon species of fish is the introduction of hybrids into fish farming practice [1].Experimental cultivation of sturgeon hybrids has been carried out for a long time in Primorsky Krai at the Luchegorskaya scientific research station (NIS) Pacific Branch of “VNIRO” (“All-Russian Scientific Research Institute of Fisheries and Oceanography”, TINRO) [2-4]. One of the objects is a caster (a hybrid of kaluga × sterlet and sterlet × kaluga) – the Far Eastern analogue of the western bester (beluga × sterlet). After confirming the fertility of custer [5], returnable hybrids were obtained, and the first results of cultivation [6] will contribute to the creation of industrial hybrids by analogy with bester breeds: “Vnirovskaya” (beluga × (beluga × sterlet) and “Aksayskaya” (sterlet × (beluga × sterlet).The advantages of hybrids are manifested in accelerated growth, increased resilience, sometimes in early puberty, and adaptability in cultivation. In practical terms, another feature of hybrids is also important – increased adaptive plasticity [7].On the one hand, hybridization refers to resource-saving technologies, on the other hand, it solves theoretical problems of compatibility of genomes of cultivated species and hybrid forms of sturgeon. The nature of genetic material inheritance is manifested in the morphology of the body structure. Using mathematical statistics it is possible to assess the effect of the influence of parental species.The aim of the study is to give a morphological characterization of the return custer hybrids. Materials and methodsMorphometry data were collected during the annual bonitation in October 2020. The object of the study were six-year-olds (5+) of the return hybrids (K × St) × K, (K × St) × St and (St × K) × K generation 2015 (in the left part, the mother&#39;s material is used, and in the right, the father&#39;s. For example: (K × St) × K; (K × St) – ♀, K – ♂. All the individuals were grown under the same conditions in the cages of the research station (NIS) of the Pacific Branch of TINRO in the village Luchegorsk (Primorsky Krai).Morphological studies were  carried  out  according to existing schemes and techniques on live fish on the left side of the body [8, 9]. The number of abdominal and lateral scutes was calculated from both sides of the body. There were studied 17 plastic and 4 meristic characters. In total, 20 copies of each hybrid were measured.The confidence interval was calculated using the formula:D=x±Sx* t p   ,where D – average of the general population; х – sample average; Sx – standard error; tp – Student&#39;s criterion for probability P = 99.9 %.The similarity of features was evaluated using transgression according to the formula:Т= n 1 [0,5±φ x 1 ]+ n 2 [0,5±φ x 2 ] n 1 + n 2 where x 1 = x 2 -3 S 2 - x 1 S 1   , x 2 = x 1 -3 S 1 - x 2 S 2    – average values of the attribute, x 1  , x 2    – standard deviations of these features, 3 and 0.5 – numeric values provided by the formula; n1 and n2 – total number of observations in the first and second distribution (samples); φ(x) – the second function of the normalized deviation.Using the Student&#39;s criterion, the existence of differences between the two samples was determined: Т st = x 1 - x 2 S x 1 2 - S x 2 2   ,where   x 1   – squares of the standard error.Using the difference coefficient CD which is the ratio of the difference of the averages to the sum of the standard deviations of the samples and is determined by the formula: CD= x 1 - x 2 S 1 + S 2  .Statistical processing was performed using the MS Office Excel program. ResultsThe studied parameters (Table 1) for all objects according to the coefficient of variation (Cv) varied from 1.86 (K × St) × St – SL (body length from the end of the snout to the end of the middle rays) to 23.39 (K × St) × K – W (body weight) %. Table 1 Morphometric indicators of recurrent hybrids and their descriptive statistics Character(КхSt)хК ( n = 20 ex.)(КхSt)хSt ( n = 20 ex.)(StхК)хК ( n = 20 ex.)  x±mConfidence interval at P = 99.9%δCV, %x±mConfidence interval at P = 99.9%δCV, %x±mConfidence interval at P = 99.9%δCV, %    W, g7762,5±406,016185,98-9339,021815,7123,397950±408,516363,75-9536,251826,9222,986797,5±326,965527,93-8067,071462,221,51  TL, sm117,96±2,13109,69-126,239,538,08116,83±2,16108,43-125,229,678,27109,61±1,58103,47-115,747,066,44  In % of TL  SL88,31±0,5386,25-90,362,372,6887,63±0,3686,22-89,051,631,8689,11±0,7784,98-96,673,453,87  FL84,34±0,5582,22-86,472,442,983,92±0,3882,44-85,41,712,0484,55±0,7578,51-92,863,353,96  aD61,77±0,3860,27-63,261,722,7960,5±0,3859,04-61,961,682,7861,39±0,4959,07-66,892,183,55  aV54,01±0,3652,62-55,391,592,9552,42±0,2951,3-53,541,292,4653,42±0,4850,46-57,812,154,03  aA68,63±0,6766,01-71,253,024,467,46±0,7564,54-70,383,364,9869,73±165,4-85,494,486,43  aP21,72±0,3120,53-22,911,376,3121,16±0,3119,96-22,351,376,4821,7±0,2419,88-24,111,074,92  C20,05±0,318,9-21,211,336,6219,61±0,3318,32-20,91,487,5619,85±0,2318,23-22,321,035,2  pl10,42±0,110,02-10,820,464,410,46±0,169,86-11,060,76,6510,58±0,179,71-13,330,767,19  In % of С  R34,03±0,8430,79-37,283,7410,9839,7±0,637,37-42,042,696,7840,48±0,5934,95-43,752,656,55  OP46,14±1,042,24-50,044,499,7451,9±0,5249,86-53,942,354,5252,49±0,6746,12-57,692,985,68  O4,34±0,113,92-4,770,4911,24,19±0,133,69-4,70,5813,774,40±0,073,6-4,810,317,13  BC37,8±0,9534,11-41,494,2511,2542,18±0,9838,36-464,410,4441,31±0,9135,6-48,084,089,87  rc19,09±0,815,97-22,223,618,8523,1±0,620,77-25,432,6811,6223,1±0,618,52-26,962,6911,63  rr32,95±0,9129,43-36,474,0512,338,71±0,4936,82-40,592,175,638,8±0,6532,52-43,522,897,44  rl13,85±0,511,92-15,792,2316,0915,61±0,314,46-16,751,328,4615,7±0,2413,59-18,061,096,95  lc15,48±0,513,53-17,442,2514,5517,13±0,4115,55-18,711,8210,6317,8±0,5311,63-23,362,3613,26  SO31,08±0,5229,05-33,112,347,5330,3±0,5628,11-32,492,528,3231,01±0,4127,91-35,651,835,89  Meristic сharacters  Sl139,95±0,6137,57-42,332,746,8743,9±0,9640,19-47,614,289,7539,5±0,5737,29-41,712,546,44  Sl240,3±0,6737,72-42,882,987,3844,1±0,7539,69-48,513,357,6138,6±0,6436,11-41,092,877,44  Sv19,55±0,268,56-10,541,151210,1±0,188,71-11,490,797,810,0±0,219,2-10,80,929,18  Sv29,35±0,178,7-100,757,9710,2±0,178,5-11,90,777,5310,1±0,259,13-11,071,1211,08   * X±m – average value and the error of the average; n – number of instances; δ – standard deviation; Cv % – coefficient of variation. Characters designations: TL — total body length; W – body weight; SL – body length from the end of the snout to the end of the middle rays; FL – body length from the end of the snout to the roots of the middle rays; aD — antedorsal distance; aV – anteventral distance; aA – antenanal distance; aP – antepectoral distance; C – the length of the head ; pl – the length of the caudal stem; R – the length of the snout (from the end of the snout to the front edge of the eye); OP – the orbital space; O – the horizontal diameter of the eye; BC – the largest width of the head; rc – the distance from the end of the snout to the line passing through the middle of the base of the middle antennae; rr – the distance from the end of the snout to the cartilaginous arch of the mouth; rl – the distance from the base of the middle pair of antennae to the cartilaginous arch of the mouth; lc – the length of the largest tendril; SO – the width of the mouth; Sl1, Sl2 – the number of scutes of the lateral row, respectively, on the left and right, Sv1, Sv2 – the number of scutes of the abdominal row, respectively, on the left and right. Table 2The value of the Student&#39;s criterion when comparing the averages of the three hybrids with each other(p < 0.1 = 2.711, p < 0.01 = 3.566 with 38 degrees of freedom) nsgression indicator for pairwise comparison of morphological features of three hybrids Character(K × St) × K-(K × St) × St(K × St) × K-(St × K) × K(K × St) × St-(St × K) × KW, g88.0699.8199.94TL, sm99.9599.9199.92In % of TLSL98.2899.5391.95FL99.1799.4295.50aD99.72100.0098.54aV99.6999.9995.41aA98.0899.9498.00aP99.5399.9898.67C99.6799.9398.66pl97.3298.6399.72In % of СR80.9785.9799.66OP85.5380.6899.22O98.2499.8794.90BC98.4197.6399.92rc93.2594.3499.87rr87.0178.4999.34rl86.9591.9599.59lc97.6397.6098.93SO99.5699.9498.53Meristic сharactersSl199.9292.0499.88Sl2100.0095.98100.00Sv198.8697.1599.82Sv295.4096.9299.21  In hybrids (K × St) × St and (St × K) × K transgressed R – 85.97, OP – 80.68, and rr – 78.49%. The remaining indicators did not exceed the value above 91.95%The index of the CD difference coefficient did not exceed 1.28 for any of the studied parameters. DiscussionThe return hybrids based on K × St significantly differed in terms of indicators (R, rr, OP, rc, rl, Sl) and Table 1 shows how the average relative indicators from the head change when using a male sterlet (they increase with a smaller head size) in comparison with the use of paternal kaluga material.When comparing the hybrid (St × K) × K with each of the forms of K × St, differences appear separately either on the characters of the head or on the side scutes. Apparently, at the sterlet × kaluga stage, certain morphological characteristics are fixed, and with the addition of “blood” from the male, only some of them are enhanced. In the work on the morphological features of casters at the age of two years (1+) [10], the differences of reciprocal hybrids are considered, while the influence of sterlet on the formation of head characters is stronger than that of kaluga. Our work indicates that, in general, the patroclinic nature of inheritance of the studied indicators is preserved at the stage of recurrent crossing. It is necessary to conduct an analysis of recurrent and reciprocal hybrids with an assessment of similarity using a hybrid index. Conclusions1) A couple (K × St) × K and (K × St) × St can be reliably distinguished (student &#39;s criterion, transgression, confidence variant) by indicators such as the length of the snout – R and the distance from the end of the snout to the cartilaginous arch of the mouth – rr.2) Hybrids (K × St) × K and (St × K) × K can be reliably distinguished (student &#39;s criterion, transgression) by the length of the snout – R, the orbital space OP and the distance from the end of the snout to the cartilaginous arch of the mouth – rr.3) Pair (K × St) × St and (St × K) × K are reliably distinguishable by the Student&#39;s criterion only by side scutes.