Supporting information S1.
Introduction
Swimming respirometry was performed on Nile tilapia Oreochromis niloticus Peters
1833, to calculate their net cost of transport (Cn) at 30 °C. In particular, experiments were
performed using the same swim tunnel respirometer used to measure swimming respirometry
of banded knifefish Gymnotus carapo L. 1766 at the same water temperature (McKenzie et
al., 2012). Thus, the Cn data can be directly compared with data from that study, with the
results shown in Figure 3 of the manuscript associated with this supplementary material.
Materials and methods
Experimental animals
Oreochromis niloticus of unknown sex with a body mass of approximately 30 g and
total length of approximately 115 mm were obtained from a commercial supplier and
transported to the Universidade Estadual Paulista (UNESP) campus in Rio Claro, São Paulo
state, Brazil. They were maintained in indoor 1.5 m3 tanks provided with a recirculating flow
of biofiltered freshwater at 30 °C, under a natural photoperiod, for at least two weeks prior to
use in experiments. They were fed each day with commercial pellets, but individuals were
fasted for at least 36h prior to use in experiments. Experiments were performed in accordance
with United Kingdom Home Office regulations for animal experimentation, which also
complied with the guidelines for animal experimentation at UNESP, Rio Claro, Brazil. All
experiments were performed at 30 ± 0.1 °C.
Swimming respirometry
Swimming respirometry was performed with a Steffensen-type swim-tunnel
respirometer constructed of Plexiglas (volume 13.4 l), designed to exercise fish in a nonturbulent water flow with a uniform velocity profile. The swim tunnel has been described in
detail previously (McKenzie et al., 2007a). Individual O. niloticus were gently wrapped in a
moist cloth and measured for their mass (to the nearest 0.1 g) and total length (to the nearest 1
mm), and placed in the respirometer in the evening. They were left to recover from the
handling overnight, in a water current at l bodylength s-1 (BL s-1). At this speed they
maintained station with gentle pectoral fin sculling movements (McKenzie et al., 2003). The
following morning they were exposed to stepwise increments in swimming speed, each of 1 L
s-1 every 30 min, until they fatigued. All swimming speeds were corrected for the blocking
effect of the fish (Bell & Terhune, 1970). Critical swimming speed (Uc, BL s-1) was
calculated as described previously (Brett, 1964), using an equation which adds the velocity of
the most recently completed increment to the product of the incremental increase in velocity
and the proportion of the final increment completed before fatigue.
Measurements of O2 uptake from the water (MO2w, in mg O2 kg-1 h-1) were made at
each swimming speed by intermittent stopped-flow respirometry (Steffensen et al., 1984;
Steffensen, 1989) over a 15 min cycle, providing two measures of MwO2 for each swimming
speed during the swim test. Details of this method have been provided previously (Chatelier,
McKenzie & Claireaux, 2005; McKenzie, Pedersen & Jokumsen, 2007b; Jourdan-Pineau et
al., 2010). Water oxygen concentration was recorded continuously using an optical oxygen
probe and meter (Fibox, Pre-sens GmbH, www.presens.de) and MO2w calculated
automatically with Loliresp software (Loligo Systems, www.loligosystems.com), considering
the rate of decline in oxygen concentration, the water volume in the swim tunnel and the mass
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of the fish (Steffensen, 1989). Blank tests were run at the end of each day, to correct for the
contribution of bacterial metabolism to MO2w. This was never greater than 10% of MO2w by
the fish.
For each individual fish, a least-squares exponential regression was applied to the
relationship between swimming speed and MO2w. Extrapolation back to the y-intercept, a
notional swimming speed of zero, was employed to derive an estimate of standard metabolic
rate (Rs, Brett, 1964; Fry, 1971). Net metabolic cost of transport (Cn) was calculated at each
swimming speed by subtracting Rs from mean MO2w and dividing by swimming speed in BL
(Beamish, 1978; McKenzie et al., 2003).
Results and Discussion
Experiments were completed on six O. niloticus, with a mean (± S.E.) mass of 30 ± 6
g and total BL of 115 ± 9 mm. They achieved a mean Uc of 4.8 ± 1.4 BL s-1. The
relationship between swimming speed and MO2w showed a typical exponential increase from
which the mean Rs could be estimated. These data allowed a satisfactory calculation of Ct,
with the results shown in Figure 3 of the accompanying manuscript.
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