4 | DISCUSSION
Our study demonstrates differences in our ability to detect invasive and
native larval fish presence accounting for various environmental
conditions. Detection probabilities for bigheaded carp were lower than
native taxa and required more samples to detect their presence.
Detection probability estimates increased with water volume filtered for
all species, but detection also varied among habitats for carp and
freshwater drum, indicating spatial differences in sampling efficiency.
Similar to detection, bigheaded carp occupancy was generally lower than
native species, suggesting they have a narrower range of environmental
conditions suitable for reproduction. Temporal and thermal environmental
covariates improved occupancy probabilities among all species, while
hydrological covariates were only beneficial to bigheaded carp
occupancy.
Occupancy and detection models indicate some habitat separation among
taxa. On average, bigheaded carp and drum detection probabilities were
higher in thalweg and channel border habitats, whereas detection
probability of shad and freshwater drum was similar among habitats.
Pelagophil fishes, such as bigheaded carp and drum, reproduce in open
water conditions (Balon, 1975; Welcomme et al., 2006) associated with
the higher velocity thalweg and channel border river channel habitats.
Larval fishes in these habitats are likely younger and passively
drifting with the current (Holland, 1986), potentially making them
easier to capture. Alternatively, larval bigheaded carp can migrate out
of the main channel to areas with lower velocity at 5 to 6 days of age
and larger, more developed larvae that are better swimmers may be more
likely to avoid the ichthyoplankton net (Chapman and George, 2011; Roth
et al., 2023). In contrast to detection, occupancy of carp and
freshwater drum was similar among habitats, suggesting that all habitats
are used by larval carp and drum. Thus, if the objective is to simply
document bigheaded carp reproductive success, focusing sampling efforts
in the thalweg would be more efficient. Alternatively, shad and percid
detection was similar among habitats but occupancy was higher in
backwater habitats than thalweg and channel borders, consistent with
conventional reproductive guild classification for both shad
(litho-pelagophil) and percids (lithophil; Simon, 1998). While not
directly considered in this study, larval habitat selection can change
during later developmental stages due to gas bladder inflation and
increased horizontal mobility (Chapman and George, 2011). While we
limited our occupancy and detection models to sub-juvenile stages, there
may exist occupancy and detection habitat variation among sizes, ages,
and developmental stages due to changes in mobility.
Native and invasive larval occupancy both varied temporally and were
influenced by environmental conditions. Bigheaded carp reproduce in
rivers during May through June when water temperatures exceed 17°C, with
peak reproduction occurring at 22 to 26°C (Schrank et al., 2001), though
protracted spawning documented into the fall can occur in some systems
(Coulter et al., 2013; Papoulias et al., 2006). Freshwater drum
typically spawn between May and June in the Upper Mississippi River
(Butler, 1965) when water temperatures are between 18 to 25°C (Swedberg
& Walburg, 1970). Shad spawn over a broad temporal window with
reproduction occurring from late April into early August with water
temperatures between 10 to 21°C (Becker, 1983), whereas percids,such as walleye, spawn the earliest from late April to early May with
water temperatures ranging from 5 to 10°C (Becker, 1983; Bozek et al.,
2011). Similarly, we found maximum occupancy estimates corresponded with
Julian dates for bigheaded carp (June 19th),
freshwater drum (July 3rd), and shad and percids (May
24th) that were typically at the upper limits of their
documented spawning periods. This trend was mirrored in maximum
occupancy occurring from freshwater drum when water temperatures was
23.9°C, whereas the water temperature for carp (20.7°C) was well within
their recorded temperature limits and even lower than the cited
temperature range for peak reproduction (22-26°C; Schrank et al., 2001).
Maximum bigheaded carp occupancy in the Upper Mississippi River
occurring below the peak reproduction threshold is likely due to
stability of water temperatures in this region throughout our sampling
period (mean water temperature of 22°C from April to August). In
addition to mean temperature, increasing temperature variation led to
lower occupancy probabilities for carp and drum, but an increase in
occupancy probability for shad/percids. Fluctuations in water
temperature can negatively influence bigheaded carp reproduction due to
disruption in oogenesis (Majdoubi et al., 2022) and can reduce egg
viability of other fishes (Van Der Kraak & Pankhurst, 2011), indicating
post-spawning temperature stability could promote larval survival.
Larval occupancy among taxonomic groups was primarily affected by
temporal and thermal variation; however, bigheaded carp occupancy also
increased with mean river discharge and may have been negatively
affected by variability in discharge. Discharge and water temperature
are central catalysts of bigheaded carp reproduction (Camacho et al.,
2023; Kolar et al., 2007; Lohmeyer & Garvey, 2009; Schrank et al.,
2001). Adult bigheaded carp move upstream in spring with increasing
discharge and spawn during peak flows when water temperature exceeds
17°C (DeGrandchamp et al., 2007; Kocovsky et al., 2012; Schrank et al.,
2001). Effects of variation in discharge on bigheaded carp reproduction
are less understood. With increasing variation in discharge, our models
suggested a slight negative effect on bigheaded carp larval occupancy,
although the slope overlapped zero. Schaick et al., (2023) observed a
similar relationship between larval bigheaded carp densities and
variable discharge and theorized sustained, high magnitude discharged
events were preferred for bigheaded carp reproduction. While discharge
is also often cited as a major driver of reproduction for lotic fishes
(Dudley & Platania, 2007; King et al., 2016; Humphries et al., 2002),
we did not find an effect of discharge on native larval fish occupancy,
potentially because peaks in discharge were not synchronized with
appropriate spawning temperatures or because native taxa are more
adapted to flow regimes (Lytle & Poff, 2004). Alternatively, discharge
may not affect the occurrence of reproduction (e.g., occupancy), but
could affect the magnitude of reproduction (e.g., larval densities) and
still be responsible for large versus small year-classes (Weber et al.,
2021).
Understanding when, where, and under what conditions larval fishes are
present is strongly dependent on successfully capturing them given they
occur at a site. Drum, shad, and percids had higher detection
probabilities than bigheaded carp. Adult bigheaded carp are difficult to
capture (Bouska et al., 2017; Collins et al., 2015), but our results are
the first to document challenges associated with lower capture success
of invasive larvae compared to native taxa. Low detection of bigheaded
carp larvae can make it difficult to document reproductive events,
particularly along invasion fronts, resulting in misinformed population
status assessments with implications for management decisions. For
instance, efforts are underway in the Upper Mississippi River to install
barriers to slow or stop adult upstream movements into areas where
reproduction has not yet been documented. Sampling effort strongly
influenced our ability to detect larval fishes, but effects varied among
taxa. Biologists can improve larval detection probabilities by 1)
focusing sampling in the thalweg, 2) increasing the water volume
filtered per tow, and 3) increasing the number of tows collected per
site visit. While we collected three tows per site visit, cumulative
detection curves indicated we would need to collect 14 thalweg/channel
border samples to achieve 90% detection probability of bigheaded carp
larvae. Alternatively, increasing the water volume filtered by 33%
would reduce the sample size to 9 thalweg/channel border tows. These
results provide an adaptable and flexible framework to determine how
many samples to collect and how much water to filter based on river
conditions (e.g., debris load) and acceptable uncertainty in bigheaded
carp presence. Further, sampling could also be adjusted to determine
successful reproduction through targeted sampling based on habitat,
temporal, thermal, and hydrological effects on occupancy to optimize
future sampling to assess reproduction of cryptic invaders.