Investigation of the growth potential and ecosystem impact of intensively farmed Atlantic salmon fed on experimental diets.

Abstract

There are increasing concerns regarding the environmental impacts and sustainability of intensive fish farming. In particular, criticism has centred on the use of fish meal and fish oil in the diets of farmed carnivorous fish species such as Atlantic salmon. If the industry is to continue to expand, reduction in the levels of fish meal and fish oil incorporated into diets and/or the use of alternative terrestrial sources of protein and oil must occur. The present study assesses several diet types containing different inclusion levels and /or sources of protein and oil in terms of growth and performance as well as assessing the diets in terms of sustainability and their potential to impact on the marine environment. In two nutritional studies, Atlantic salmon fed a low protein (LP) diet achieved similar growth and performance compared to fish fed a normal commercial diet (control diet) in both studies. Growth rates of fish fed a diet containing partial replacement of fishmeal with corn gluten and fish oil with rapeseed oil (SUS) were better than those of fish fed the control and LP diet and were similar to those of fish fed a high energy; nutrient dense (ND) diet. The amount of wild fish required to produce 1 Kg farmed salmon based on fish meal and fish oil inclusion levels were lowest for fish fed the SUS diet (1.3 kg) whilst fish fed LP diets had a lower conversion value compared to both the ND and control diets based on fish meal inclusion levels only. These results suggest there is potential for aquaculture to be more environmentally sustainable by reducing the amounts of marine fishmeal and oil used in diets fed to intensively farmed Atlantic salmon. In terms of dissolved wastes, fish fed a nutrient dense diet had higher feedrelated concentration peaks of ammonia detected which occurred earlier compared to fish fed other diet types. In contrast, fish introduced to low protein diets at different sizes throughout the marine phase of production had consistently lower concentration peaks of ammonia detected compared to fish fed a standard commercial ration. All groups fed a low protein diet had between 17 and 28 % less ammonia detected as a feed related concentration peak compared to the control group. The area under each concentration peak of ammonia ranged from 43.6 to 88.8 % ± 60 min of total ammonia detected over time for all diets. These results show that feeding fish diets containing lower inclusion levels of fish meal resulted in lower feed related concentration peaks of ammonia being detected. However most of the ammonia was excreted over a short time period and its potential to impact on the marine environment was assessed using mesocosm studies. In the first of two studies, mesocosms were fertilised with NH4CI based on ammonia concentration peaks from either 500 or 1000 T rainbow trout production (LN and HN enclosures respectively). These enclosures had consistently more phytoplankton present than the control enclosures. There was evidence of rapid uptake of excess nutrients with the HN enclosures having more cells present than the LN enclosures. In the second experiment, enclosures were fertilised based on ammonia concentration peaks detected from Atlantic salmon fed a low protein (LP enclosures) or a nutrient dense diet (ND enclosures). Both had consistently more cells present than the control enclosures.. These results suggest that increases in phytoplankton communities may occur as a result of a single pulsed release of ammonia simulating discharge from intensively farmed fish. The predicted rate of dissolved nitrogen production was calculated from fish introduced to low protein diets throughout a complete marine production phase using a mass balance model. Fish introduced to low protein diets at 330, 800 and 1600 g had lower dissolved N discharge rates (22.51, 22.02 and 21.07 Kg'1 NT'1 Production respectively) compared to fish fed a standard commercial ration (23.32 Kg-1 NT-1 Production). These results show that there is potential to maximise use of low protein diets, which would result in less ammonia excretion and reduce the potential risk to impact on the marine environment. In an attempt to accurately quantify waste outputs from intensively farmed Atlantic salmon a custom-made tarpaulin was designed. Initial studies have shown that there is potential to collect data on waste outputs from fish reared in the marine environment whilst taking in to account seasonal and daily fluctuations in water temperature and salinity. The system can be used to directly compare different feed types and feeding strategies. It has been shown that changing the macronutrient inclusion level and sources results in differences in the physical characteristics of extruded feeds. High energy diets and substitution diets have similar settling velocities compared to a standard commercial diet but produce lower environmental impacts when modelled for solid waste impacts. Low protein pellets have slower sinking rates but generate more waste due to a higher FCR. The data from these results should be used in conjunction with other data for a range of feeds and environmental conditions to employ a “look-up table” approach to differentiate between diets when modelling waste dispersion.