With the rapid global expansion and
increased production of aquaculture,
increases in aquatic feed production are
challenged by the availability of traditional
ingredients such as fishmeal and fish oil,
and environmental sustainability. Therefore,
alternative ingredients are being
explored to replace traditional ingredients
to meet the demands of the fast-growing
feed industry. For example, utilization of by-products from different industries in
aquatic feeds is becoming attractive.
Beyond the nutritional composition of an ingredient and its effect on palatability, digestibility is often a concern in aquatic feeds. In a study funded through a grant from the U.S. Department of Agriculture Agricultural Research Service and a cooperative agreement with the University of Alaska Fairbanks, the authors determined the digestibility of six fisheries by-products in shrimp feed.
Alaska has the largest number of fisheries
in any United States state. Its annual
fisheries production totals 1.84 mmt, and
processing generates significant amounts
of by-products. Previous research by the
authors demonstrated that some fisheries
by-products contained significant
amounts of nutrients and exhibited a
stimulating effect on shrimp fed plant
For this digestibility study, six Alaskan fisheries by-products from processing plants in Kodiak, Alaska, were supplied by the University of Alaskas Fishery Industrial Technology Center (Table 1). A reference diet containing 40.0% protein and 9.0% lipid was formulated with 34.2% menhaden fishmeal, 32.7% whole wheat, 12.5% soybean meal, 6.0% vital wheat gluten, 5.0% brewers yeast, 2.5% squid meal, 2.0% soy lecithin, 1.6% menhaden oil, 1.0% chromic oxide and 4.5% other ingredients, including vitamins and minerals. Chromic oxide was used as a marker to estimate digestibility.
The test diets were formulated by replacing the reference diet with 30% byproduct. The diets were milled to a pellet size of 2.4 x 4 mm.
The digestibility trial was conducted in an indoor system with flow-through water and a photoperiod with 12 hours light and 12 hours dark. Four replications were used for each dietary treatment. In trial 1, 6-g shrimp were stocked at 100/tank, and in trial 2, 14-g shrimp with 75 shrimp/tank were stocked. Shrimp were fed 10% of body weight for two hours before fecal samples were collected. Water quality was monitored during the trials, with temperature at 26.5 0.2 C, salinity at 31.0 0.3 ppt, dissolved oxygen at 6.0 0.3 mg/L, pH at 7.8 0.1 and total ammonia nitrogen below 0.08 mg/L.
Proximate composition analysis of the
tested by-products showed that the
salmon livers and milt meal had higher
protein levels than menhaden fishmeal
(Table 1). The rest of the by-products
had lower protein levels than the fishmeal,
but still contained significant levels
of crude protein ranging from 35 to 42%.
The crude protein level for the black cod viscera could be increased from 42 to 50% if moisture could be removed from the product. Some by-products, such as the arrowtooth heads and viscera, and the cod viscera, were found to be rich lipid sources. The by-products from crab carapaces and viscera, and pollock bones had very high ash content. All the by-products except pollock bones and crab carapaces/ viscera contained higher gross energy than the fishmeal.
The apparent digestibility coefficients
(ADCs) of the test diets showed that the
diets containing salmon livers or milt had
the same digestibility as the reference diet,
except that the lipid ADC was lower than
for the reference diet (Table 2). The
ADCs of test diets containing pollock
bones or the crab carapaces and viscera
were significantly lower than the ADC for
the reference diet. The ADCs of nutrients
for the diets with arrowtooth heads and
viscera or black cod viscera were similar to
the ADC of the reference diet.
Among all by-products, the ADC for crude protein was the highest for the salmon milt, followed by those for the black cod viscera and the arrowtooth heads and viscera, which had values similar to that for the salmon liver (P > 0.05) (Figure 1). The pollock bone and crab carapace and viscera treatments exhibited significantly lower ADC values for crude protein than the other by-products did.
The ADC values for crude lipid were significantly lower for the pollock bones and salmon livers than the other by-products (Figure 2). Although not presented, the ADC values for gross energy were lower for the pollock bone and crab carapace/ viscera treatments than those for the remaining by-products used in the test.
Apparent Digestibility Coefficents (%)
The study demonstrated that byproducts
from salmon livers and milt,
black cod viscera, and arrowtooth heads
and viscera were easily digested by Pacific
white shrimp. These by-products are also
rich in protein and/or lipids.
Previous studies have shown that supplementation of these by-products in shrimp feed stimulated feeding in shrimp fed a plant protein-based diet. Therefore, based on evaluation of the proximate compositions of the by-products and their effects on palatability and digestibility in shrimp, the by-products can be considered good candidates as ingredients or additives for shrimp feed. Investigation of their effects on the growth performance of shrimp will be needed to further support this conclusion.
The low digestibility of pollock bones and crab carapaces and viscera could be due to the high level of ash in these byproducts. The authors previous studies also showed that these by-products had no stimulating effect on shrimp feeding. Therefore, under current conditions, the by-products are not good candidates for ingredients in shrimp feed.
Apparent Digestibility Coefficents (%)