Stable isotope discrimination factors of omnivorous fishes: influence of tissue type, temperature, diet composition and formulated feeds

Abstract

Application of stable isotope data to trophic studies requires understanding of factors influencing the isotopic discrimination factor (D) between consumers and their prey resources. This is missing for many omnivorous species, despite their diet and environment potentially impacting D. The effects of temperature, diet (including formulated feeds) and tissue type on D 13 C and D 15 N were thus tested experimentally. A temperature experiment exposed three species to identical diets at 18 and 23°C, whereas a diet experiment exposed one species to four diets at 18°C. At 23°C, C:N ratios, D 13 C and D 15 N were generally elevated versus 18°C. After lipid correction, tissue/species-specific differences at 23°C in D 13C and D 15 N were up to 0.73 and 0.54% higher, respectively. Across the four diets, there were also significant differences in D 13 C and D 15 N between a natural diet and diets based on formulated feeds. D 13 C and D 15 N of muscle were 1.51 to 2.76% and 3.13 to 5.44%, respectively. Highest D for both isotopes was from a formulated feed based on plant material that resulted in lower dietary protein content and quality. Thus, diet and environment fundamentally affected the isotopic discrimination factors and these factors require consideration within trophic studies based on stable isotopes.

Effects of temperature on tissue–diet isotopic spacing of nitrogen and carbon in otolith organic matter

“…A consistent effect of temperature on carbon tissue-diet isotopic spacing has been observed in previous studies, in which isotopic spacing increases with increasing temperature (Barnes et al 2007;Britton and Busst 2018). Although we found differences in d 13 C in muscle and heart tissues among temperature groups, the pattern did not represent a systematic change and was not significant.…”

“…Barnes et al (2007) found a negative relationship between Dd 15 N values and temperature (4.41% at 118C and 3.78% at 168C) in white muscles of European sea bass. However, Britton and Busst (2018) found a positive relationship between tissue-diet isotopic spacing and temperature (18 v. 238C). Britton and Busst (2018) experimented with three species of Cyprinidae, and found significantly higher tissue-diet isotopic spacing at the highest temperature for all tissues analysed (muscle, fin and scale).…”

“…However, Britton and Busst (2018) found a positive relationship between tissue-diet isotopic spacing and temperature (18 v. 238C). Britton and Busst (2018) experimented with three species of Cyprinidae, and found significantly higher tissue-diet isotopic spacing at the highest temperature for all tissues analysed (muscle, fin and scale). Sweeting et al (2007a) tested muscle, heart and liver across temperatures and similarly found an effect of temperature on the incorporation of nitrogen.…”

“…Soft tissue stable isotope enrichment Soft tissues are widely used in dietary studies of nitrogen and carbon isotope analyses and can provide a good indication of the diet at a given time (Verweij et al 2008;Elsdon et al 2010). However, tissue enrichment factors and turnover rates differ among species (Sweeting et al 2007a(Sweeting et al , 2007b, environmental temperatures (Bosley et al 2002;Barnes et al 2007;Britton and Busst 2018), diets (Ankjaerø et al 2012) and trophic levels of fish (Hussey et al 2014). For example, muscles isotope values, the most commonly used in diet studies of fish, exhibit a wide range of enrichment factors, ranging from 0.8 to 5.6% in d 15 N and from À0.39 to 3.7% in d 13 C, depending on species (Sweeting et al 2007a(Sweeting et al , 2007b.…”

“…Temperature has been found to have a significant effect on both nitrogen and carbon tissue-diet isotopic spacing in several species (Barnes et al 2007;Sweeting et al 2007aSweeting et al , 2007bBritton and Busst 2018). The effects of temperature on nitrogen fractionation are likely related to: (1) higher metabolic rates at high temperature affecting energy demands and potentially nitrogen-use efficiency; (2) changing metabolic rates affecting nitrogen excretion rates; and (3) self-catabolism in chronically nutritionally stressed fish.…”