The effects of stocking density and low level sustained exercise on the energetic efficiency of rainbow trout (Oncorhynchus mykiss) reared at 19 °C
Highlights
► We examine effects of density on growth and feed conversion in rainbow trout. ► We examine effects of water current on growth and bioenergetics. ► Density negatively affected growth by affecting oxygen uptake and energy metabolism. ► The presence of water current improved energy metabolism. ► Water current may improve welfare by inducing schooling behaviour.
Introduction
Intensive fish farming inevitably involves rearing animals at densities much higher than they would experience under wild conditions. As with other production animals, husbandry practices and their relation to welfare are of major public and scientific concern (cf. reviews by (Ellis et al., 2002, Huntingford et al., 2006, Adams et al., 2007, Ashley, 2007, Turnbull and Kadri, 2007, Volpato et al., 2007)). Earlier studies have provided considerable evidence for negative impacts of high rearing densities on production as well as welfare parameters in rainbow trout. However, the causative mechanisms are unclear, making it difficult to establish threshold guidelines for rearing density. This is reflected in large discrepancies between recommendations for rainbow trout rearing density, which varies from as little as 2 kg m− 3 to 80 kg m− 3 in North America and Europe (Ellis et al., 2002). However, in many places stocking density is also based on water flow rate, i.e. the carrying capacity of the system is taken into account.
In terms of welfare, the negative effects of high rearing density include a higher frequency of fin-bites, fin erosion, abrasion, and gill damage as well as increased stress levels and reduced immune capacity (Ellis et al., 2002). It is clear, however, that to some extent these negative effects of high rearing densities relate to the capacity of the rearing facilities, both in terms of oxygen supply and removal of waste products such as CO2 and ammonia. In the present study, our focus was on parameters related to fish production e.g. mortality, feed intake, feed conversion ratio, and specific growth rate.
Several causative mechanisms have been proposed to explain why high densities negatively affect growth and feed utilization. The biochemical, behavioural, and physiological changes induced by high stress levels are presumed to be energetically costly, affecting the amount of energy available for growth (Barton and Iwama, 1991, Pickering, 1992, Vijayan and Leatherland, 1988). High rearing density in itself may further reduce access to feed, thereby reducing feed intake and directly affecting growth (Alanara and Brannas, 1996, Boujard et al., 2002, Marchand and Boisclair, 1998). An increased metabolic cost of feeding will also indirectly affect the feed conversion ratio because a smaller fraction of the energy obtained from the feed is available for somatic growth.
An increased metabolic rate at high density may also reflect social interactions amongst individuals and behaviour which significantly may influence the energetics of salmonids. Territorial actions dominate at low densities, whilst medium to high densities cause a shift towards dominance-based social hierarchies, with the occasional occurrence of spontaneous schooling activity (Adams et al., 1995, Alanara and Brannas, 1996, Christiansen et al., 1992, Jorgensen et al., 1993, Linner and Brannas, 2001, Mccarthy et al., 1992). The prevailing behaviour at a given density is species specific and is also moderated by the presence of water current. Water current may have several beneficial effects, perhaps most importantly by encouraging schooling behaviour and thus reducing aggressive behaviour and stress. This has been extensively studied in Arctic charr (Adams et al., 1995, Brannas, 2009, Christiansen and Jobling, 1990, Christiansen et al., 1992), but is presumed applicable to many species (Davison, 1997). In general, although depending on the degree of severity, exercise is considered beneficial to a number of physiological processes including growth and feed conversion (Davison, 1997, Jobling et al., 1993). The presence of a water current has also been shown to improve feed distribution (Jobling et al., 1993, Jorgensen et al., 1996) and facilitate the recovery from stress (Milligan et al., 2000, Veiseth et al., 2006). Overall, these studies could suggest that the negative impact of high stocking density could be alleviated to some extent by the presence of water current.
In the present study, a temperature of 19 °C was chosen. Although this is above the thermal optimum for rainbow trout (~ 15 °C) it is nevertheless relevant, especially for trout farms using recirculation technology and minimal water intake. As a consequence of increased hydraulic retention times, temperatures are likely to increase, particularly during warm summer months. Danish model trout farms have reported peak temperatures during summer months as high as 24 °C (Skov et al., 2011). Once temperatures exceed the thermal optimum, oxygen consumption rapidly increases and swimming performance and metabolic scope are negatively affected (Brett, 1971, Farrell, 2002, Farrell, 2007, Jain and Farrell, 2003, Randall and Brauner, 1991, Taylor et al., 1996). A reduction in metabolic scope means relatively less energy available for feeding and growth, i.e. feed intake may decrease and feed conversion ratio increase, and hence negatively affect the economy for fish farmers.
The purpose of the present study was to investigate the effects of rearing density at 19 °C on specific growth rate (SGR) and feed conversion ratio (FCR) as well as their interrelationship to exercise. By use of a unique experimental facility for instantaneous measurements of oxygen uptake, effects of rearing condition on metabolic rate could be measured on an hourly basis, throughout the entire growth period. The simultaneous registration of feed intake and metabolic expenditure, allowed us to calculate the relative energetic efficiency of fish reared under the different experimental conditions. The aim was to provide some basic data to explain any observed effects of rearing density and water current on growth and feed conversion ratio. Two rearing densities were used; a low (~ 25 kg m− 3) and a high (~ 100 kg m− 3), in combination with either static water or in a current corresponding to a swimming speed of 0.9 body lengths s− 1.
Section snippets
Experimental animals and holding condition
All-female rainbow trout, with an average body mass of ~ 100 grammes and approximately 8 months old, were obtained from a Danish trout farm (Fousing Dambrug, Pilgårdsvej 6, Ølby, DK — 7600 Struer, Denmark), and transported to the aquaculture facilities of the Technical University of Denmark, at the North Sea Research Centre. The fish were initially kept in large (3000 L) circular outdoor tanks for a 3 week quarantine period (15 ppt saltwater, 10–12 °C, water renewal rate ~ 15 L min− 1), to allow for
Mortality
Mortality during the growth trial was low for all experimental groups, although significantly higher (p < 0.001) in high density groups. Average mortality (% ± SE) was 0.57 ± 0.15 and 1.76 ± 0.15 in low and high density groups, respectively. Water current had no significant effect on mortality.
Rearing density
The average densities at the beginning of each growth period and subsequent biomass increases are given in Table 1. The initial density for each three-week growth period was 19 and 75 kg m− 3 in low and high density
Discussion
In the present study at 19 °C, the presence of a gentle current had clear beneficial effects, reducing RMR and promoting growth. The overall effects of rearing density on SGR and feed conversion ratio at 19 °C are in agreement with a previous study at 14 °C performed in the same rearing system (McKenzie et al. submitted) as well as earlier findings (Ellis et al., 2002).
Conclusion
The present study showed that high rearing density resulted in significantly lower specific growth rate and increased feed conversion ratio in rainbow trout reared at 19 °C. Although, the presence of a water current caused only small non-significant improvement, during the entire 9-week growth trial, it did have positive effects on energetic budgets. These were calculated for periods of similar mass gain, using feed/energy intake and hourly measurement of oxygen uptake, and showed that whilst
Acknowledgements
This study was funded by the Danish Ministry of Food, Agriculture and Fisheries under grant number FFS05-7, Strategies to improve health and welfare in rainbow trout farming.
BioMar A/S kindly provided the feed used in the experiment.
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