Effects of temperature on specific dynamic action in Atlanticcod Gadus morhua

Bjørn Tirsgaard • Jon Christian Svendsen •

John Fleng Steffensen

Received: 1 June 2014 / Accepted: 18 October 2014

� Springer Science+Business Media Dordrecht 2014

Abstract Growth requires that energy is directed

towards ingestion, digestion, absorption and assimila-

tion of a meal; energy expenditures are often

expressed as the specific dynamic action (SDA).

While SDA is an important part of fish energy budgets

and strongly affected by water temperature, temper-

ature effects are not known across a wide temperature

range in Atlantic cod Gadus morhua. The objective of

this study was to examine effects of temperature (2, 5,

10, 15 or 20 �C) on the energetic cost and time used for

SDA in juvenile G. morhua by intermittent flow

respirometry. At each temperature, G. morhua were

fed a meal of herring (Clupea harengus) correspond-

ing to 5 % of the body mass. Standard metabolic rates

measured pre-feeding and post-feeding metabolic

rates were measured to determine SDA. The study

showed that SDA coefficients (%, SDA energy divided

by meal energy) were significantly lower at 2 and

10 �C (5.4–6.3 %) compared to 5, 15 and 20 �C

(10.4–12.4 %), while SDA duration increased signif-

icantly from 80 h at 10 �C to 130–160 h at 2, 15 and

20 �C and reached a maximum of 250 h at 5 �C. The

significant decrease in SDA duration at 10 �C com-

bined with a low SDA coefficient suggests that water

temperatures close to 10 �C may represent the opti-

mum temperatures for SDA in this population of G.

morhua. Our results suggest that SDA is not a simple

function of temperature, but may vary with tempera-

ture in a more complex fashion.

Keywords Digestion � Gadus morhua � Metabolic

rate � Respirometry � SDA � Temperature

Introduction

Atlantic cod (G. morhua) is distributed over a wide

area and is known to inhabit a broad thermal niche

(-1.5 to 19 �C) and because water temperature is

known to affect enzymatic activity, basic body

functions including growth typically change with the

water temperature in ectothermic species. Mainte-

nance and growth of fishes require that energy is

directed to feeding and processing nutrients, often

expressed as the specific dynamic action (SDA). SDA

integrates energy expenditures associated with inges-

tion, digestion, absorption, biosynthesis, nutrient

turnover and assimilation of a meal (Jobling 1985,

1994) and is usually determined using respirometry as

the excess post-feeding oxygen consumption (Jordan

and Steffensen 2007).

B. Tirsgaard (&) � J. F. Steffensen

Marine Biological Section, Biological Institute,

University of Copenhagen, Strandpromenaden 5,

3000 Helsingør, Denmark

e-mail: btirsgard@gmail.com

J. C. Svendsen

Interdisciplinary Centre of Marine and Environmental

Research, University of Porto, Rua dos Bragas 289,

4050-123 Porto, Portugal

123

Fish Physiol Biochem

DOI 10.1007/s10695-014-0004-y

SDA is important to study because the energy

allocated to SDA may constitute a significant fraction

of the energy uptake (Soofiani and Hawkins 1982;

Ware 1982; Castro-Santos and Haro 2005). Previous

studies have shown that the amount of energy

allocated to SDA (SDA cost; SDAcost) and the

duration of the SDA response (SDA duration; SDAdur)

vary with meal size (Fu et al. 2005a, b; Jordan and

Steffensen 2007), diet composition (Shi-Jian et al.

2005), feeding frequency (Fu et al. 2005a) and body

mass (Von Herbing and White 2002; Pirozzi and

Booth 2009). Moreover, SDAdur and the SDA coef-

ficient (SDAcoef; SDAcost divided by meal energy)

may vary with water oxygen tension (Jordan and

Steffensen 2007).

Several studies have investigated the influence of

temperature on SDA variables; the majority of the

studies has reported that SDAdur decreases with

temperature in ectotherms (Jobling and Davies 1980;

Wang et al. 2002; Secor and Boehm 2006; Luo and

Xie 2008). However, a few studies have not reported

this pattern. For example, an optimum temperature at

4 �C for the SDAdur was found in the teleost Patago-

notothen sima (Vanella et al. 2010), when tested at 2, 4

and 10 �C. In addition, an increasing SDAdur with

increasing temperature has been observed in two

species of killifish Aphyosemion cameronense and A.

exiguum (McKenzie et al. 2013). In contrast to the

effects of temperature on SDAdur, recent reviews have

considered temperature to have negligible effects on

the energetic cost of digesting a meal, measured as

either the SDAcost or the SDAcoef in ectotherms

(McCue 2006; Secor 2009). However, a small number

of studies have reported increasing SDAcoef with

increasing temperatures in Atlantic cod Gadus morhua

(Soofiani and Hawkins 1982), gilthead sea bream

Sparus aurata (Guinea and Fernandez 1997) and

southern catfish Silurus meridionalis (Luo and Xie

2008). In a leech species Nephelopsis obscura,

Kalarani and Davies (1994) found an optimum

temperature with the lowest SDAcost at 15 �C.

Numerous studies have linked the growth potential

of fish to their metabolic scope (MS) (Portner and

Knust 2007; Farrell et al. 2008; Neuheimer et al.

2011). MS is the difference between the standard

metabolic rate (SMR) and the maximal metabolic rate

(MMR) and reflects the energy available for all

activities beyond SMR including SDA (Fry 1971;

Portner and Farrell 2008; Portner 2010). At increasing

temperatures, both SMR and MMR increase but with

MMR increasing at the highest rate at low tempera-

tures thus resulting in an increasing MS. If the

temperature increases further, SMR continues to

increase, whereas MMR peaks and declines as MMR

approaches the upper thermal tolerance (Portner 2002;

Portner and Farrell 2008) resulting in MS being

maximized at an intermediate temperature with a

decline at temperatures above and below (Frederich

and Portner 2000; Portner 2001). Because oxygen is

used in most energy demanding processes, a reduction

in MS may affect all higher functions (muscular

activity, behaviour, growth and reproduction) and

might thereby shape the long-term fate of species

(Portner and Knust 2007).

Detailed knowledge about the effects of tempera-

ture on ectothermic animals is clearly important to

fully understand their physiological performance and

behaviour. The aim of the present study was to

improve the understanding of the effects of temper-

ature on different SDA variables in juvenile G.

morhua and to provide a better empirical basis for

bioenergetic models commonly used to estimate

growth and survival rates and stock dynamics in fish

(Hansson et al. 1996; Essington et al. 2001; Perez-

Rodrıguez and Saborido-Rey 2012). Specifically, we

used intermittent flow respirometry to measure oxygen

consumption rates (MO2) in G. morhua and test two

predictions: (1) SDAdur will decrease with tempera-

ture; (2) SDAcost and SDAcoef will not vary with

temperature. To this end, juvenile G. morhua were

captured in the wild and SDA variables were deter-

mined at 2, 5, 10, 15 and 20 �C.

Materials and methods

Experimental animals

A total of 33 G. morhua [173.9 ± 5.8 g (mean ± SE)]

were captured using fish traps in Øresund near

Helsingør in Denmark and held in 0.7 m3 flow-

through tanks (10 ± 1 �C; salinity: 31 ppt) at the

Marine Biological Section, University of Copenhagen,

Denmark. G. morhua were randomly divided into five

groups, and water temperature was modified by

1 �C day-1 to reach the five test temperatures of 2,

5, 10, 15 and 20 �C (±0.3 �C). G. morhua were then

acclimated to the stable test temperature for at least

Fish Physiol Biochem

123

2 weeks prior to experimentation. G. morhua were fed

herring Clupea harengus three times a week. The

average body mass and sample size for the experiment

were as follows: 2 (163.9 ± 16.4 g, N = 7), 5

(180 ± 9.5 g, N = 7), 10 (169.1 ± 4 g, N = 7), 15

(195.8 ± 18.7 g, N = 6) and 20 �C (162.2 ± 9.7 g,

N = 6).

Respirometry

The experimental set-up consisted of four static

respirometers (each 3.25 L) and a mixing pump,

submerged in a 100-L opaque tank, filled with salt

water (31 ppt) and maintained at the appropriate test

temperature (either 2, 5, 10, 15 or 20 ± 0.1 �C).

Normoxic conditions in the tank were secured using

air stones. To ensure water quality, water in the tank

was continuously replaced (2 % h-1) and simulta-

neously recirculated through a loop consisting of a

separate biofilter and a UV sterilizer (UV-10000;

Tetra Pond, Melle, Germany). Respirometers were

situated in a separate room with no other ongoing

experiments to minimize any disturbance of the fish.

Measurements of MO2 (mg O2 kg-1 h-1) were

taken every 10 min using computerized intermittent

flow respirometry allowing long-term ([48 h) repeated

measurements (Steffensen 1989). Each static respirom-

eter was fitted with two outlet and inlet ports at each end

as described previously (Svendsen et al. 2012) and a

vertical chimney allowed feeding within the respirom-

eter (Jordan and Steffensen 2007). The repeated

respirometric loops consisted of a 5-min flushing phase

during which a pump flushed the respirometer with

ambient water through one set of ports. The second set

of ports and a pump constituted a recirculating loop,

which secured re-circulation of water in the respirom-

eter in a closed circuit phase for 5 min, divided into a

waiting phase (1 min) and a measurement phase

(4 min). To secure water oxygen levels above 80 %

air saturation (O2sat) at all times, the measurement

phase was shortened to 3 min in the warmer treatments

(15 and 20 �C).

To determine MO2, water oxygen partial pressure

was measured in the re-circulated loop at 1 Hz using

fibre optic electrodes (PreSens, Regensburg, Germany

and Firesting, PyroScience, Aachen, Germany). The

pumps flushing the respirometers were controlled by

AutoResp software (version 2.1.3; Loligo Systems,

Tjele, Denmark) or AquaResp software (www.

aquaresp.com) that also calculated instantaneous

MO2, from the decrease in oxygen partial pressure

within the measurement phase using standard equa-

tions described by Schurmann and Steffensen (1997).

Preliminary testing demonstrated that the set-up

ensured that the coefficient of determination (r2)

associated with each MO2 measurement was always

[0.95, identical to previous studies (Svendsen et al.

2013). Corrections of background respiration (i.e.

microbial respiration) followed Capossela et al.

(2012).

Experimental Protocol

G. morhua used in the experiments were fasted for

5–10 days prior to respirometry to ensure a post-

absorptive state. The fasting period was 10 days for

the 2 and 5 �C treatments and shorter for the warmer

treatments. Before the experiment, G. morhua’s body

mass (to nearest 0.1 g) was determined, and individual

fish were transferred to the respirometers and left for

24 h to acclimate, while MO2 data were collected

continuously. Four fish were tested simultaneously in

the set-up with test temperatures modified randomly

between each trial. After acclimation, 24 h were used

to estimate SMR, as described below. Subsequently,

each G. morhua were fed a piece of C. harengus fillet

corresponding to 5 % (±0.1 %) of the body mass. C.

harengus used in the present study was from the same

season (autumn) as the C. harengus used by Jordan

and Steffensen (2007). The energetic value

(9.92 kJ g-1) reported by Jordan and Steffensen

(2007) was therefore used to estimate meal energy.

After ingestion, MO2 data were collected until post-

feeding MO2 converged with SMR ? 5 % as

described below. All experiments were carried out in

strict accordance with the 2010/63/EU directive of the

European Parliament concerning the protection of

animals used for scientific purposes.

Data acquisition

All measurements of MO2 were body mass adjusted

following previous studies (Steffensen et al. 1994;

Frisk et al. 2012). Specifically, measurements were

taken to a body mass of 174 g (average body mass)

using the equation:

Fish Physiol Biochem

123

MO2ð174Þ ¼ MO2 �M

174

� �ð1�AÞð1Þ

where MO2(174) = oxygen consumption rate of a

174 g fish, MO2 (mg O2 kg-1 h-1) = oxygen con-

sumption rate of a fish with body mass = M, and

A = the mass exponent describing the relationship

between metabolic rate and body mass. A mass

exponent of 0.87 was used based on a previous study

on G. morhua within the same temperature range

(2–20 �C) (Tirsgaard et al. 2014).

SMR and SDA were calculated using an algorithm

written in R (Team 2012) and the quantreg package

(Koenker 2005, 2011). Separate nonparametric quan-

tile regressions (function rqss) were fitted to the 24 h

pre-feeding MO2 data and the post-feeding MO2 data

to estimate SMR and SDA, respectively (Dorcas et al.

2004; Dupont-Prinet et al. 2010, 2013). SDA was

deemed terminated when the quantile fit reached

SMR ? 5 % (Fig. 1), similar to Jordan and Steffensen

(2007). The parameter s [tau, analogous to the quantile

(q)] was adjusted to 0.25 for SMR and SDA estimates

such that the same proportions of MO2 values were

below the fitted curve for the two different estimates.

G. morhua MMR was estimated for each fish at the

five test temperatures (2, 5, 10, 15 and 20 �C) based on

(Tirsgaard et al. 2014). The metabolic scope (MS) was

calculated as MMR–SMR (mg O2 kg-1 h-1). Five

SDA variables were calculated to describe the post-

feeding MO2 following Secor (2009):

1. SDAscope, i.e. the maximum value of MO2 above

SMR during the SDA course, assumed to occur

before 100 h post-feeding (Fig. 1);

2. tpeak, i.e. the time course from feeding to SDAscope

(Fig. 1);

3. SDAdur, i.e. the time from feeding until MO2

converged with SMR ? 5 % (Fig. 1);

4. SDAcost, i.e. the post-feeding integrated excess

oxygen consumption (mg O2 kg-1) above SMR;

5. SDAcoef (%) calculated as the SDAcost (mg O2

kg-1) converted to energy units using an oxycal-

orific coefficient of 14.06 kJ g O2-1 (Gnaiger

1983) and divided by the full energy content of

the meal.

Statistical analysis

The five SDA variables and body mass were grouped

according to temperature treatment and examined for

differences using a one-way ANOVA. The test was

followed by pairwise multiple comparison procedures

(Holm–Sidak) when appropriate. Data were log10-

transformed to meet assumptions of a normal distri-

bution of data and homogeneity of variance. Results

were considered significant at p \ 0.05. All values are

reported as mean ± SE unless otherwise noted. Data

analyses were carried out using SigmaPlot (version

11.0; Systat Software, Chicago, IL, USA). Correla-

tions between water temperature and SMR and

SMR ? SDAscope were fitted using exponential

regression lines, whereas linear lines were applied

between water temperatures and MS, MMR and the

remaining SDA variables.

Results

The average body mass for each temperature treatment

did not differ between the five temperature treatments

0 20 40 60 80

Time (hours)

SMR = 63.1

050

100

150

200

Duration = 75 h

SDAscope = 33.5 mg O2 kg−1 h−1

MO

2(m

gO

2kg

−1

h−1 )

−20

tpeak = 13 h

SDAcost = 1389 mg O2 kg−1

Fig. 1 An example of metabolic rates (mg O2 kg-1 h-1) in

Atlantic cod G. morhua (187 g) at 10 �C as a function of time

(h), prior to feeding (-24–0 h, black solid triangles) and after

feeding (0–95 h, black triangles). A quantile regression

(q = 0.25) is used to estimate the SMR from pre-feeding

MO2, indicated by the black horizontal line, whereas SDA

(specific dynamic action) is estimated from post-feeding MO2.

The fish is fed a C. harengus fillet corresponding to 5 % of the

wet body mass at 0 h (vertical dashed line) and then left

undisturbed while the metabolic rate is recorded continuously.

SDA is considered terminated when the regression converged

with the SMR ? 5 %. The corresponding duration is 75.5 h and

reflected SDAdur. SDAcost is estimated by integrating the area

between the curve and SMR (marked area). SDAscope (mg O2

kg-1 h-1) and tpeak (h) (see text for details) are indicated by the

arrow

Fish Physiol Biochem

123

(p = 0.65). SMR increased significantly with temper-

ature at each of the five tested temperatures (all

p \ 0.003) (Fig. 2). MMR increased significantly

with temperature (p \ 0.05) except between 10 and

15 �C (p [ 0.05) (Fig. 2). The tpeak did not differ

between treatments (p [ 0.05), whereas SDAscope

increased with temperature, although the trend was

only significant (p \ 0.05) between 2 and 20 �C. MS

as a function of temperature was bell-shaped with the

MS peaking around 10 �C (Fig. 2) and was signifi-

cantly different between all temperatures (p \ 0.013).

Similarly, the SDAdur varied significantly between

treatments. The shortest SDAdur was observed at

10 �C (p \ 0.001), followed by 20 �C, 15 �C, 2 �C,

while the longest SDAdur occurred at 5 �C (p \ 0.004;

Fig. 3a). Thus, SDAdur clearly dipped at 10 �C,

allowing for a faster SDA at this temperature (Fig. 3a).

The SDAdur at 5 �C was three times longer than the

SDAdur at 10 �C, revealing the steepest change in

SDAdur as a function of temperature (Fig. 3a). Both

SDAcost and SDAcoef varied significantly with tem-

perature (p \ 0.05) and revealed comparable

responses to temperature (Fig. 3b, c). At 5, 15 and

20 �C, the values were higher than at 2 and 10 �C

(Fig. 3b, c).

Discussion

This study provides evidence that SDAcost, SDAcoef

and SDAdur vary with temperature in G. morhua. We

found that SDAcost and SDAcoef were significantly

decreased at 2 and 10 �C compared to 5, 15 and 20 �C.

Furthermore, the study showed that SDAdur was

significantly shorter at 10 �C and longer at 5 �C

compared to 2, 15 and 20 �C. The significant decrease

in SDAdur at 10 �C combined with a low SDAcoef

suggests that water temperatures close to 10 �C may

represent the optimum temperatures for SDA in this

population of G. morhua.

Only few studies have analysed the effect of

temperature on SDA in G. morhua. A previous study

by Soofiani and Hawkins (1982) measured the effect

of repeated feeding events (every second day) on

oxygen consumption rates and SDAcoef at tempera-

tures between 7 and 18 �C in G. morhua, while

Saunders (1963) compared SDAdur at 10 and 15 �C in

G. morhua provided abundant food (C. harengus

ad libitum) for 1 h. The present study is the first to

compare SDAdur, SDAcost and SDAcoef across a

markedly wider temperature range in G. morhua.

Within the temperature range from 2 to 20 �C, we

found that G. morhua has an optimum temperature for

SDAdur around 10 �C, whereas SDAcost and SDAcoef

were lowest at 2 and 10 �C (Fig. 3a–c). It is difficult to

compare our results directly with Soofiani and Haw-

kins (1982), because they employed repeated feedings,

whereas we only fed G. morhua once. However,

Soofiani and Hawkins (1982) showed an increase in

the SDAcoef at temperatures above 10 �C similar to the

present study. In contrast to the present study,

Saunders (1963) found the time used for digesting a

meal to be shorter (4–5 days) at 15 �C compared to

10 �C (7 days). However, Saunders (1963) provided

abundant of food for 1 h without determining the

actual meal size and could not estimate SDAcoef. It is

therefore possible that the shorter SDAdur Saunders

(1963) found at 15 �C was caused by a lower feed

intake compared to G. morhua at 10 �C. A difference

in feed intake was also suggested by Saunders (1963)

as a possible explanation along with a possible

temperature effect on SDA.

The variables SDAdur, SDAcost and SDAcoef are

affected by many factors, including meal size, diet

composition, feeding frequency, body mass, temper-

ature and environmental oxygen level (Soofiani and

Fig. 2 The effect of temperature on the standard metabolic rate

(SMR, black solid triangles), the maximum post-feeding MO2

(SMR ? SDAscope, black solid squares), the metabolic scope

(MS, times), maximum metabolic rate [MMR, calculated using

(Tirsgaard et al. 2014), black solid circles] in juvenile Atlantic

cod G. morhua. Results of statistical analyses are omitted from

the figure to improve clarity (details are provided in ‘‘Results’’

section). Correlations between water temperature and SMR and

post-feeding MO2 are indicated using solid exponential least

square regression lines. Effects of water temperature on MS and

MMR are indicated using dashed linear lines. Values are

mean ± SE, N = 7 at 2, 5 and 10 �C and N = 6 at 15 and 20 �C

Fish Physiol Biochem

123

Hawkins 1982; Von Herbing and White 2002; Fu et al.

2005a, b; Shi-Jian et al. 2005; Jordan and Steffensen

2007; Pirozzi and Booth 2009), and a direct compar-

ison between studies is rarely feasible. This study

found that temperatures around 10 �C provide the

shortest SDAdur. Is this result supported by previous

studies? Among previous SDA studies on G. morhua,

the approach used by Jordan and Steffensen (2007)

resembles the present study the most. Feeding G.

morhua (150 g) with a fillet of C. harengus (5 % of

body mass) at 10 �C, Jordan and Steffensen (2007)

reported that SDAdur is 95 h, which is close to our 80 h

(Fig. 3a). Likewise, the study by Behrens et al. (2012)

used approaches similar to the present study. Behrens

et al. (2012) found that SDAdur is 65 h in G. morhua

fed a single meal of pollack Pollachius virens

corresponding to 2 % of the body mass at 6.5 �C.

According to Jordan and Steffensen (2007), increasing

meal size from 2.5 to 5 % doubles the SDAdur in G.

morhua. Hence, extrapolating the 2 % meal size used

by Behrens et al. (2012) to a 5 % meal size causes the

SDAdur to increase to 163 h at 6.5 �C (i.e. 65 h 9 5/

2 %). Using this extrapolated value (160 h) for a

comparison with the present study reveals a much

shorter SDAdur at 10 �C (95 h; (Jordan and Steffensen

2007) and 80 h; present study). Thus, Behrens et al.

(2012) provide indirect evidence that SDAdur at 6.5 �C

is substantially longer (C65 h) than at 10 �C.

Although a comparison using an extrapolated value

and a different diet (P. virens) should be performed

cautiously, the data are consistent with the observation

that SDAdur is substantially shorter at 10 �C than at

colder temperatures as found in the present study

(Fig. 3a).

It may be relevant to compare the present SDA

study with studies on gastric evacuation rates. Gastric

evacuation and intestinal transit normally take longer

at lower temperature so it would be expected that

nutrients from a given meal would be absorbed over a

b Fig. 3 Time (h) used for SDA (SDAdur) (a), cost (mg O2 kg-1)

of SDA (SDAcost) (b), and the percentage of ingested energy

used for SDA (SDAcoef) (c) in juvenile Atlantic cod G. morhua

fed a meal of C. harengus filet corresponding to 5 % of the wet

body mass at 2, 5, 10, 15 and 20 �C (black solid circles, the

present study). Values are mean ± SE; N = 7 at 2, 5 and 10 �C

and N = 6 at 15 and 20 �C. For the present study, different

letters indicate significant differences between temperatures

(p \ 0.05). For comparisons, data from related studies on G.

morhua are included [black solid triangles (Jordan and

Steffensen 2007), black solid squares (Behrens et al. 2012),

black squares (Soofiani and Hawkins 1982) and black circles

(Perez-Casanova et al. 2010)]. Across temperatures, multiple

data points are connected by linear lines, either solid (the present

study) or interrupted [(Soofiani and Hawkins 1982) and (Perez-

Casanova et al. 2010)]. Filled symbols (black solid circles, black

solid triangles and black solid squares) indicate that G. morhua

are fed with a natural diet (P. virens or C. harengus), whereas

open symbols (black squares and black circles) indicate that G.

morhua are fed artificial aquaculture pellets

(a)

(b)

(c)

Fish Physiol Biochem

123

longer period of time at low temperature. This would

be predicted to give a longer SDAdur and, presumably,

a flatter SDA profile at low temperatures. In the

present study, a depression of the SDA profile was

evident due to a decreasing SDAscope with decreasing

temperature; however, the temperature effect was only

significant between 2 and 20 �C. On the other hand,

opposite most other studies on ecthothermic animals

(Jobling and Davies 1980; Wang et al. 2002; Secor and

Boehm 2006; Luo and Xie 2008), the SDAdur did not

decrease with increasing temperature in the present

study, but revealed an optimum SDAdur at 10 �C

(Fig. 3a). A similar optimum temperature has been

found for the gastric evacuation rate in G. morhua

between 11.3 and 15.6 �C (Tyler 1970; Singh-Renton

and Bromley 1996; Andersen 2012). Studies by Smit

(1967) indicate that at low temperatures, the observed

increase in digestion rate with temperature is caused

by increased enzyme activity and thus an increase in

secretion of gastric juice of constant pepsin concen-

tration. At temperatures above the optimum for gastric

evacuation, Andersen (2012) argued that the decrease

in gastric evacuation rate may not be caused by a

decrease in enzyme activity but rather a restriction in

the cardiovascular system as reported by (Lannig et al.

2004). Similarly, both the highest MS and the lowest

SDAdur occurred at 10 �C in the present study. Some

of the differences identified in the optimum temper-

ature between gastric evacuation studies and the

present SDA study could be due to the 5 �C test

intervals used in the present study. Moreover, SDA

studies include post-absorptive processes such as

biochemical transformation of food and de novo

protein synthesis. The two processes represent a major

contributor to SDA (Brown and Cameron 1991;

Bureau et al. 2002). Measurements of in vivo rates

of protein synthesis in isolated hypatocytes (i.e. liver

cells) from rainbow trout Oncorhynchus mykiss

(Walbaum) have demonstrated that the highest rate

occurs around the optimum temperature for growth

(Hokanson et al. 1977; Pannevis and Houlihan 1992).

The optimum temperature for growth in juvenile G.

morhua (109 g) is 10 �C according to (Bjornsson et al.

2001), comparable to the temperature with the shortest

SDAdur (10 �C) observed in the present study. Corre-

spondingly, we conclude that water temperatures close

to 10 �C represent optimum temperatures for SDAdur.

When comparing the energetic cost of digesting a

meal between studies, SDAcoef is often the most useful

variable, because it corrects for variation in energy

intake. However, many factors still affect the SDAcoef

[e.g. diet composition, apparent digestibility coeffi-

cient (ADC) and experimental conditions including

data analysis method; (Satoh et al. 2004; Jordan and

Steffensen 2007; Behrens et al. 2012)], as indicated by

the substantial variation in SDAcoef among studies on

G. morhua (Fig. 3c). Therefore, a direct comparison

between studies is rarely feasible. Regardless, trends

in previous studies are indicative of SDAcoef dipping

around 10 �C in G. morhua comparable to the present

study. Perez-Casanova et al. (2010) found an increase

in the SDAcoef when temperature was decreased from

11 to 5 �C and 2 �C. Likewise, Soofiani and Hawkins

(1982) found a slight decrease in SDAcoef when the

temperature increased from 7 to 10 �C and a

substantial increase at temperatures above 10 �C in

G. morhua fed to satiation. Finally, in two studies

feeding G. morhua natural diets (C. harengus and P.

virens), Jordan and Steffensen (2007) measured a

lower SDAcoef at 10 �C (9.7 %) than Behrens et al.

(2012) at 6.5 �C (14.9 %) (Fig. 3c). Thus, despite

considerable variation in the SDAcoef values reported

and the test methods used, there are trends indicating a

lower SDAcoef around 10 �C in G. morhua, with an

increase in the SDAcoef at both lower and higher

temperatures (Fig. 3c). The previous studies do not,

however, contribute to explaining the low SDAcoef

measured at 2 �C in the present study.

A comparison of data between 2 and 5 �C showed

that SDAdur, SDAcost and SDAcoef decreased at 2 �C in

G. morhua (Fig. 3a–c). In contrast, previous studies

have indicated that SDAdur is increasing with decreas-

ing temperatures (Jobling and Davies 1980; Wang

et al. 2002; Secor and Boehm 2006; Luo and Xie

2008), whereas temperature typically has little effect

on the SDAcost and SDAcoef [see reviews by McCue

(2006) and Secor (2009)]. While our study may

indicate that SDA in G. morhua is more efficient at

2 �C than at 5 �C, we suggest that the lower SDAdur,

SDAcost and SDAcoef found at 2 �C compared to 5 �C

reflect a lower apparent digestibility coefficient

(ADC). Lower ADC has been reported in fish (Windell

et al. 1978; Olsen and Ringø 1998; Satoh et al. 2004;

Pace 2013) and lizards (Harlow et al. 1976) at low

temperatures, likely due to decreasing activities of

protease enzymes (Kofuji et al. 2005). A decreased

ADC elevates the amount of food passing through the

guts not fully digested (Jobling 1994), likely lowering

Fish Physiol Biochem

123

the amount of energy needed for nutrients reabsorption

and protein synthesis, resulting in the lower SDAdur,

SDAcost and SDAcoef observed at 2 �C in the present

study (Fig. 3a–c). Nevertheless, it remains a hypoth-

esis that low values of SDAdur, SDAcost and SDAcoef at

2 �C are explained by a drop in ADC and future studies

should address the potential influence of ADC varia-

tion, for example, by measuring energy intake, SDA

variables and energy content in faeces over the full

SDAdur covering a wide range of temperatures, in

particular at low temperatures. In this way, both the

cost and the duration of SDA may be estimated in

combination with accurate measurements of the actual

energy absorbtion, which will allow an assessment of

ADC variation in relation to temperature.

In summary, the present study indicates that SDA in

G. morhua may not be a simple function of temper-

ature within the temperature range of 2–20 �C. The

study showed that SDAscope increase with tempera-

ture, whereas SDAdur is shortest around 10 �C and

SDAcost and SDAcoef are lowest at 2 and 10 �C. We

suggest that the surprisingly low value of SDAcoef

found at 2 �C could be caused by a lower ADC, which

should be investigated in future studies. The signifi-

cant decrease in SDAdur at 10 �C combined with a low

SDAcoef suggests that water temperatures close to

10 �C may represent optimum temperatures for SDA

in this population of G. morhua.

Acknowledgments This study was part of the project

SUNFISH (sustainable fisheries, climate change and the North

Sea ecosystem), and financially supported by the Danish

Council for Strategic Research [Grant Number 09-063096]

and Department of Biology, University of Copenhagen [Grant

Number 102-0218/11-5550]. Jon Christian Svendsen was

supported by the Foundation for Science and Technology

(FCT) in Portugal (Grant Number SFRH/BPD/89473/2012). We

thank an anonymous reviewer for helpful and constructive

comments on an earlier version of the manuscript.

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