5 | DISCUSSION
By comparing historical and contemporary food webs in the San Juan
River, we gained insight into trophic relationships among species, which
supports a prior hypothesis that resources could be limiting the
reestablishment of Colorado Pikeminnow in this modified system (Franssenet al ., 2007; Franssen et al ., 2019). Our results indicate
the fish community reduced its overall trophic resource use, potentially
increasing competition for resources, and Colorado Pikeminnow
experienced a reduction in resource use likely due to a shift to feeding
lower in the contemporary food web. Together, these results suggest
trophic resource availability may be reducing prey fish populations, and
in conjunction with the absolute loss of Roundtail Chub, could be
hindering Colorado Pikeminnow’s ability to completely transition to
piscivory and limit their ability to persist in the San Juan River.
Our results are consistent with other studies that demonstrated
decreased resource use by fishes following river regulation (DeLonget al ., 2011; Turner et al ., 2015; Thoms and DeLong,
2018). Here we were also able to identify species turnover per se (loss
of Roundtail Chub and addition of Red Shiner) as having a limited effect
on this trophic contraction. The reductions in trophic resource
diversity could stem from the loss of upstream inputs, limited
floodplain exchange, and overall habitat simplification which can reduce
primary basal and secondary consumer diversity and abundance (Vannoteet al ., 1980; Junk et al ., 1989; Turner et al .,
2015; Ward et al ., 2002; Winemiller et al ., 2010; Zeni and
Casatti, 2014). Additionally, deviations of the natural flow regime can
shift macroinvertebrate communities (Bunn and Arthington, 2002),
potentially altering sources of carbon incorporated into the food web
(e.g., relative abundance of allochthonous shredders vs. autochthonous
scrapers). The changes we observed in the San Juan River fish
community’s trophic structure indicated resource sharing increased which
has the potential to affect population dynamics of native fishes.
Results assessing changes to trophic niche overlap indicated competitive
interactions among all species potentially intensified between time
periods. Invertebrate size, abundance, and diversity can substantially
decrease after dam construction and all fishes included in our analysis
likely forage on invertebrates at some period of their lifetime (Bunn
and Arthington, 2002; Zeni and Casatti, 2014; Caldwell et al .,
2018). While changes to the San Juan River invertebrate community are
unknown, if invertebrate numbers are insufficient, competition for this
prey could reduce overall fish abundances (Van Poorten et al .,
2018). Indeed, the densities of invertivorous Flannelmouth Sucker,
Bluehead Sucker, Speckled Dace, and Red Shiner have declined in the San
Juan River over the last two decades following ongoing deviations from
the natural flow regime (Pennock et al ., 2022). Given the
surprising and significant increase in niche overlap by Colorado
Pikeminnow into all other species, this may also be indicative of a
paucity of fish prey given their presumed piscivorous nature (Vanicek
and Kramer, 1969; Franssen et al ., 2007; Franssen et al .,
2019). Bioenergetics modeling assessing production and current fish prey
abundances could be a useful path forward in quantifying the ability of
the system to support Colorado Pikeminnow, however, to be information
such modeling may need to include ontological prey preferences and
Colorado Pikeminnow predation rates.
Competition for resources can cause intraspecific individual
specialization (Araújo et al. 2011; Bolnick et al. 2010), which may
explain some trophic dispersion changes of fishes between time periods.
Two trophic generalists, Flannelmouth Sucker and Fathead Minnow, able to
consume algae and detritus in addition to invertebrates increased their
relative niche breadth (i.e., standard ellipse area) between time
periods. Although this diet flexibility could be advantageous in
situations where resources are limited, if specialization is towards
resources that are less energetically efficient, population abundances
could decline (Floeter et al ., 2004). Colorado Pikeminnow and
Speckled Dace deceased their diversity of resource use, which could be
in response to environmental change because they are habitat (Speckled
Dace; Gido et al ., 1997) or trophic (Colorado Pikeminnow; Vanicek
and Kramer, 1969) specialists. Thus, these species may be unable to
adapt when trophic resources when they become limited (Bolnick et
al ., 2010; Araújo et al ., 2011; Sanchez-Hernandez et al .,
2021).
Results from the stable isotope mixing models suggested Colorado
Pikeminnow shifted towards increased insectivory in the current food
web. Although we introduced uncertainty into our finding by estimating
isotopic variation of historical invertebrates, we think our results are
plausible because complete piscivory has been documented for
wild-spawned Colorado Pikeminnow >150 mm TL elsewhere in
the Colorado River Basin (Vanicek and Kramer, 1969) and the
δ15N signature of the two largest historical Colorado
Pikeminnow sampled here also indicated high piscivory rates.
Additionally, contemporary consumption of invertebrates was described in
a previous investigation where the diet of Colorado Pikeminnow 100 –
350 mm TL in the San Juan River was estimated to be
~25% invertebrates (Franssen et al ., 2019).
However, due to similar isotopic signatures among prey fishes, modeled
results had relatively high levels of uncertainty. Further insights into
prey consumption through stomach analysis may be required to identify
potential prey preferences of Colorado Pikeminnow and confirm the
relatively high predicted consumption of invertebrates in the current
food web.
Combined, cur results suggest Colorado Pikeminnow fed on relatively few
fish prey contemporarily compared to historic conditions possibly due to
reduced prey fish abundances. Although historical fish prey abundances
that supported Colorado Pikeminnow in the San Juan River are unknown, a
shift to greater insectivory could have implications for growth and
survival of stocked individuals that infrequently recruit to the adult
life stage (Clark et al ., 2018; Franssen et al ., 2019).
For piscivorous fishes like Colorado Pikeminnow, the ability to move
through ontogeny from an invertebrate diet to more energetically
efficient fish prey is necessary for rapid growth (Mittelbach and
Persson, 1998) and subsequent survival (Nilsson and Brönmark, 2000;
Persson et al ., 1996).
For piscivores like Colorado Pikeminnow, the establishment of some
nonnative species could be useful in subsidizing a reduced prey base
(Boltovskoy et al ., 2018; Maerz et al ., 2005). For
example, our mixing models suggested Colorado Pikeminnow consumed a
significant quantity of nonnative Fathead Minnow in both periods and
nonnative Red Shiner in the contemporary period. Increased densities of
these small-bodied nonnative fishes have been correlated with higher
body condition in Colorado Pikeminnow suggesting they could be an
important food source (Osmundson in review ). Thus, managers may
need to consider how reductions in these prey species could impact the
availability of trophic resources for Colorado Pikeminnow.