en aquaculture aquaculture پژوهشي Orginal research papers <span lang="TR" style="font-size:11.0pt"><span style="font-family:&quot;Calibri&quot;,sans-serif">Mass selection is an effective strategy for improving growth traits in the <i>Penaeus vannamei</i>. In this study, two populations with distinct genetic backgrounds (Molokai and Mix) were subjected to mass selection, and growth and morphological traits including body weight, total length, muscle length, carapace length and width, and third abdominal segment depth were measured.</span></span> <span lang="TR" style="font-size:11.0pt"><span style="font-family:&quot;Calibri&quot;,sans-serif">Genetic parameters, phenotypic correlations, and selection responses were estimated to assess population structure and selection efficiency. The Mix population, due to higher genetic heterogeneity and potential heterosis effects, exhibited higher heritability (0.41&plusmn;0.07 to 0.56&plusmn;0.09) and faster response to selection. In contrast, the Molokai population, with a more fixed genetic base and greater phenotypic uniformity, showed lower heritability (0.13&plusmn;0.03 to 0.38&plusmn;0.07) but maintained long-term trait stability. Relative genetic gain for growth traits was higher in the Mix population (4.06% to 11.49%) compared to Molokai (1.17% to 5.58%), highlighting the efficiency of mass selection in genetically diverse populations. Mean female body weight was approximately 23.5% and 23% higher than males in the Molokai and Mix populations, respectively. Strong positive correlations (r up to 0.97) were observed between growth and morphological traits, enabling the development of multivariate selection indices to enhance selection accuracy. Overall, these findings suggest that strategic use of hybrid populations in early breeding stages can achieve faster genetic improvement, while fixed populations play a crucial role in maintaining long-term stability of economically important traits.</span></span> Body weight, Broodstock, Genetic parameters, Mass selection, Morphological traits, Whiteleg shrimp 821 837 http://jifro.ir/browse.php?a_code=A-10-4285-2&slc_lang=en&sid=1 R. Pasandideh Rezapasandideh63@gmail.com 100319475328460059587 100319475328460059587 Yes Iranian Shrimp Research Center, Iranian Fisheries Science Research Institute, Agricultural Research, Education & Extension Organization (AREEO), Bushehr, Iran M.A. Nazari ma.nazari89@gmail.com 100319475328460059588 100319475328460059588 No Iranian Shrimp Research Center, Iranian Fisheries Science Research Institute, Agricultural Research, Education & Extension Organization (AREEO), Bushehr, Iran V. Yeganeh v.yeganeh@gmail.com 100319475328460059589 100319475328460059589 No Iranian Shrimp Research Center, Iranian Fisheries Science Research Institute, Agricultural Research, Education & Extension Organization (AREEO), Bushehr, Iran