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Simple Summary: Due to the underdeveloped digestive tracts of larval fish, predigested fish protein has been included in feeds to help increase dietary protein utilization. The predigested protein has been produced from various protein sources and broken down using different methods. This has ultimately led to variable results when included in larval diets, and there is still a lack of an optimal dry diet for larval fish. This study tested the use of adult Walleye muscle as a protein source to provide the optimal dietary amino acid composition for same-species larvae. The muscle was also broken down with enzymes from the adult Walleye gut to predigest protein in a way that mimics the digestion of live Walleye. This protein source was tested against protein produced from muscle and/or enzymes from Nile Tilapia, as a non-species-specific source. The dietary inclusion of predigested protein produced from same-species muscle and digestive enzymes led to increased growth and amino acid absorption in larval Walleye. However, reduced survival in fish fed with the predigested protein suggests that further research is required to improve the pellet stability of the larval diets and reduce the loss of nutrients to the water. Fish protein hydrolysates used in larval diets have been prepared from a variety of fish species, with different enzymes used to hydrolyze the protein. This study's objectives were to determine the effect of the dietary inclusion of fish muscle hydrolysates obtained from species-specific muscle/enzymes—versus hydrolysates produced from muscle/enzymes of a different species—on the growth performance, survival, skeletal development, intestinal peptide uptake, and muscle-free amino acid (FAA) composition of larval Walleye (Sander vitreus). Eight protein products were obtained for this study, comprising an unhydrolyzed and hydrolyzed product from each combination of muscle/enzymes from Walleye and Nile tilapia (Oreochromis niloticus). Four diets were produced, and the dietary protein was provided in a 50/50 ratio of unhydrolyzed and hydrolyzed protein from the respective muscle/enzyme combination. Four groups were fed one of the corresponding formulated diets, and two groups of larvae, fed a commercial starter diet and Artemia, respectively, served as reference groups. Larval Walleye fed the diet containing protein produced with the species-specific muscle and enzymes had a significantly higher weight after the study—30% higher than any other group. A significant interaction effect between muscle and enzyme sources on the growth of Walleye larvae was observed. The species-specific combination also led to a significant increase in postprandial FAA and indispensable amino acid concentrations in muscle. No significant differences were observed between the hydrolysate-fed groups in survival, deformity occurrence, or peptide uptake. Each hydrolysate-based diet significantly reduced skeletal deformities and survival compared to the commercial diet. The results of this study suggest that species-specific muscles and enzymes produce a more optimal dietary protein source for larval fish than non-species-specific products. Further research should focus on improving the physical properties of the formulated diets to reduce possible leaching of hydrolyzed protein and improve the survival of fish larvae. [ABSTRACT FROM AUTHOR]

Objective: Corn‐fermented protein products (CFP) from the bioethanol industry have strong potential as alternatives to traditional plant protein sources in finfish diets. This study examined the culture performance, apparent digestibility, intestinal histology, and expression of gut inflammation markers in juvenile Rainbow Trout Oncorhynchus mykiss fed experimental diets with graded levels of soybean meal and CFP. Methods: Four experimental diets were formulated to contain 40% protein and 20% lipids, and the diets incrementally replaced soybean meal with CFP (0% [Control], 25%, 50%, or 100% replacement). Juvenile Rainbow Trout (48.9 ± 1.3 g) were randomly distributed to 12 100‐L tanks within a recirculating aquaculture system (30 fish/tank). Fish were fed one of the four experimental diets on a restricted feeding regime (1–3% biomass/day), and tanks were weighed every 14 days for 70 days. Result: The mean weight, specific growth rate, feed conversion ratio, survival, and proximate composition of fish were not significantly different (α = 0.05) between the diet groups after 70 days of feeding (139.6 ± 7.5 g final weight). Apparent protein digestibility was significantly higher in the 50% CFP diet group than the Control group (80.3 ± 1.2% and 77.6 ± 1.2%, respectively). Histological analysis of distal intestines showed significantly larger villi lengths and villi length‐to‐width ratios in fish fed the 100% CFP diet, relative to fish fed the Control and 25% CFP diets. The expression of gut inflammation markers tumor necrosis factor alpha (TNF‐α), interleukin (IL)10, and matrix metalloproteinase‐9 (MMP9) trended lower in the 50% and 100% CFP diet groups compared to the Control group. The expression of the nutrient absorption markers PepT1 and FABP2 trended higher in the 50% and 100% CFP diet groups relative to the Control group. Conclusion: The results of this study suggested that the replacement of soybean meal with CFP in Rainbow Trout grow‐out diets did not impair growth performance and improved overall protein digestibility and gut health. Impact statementThis study evaluated the use of a value‐added corn ethanol by‐product as a protein and fat source in Rainbow Trout feeds. This new ingredient could be more affordable than traditional plant protein and fat sources in trout feeds and would not divert a potential human food crop (soybean) towards animal feeds. The study found that replacing soybean‐based protein and fat sources in trout feeds with the value‐added corn ethanol by‐product did not affect fish growth rates but did improve gut health and protein digestibility. These results support the use of this new ingredient in trout feeds and the study offers a good launch point for future research on the topic. [ABSTRACT FROM AUTHOR]

Gender identification of fish species is carried out mainly by examining external morphological characteristics, which in general, it is very complex and not always a reliable approach. Electrophoresis of plasma proteins can be used as an alternative and useful molecular tool for a more precise sex determination. The presence of female specific proteins in the plasma is a starting point for the application of this technique. In this study, reducing discontinuous sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was applied to analyze plasma proteins of male and female koi carp (Cyprinus carpio haematopterus). Image analyses of electrophoregrams with resolved plasma proteins by SDS-PAGE showed that it is an appropriate technique to discriminate male from female samples. It is based on the presence of apolipoprotein B-100 which can be used as a suitable marker. Further amino acid characterization of apolipoprotein B-100 confirmed that it is a specific protein for female individuals.