Vertebral deformities remain a significant challenge for pikeperch ( Sander lucioperca ) aquaculture. Their occurrence in fingerlings further reduces the already small number of fish suitable for grow-out, limiting pikeperch aquaculture overall. Mechanical stress has proven to influence deformation rates, and recently, the overlap of grading procedures with crucial developmental steps of spine formation has been described. In this context, this study investigates the occurrence of vertebral deformities during the times of grading in pikeperch early life stages. Sampling followed the first three manual grading steps and included sampling of different size groups resulting from the sorting steps. Using morphometric comparisons as well as morphological analyses of cleared and stained specimens, small and large groups were compared. Morphometric data confirmed the grading to result in distinct size groups; however, cannibalism could not entirely be removed. Differences between the maximum body widths and applied grid spacings suggest that mechanical pressure was exerted during sorting. The morphological examinations revealed an increase in vertebral compressions and fusions over time. Notably, transversally deformed vertebral/notochord areas were observed in the first samplings, indicating stress on the developmental precursors from an early point onwards which potentially leads to long-term effects and spine deformities. Later increases in deformation rates were timed with the progressing ossification in these stages. The gained insights from this farm example demonstrate the potential of grading procedure adaptations which can aid in reducing spinal deformation occurrences. Exemplary protocol adaptations could enhance pikeperch larviculture, improve animal welfare, and increase the economic efficiency.

Reliable fish cell models are essential for studying cellular processes under controlled in vitro conditions and thereby reducing the need for animal experiments. In this study, we established and characterized OMYlar5, a novel larval cell line derived from rainbow trout (Oncorhynchus mykiss). OMYlar5 demonstrated sustained proliferative capacity over at least 60 passages, indicating spontaneous immortalization and its suitability for long-term applications in fish biology and aquaculture research. To investigate temperature-dependent cellular responses, cells were cultured at 18, 20, and 22 °C, and changes in growth, morphology, cytoskeletal organization, stress responses, and gene expression were analyzed. Increased temperature enhanced cell proliferation, whereas the expression of the proliferation-associated genes pcna and c-myc remained unchanged. Temperature-dependent alterations in cell morphology and F-actin organization were observed, with enhanced actin bundle formation at 22 °C. However, no significant induction of Hsp70 or Hsp90 was detected at the transcript or protein level, suggesting that the tested temperature range was not associated with detectable induction of these heat shock proteins. Furthermore, OMYlar5 cells expressed nanog, nes, pax3, and sox9, whereas pou5f1 and sox2 were expressed at very low levels. These expression patterns provide information on the molecular characteristics of the cell line. Overall, OMYlar5 may provide a useful in vitro model for investigating temperature responses, cellular physiology, and developmental processes in rainbow trout.