Discussion:
Metabolic rate can be viewed as the most fundamental biological rate,
explaining the rate at which organisms take up, transform, and expend
energy (Brown et al. 2004). Thus, it must be assumed, that the
individual metabolic rate has profound implications for shaping the
TDFs. To the best of our knowledge, this framework has only been applied
to the study of TDFs in small mammals (i.e. mice and rats: MacAvoy et
al. 2006, MacAvoy et al. 2012), or birds (Ogden et al. 2004). However,
in ectothermic teleosts, where metabolism is comparably slower, this
process was assumed to be negligible. This view needs a revision, as our
results show a clear and gradual relationship between
Δ13C and metabolic rate on the individual
level for muscle tissue. However, we could not detect any association
between metabolic rate and Δ15N in muscle tissues, nor
to TDFs in liver tissue. In accordance with Herzka (2005), our results
highlight the need for establishing different TDFs for specific
ontogenetic stages to allow more precise interpretation of isotopic
data.
Trueman et al. (2005) reported differences in individual growth rates of
Atlantic salmon (Salmo salar ) to be associated to variable TDFs.
The authors suggested that this pattern could be explained by
intraspecific differences in metabolism, but direct measurements of
metabolic rates were not included. In many studies of TDFs in ecothermic
species, isotopic change has been attributed to growth rather than
metabolism (Hesslein et al. 1993, Bosley et al. 2002), but see (Herzka
et al. 2001, Tarboush et al. 2006, Sun et al. 2012). Unfortunately, we
are not able to separate the isotopic change into contributions of
growth and metabolic rate sensu Fry and Arnold (1982) as we did
not track the precise individual weight increase. .We approached this
issue by correcting our data to potential variation from growth by using
the residuals from the regression of TDFs on size (total length at the
end of the experiment). Here, the strongest differences in
size-corrected TDF could be detected comparing juvenile to adult perch.
Besides having the highest SMR, juveniles were also characterized by the
strongest approximate weight increase (Table 1). When measuring
individual metabolic rate immediately before assessing their TDFs we
found fine scale differences that correlated strongly with their
Δ13C isotopes indicating that changes in
metabolic rates could translate into variations in TDFs (Kleiber 1932,
1947, Boecklen et al. 2011).
Results of posthoc comparisons of SMR between perch with an initial
weight of 20-30 g caught in different habitats (pelagic and littoral)
were not significant, but when analyzed separately, a significant
difference appears with pelagic having a higher SMR (t-test: t7.633 = -2.406; P= 0.044), similar to previous results
(Andersson et al. unpublished data). Generally, it is known that such
intra-specific differences in metabolic rates within adult individuals
exist in many species, including fish (Biro and Stamps 2010). However,
less is known about differences between individuals living in different
habitats. In many Swedish lakes including Erken, which is the origin of
the perch used in this study, littoral and pelagic perch of the
intermediate class size differ in their individual specialization for
respective food items, which is even translated into adaptations of
their morphology. While pelagic perch predominately ingest pelagic
zooplankton and have a more streamlined body form, littoral perch
include benthic macroinvertebrates in their diet to a higher degree and
are characterized by a deeper body (Svanbäck and Persson 2004, Marklund
et al. 2019). Potentially, habitat-specific differences in activity
levels could be related to the differences found in SMR (Kahilainen et
al. 2014). Pelagic perch need to be endurance swimmers in order to catch
the smaller fast-moving prey, while littoral perch forage on larger prey
items of lower mobility (Svanbäck and Eklöv 2004). Future research is
needed to resolve the underlying causes for the differences found in SMR
between littoral and pelagic perch. Interestingly, average
Δ13C in muscle tissue of 20 – 30 g pelagic
perch was lower compared to littoral perch of the same weight class
(Table 1) but this difference was not significant. Thus, our data
reflect a trend that the inverse relationship (i.e. elevated SMR leads
to lower Δ13C), holds true not only between
juveniles and adults, but also between the habitat-specific individuals
of the same weight class.
While the effect of SMR was strong for Δ13C
in muscle tissue, and a trend (though not significant) was visible for
the association of SMR and Δ15N in muscle tissue, we
did not observe any relationship of SMR with the TDFs of liver tissue,
indicating that individual metabolic rate has a stronger effect of
tissue types of slower isotopic turnover. Generally, liver had lower
TDFs (Δ13C: 1.1 ‰ ± 0.4;
Δ15N: 1.1 ‰ ± 0.5) compared to the TDFs of muscle
tissues (Δ13C: 3.7 ‰ ± 0.5;
Δ15N: 1.3 ‰ ± 0.6), showing relatively little change
in the isotope values between consumer and prey. Our results are in line
with the findings of other studies on tissue-specific difference in TDFs
in fish (Buchheister and Latour 2010, Matley et al. 2016), and the
observed pattern might be due to different biochemical composition of
the tissue types, e.g. the abundances of specific amino acids (Pinnegar
and Polunin 1999).
Our values of Δ15N in muscle tissue are relatively low
compared to the highly cited average value of 3.4 ‰ (Post 2002).
However, consumers raised on a invertebrate diet typically show a lower
TDF compared to consumers raised on a high-protein diet (McCutchan et
al. 2003). Protein is the principal source of energy for perch, which
are ammoniotelic organisms, and thus characterized by a relatively high
N use efficiency that is linked to lower Δ15N (Trueman
et al. 2005).
In contrast, our derived values of Δ13C for
muscle tissue were rather high compared to the typically assumed value
of 0.3 ‰ (Post 2002) or 0.4 ‰ (McCutchan et al. 2003). However, previous
studies have reported similarly high values (e.g.Pinnegar and Polunin
1999, Barnes et al. 2007, Busst and Britton 2016). Vollaire et al.
(2007) reported Δ13C of 4.02 ± 0.13 ‰ for
juvenile perch feeding on artificial diet. A potential reason for
variation in Δ13C could be a process termed
“isotopic routing”, where resource constituents, such as proteins,
lipids and carbohydrates are allocated to different tissue types
(Schwarcz 1991). Furthermore, the organisms lipid content may lead to
substantial variation, as lipids are generally13C-depleted (DeNiro and Epstein 1978, Focken and
Becker 1998) However, as C/N in the perch used in this study was
generally low (3.2 ± 0.1, average ± SD), lipid correction is not
recommended (Kiljunen et al. 2006). Altogether, we thus agree with Wolf
et al. (2009) and acknowledge that further studies are urgently needed
to understand variation of Δ13C in fish.
Variation in Δ15N, specifically in liver tissue was
higher compared to the variation in Δ13C. We
assume that this variability can be attributed to the fact that the
δ15N of the diet was rather high (14.0 ‰ ± 3.7).
Commercially raised Chironomids are maintained in big flumes and
cannibalism might occur which would result in higher
δ15N of individual organisms. Another aspect that
could potentially influence the variation in Δ15N, but
also in Δ13C is the individual food intake,
which was shown to influence TDFs (Bosley et al. 2002, Barnes et al.
2007). In our study, perch shoals were fed at high feeding rates
(approximately 15% of the individual wet weight–day), and left-over food was removed. However, this
does not imply that all fish individuals fed until satiation. Strong
hierarchies exist in perch shoals (Magnhagen 2012) and were observed in
some of our tanks. Dominance behavior of single individuals might have
prevented subordinates the access to food. This artefact of the
experimental design adds to the previously mentioned confounding aspects
and highlight again the complexity and difficulty involved in
experimentally accessing general and widely applicable TDFs.