Discussion:
Minnows caught in Ånnsjön generally ingested more zooplankton, compared
to minnows caught in the adjacent streams, which ingested benthic
invertebrates and terrestrial insects to a higher degree. Thus, my
results are in line with previous findings on minnow diet (22, 42).
Furthermore, morphology differed between minnows caught in the lake and
streams. This habitat-specific body shape was associated with the use of
a specific set of resources, indicating that minnow morphology is an
adaption to a specific foraging mode. While the streamlined morphology
of lake minnows can be seen as an advantage to swim in a uniform
environment, stream minnows were characterized by a downward pointed
snout and a deep body that could help feeding on benthic prey while
maneuvering in the structurally complex stream habitat (20, 21), where
zooplankton is typically scarce (33).
Resource polymorphism will lead to intraspecific divergence within a
single population (15, 16), and can be seen as an early stage of
speciation (6). As predicted from niche evolution theory,
morphologically divergence will reduce competition as less prey items
are shared (43, 44), and this pattern could also be demonstrated
empirically in the perch-roach system (45). Intraspecific
differentiation may initially emerge from phenotypic plasticity (46),
and depending on the stability of the selective regime, divergent
phenotypes may become genetically fixed (47, 48). Unfortunately, genetic
data for the minnows of this study is not available. Future studies to
investigate the level of genetic differentiation are needed to
characterize the position of the morphotypes of the European minnow in
Sweden along the specification continuum that could vary from adaptive
variation to complete and reproductive isolation (6, 49). However,
results of morphological divergence, but also resource use showed a
strong variation within the habitats, between the different locations.
Individuals caught at location S3 were feeding to a greater extent on
terrestrial insects, compared to individuals caught in the other two
stream locations. Furthermore, individuals caught at location L3 were
feeding to a greater extent on benthic invertebrates and ingested fewer
zooplankton than individuals caught at L1 and L2 and body shape were
more similar to stream minnows. Such variation indicates a strong degree
of plasticity in resource-morph formation, which would suggest that
differences between minnows of the two adjacent habitats might not be
genetically fixed. In contrast to the other lake habitats, water plants
were abundant at L3, which could provide suitable microhabitats for
benthic invertebrates. To understand the driving forces behind the
variation in the degree of divergence, estimates of prey abundances at
the different locations are needed.
In contrast to my results that are in accordance with the ones of
Ramler, Palandacic (28), Collin and Fumagalli (27) found minnows
inhabiting stream habitats in Switzerland to be more streamlined
compared to conspecifics living in lakes. However, besides attributing
these morphological adaptions to the hydrodynamic conditions occurring
in stream habitats, they further reported a high predation pressure
present in the lake habitats. A deeper body morphology can be seen as
advantageous under such kind of ecological conditions, as muscle mass
may enhance a rapid acceleration to escape predators (13, 50, 51). Only
very few studies have tried to identify the specific factors that may
shape the degree of variation in individual specialization (but see 52,
53, 54). Potentially, predation pressure could influence the strength
and direction on the correlation of minnow morphology and diet, but
further lab experiments are needed to resolve this relationship.
Scandinavian mountain lakes are characterized by a low species richness
of fish. In many of these lakes, that are often remote, minnows were
introduced as anglers used them as life bait (26). They are considered
as being invasive, due to the fact that they can reach high densities,
as for example, in Ånssjön (34). Næstad and Brittain (25) further showed
that they have the ability to modify lake food webs, thus being
responsible for a zoobenthos assemblage with a dominance of Chironomidae
and Oligochaeta, and a low abundance of Gammarus lacustris . As
lake minnows show a strong diet overlap with juvenile brown trout (24),
they are also regarded as one of the factors contributing to the reduced
recruitment and growth of the native brown trout in lake habitats (26).
My results presented herein indicate that minnows inhabiting stream
habitats may rely on different resources than the individuals inhabiting
lakes. Therefore, interspecific competition target at different species
in these contrasting habitats and patterns observed in lake habitats
cannot be directly transferred to the interactions occurring in stream
habitats. Nonetheless, the introduction of minnows into stream habitats
may also pose a similar threat for the native fish populations of the
stream, if a diet overlap would occur. Certainly, future studies need to
determine the consequences of minnow invasions on the stream food webs.