Discussion
Dietary reliance of brown trout on terrestrial prey increased in
sympatry with invasive brook trout. The increased reliance on
terrestrial prey did not appear to be influenced by the low availability
of aquatic prey, indicating that it was likely caused by the behavioural
changes of sympatric brown trout exposed to the invasive species (Lovén
Wallerius et al. 2017; Larranaga et al. 2018; Cucherousset et
al. , 2020). Higher reliance on terrestrial prey of sympatric brown
trout resulted in lower relative content of EPA in their muscle tissues.
In the sympatric populations, the relative content of ALA increased and
the relative content of DHA decreased in tissues with increasing
reliance on terrestrial prey, but the relative content of ALA and DHA in
allopatric brown trout was not related to the diet quality. Sex had no
effect on diet quality and muscle content of n-3 LC-PUFA, but larger
(and presumably older, i.e., Bowker 1995; Öhlund et al. 2008)
individuals relied more on terrestrial prey and had lower relative
content of EPA and DHA in muscles. We found that the relative content of
DHA in muscle tissues was positively related to the total brain volume.
However, brain volume did not differ between sexes and allopatric and
sympatric populations. The brain morphology was not explained by any
variable considered. These findings demonstrate that the variation in
diet quality can cause divergence of n-3 LC-PUFA content in tissues of
wild fishes and that DHA content in muscle tissues is a predictor of the
brain size.
Lipids are the densest form of energy and key to dietary energy transfer
from producers to consumers (Parrish 2009). Therefore, the dietary
intake and subsequent retention of energy in form of lipids has a
substantial effect on the physiological development of consumers (Arts
et al. 2009). However, our results indicate, in agreement with previous
laboratory studies (Ishizaki et al., 2001; Lund et al., 2012), that the
availability of structural lipids, especially of DHA, and not the energy
reserve is key for brain development in wild fishes. These findings
suggests that the specialization of dietary niche on resources that have
similar content of energy, but differ in the content of n-3 LC-PUFA
(Heissenberger et al. 2010; Scharnweber et a. 2021), can lead to
diversification of brain development. This important finding indicates
that diet quality induced differences in brain development can
eventually also influence the cognitive capacity of individuals as small
brain volume has negative effects on learning (Marhounová et al. 2019;
Kotrschal et al. 2013) and on foraging behaviour (Wilson & McLaughlin
2010). Omega-3 LC-PUFA-deprived diets have also been shown to reduce
mitochondrial efficiency of muscles (Salin et al. 2021; Závorka et al.
2021) and reduce n-3 LC-PUFA in fish eggs (Hou & Fuiman 2020).
Therefore, carefully designed future experiments in a realistic
ecological context are needed to understand the trajectory of how
dietary omega-3 PUFA impacts consumer fitness.
Effects of diet quality on the physiology and brain development of
fishes necessarily depends on temporal scales on which the consumption
of different dietary resources occurs (Murray et al. 2015; Hou & Fuiman
2020). The relatively short study (i.e. , 8 weeks of dietary
treatment) by Závorka et al. (2021) on juvenile Atlantic salmonSalmo salar showed a strong shift of DHA content in the brain,
but no significant effect on brain size and performance in a cognitive
test. On the other hand, a longer study (i.e., 21 weeks of
dietary treatment) by Lund et al. (2012) on juvenile pikeperchSander luciperca demonstrated that dietary n-3 LC-PUFA
deprivation induces leads to reductions of DHA content and size of the
brain resulting in lower cognitive capacity. The reliance on terrestrial
prey in our study was estimated based on bulk δ13C
values of fin clips, which indicate dietary carbon on the temporal scale
of several weeks (Jardine et al. 2005; Layman et al. 2012). However, a
previous study has shown that the dietary shift of brown trout sympatric
with invasive brook trout occurs early in ontogeny and remains stable
across all life stages (Cucherousset et al. , 2020). Thus, it is
possible that the observed diet differences in our study represent a
long-term dietary specialization of individuals. Brown trout could also
acquire and retain DHA from seasonal resources not included in our prey
analysis, for example via egg predation in autumn (Aymes et al.
2010; Näslund et al. 2015), but internal synthesis from precursor
molecules (i.e., ALA, but mainly EPA) contained in prey
macroinvertebrates was likely the key source of DHA (Heissenberger et
al. 2010; Twining et al. 2019; Guo et al. 2021). Interestingly, despite
higher reliance on terrestrial prey and corresponding decrease of EPA in
muscle tissues, there was no significant reduction of DHA in muscles of
sympatric compared to allopatric brown trout. A potential explanation is
that sympatric brown trout has adapted to the lower dietary intake of
n-3 LC-PUFA via elevated retention and/or synthesis of DHA (Ishikawa et
al., 2019). However, our results also indicated limits of this
adaptation as the relative content of DHA in sympatric individuals
decreased with increasing reliance on terrestrial prey. Therefore,
individuals that relied mostly on terrestrial prey were clearly unable
to compensate the impact of poor diet quality on the biochemical
composition of their tissues. This is not surprising, because the
synthesis of DHA from dietary precursors comes at a substantial
energetic cost that can lead to reduced growth rate (Murray et al. 2014;
Závorka et al. 2021). The increased metabolic cost due to the elevated
DHA synthesis could possibly explain the lower growth rate and higher
mortality of brown trout sympatric with brook trout observed in previous
studies (Öhlund et al. 2008; Závorka et al. 2017).
Sexual differences did not influence the reliance on terrestrial prey,
the relative content of n-3 LC-PUFA in muscles, and total brain volume
and morphology. The lack of sexual differences in brain size morphology
contrasts with findings of Kolm et al. (2009) who showed that females of
stream resident brown trout have overall larger brain than males. In
contrast to the landlocked populations of our study, Kolm et al. (2009)
sampled individuals in a costal stream, which mostly contained
anadromous individuals. Therefore, the gene flow between anadromous and
residential part of the population (Nevoux et al. 2019) and the higher
availability of marine derived n-3 LC-PUFA delivered to the stream by
anadromous spawners (Näslund et al. 2015) could possibly lead to the
sexual divergence in brain morphology that does not occur in landlocked
populations. The lack of sexual difference in n-3 LC-PUFA muscle content
could be explained by the fact that our study was conducted in early
summer when investment to n-3 LC-PUFA rich gonads is low compared to
late autumn when the spawning season occurs (Jonsson & Jonsson 1997).
The size range of individuals in this study corresponds to subadult and
adult life-stage (Öhlund et al. 2008) when the capacity to retain and/or
synthetize n-3 LC-PUFA internally decreases compared to the early
ontogenetic stages (Chaguaceda et al. 2020). However, we still found
that larger individuals relied more on terrestrial prey and had lower
EPA and DHA contents in muscles. This confirms that larger and older
individuals may require less EPA and DHA than juvenile freshwater fishes
(Tocher 2010; Chaguaceda et al. 2020).
In conclusion, this field study demonstrates the largely overlooked
importance of dietary intake of n-3 LC-PUFA for brain development of
wild fishes, which have so far been recognized mainly under laboratory
and aquaculture conditions (Tocher 2010; Pilecky et al. 2021). We
demonstrated effects of reduced dietary intake of n-3 LC-PUFA induced by
co-existence with an invasive species, however, other anthropogenic
factors can alter the availability of n-3 LC-PUFA for stream dwelling
fishes and other consumers even more profoundly. For example, climate
change is predicted to decrease the availability of n-3 LC-PUFA in
aquatic food-webs (Hixson & Arts, 2016). It still remains to be
determined how wild animals respond to diet quality shifts, but our
results indicate that the reduction of dietary n-3 LC-PUFA can have
negative impacts even on species which are adapted to n-3 LC-PUFA
deprived prey (Syrjänen et al. 2011; Závorka et al. 2021). Dietary
intake of omega-3 LC-PUFA have a high potential to affect fitness of
consumers (Twining et al. 2021; Pilecky et al. 2021), and therefore
further studies are needed to understand how the availability n-3
LC-PUFA affects brain development, behaviour and physiology of wild
fishes and other animals.