The expanding human population requires ever more increasing sources of quality food. The importance of adequate nutrition is a factor predicting future preferred diet composition, and the role of fish is likely to increase in human diets. The ongoing over-exploitation of natural fish resources has led to expanded development of marine and freshwater aquaculture in recent decades. Aquaculture is currently the fastest growing food-production sector, accounting about 50% of the world's food fish.
Economic, environmental, and zoo hygienic concerns have led to increased efforts to maximize water reuse and waste management and development of various types of Recirculating Aquaculture Systems (RAS). One of the most important freshwater species reared in RAS is rainbow trout Oncorhynchus mykiss, having high culinary and nutritional value obtainable by plant-based diets. Annual worldwide production of rainbow trout reached more than 770 thousand tons in 2011, approximately 40% more than in 2003, with the prospect of further growth.
The expanding use of RAS for salmonid production has led to increased demand for quality fingerlings to stock such facilities. RAS, as a closed intensive system, requires pathogen-free fingerlings of stable quality several times per year, and the need for new hatchery facilities can be assumed. The potential detrimental effects of fish farms on the environment, especially with usual hatchery placement in upper stretches of streams, may be mitigated by RAS systems for hatching and rearing fingerlings, which can reduce fresh water demands as well as the amount of effluent water.
A facility for fingerling production in conjunction with the RAS will prevent disease transfer and ensure quality and continuous production. Maintaining brood stock requires time, space, and specialized care at high costs, and purchase of certified disease free eyed eggs for hatching and rearing of fingerlings every 2-3 months is an economical alternative.
The production approach must comply with safety, environmental, and economic criteria. We developed a simple recirculating hatchery with low initial construction costs and low demands for energy and fresh water. The main goals of the study were to evaluate the usability, effectiveness, sustainability environmental safety, and potential negative/positive impacts of an RAS hatchery and to calculate the overall annual production and the potential profitability. The particular objectives were to evaluate fish performance and challenges of RAS hatchery function and to monitor water quality during rearing.
Physical and chemical conditions
Oxygen saturation during the monitored period was consistently > 85% and > 75% saturation at system inlet and outlet, respectively. Water temperature ranged from 9.1-13.9°C throughout the study with a maximum day change of 0.4°C. pH values were 7.1-7.7 throughout the study. The biofilter function was adequate for fish biomass and the amount of daily feed, with almost optimal ammonia and nitrite content and proportionally increasing nitrate content. Only in the final phase of the production cycle, when biomass in the RHS reached maximum values of 170-200 kg, were higher values of ammonia (to 2.3 mg l-1 NH4 +) and nitrite (to 4.9 mg l-1 NO2 ¯) observed.
However, due to near neutral pH and chloride content of approximately 100 mg l-1, the ammonia and nitrite levels did not affect fish health. The level of organic compounds in water was low during the production cycle, reaching maximum BOD and CODMn values of 2.5 and 4.1 mg O2 l-1, respectively. The acidneutralizing capacity was optimal at 1-2 mmol l-1. Detailed data from monitoring of collected water samples are shown in Table 1.
Production cycle, fish growth and FCR
The production period was consistently shorter than 3 months, with the hatching period accounting for 20% and each of the further two periods about 40% of the time. The highest losses were observed during the initial feeding period and included mortalities as well as serious body malformations (Table 2). During each production cycle, high growth and low FCR was observed in both the initial feeding and the rearing period. FCR was lower in the initial feeding period (Table 3).
Total yield per year
From 2011 through 2013, 11 controlled production cycles produced 694,000 fingerlings (1426 kg). At least 4 production cycles were possible annually including time for cleaning and preparation of both RHS systems. The parallel use of the systems enables completion of at least 5 cycles per year (Table 4): Incubation of eyed ova, hatching, and the initial feeding period can be carried out concurrently during the final 20-25 days of the rearing period of the previous production cycle.
Discussion
The present study evaluated a recirculating hatchery system developed for supplying a trout farm and other small local farms. The development was focused on simplicity and effectiveness of production units and economic sustainability in the view of current and future requirements for environmental sustainability. The RHS was developed as a simple facility without special structures or technologies, potentially enabling its use for large producers as well as smaller operations.
Evaluation of the RHS assessed usability and effectiveness (fish performance, possibilities and challenges in RHS function, and water quality), environmental sustainability (environmental safety, potential negative/positive impacts), and economic sustainability (production per year and the potential profitability).
The basic parameter affecting the usability and effectiveness of recirculating aquaculture systems is the fluctuation in nitrogen compound levels relative to fish biomass and the amount of feed supplied [27,28]. This parameter must be associated with a satisfactory feed conversion ratio and fish welfare [29,30]. The data obtained from water quality monitoring confirmed adequate biofilter function indicated by increase in non-toxic nitrates, low levels of ammonia and nitrite, stable pH, and low level of organic load (Table 1).
Manual feeding at short intervals and pathogen free fresh water intake provided rearing conditions with no negative impact on fish health and welfare. Conditions resulted in rapid growth and low FCR, not only with a daily feed ration of approximately 5% of fish biomass, but during the initial feeding period when feed was supplied in excess.
The positive results obtained in a simple recirculating system call into question the reported need to use additional technologies for hatcheries such as UV treatment, ozone application, microsieve filtration, oxygenation, and aeration. The RHS was fully functional without such treatments, thus is more efficient with respect to initial investment as well as operating costs. Nevertheless, potential use of additional technologies in enhance the RHS should be discussed. Added aeration and oxygenation may increase the capacity of RHS, but benefits such as increased production and increased biofiltration rate are cancelled out by increased operating costs. Oxygen level and biofiltration efficacy were found to be sufficient in the current study.
The same situation arises when the benefits and cost are compared for technologies, such as ozone treatment and UV irradiation . Elimination of these treatments requires adherence to strict zoohygiene principles, including purchase of eyed ova only from certified diseasefree providers. A possible option could be the addition of filtration, such as a simple microsieve filter or incorporation of small constructed wetland to the system to enable increase in the daily amount of feed without the need of extra power.
The RHS, as a simple unit for fingerling production can provide small farms with overall annual production of 150-300 tons with the possibility to sell a substantial portion of fingerling production to other farmers, with minimal fresh water demand and low operating costs. The use of a RHS may allow small producers to be independent of fingerling suppliers with effective, zoohygienic, and environmentally safe production.
Compared to traditional salmonid hatcheries, often located on upper stretches of streams with possible negative impact on local oligotrophic ecosystems , RHSs can be established essentially anywhere, because of low fresh water demand. In recirculating hatcheries, the amount of waste water is substantially lower and more concentrated, which allows its collection, sedimentation, and potential use as fertilizer or its treatment through an constructed wetland to prevent the trophic changes to ecosystems. The low energy demand has environmental as well as economic benefits.
Conclusion
The RHS could be a safe means of fingerling production throughout the year with positive economic and environmental benefits. The possibility of incorporating additional technologies, the effects of economic and environmental issues, as well as the use of different fish in RHS should be targets of future research.
December 2014
