A team of researchers from two universities in Malaysia in conjunction with Kentucky State University examined soybean meal as an ingredient to replace fishmeal use in the diets of farmed swimming crab. The group published their results in the journal Aquaculture.
“The aim of this study was to assess the feasibility of replacing fishmeal with SBM on the survival, growth, hemolymph/body biochemical composition as well as hepatopancreatic trypsin and histopathology,” said the researchers.
The team found that adding 20% soybean meal (SBM) to the diet improved crab growth, compared to a completely fishmeal diet, and that higher levels of the plant protein also could be used in the diet, they said.
“Results of the current study demonstrate that dietary SBM can replace fishmeal up to 40% without significantly affecting the growth or development of P. pelagicus juveniles after six consecutive molts compared to the control diet,” said the researchers. “When SBM was increased to 60% of protein in the diets, however, this led to significantly lower growth of the crabs compared to the best treatment of 20% dietary SBM.”
Why soybean meal?
The use of fishmeal in aquaculture diets for crustaceans is considered both uneconomical and unsustainable in practice, said the researchers. There is great interest in developing alternative protein ingredients for feed.
Plant proteins are often considered, and soybean meal is one of the most often examined because of its high protein content and well-balanced amino acid profile, they said. But, SBM is often unable to entirely replace fishmeal without limiting the growth of crustaceans.
In past studies, it also has been found that levels of the alternative protein that can be added to a diet vary by species, they said. “The ability of crustaceans to accept dietary SBM is highly species specific and ranges from 40% for the speckled shrimp Metapenaeus monoceros and 76% for the white shrimp Litopenaeus schmitti to relatively low levels of 22% and 25% for the kuruma shrimp, Marsupenaeus japonics and signal crayfish, Pacifastacus leniusculus,” they added.
The limitations with replacement proteins are often considered to be a lack in essential nutrients like amino acids and anti-nutritional factors (ANF), said the researchers. Large amounts of plant proteins also have been shown to decrease the protease enzyme trypsin in some species of fish and crustacean.
One method to track the nutritive condition of crustaceans is using histopathological analysis of the hepatopancreas and the related glycogen content, they said. “There appears, however, to be a paucity of information on the potential influence of dietary SBM to the hepatopancreatic digestive enzyme activity or condition in crabs,” they added.
SBM has a relatively high digestibility for crabs, despite the chance for problems, said the researchers. Findings from feeding trials with the mud crab and S. paramamosain found that soybean meal-based diets had improved digestibility compared to diets based in either cassava meal or fishmeal.
“These findings indicate that SBM could be a suitable fishmeal alternative to portunid crabs; however, to date, investigations on the feasibility of SBM to the swimming crab Portunus pelagicus have not yet been performed,” they said.
The commercial aquaculture production of P. pelagicus is still being developed, although there is a strong consumer demand, said the researchers. However, feed formulations for the species are underdeveloped, leading to a reliance on trash fish or shrimp feed that this research hopes to alter.
In the study, 180 crabs were given one of six diets for a period lasting from the first juvenile stage through the seventh stage of molt, or 81 days, said the researchers. The diets replaced 0, 20, 40, 60, 80 or 100% of the total dietary protein from fishmeal with soybean meal along with equal parts soybean oil and fish oil.
“The crude protein, crude lipid and cholesterol were generally similar among the diets, although the ash and fiber contents decreased and increased, respectively, with increasing dietary SBM,” they said.
Crabs were checked daily for mortality and a sample of crabs from each group was collected three days after reaching the C7 stage, they said. Also at that point, crabs were measured for final carapace width and weight.
Sampled crabs were assessed for trypsin activity, moisture, crude protein, cholesterol, triglycerides and phosphate, they said. A selection were checked for hematoxylin and eosin or Periodic-acid Schiff.
Crabs had decreasing survival rate with the higher inclusions of SBM in their diet, moving from a 90% survival to 23%, with the 100% SBM diet, said the researchers. No significant differences were found for whole-body moisture, crude protein or cholesterol levels for the different diets.
However, crabs getting a diet with 20% SMB had improved significant growth rate (SGR) compared to crabs getting the 100% fishmeal diet or the diets with 60, 80 or 100% SBM, they said. But, crabs getting diets with no fishmeal had the lowest SGR.
“Results of the current study demonstrate that dietary SBM can replace fishmeal up to 40% without significantly affecting the growth or development of P. pelagicus juveniles after six consecutive molts compared to the control diet,” said the researchers. “When SBM was increased to 60% of protein in the diets, however, this led to significantly lower growth of the crabs compared to the best treatment of 20% dietary SBM. Further increases to dietary SBM at 80 or 100% significantly reduced crab growth as well as decreasing their hepatopancreatic glycogen and structural integrity.”
Crabs getting the 100% SBM diets had slower molt times than those getting the other diets by the end of the experiment, they said. And, hepatopancreate trypsin activity decreased with the inclusion of SBM in the diet.
“The hemolymph cholesterol and triglycerides of the crabs significantly decreased with increasing SBM, which became significantly lower for crabs fed ≥ 40% SBM and ≥ 80% SBM, respectively,” said the researchers. “No significant relationship was detected between dietary SBM and hemolymph phosphate although this was significantly lower for crabs fed the 100% SBM diet compared to the control diet.”
Hepatopancreatic tubules were similar for crabs getting diets with 0, 20 and 40% SBM, but some structural damage was detected for other diets.
Title: Assessing the feasibility of dietary soybean meal replacement for fishmeal to the swimming crab, Portunus pelagicus, juveniles
Authors: Sofea Taher, Nicholas Romano, Aziz Arshad, Mahdi Ebrahimi, Jun Chin Teh, Wing-Keong Ng, Vikas Kumar