DISPELLING ANTIBIOTIC DRUG USE MYTHS: OXYTETRACYCLINE DIHYDRATE DOES NOT ENHANCE FISH GROWTH PERFORMANCE

Antibiotics were developed for therapeutic purposes in humans and animals, but these products have been used for years in animal agriculture for non-therapeutic purposes, i.e., to promote growth and improve feed efficiency.  There are no FDA-approved prophylactic or production claims for the use of antibiotic drugs in fish, but it is a common misconception that antibiotics are widely used for these purposes in U.S. aquaculture.  Not only are such uses illegal, it is generally understood among fish culturists that such drugs are not effective growth promoters in fish, thereby removing any incentive to illicitly use antibiotics this way.  Unfortunately, there is very little information available that unequivocally demonstrates the effect (or lack thereof) of antibiotic administration on growth performance of fish, whether administered at the therapeutic or subtherapeutic doses.  

Accordingly, we conducted a series of experiments to determine whether administration of an antibiotic in feed with known growth-promoting effects in terrestrial livestock has a similar effect when applied to representative taxa of fish.  Specifically, we evaluated the effect of oxytetracycline dihydrate (OTC) treatment on the health and growth performance of Channel Catfish Ictalurus punctatus, Hybrid Striped Bass Morone chrysops ♀ × M. saxatilis ♂, Nile Tilapia Oreochromis niloticus.  OTC products with FDA-approved production claims in terrestrial livestock are typically applied at doses substantially lower than the approved therapeutic doses for the same products.  Thus, commercially available aquaculture feed appropriate for the selected taxa was used as-is or medicated with OTC to achieve target doses of 80 mg/kg fish/d (a typical therapeutic dose for OTC treatment of fish) or 16 mg/kg fish/d (20% of the therapeutic dose) when fed at 3% body weight/d.  Juvenile Channel Catfish (5.4 ± 0.8 g), Hybrid Striped Bass (27.0 ± 0.2 g), and Nile Tilapia (53.5 ± 0.6) were stocked in a recirculation system (10 fish per tank) comprising 12, 150-L tanks and provided with continuous aeration, mechanical, and biological filtration (trials conducted sequentially).  Dietary treatments were randomly assigned to quadruplicate tanks (N = 4) and offered once daily, with rations adjusted every 2 weeks based on group weights.  

After 8 weeks, fish growth and feed conversion efficiency were unaffected by dietary treatment.  Few significant treatment effects were observed, but those noted suggested inferior performance among fish fed medicated feed.  Collectively, our results indicate there is virtually no incentive to misuse OTC for the purpose of enhancing growth or conversion efficiency in fish.