50 



(1986), should minimize concommitant introductions of parasites and pathogens. Re- 

 quirement of such precautions might be mandated in legislation. 



Concentrations of fish in aquaculture operations create conditions for outbreak of 

 disease. The relative importance of disease transmission from cultured stocks to 

 wild stocks and vice versa is a contentious issue. 



Wastes from aquaculture operations. Major soluble end products in the digestion 

 process are ammonia and orthophosphate. Although ammonia is toxic in the milli- 

 gram per liter range, dilution effects in marine environments keep ammonia toxicity 

 from becoming problematic. However, efiluents from aquaculture facilities into 

 freshwaters or into enclosed bays with limited circulation can pose problems of am- 

 monia toxicity. The nitrogen in ammonia and the phosphorus in orthophosphate can 

 be utilized as nutrients by algae, and large loadings can pose eutrophication prob- 

 lems — such as algal blooms, dissolved oxygen depletion, turbidity, and suppression 

 of rooted aquatic plants — in freshwater systems and in enclosed bays where dilution 

 is insufficient. 



Cage culture of fishes generates large amounts of solid wastes in the form of feces 

 and uneaten feed. About onefourth of the feed eaten ends up as feces, and from 5- 

 20 percent remains unutilized (Getchell 1988). The wastes generally are deposited 

 in the immediate area of the culture site. Accumulation of wastes can alter benthic 

 ecology (i.e., that of bottom-dwelling organisms) by changing the physical and chem- 

 ical environment or by changing or reducing the numbers and species resident be- 

 neath net pens or downstream from effluents (National Research Council 1992). If 

 deposition is sufficient, chemical changes in the sediment will favor opportunistic 

 species that can tolerate low dissolved oxygen levels and higher sulfide and ammo- 

 nia concentrations. Waste tends to accumulate beneath pens in sites of less than 

 15 meters depth and low current velocities (Weston 1986). Models based on depth, 

 current velocity, loading rates, and other factors are now available to select sites 

 where impacts of aquaculture will be minimal (Weston and Go wen 1988). A mecha- 

 nism for assuring careful attention to siting of marine aquaculture operations might 

 be embodied in new legislation. 



Use of feed additives and antifouling agents. Antibiotics may be added to fish 

 feeds to reduce mortality from bacterial fish diseases such as vibriosis and furun- 

 culosis. These antibiotics are used in marine aquaculture as prophylaxis and as 

 therapeutics for disease outbreaks. Concerns about antibiotics stem from three po- 

 tential environmental effects (Whitely and Johnstone 1990): 



• Development of drug-resistant strains of bacteria. Drug resistant bacteria were 

 found in the efTluent of an intensive fish culture pond in Japan (Aoki and Kitao 

 1985). Of further concern, drug resistance was shown to be transferable from a fish 

 pathogen to a human pathogen in vitro (Toranzo et al. 1984). 



• Accumulation of antibiotics in sediments and subsequent inhibition of microbial 

 decomposition. Jacobsen (1989) reported oxytetracyclin in the sediments beneath net 

 pens in Norway. Accumulation of an antibiotic is sediment depends on many factors, 

 including its solubility, half-life, and concentration in seawater. Because the most 

 commonly-used antibiotic, oxytetracycline, is highly soluble and has a short shelf 

 life, release of pharmaceutical compounds from fish farms seems unlikely to pose 

 environmental problems (National Research Council 1992). 



• Accumulation of antibiotics in fish and shellfish. For salmonids given 

 oxytetracyclin, recommended withdrawal times are 60 to 90 days, depending on 

 water temperature (Jacobson 1989). Little information is available on clearance 

 times in non-salmonid farmed fish (National Research Council 1992). 



A fourth, related concern about antibiotics is the possible impact on human con- 

 sumers of antibiotic residues in fish and shellfish. Many of our fisheries products 

 are imported from other countries, where regulation of antibiotic treatments and 

 withdrawal times are not regulated as stringently (National Research (^ouncil 1992). 

 Although cooking destroys most oxytetracline residues in salmonids, little informa- 

 tion is availiable about residues in other fishes. Authorization of research targeted 

 toward broadening our understanding of the implications of feed additives on ap- 

 pearance of drug resistant bacterial strains and on on food safety of aquaculture 

 products would be well justified. 



Anti-fouling agents have been used to retard the growth of organisms on nets or 

 predator control nets of floating net-pen operations. The antifouling agent 

 tributyltin (TBT) has been shown to be exceptionally toxic to shellfish larve 

 (Minchin et al. 1987, Davies et al. 1988). The National Marine fisheries Service has 

 demonstrated that salmon held in TBT-treated nets accumulated residues of the 

 chemical in their tissues (Getchell 1988). The State of Maine has banned the use 

 of TBT in antifoulant paints and dips for marine use, suggesting consideration of 

 a similar ban at the federal level. 



