The Giant Trevally ( Caranx ignobilis ), also known as the lowly trevally, barrier trevally, giant kingfish, or ulua, is a fascinating and ecologically significant fish. This article delves into the dietary habits of this apex predator, exploring its feeding patterns, nutritional requirements, and the impact of diet on its growth and health.
Giant Trevally: An Apex Predator
The Giant Trevally holds the impressive status of an apex predator in most of the regions it inhabits. This is due to a combination of factors, including its opportunistic hunting strategies and adaptability. The species ranks as the largest in its genus, with rare individuals sometimes measuring up to 5.57 ft (1.7 m) in length and weighing as much as 176 lb (80 kg).
Distribution and Habitat
The Giant Trevally has a comparatively wide distribution range, inhabiting specific sections of both the Pacific and Indian Oceans. This substantial range extends along the coasts of three continents, specifically in tropical and subtropical waters, including hundreds of islands and archipelagos. It thrives in a wide range of both inshore and offshore marine environments, including shallower waters, bays, estuaries, and depths of between 33 and 328 feet. Adults tend to travel in solitude and stay near reefs or other stable ecosystems, while juveniles school closer to inshore environments like estuaries.
Feeding Habits and Diet
The feeding patterns of the Giant Trevally understandably vary from region to region, and its diet does also. They are strong swimmers and fierce predators who hunt most frequently in the early morning hours or late afternoon. As opportunistic feeders, they consume whatever is easiest to obtain. The majority of their prey includes fish, cephalopods, crustaceans, and mollusks. They will also eat the occasional eel, bird, or juvenile turtle. They can forage alone or in groups and are agile hunters, displaying aggressive behavior and biting indiscriminately when they think they have found prey. They are often seen traveling and foraging near sharks, seals, dolphins, and other large ocean predators.
Nutritional Requirements and Formulated Feeds
Trevally (Caranx spp.) is an important commercially aquacultured genus in the family Carangidae. One hundred forty-six trevally species have been recorded worldwide, distributed in tropical, subtropical, and temperate marine waters. They are carnivorous and prey mostly on pelagic fishes, crustaceans, and cephalopods. The giant trevally (Caranx ignobilis) is likely the largest species in this genus and has been commercially cultured in Vietnam because it grows fast, fetches high market prices, and adapts well to sea-cage farming.
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Currently, trash fish is still the main feed for giant trevally in Vietnam, leading to high risks of environmental pollution and pathogen introduction, as well as unsustainability due to an unstable supply. There are no specific formulated feeds for giant trevally because of the lack of information on its nutritional requirements. Thus, nutritional information key for the production of balanced formulated feeds for giant trevally is a first and essential step towards its sustainable farming. Determining the optimum protein requirement is crucial to establish cost-effective and balanced diets for the species.
A balanced protein/energy (P/E) ratio in aquafeeds is a top priority in fish nutritional studies. P/E ratios have been reported for several marine carnivorous fish. However, protein and lipid requirements for the optimal growth of juvenile giant trevally are not well documented.
Effects of Dietary Protein and Lipid Levels on Growth
A study was conducted to assess the effect of protein and lipid levels with different P/E ratios on growth, feed utilization, and body composition for juvenile giant trevally. Six experimental diets were prepared with three dietary protein levels (42, 47, and 52 percent) and two lipid levels (10 and 18 percent), labeled as P42:L10, P42:L18, P47:L10, P47:L18, P52:L10, and P52:L18. Fishmeal was used as the main protein source. The corresponding calculated protein-to-energy (P/E) ratios for the diets were 20.23 grams per MJ, 18.73 g/MJ, 22.31 g/MJ, 20.81 g/MJ, 24.63 g/MJ, and 22.12 g/MJ, respectively.
At all dietary lipid levels, the final body weight (FBW) and specific growth rate (SGR) of juvenile giant trevally improved with increasing dietary protein from 42 percent to 52 percent, as reported for other carnivorous fish that require high levels of adequate quality protein for growth. Fish exhibited the highest growth performance with the P52:L10 diet, thus suggesting that the diet containing 52 percent protein and 10 percent lipid with a P:E ratio of 24.63 g/MJ is the best diet for the growth of juvenile giant trevally.
Increasing lipid levels in the experimental diets, from 10 to 18 percent in the 52 percent protein diets, contributed to a decrease in the growth performance of fish, suggesting that 18 percent lipid is excessive for giant trevally fed 52 percent protein diets. A slight protein sparing for energy occurred for the 42 percent protein diets, where the SGR improved with increasing dietary lipid from 10 to 18 percent. In low-protein diets, increased energy from non-protein sources is likely to spare protein allocation for growth and maintenance. Protein sparing by energy, sourced from lipids, has been reported in several fish species.
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Carnivorous fish can easily digest energy from lipid sources rather than from carbohydrates; however, excess dietary lipids can lead to a decrease in fish growth performance. The results showed that 18 percent was an excessive lipid level for juvenile giant trevally fed the 52 percent protein diets. The excess dietary lipid levels could inhibit the de novo synthesis of fatty acids and decrease their absorption.
The data also showed increased whole-body protein by the increase in dietary protein and lipid levels, indicating high dietary protein improved the protein synthesis in fish tissues, and that high lipid levels provided more efficient utilization of dietary protein for protein synthesis, evidence of the protein-sparing effect of energy.
The whole body lipid value was significantly influenced by the interaction between dietary protein and lipids, as high protein and lipids resulted in high lipid accumulation. Fish fed a 52 percent protein and 18 percent lipid diet had the highest body fat composition. But excessive lipid accumulation can affect the health status of the animals due to the possible link to changes in some enzymes. Higher lipid deposition in the liver can lead to negative effects on the health of fish, resulting in higher levels of oxidative stress.
The different experimental diets affected amino acid levels in the fish: the P52:L10 diet resulted in the highest levels of the amino acids isoleucine, lysine, methionine, and threonine; fish fed the 52 percent protein diets had higher leucine, valine, arginine, alanine, aspartic acid, glutamine, and serine levels than lower dietary protein, whereas the 10 percent lipid diets contributed to increased levels of leucine, valine and arginine, alanine, and glutamine in the whole body.
Perspectives on Optimal Diet
Based on the study results, growth performance in terms of body length, final body weight, and specific growth rate for experimental juvenile giant trevally were the highest when the fish were fed the diet with P52:L10 and P/E of 24.63 g/MJ. The same diet also resulted in the highest effective feed utilization in terms of feed intake and feed conversion ratio.
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The Role of Activated Charcoal in Digestion
Caranx ignobilis is a commercially important marine fish in Indonesia, with culture initiated in Aceh Province. Giant trevally in culture systems are currently fed waste fish and a commercial diet. Activated charcoal is commonly added to the diet to increase digestibility and trigger growth in fish.
A study tested four charcoal sources in the diet to evaluate the morphology of the gut and intestine of giant trevally. The experimental groups were: (A) the experimental diet without charcoal, (B) the experimental diet with 2% charcoal from coconut shell, (C) the experimental diet with 2% charcoal from mangrove wood, (D) the experimental diet with 2% charcoal from rice husk, and (E) the experimental diet with 2% charcoal from kernel palm shell.
Analysis of variance showed that adding charcoal to the diet had significant effects on the length and width of the foveola gastrica and villous intestine. The greatest length and width of the foveola gastrica was recorded in fish fed an experimental diet of rice husk charcoal with average values of 311.811 ± 9.869 µm and 241.786 ± 10.394 µm, respectively. The greatest length of intestinal villous was found in fish fed the mangrove wood charcoal diet, with a value of 135.012 ± 5.147 µm, but this length was not significantly different to that in fish fed rice charcoal and kernel palm shell charcoal.
Optimal development of the alimentary tract was recorded in giant trevally juveniles fed the experimental diet containing rice husk charcoal. This was presumably due to the high hemicellulose, cellulose, and lignin contents in the rice husk charcoal. The microscopic observations showed that the intestinal villi of the fish fed the diet with activated rice husk charcoal had a more pointed shape compared to other treatments, in which the villi tended to be round and blunt. An increase of intestinal villus size is related to nutrient absorption capacity.
The morphology of the intestinal villi of fish fed a diet without activated charcoal was wider and shorter than that of fish fed the diets with activated charcoal. This was probably due to impaired intestinal mucosal integrity, causing interference in nutrient absorption.
Conservation and Threats
Since the species is large and relatively long-lived, it is more sensitive to fishing pressures. The good news is that populations of giant trevally appear to be healthy and stable overall. In populated areas where recreational fishing is common, populations have been depleted, but not to levels that threaten or endanger the species. Future threats to the species include habitat loss, physical and chemical pollutions, and changes to the environment like warming or acidifying waters.