1 | INTRODUCTION
Aquatic invasions have become common and are one of the greatest threats
to aquatic ecosystems worldwide ( Alexander et al., 2015; Gherardi,
2007; Sala et al., 2000) by altering nutrient cycling (Vanni, 2021),
food webs (DeBoer et al., 2018), and abundance and distribution of
native species (Gallardo et al., 2016; Weber & Brown, 2011). Currently,
bigheaded carps (Hypothalmichthys moltrix andHypopthalmichthys nobilis ) are some of the most problematic
aquatic invasive species that have spread rapidly and broadly in North
America with adverse ecosystem effects (DeBoer et al., 2018; Solomon et
al., 2016; Tillotson et al., 2022). Pelagic planktivores native to China
and a small portion of eastern Russia (Kolar et al., 2007), bigheaded
carp were introduced to the United States for aquaculture and sewage
treatment resulting in their eventual escapement to natural waters where
populations rapidly increased (Kolar et al., 2005). Since their initial
escapement in the early 1980s, the Mississippi River has served as an
invasion highway throughout the central United States. The Upper
Mississippi River (UMR) is currently one of the primary invasion fronts
for bigheaded carp in the Midwestern United States due to a series of
locks and dams that have slowed their upstream expansion (Fritts et al.,
2021; Tripp et al., 2013; Whitledge et al., 2019). Lock and Dam 19
specifically represents a major barrier to their northern expansion (
Larson et al., 2017; Fritts et al., 2021; Tripp et al., 2013), although
adults have been captured as far north as pool 2 (U.S. Geological
Survey, 2022).
While adult bigheaded carp have been captured up to pool 2, reproduction
has only been documented to pool 16 (Camacho et al., 2023; Larson et
al., 2017). Fish reproductive phenology is the product of many
environmental conditions (Krabbenhoft et al., 2014; Yang et al. 2021)
and quantifying fish reproduction provides critical temporal, spatial,
population, and community metrics in relation to environmental
conditions (Pritt et al., 2015; Quist et al. 2004). Seasonality in
concert with water temperature is often considered necessary for
successful reproduction by triggering the release of gametes (Pankhurst
& Porter, 2003; Werner, 2002). Bigheaded carp require water
temperatures > 17°C to initiate reproduction, with
reproductive activity occurring up to 30°C (DeGrandchamp et al., 2007;
Kocovsky et al., 2012; Schrank et al., 2001). Alternatively,
reproduction of native fishes is generally initiated at cooler
temperatures and occurs over a more narrow range (e.g., walleyeSander vitreus , 5 to 10°C; Bozek et al., 2011; gizzard shadDorosoma cepedianum , 10 to 21°C; Becker, 1983; freshwater drumAplodinotus grunniens , 18 to 25°C; Swedberg and Walburg, 1970).
Beyond temperature, changing river discharge strongly affects
reproductive activity in many invasive and native lotic fishes
(Humphries et al., 2002; King et al., 2016; Dudley & Platania, 2007)
including bigheaded carp (e.g., Kolar et al., 2007; Schrank et al.,
2001).
Beyond environmental conditions, spatial variation in riverine habitats
can influence where invasive and native fish reproduce (Camacho et al.,
2023; Kolar et al., 2007). In rivers, spawning habitat relates to
channel position such as channel border, thalweg, and backwaters due to
differences in river discharge and species spawning requirements. Larval
habitat use often corresponds to their reproductive guild (Holland,
1986). Some species such as bigheaded carp and freshwater drum spawn in
the open water environments of the thalweg where eggs passively drift
until they hatch and reach sizes that allow increased mobility to escape
the current (Becker, 1983). Other species reproduce in the lower
velocities in channel borders and backwaters that provide cover for eggs
and developing larvae (e.g., walleye) or passively drift downstream
(e.g., gizzard shad; Holland, 1986; Simon, 1998). Consequently, while
most lotic larval fish assessments focus on thalweg collections, larval
occupancy may vary among taxa and habitats.
Given the network of environmental conditions for fish reproduction,
along with the uncertainty of ichthyoplankton sampling, determining
occurrence of riverine fish reproductive events can be difficult. High
spatio-temporal variability in larval catches is common (Cyr et al.,
1992; Leonardsson et al., 2016; Michaletz & Gale, 1999; Weber et al.,
2021). Therefore, it is difficult to determine when, where, and under
what conditions fish reproduction occurs, particularly when detection
probability is imperfect. Additionally, adult bigheaded carps are
difficult to capture compared to adult native fishes (Bouska et al.,
2017), but whether or not differences in sampling efficiency also exist
at the larval phase is unknown. Occupancy (Ψ) and detection (p )
modeling offers a quantitative method of estimating the true presence of
species in a system by accounting for imperfect detections based on
discrete encounters over temporal or spatial scales (MacKenzie et al.,
2002). Within fisheries, occupancy and detection modeling has been
primarily used to describe distribution of adult fishes (Potoka et al.,
2016; Schumann et al., 2020; Sullivan et al., 2018;), but models are
plastic and have been adapted to evaluate age-0 fish habitat use
(Burdick et al., 2008; Falke et al., 2010, 2012), timing of reproduction
events associated with environmental conditions (Falke et al., 2010;
Peoples and Frimpong, 2011; Pritt et al., 2014), and recruitment of
cryptic invaders (Weber & Brown, 2019). These models also provide
sample size requirements to achieve desired detection probabilities,
improving sampling designs and monitoring programs (Kelly et al., 2021;
Kuehne & Olden, 2016; Rodtka et al., 2015). Consequently, occupancy
models can serve as a useful tool for assessing reproductive dynamics of
bigheaded carp along leading edges of invasion where reproduction may be
limited, and spawning events can be brief and difficult to detect.
Our objectives were to estimate occupancy and detection probability of
bigheaded carp larvae and native fish taxa in relation to spatial and
environmental variation. Next, we quantified cumulative detection
probabilities to estimate sampling effort required to detect bigheaded
carp larvae compared to native species. First, we hypothesized bigheaded
carp larval occupancy would be higher in the thalweg compared to other
habitats and positively related to water temperature and river
discharge. Second, we hypothesized bigheaded carp larval occupancy would
be lower than native fishes and lower in Mississippi River pools along
the leading edge of population expansion than already established native
fish populations. Finally, we hypothesized larval detection
probabilities would be lower for bigheaded carp than native taxa.