International Aquafeed Article: Transfering vitamin C from fish to embryos

The May/June 2012 issue of International Aquafeed magazine is out now. It's crammed full of news and features from the aquaculture world.

If you don't have a print copy, read the full magazine online here (our Flash document does not run on iPhones or iPads)

One of the most interesting articles from this issue is 'Transfering vitamin C from fish to embryos' by Nagaraj G. Chatakondi, PhD, National Warmwater Aquaculture Center, USA

The full text follows but you can also view the article as it appears in the magazine (with pictures and tables) here (pdf format, runs on iPhones or iPads)

Transfering vitamin C from fish to embryos

by Nagaraj G. Chatakondi, PhD, National Warmwater Aquaculture Center, USA

Beneficial effects of ascorbic acid supplementation to broodstock of a select aquaculture species is well documented. At the present levels of feeding, dietary means of vitamin C does not meet the requirements for maturation, reproduction and needs of early life stages of larvae. 

In addition, this nutrient is water soluble and readily gets accumulated by other organs before reaching the ovary.  For practical reasons, it is not possible to attain the desired level of a nutrient by conventional methods, hence innovative approaches are needed.   Mass transfer of nutrients via injection into broodstock is a novel method.  

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Two routes of maternal transfer of vitamin C in mature channel catfish (Ictalurus punctatus) prior to hormone-induced spawning were explored as a strategy to incorporate the vitamin and to determine its effect on reproduction and progeny performance.  

The results of this study suggest injecting vitamin C prior to hormone-induce spawning, invokes transfer to eggs, improves reproductive performance, and may subsequently improve ontogeny performance.  

However, the effect of vitamin C diminished with age and also in more natural conditions.  Our goal was to achieve predictable fish production of robust quality for healthy, efficient, higher surviving and able to adapt to common stressors and pathogens.  

Improvements can be made in this area by new knowledge-based advances in nutrient delivery systems that may create large improvements in terms of production, feed conversion and survival in aquaculture production.

Importance of vitamin C

Ascorbic acid is an essential micronutrient in the diet of teleost fish, which do not have gulonolactone oxidase activity. Vitamin C is needed for post-translatory hydroxylation of proline and lysine moieties in collagen, mineral metabolism to improve stress response and immunity, detoxification reactions, steroid synthesis and vitellogenesis. 

Egg ascorbic acid deposition levels may easily be tailored by feeding broodfish with elevated levels of ascorbic acid before and after vitellogenesis. The accumulation of essential nutrients in eggs is dependent on the nutrient reserves in the female fish and therefore on the dietary intake of broodfish in the period preceeding and during gametogenesis. Hence, broodfish nutrition consisting of essential nutrients is important.  

The earliest steps in embryonic development are dependent on and driven by maternal factors deposited in the oocyte during oogenesis.  Maternal factors are stored in the form of specific mRNAs, proteins, hormones and other biomolecules.  At egg activation and fertilization, these factors become available for embryogenesis, sometimes after a process of activation involving translation or protein modification.  

It has been documented that vitamin C or ascorbic acid deficiency in larval fish has been associated with hyperplasia of collagen and cartilage, scoliosis, lordosis, internal hemorrhages, resorbed opercules and abnormal support cartilage in gills, spine and fins with deformities of the jaw and snout.  

Based on recent research, vitamin C needs for reproduction and early life stages of fish are 10 times the recommended dose for raising young adult fish. These high levels cannot be met by dietary administration to broodfish because the nutrient is water soluble and readily absorbed / utilized by other organs during oocyte development.

It has been demonstrated in several species that nutrients in broodfish diet are transferred to oocytes through uptake of extra-ovarian substances from the maternal blood. Also, there was up to a 82.4 per cent loss of ascorbic acid of the prepared commercial diet. A 3.8 – 8.3-fold increase of vitamin C in the diet generally results in 56 to 71.9 per cent increase of total ascorbic acid in the eggs respectively. The fry produced from parents fed with elevated levels of vitamin C tend to have higher growth performance as compared with control groups.   Thus, there is a need in enhance ascorbic acid in the  broodfish. A diet with vitamin C content adequate for normal growth may not be sufficient for broodfish when the goal is to transfer ascorbic acids to embryos.

Reproduction and arval performance

Broodfish diet has a major influence on fecundity and egg quality. It has also been demonstrated that the nutritional status of broodfish can affect offspring quality. The accumulation of essential nutrients in eggs is dependent on 1) the nutrient reserves in the female fish and 2) the dietary intake preceding gonadogenesis. Vitamin C is needed for maturation, reproduction and larval metamorphosis. Beneficial effects include increased fertility, fecundity and egg quality.  

Nutrients in broodfish diet are transferred to oocytes through uptake of extra-ovarian substances from the maternal blood. Immersion enrichment of eggs is another approach to introduce compounds and nutrients into eggs. Immersion enrichment followed by feeding fry with vitamin C enhanced feed was also found to be an effective method. 

Injecting vitamin C in to broodfish during artificial- induced maturation improved reproduction and progeny performance. Efforts are underway to develop procedures to effectively and stably accumulate vitamin C in eggs by broodstock injections (Table 1).

Vitamin C needs of channel catfish  

Channel catfish is the single largest aquaculture fish species cultured in the United States.  Based on 2011 data, approximately 335 million pounds of catfish were processed, a reduction of over 50 per cent compared to the best production in 2003 (2012 USDA NASS). The industry is currently struggling to keep pace with the increasing cost of feed, fuel, production inefficiencies, foreign imports and economy. Adopting hybrid catfish (channel catfish female x blue catfish, I. furcatus male), hybridisation can be used to improve productivity immediately by producing fish that exhibit hybrid vigor.  

Based on numerous laboratory and field trials, hybrid catfish are superior in growth rate, feed conversion, survival, seinability and processing traits compared to commonly raised channel catfish.  

A decade ago, producing commercial quantities of hybrid catfish was believed to be unattainable. Natural hybridisation is rare and artificial spawning of channel catfish has been historically low and with no effective ovulating agents available.  

However, in the last 10 years, consistent and marked improvements were made in all the phases of artificial spawning and the hatchery production of hybrid catfish embryos. Improved production and consistent superior performance of hybrid catfish in commercial earthen production ponds has rejuvenated the industry with unprecedented optimism. 

Presently, a third of catfish farmers raise hybrid catfish in production ponds and hybrid catfish account for approximately 25 per cent of all the total catfish processed in 2011.

Our goal is to achieve predictable fish production of robust quality for healthy, fast growing, survival and adapt to common stressors and pathogens and to varying environmental conditions. Improvements can be made in this area by new knowledge-based advances in live food production or nutrient delivery systems that may create large improvements in terms of production, survival and processing yield.

Preliminary findings

Broodstock preparation is the primary requisite for hormone-induced spawning of channel catfish in the production of channel x blue hybrid catfish. Hence, broodfish management techniques must be geared towards attaining maximum production of high-quality eggs and larvae because variable egg quality is one of the limiting factors in fish hatcheries. 

Broodstock diet has been considered as one of the factors affecting fecundity, egg, and larval quality in fish. The accumulation of essential nutrients in eggs are dependent on the nutrient reserves in the female fish, and consequently on the dietary nutrient input of broodstock in the period preceding gonadogenesis. When eggs absorb water, it is possible to introduce compounds such as vitamins and minerals into the eggs with the water solution before water hardening.   

It was hypothesised that injecting female broodfish prior to hormone-induced spawning would result in mass transfer of nutrients to improve maturation, ovulation, and subsequent progeny performance. Preliminary studies confirmed accumulation of vitamin C in ovarian tissue and invoked a positive response to ovulation, fecundity and egg quality. Mass transfer of vitamin C to the eggs improved growth and reduced  mortalities following Edwardseilla ictaluri disease challenge. 

It appears that mass transfer of vitamin C to eggs is attained by injecting broodfish prior to hormone-induced spawning to improve progeny performance.