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.