Fish

Brook charr (26.2 ± 0.6 cm, total length, L_T, mean ± SD, N = 10) were caught by means of angling with a barbless hook from a private fish pond (Nashwaak Village, New Brunswick, Canada). Upon transport to the aquaculture facilities at the University of New Brunswick (Fredericton campus, Fredericton, New Brunswick, Canada), they were kept in dechlorinated municipal water for 7–15 days prior to experimentation at 12.2°C (range ± 0.9°C) at a low current speed in the tanks (0–10 cm s⁻¹). Fish were fed a salmonid grower diet (Corey Feed Mills Ltd., Fredericton, NB, Canada) ad libitum. Fish fed and behaved naturally in captivity. The experiments were conducted between July and August, 2008.

Ten fish were used in both U_pref and U_opt trials. For five fish, U_pref was measured first followed by U_opt, and for the remainder, U_opt was determined before transfer to the tilted flume to determine U_pref. After a fish had completed both tests, each fish was anaesthetized in a 1% solution of tertiary amyl alcohol. Body mass and total length L_T were measured, averaging 200 ± 13 g and 26.2 ± 0.6 cm, respectively (mean ± SD).

Preferred swimming speed (U_pref)

U_pref was determined in a Plexiglas raceway with gradually increasing water speed due to a 3.5° inclination (Fig. 1, see also Tudorache et al. ²⁰¹⁰ for details). The raceway was 450 cm long, 12.5 cm wide and 50 cm deep. The 3.5° inclination resulted in an upstream water depth of 5 cm and water speed of 110 cm s⁻¹ and a downstream water depth of 40 cm and speed of 10 cm s⁻¹. Water speed was measured every 10 cm (flow probe HFA-U276, Hoentzsch GmbH, Weiblingen, Germany) and showed a steady decrease towards the downstream end of the raceway. With the exception of the upstream 40 cm of the raceway, the water stream was free of turbulences, according to a dye test, which was performed by dripping a few drops of ink in the water, and the turbulences noted in the upper 40 cm were minimal. The downstream end was fitted with a holding tank, 75 cm long, 50 cm wide and 50 cm deep (Fig. 2). Water was pumped from the downstream tank to the upstream end of the flume by means of a submersible pump (Alita Model PV-800; Aracadia, CA, USA). The total volume of water was 250 l. Water temperature was maintained at 15°C with an in-line chiller (Aqua Logic Model DS-7; San Diego, CA, USA) and dissolved oxygen concentration did not fall below 7.5 mg l⁻¹. Eight cameras (SNG SED-CAM-YC26S, Sharpe Electronics Corp., Osaka, Japan), connected to a digital video recorder (EDVR 16 D3, Everfocus, Taipei, Taiwan, see Tudorache et al. ²⁰¹⁰ for comparison), were mounted ca 1 m below the raceway so they could record images of a section of 70 cm with overlapping fields. A grid on the bottom over the entire length of the raceway facilitated orientation and scaling.

Fish were introduced to the downstream holding tank at the low velocity end of the raceway 16 h before the experiment. Access to the raceway was blocked with a grid. After the acclimation period, the grid was removed and fish locations and behaviour were recorded for the following 4 h. At the end of the experiment, fish were gently removed from the raceway by means of netting (hand net, 30 × 15 × 15 cm) for recovery prior to measuring U_opt or for mass and length measurement.

Video footage in mp4 format (25 fps) was analysed using the program ImageViewer (Everfocus, Taipei, Taiwan). Locations of the centre of mass (determined according to Tudorache et al. ²⁰⁰⁸) of each fish were recorded along the raceway. Swimming speeds (i.e. position holding against water speeds) were categorized in intervals of 5 cm s⁻¹ and indicated as U ± 2.5 cm s⁻¹, so that, for example, all speeds between 7.75 and 12.25 cm s⁻¹ were categorized as 10 cm s⁻¹, between 12.5 and 17.5 as 15 cm s⁻¹, etc. If a fish spent more than 3 s within an interval, then it was defined as being swimming at a ‘steady’ speed. Shorter durations or variable ground speeds were defined as ‘transient’ associated with fishes changing location along the raceway. These observations resulted in time (min) spent swimming at specific current speed intervals.

The choice for 3 s as a threshold between steady and transient swimming behaviour is arbitrary. A different time interval would lead to different proportions of total time spent in steady and transient swimming. However, the distributions of time spent at different speeds would be resulting in similar values for U_pref.

Optimal swimming speed (U_opt)

A 35 l Blazka-type respirometer (as described by Kutty and Saunders ¹⁹⁷³) was used to determine U_opt. The inner tunnel (108 cm length × 12.8 cm diameter) served as a swim chamber in which the fish were contained by means of screens located at either end. Oxygen concentration of the water was measured with a probe (Oxyguard Standard Probe, Oxyguard International A/S, Brikerød, Denmark) that was inserted in the outer tunnel connected to a data collecting system (Oxyguard Multichannel System) and data-logger system (Oxyguard Multilog) connected to a PC. Data were collected with Windows Hyperterminal.

Fish were allowed 2 h acclimation in oxygen-saturated (>10 mg l⁻¹) flow-through water at 10 cm s⁻¹at 15°C after transfer into the inner tunnel. An acclimation of 2 h is sufficient for repeatable performance measures in increasing velocity trials (Farrell et al. ¹⁹⁹⁸; Jain and Farrell ²⁰⁰³). After acclimation, oxygen consumption was measured from changes in dissolved oxygen levels over 20-min period when water throughput was turned off followed a 30-min period flushing period to replace the oxygen used. Oxygen levels never dropped below 7.5 mg l⁻¹. Oxygen consumption was measured at current speeds of 10, 20, 30, 40 and 50 cm s⁻¹ imposed in random order to avoid a possible habituation effect on the results. However, direct observation of the fish in the swimming tunnel revealed an unstable swimming behaviour, characterized by deployment of pectoral fins, irregular position holding and exploring the tunnel, at 10 cm s⁻¹, therefore the [\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{M} $$\end{document}] O₂ data at 10 cm s⁻¹ were not taken into account.

Various models have been used to describe the relationship between metabolic rate and swimming speed: linear, power and exponential (Webb ¹⁹⁹⁷). Equations using various models were explored and that with largest r² was selected as that with the best fit. This was a polynomial function for these data: (r² = 1)

where [\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{M} $$\end{document}] O₂ is oxygen consumption rate (mg g⁻¹ h⁻¹), SMR is standard metabolic rate (mg g⁻¹ h⁻¹) and determined by extrapolating the curve to zero swimming speed (Tudorache et al. ^2007a), U is swimming speed (cm s⁻¹) and a, b and c are constants.

COT was determined by dividing [\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{M} $$\end{document}] O₂ values by the swimming speed. The U_opt (m s⁻¹ and bl s⁻¹), where the COT (mg O₂ kg⁻¹ m⁻¹) reached a minimum (Tucker ¹⁹⁷⁰), was determined by plotting the polynomial trend line through COT values versus swimming speeds per individual fish. The point on this trend line with the lowest COT (COT_min) was calculated by setting the first derivative to zero (Palstra et al. ²⁰⁰⁸).

Statistics

Normality of the data used for analysis was determined by Kolmogorov–Smirnov tests. U_pref data >5% steady swimming time were compared with each other using one-way ANOVA (P < 0.05). U_opt and U_pref were compared with each other using the non-parametric Wilcoxon matched pairs test with a significance level of 0.05. Results are as means ± standard deviation (SD).

Preferred swimming speed (U_pref)

After opening the grid, fish spent about 10 min in the holding tank before entering the raceway. All fish tested entered the raceway eventually spontaneously, without external motivation. Then, the fish spent about 20–30 min exploring the raceway, especially upstream, and below the upstream water entrance where water flow was turbulent.

Fish spent 178.0 ± 12.1 min (mean ± SD; 73.6 ± 4.4% of the total time) at a steady speed and 63.6 ± 10.2 min (26.3 ± 4.4% of the time) transitioning from steady swimming at various locations. Most time was spent at swimming speeds of the speed category of 20 cm s⁻¹ (0.78 ± 0.02 bl s⁻¹) and 25 cm s⁻¹ (0.97 ± 0.02 bl s⁻¹; each representing approximately 25% of the steady speed time, statistically not different, one-way ANOVA, P > 0.05), followed by 15 cm s⁻¹ (10%) and 30 cm s⁻¹ (5%) (Fig. 3). Fish did not swim at other speeds for more than 5% of the time spent in steady swimming; therefore, those data were not considered for analysis.

Optimal swimming speed (U_opt)

After introduction in the respirometer, swimming at speeds of 10 cm s⁻¹, swimming was not steady for the entire period of swimming at 10 cm s⁻¹ during which fish explored the respirometer. Only at a swimming speed of 20 cm s⁻¹ did swimming behaviour become steady.

Figure 4 shows [\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{M} $$\end{document}] O₂ values plotted polynomially against swimming speeds. The best-fit equation revealing [\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{M} $$\end{document}] O₂ to U was the following: [\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{M} $$\end{document}] O₂ = −0.0104U³ + 1.010U² − 18.671U + 258.81 (r² = 1; P < 0.05). COT_min at a U_opt averaged 25.9 ± 8.9 cm s⁻¹ (1.01 ± 0.35 bl s⁻¹) is 0.97 ± 0.06 mg g⁻¹ m⁻¹.

When comparing U_opt with the U_pref categories with the highest time values, i.e., 20 and 25 cm s⁻¹, Wilcoxon matched pairs test revealed a significant difference between U_opt and U_pref of 20 cm s⁻¹ (P < 0.05) but no difference between U_opt and U_pref of the speed category of 25 cm s⁻¹ (P > 0.05).

Perspectives

Rearing aquaculture fish under exercise conditions improves growth and food conversion efficiencies and thus leads to better economic returns (Davison ¹⁹⁹⁷). It enhances muscle growth (Davison and Goldspink ¹⁹⁷⁷; Johnston and Moon ¹⁹⁸⁰; Totland et al. ¹⁹⁸⁷) and flesh quality (Totland et al. ¹⁹⁸⁷; Tsuchimoto et al. ¹⁹⁸⁸), reduces aggressive interactions between fish (Adams et al. ¹⁹⁹⁵; Christiansen and Jobling ¹⁹⁹⁰; Christiansen et al. ¹⁹⁹¹, ¹⁹⁹²; East and Magnan ¹⁹⁸⁷; Jobling et al. ¹⁹⁹³) and leads to a reduction in stress response (Woodward and Smith ¹⁹⁸⁵; Young and Cech ¹⁹⁹³, ¹⁹⁹⁴). It is advisable to keep water speeds applied in aquaculture facilities near U_opt, as U_opt is the swimming speed with the lowest COT (Videler ¹⁹⁹³). However, there is no indication, yet, that fish given the choice between swimming speeds would volitionally chose for swimming velocities near U_opt, the U_pref, a fact that is confirmed by the present study.

Volitional swimming tests have been shown to be a good alternative to forced-swimming tests, due to the effect of the forced character on the results (see Farrell ²⁰⁰⁷ for a review). The present study presents an alternative measurement of U_opt, merely based on behavioural observation. It is less invasive as traditional U_opt tests and based on volitional swimming instead of the traditional forced-swimming tests. This volitional swimming test can be used to compare different species, strains, size classes or ploidies, in order to find perfect rearing conditions for aquaculture fish. Also, it can be argued biomechanically that different muscle types have different optimal swimming speeds for growth (Davison ¹⁹⁹⁷). Therefore, it would be advantageous to present cultured fish with the opportunity of choosing between different water speeds. During the present study, fish in the raceway chose one speed interval at which they remained steady, for the longest period during the observation time of 6 h. However, this is only a short-time window, and it is possible that other speeds would have been chosen, depending on the time of the day, feeding regime, rearing density and other factors typical for an aquaculture environment. Additionally, it has been shown by Weihs (^1973a) that U_opt, measured in a traditional way using forced-swimming tests, depends strongly on physiological and environmental conditions, and in the recent past behavioural, factors are increasingly made responsible for physiological parameters of fish swimming (Farrell ²⁰⁰⁷).

When building aquaculture facilities, a variation of water velocities can be presented for fish to choose. Possibly, a tilted rearing tank can be considered, based on the design of the flume in the present study. Different positions with similar water velocities, including dead zones, turbulent zones and high velocity zones within these tanks can help to reduce agonistic behaviour and lead to a more efficient and less stressful rearing situation for aquaculture fish. The results can be a significant increase in flesh quality and therefore economically interesting.

In conclusion, this study presents a new, volitional-based swimming speed, the preferred swimming speed, which should be explored in more detail in future studies and which can be taken into account for the improvement of aquaculture applications.