Front. Environ. Sci. Frontiers in Environmental Science Front. Environ. Sci. 2296-665X Frontiers Media S.A. 1754403 10.3389/fenvs.2026.1754403 Original Research Flow ecology of invasive suckermouth catfish in urbanized ridge-to-reef systems on O’ahu, Hawai’i Grabowski et al. 10.3389/fenvs.2026.1754403 Grabowski Timothy B. 1 * † Project administration Writing – original draft Data curation Supervision Formal Analysis Methodology Visualization Investigation Conceptualization Writing – review and editing Funding acquisition Resources Tsang Yinphan 2 † Investigation Conceptualization Writing – review and editing Visualization Formal Analysis Funding acquisition Bartz Danielle 3 Investigation Writing – review and editing Data curation Project administration Methodology Yap Cory 2 4 Methodology Investigation Writing – review and editing Resources Falke Jeffrey 5 Conceptualization Funding acquisition Writing – review and editing Bellmore J. Ryan 6 Funding acquisition Conceptualization Writing – review and editing Fellman Jason B. 7 Writing – review and editing Funding acquisition Conceptualization U.S. Geological Survey, Hawai’i Cooperative Fishery Research Unit, University of Hawai’i at Hilo, Hilo, HI, United States Department of Natural Resources and Environmental Management, University of Hawai’i at Mānoa, Honolulu, HI, United States Marine Biology Graduate Program and Hawai’i Cooperative Fishery Research Unit, University of Hawai’i at Mānoa, Honolulu, HI, United States Office of Community Science, ‘Iolani School, Honolulu, HI, United States U.S. Geological Survey, Nevada Cooperative Fish and Wildlife Research Unit, University of Nevada, Reno, Reno, NV, United States Pacific Northwest Research Station, USDA Forest Service, Juneau, AK, United States Program on the Environment and Alaska Coastal Rainforest Center, University of Alaska Southeast, Juneau, AK, United States Correspondence: Timothy B. Grabowski, tgrabowski@usgs.gov

ORCID: Timothy B. Grabowski, orcid.org/0000-0001-9763-8948; Yinphan Tsang, orcid.org/0000-0002-0593-4916

13 02 2026 2026 14 1754403 25 11 2025 07 01 2026 26 01 2026 Copyright © 2026 Grabowski, Tsang, Bartz, Yap, Falke, Bellmore and Fellman. 2026 Grabowski, Tsang, Bartz, Yap, Falke, Bellmore and Fellman https://creativecommons.org/licenses/by/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Introduction

The effects of flow regimes on the ability of invasive species to establish and maintain populations in the ridge to reef (R2R) systems common to oceanic islands is not well understood. The hydrology of the relatively short, high-gradient, and flashy R2R streams of oceanic islands may be extremely different from that of the continental watersheds invasive species originate from and thus may exert a stronger influence on their ecology. Our objective was to evaluate the effects of annual variability in flow conditions on the growth and recruitment of invasive armored Suckermouth Catfish Hypostomus c.f. watwata in Hawaiian streams.

 Methods

Suckermouth Catfish were captured from three streams of the Ala Wai Watershed on O’ahu. We then measured, weighed, and extracted the lapilli from each fish. We used back-calculated lengths at age to estimate the effects of interannual variability in flow on growth and recruitment.

 Results

Individuals ranged from 0–16 years old and grew rapidly in their first 3 years after which growth slowed substantially. The growth of Suckermouth Catfish was positively influenced by flow conditions indicative of wetter years and more stable flow and negatively influenced by flow conditions indicative of drier years and more variable flows. However, the specific annual flow metrics most strongly influencing growth varied by stream. Similarly, recruitment was positively influenced by higher winter flows with lower daily variability.

 Discussion

The observed effects of flow on Suckermouth Catfish growth and recruitment suggests that they are not particularly well suited for the flow conditions characteristic to R2R systems and that anthropogenic alterations to the hydrology and physical in-stream habitats may have enabled the species to be more successful on O’ahu than it would have been otherwise.

 age and growth ecohydrology invasive species otoliths recruitment The author(s) declared that financial support was received for this work and/or its publication. Funding for the project was provided by the Pacific Islands and Alaska Climate Adaptation Science Centers (contract #G21AC10774), and the Hawai’i Cooperative Fishery Research Unit and the Nevada Cooperative Fish and Wildlife Research Unit. The Hawai’i Unit is jointly sponsored by the U.S. Geological Survey (USGS), University of Hawai’i System, Hawai’i Department of Land and Natural Resources, and U.S. Fish and Wildlife Service (USFWS). The Nevada Unit is jointly sponsored by the USGS, the University of Nevada - Reno, the Nevada Department of Wildlife, USFWS, and the Wildlife Management Institute. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. section-at-acceptance Freshwater Science Introduction

Alteration of flow regimes due to anthropogenic climate change has the potential to fundamentally change the ecological state of watersheds worldwide. Shifts in watershed flow regimes alter nutrient and sediment transport, species composition and abundance patterns, and ecological and cultural services (Palmer et al., 2009). While these effects are reasonably well understood in continental sub-basins, there has been little effort to examine the effects of flow regime alteration at the scale of the entire watershed, typical of ridge-to-reef (R2R) systems, such as the Pacific Islands (except see Wiegner et al., 2009). This is problematic since these systems harbor relatively high levels of biodiversity and endemic species; serve as important conduits of nutrient delivery to sensitive coastal environments; and are culturally and economically important to adjacent communities. For example, the State Wildlife Action Plan (Hawai’i Department of Land and Natural Resources, 2015) for Hawaiian stream fishes identifies habitat degradation associated with altered stream flows as a primary threat to native fishes. Further, coral reef management in Hawai’i is transitioning to R2R models to mitigate nutrient transport to nearshore waters such as seen in West Maui (Oleson et al., 2017). In Hawai’i, R2R watersheds support a suite of endemic (Chace, 1983; McDowall, 2003; Lindstrom et al., 2012; Christensen et al., 2021; Mathews et al., 2025) and culturally important (Titcomb and Pukui, 1972; Costa-Pierce, 1987; Maly and Maly, 2003; Winter et al., 2018) amphidromous gobies, shrimp, and snails; however, there are concerns that shifting hydrologic patterns and invasive species (Fitzsimons et al., 1997; Brasher et al., 2006; Layhee et al., 2014; Moody et al., 2021; Hain et al., 2019) will undermine the capacity for these watersheds to support their endemic fauna as seen in other systems (Sergeant et al., 2017; 2020). This could occur by altering stream thermal regimes, food availability or nutrient export to coastal ecosystems.

Recruitment, growth, and mortality are the three primary factors that determine the size and structure of fish populations. These primary factors interact with each other and are influenced by environmental conditions, such as temperature and flow regime. Predicting and modeling population responses to climate and landscape perturbation is therefore dependent on accounting for these environmental influences. In more typical systems, flow regime alterations can result in reduced growth rates and recruitment of native specialist species and favor the growth and recruitment of non-native generalists (Alexandre et al., 2013). However, understanding the influence of flow regimes and other environmental factors on fish population dynamics has relevance beyond the conservation and management of fish populations. Changes in a fish assemblage and growth of a fish population size can locally influence the retention and transport of nutrients in typical watersheds (Capps and Flecker, 2013; Rubio et al., 2016).

We examined the relationships between annual flow metrics and the growth and recruitment of an introduced armored suckermouth catfish, or loricariid, species, Suckermouth Catfish Hypostomus c.f. watwata in Hawaiian streams. Loracariids are native to South America, but various species have been widely introduced throughout North America and Asia, largely due to aquarium releases. Once established, Suckermouth Catfish can disrupt the nutrient dynamics of the receiving waterbody due to their ability to sustain large populations that tend to congregate during daylight hours (Capps and Flecker, 2013; Rubio et al., 2016). Further, as part of their spawning activity, the species creates burrows into the stream bank, which destabilizes them and leads to changes in sediment load and inputs of terrestrial material (Hoover et al., 2004; Nico et al., 2009). In Hawai’i, Suckermouth Catfish were originally found in the lower reaches of Mānoa Stream on O’ahu in the 1980s and have since become established in numerous watersheds on the island (Sabaj and Englund, 1999; Yamamoto and Tagawa, 2000) where they may comprise a significant proportion of the biomass. Sabaj and Englund (1999) identified the individuals on O’ahu as likely belonging to the H. watwata group based on coloration patterns and morphology; however, there remains considerable uncertainty as to the exact identity of the Hypostomus species in Hawaiian streams. Regardless, Suckermouth Catfish is regularly targeted in removal events and generally captured in large numbers. Therefore, our objective was to evaluate the effects of annual variability in flow conditions on the growth and recruitment of invasive loricariid armored catfish in Hawaiian streams.

 Methods Study area

Makiki, Mānoa, and Pālolo valleys comprise the highly urbanized Ala Wai Watershed (Figure 1). The Ala Wai Watershed is one of the most densely populated areas in Hawai’i with about 200,000 residents within a total land area of only 42.2 km2. Roughly one-third of the upper headwater regions are forested, while the remainder is moderately to heavily urbanized, consisting of residential and commercial neighborhoods. Of the three streams, Mānoa is the least modified with its channel widened and banks earthen-shaped or hardened with concrete walls only in the middle and lower reaches, whereas Pālolo and Makiki have been heavily modified for flood control throughout their runs. There are a total of six U.S. Geological Survey (USGS) stream gauges within Ala Wai Watershed with more than 10 years of hourly to daily stream records. We adopted the Hydrological Index Tool (Olden and Poff, 2003; Poff, 1996) to calculate hydrologic metrics to describe seasonal and annual flow characteristics of stream reaches representing five flow regimes, including magnitude, frequency, duration, timing, and rate of change (Poff et al., 1997) The selected study area is of interest to local community groups who are restoring this watershed to mitigate the altered stream ecosystem, as well as better care for and understand the waterways in facing the anticipated flood risks. Further, these streams are of interest to the Hawai’i Department of Land and Natural Resources - Division of Aquatic Resources because of the high levels of invasive species present in these streams that threaten populations of imperiled endemic stream fishes.

Map of the Ala Wai Watershed on the island of O’ahu, Hawai’i showing locations in Mānoa, Makiki, and Pālolo Streams where Suckermouth Catfish Hypostomus c.f. watwata were collected during 2022–2023. The locations of the stream gages that provided the discharge data used in this study are also indicated. Imagery from Google Earth and represents a composite of data from Esri, DigitalGlobe, GeoEye, Earthstar Geographics, NES Airbus DS, USDA, USGA, AeroGRID, IGN, and the GIS User Community.

Satellite map showing the Ala Wai Watershed on Oahu, Hawaii, outlined in orange, with blue lines for streams, yellow circles for fish collection sites, and various green and purple triangles for stream gage locations near urban landmarks like the University of Hawaii at Manoa, Ala Moana Beach, and Waikiki Beach. Inset map shows the watershed’s location within the Hawaiian Islands. Key in the lower right explains symbol meanings. Sample collection

To evaluate the effect of annual hydrograph characteristics (Poff et al., 1997; Clilverd et al., 2019) on the relative year class strength and growth rates of Suckermouth Catfish, we collected fish from Makiki, Mānoa, and Pālolo Streams in the Ala Wai Watershed on O’ahu. Capture efforts were conducted in collaboration with an ongoing outreach effort, Paʻēpaʻē o Waikolu, housed at ʻIolani Schoolʻs Department of Community Science that develops and implements classroom curriculum and field protocols for use in K-12 public and private schools in the Honolulu area emphasizing environmental awareness of freshwater stream ecosystems. Suckermouth Catfish were captured during invasive species removal events conducted by grades K-12 school children under the supervision of Paʻēpaʻē o Waikolu personnel (C. Yap, A. Charuk), teachers, and project personnel. Students were positioned at the downstream end of a survey reach, each with a 0.92 × 1.22 m pushnet with 0.48-cm mesh, referred to as ‘ōpae nets in Hawaiian. Students were positioned next to each other to create a barrier with their ‘ōpae nets. Teams of students would then enter the water approximately 15 m upstream of the barrier with weighted seines and drive fish into the downstream barriers.

Captured Suckermouth Catfish were euthanized in a >400 mg/L aqueous solution of clove oil (eugenol; AVMA, 2020), measured to the nearest mm total length (TL) and weighed to the nearest 1 g. Sagittal and lapillar otoliths were extracted and stored dry prior to being prepared as described in Long and Grabowski (2017). Otoliths were embedded in epoxy and a 0.5-mm transverse section, inclusive of the nucleus, was removed using a low-speed isometric saw (Beuhler, Lake Bluff, Illinois). The resulting sections were mounted onto glass microscope slides with CrystalBond thermoplastic cement (Aremco Products Inc., Valley Cottage, New York) and wet sanded with increasingly fine grit sandpaper until the nucleus was exposed. The section was then flipped over and polished similarly on the other side. The polished section was then photographed using a compound light microscope equipped with a 14-megapixel digital camera. The age of each individual was determined by counting annuli and the radius of the otolith at each annuli (Figure 2) and the total radius were measured using Fiji v 2.14 (Schindelin et al., 2012).

Photograph of thin cross section of the lapillus of a Suckermouth Catfish Hypostomus c.f. watwata captured from the Ala Wai Watershed on O’ahu, Hawai’i during sampling conducted on 11 May 2022. This fish was 170 mm total length and estimated to be 4 years old. Presumed annuli are indicated by circles.

Microscope image of a fish otolith, or ear stone, showing concentric growth rings and four red dots aligned along the right edge, indicating annuli. Data analysis

Back-calculated lengths at age were estimated using the direct proportion method (Francis, 1990; Shoup and Michaletz, 2017) Equation 1: T L t = T L c R t R c where TL t is the back-calculated length at time t, TL c is the total length at capture, R c is the otolith radius at capture, and R t is the otolith radius at time t.

A Von Bertalanffy growth function (VBGF) was fitted to the back-calculated length at age data using a single level hierarchical nonlinear model to account for the lack of independence associated with back-calculation (Ogle et al., 2017). The VBGF is typically formulated as shown in Equation 2: L t = L ∞ 1 − e − k t − t 0 where L t is length at time t, L ∞ is the average asymptotic length of the population, k is the growth coefficient, and t 0 is the theoretical time when length is zero. The resulting VBGF was used to generate predicted TL at age to calculate a relative growth index (RGI; Quist et al., 2003) value for each back-calculated length at age as shown in Equation 3: R G I = b c T L t − p T L t p T L t   ×   100 where pTL t and bcTL t are the predicted and back-calculated TL at time t respectively. The RGI values were then used as the response variable to assess the effect of annual flow characteristics on growth (Jacquemin et al., 2015; Groeschel-Taylor et al., 2020) as described below.

A catch-curve based method was used to assess mortality (Miranda and Bettoli, 2007). An age-length key was generated using the age estimates collected as described above and used to generate age-frequency distributions for the sampled populations of Suckermouth Catfish (Isely and Grabowski, 2007). These distributions were used to generate catch curves for each sampled population, and the residuals of each cohort from a regression fitted to the descending leg of these catch curves was used as an index of recruitment to compare the relative strength of cohorts within each sampled population (Maceina, 1997; Isermann et al., 2002; Maceina, 2003) relative to annual flow characteristics as described below.

Streamflow data were acquired from the six USGS gages within the Ala Wai Watershed with continuous records in the period of 2007–2023 (Figure 1). We adopted EflowStats (https://github.com/DOI-USGS/EflowStats), an R package by USGS, to reimplement the Hydrologic Index Tool (HIT; Henriksen et al., 2006) for calculating 171 biological relevant hydrologic indices; additionally, EflowStats added-calculates 7 statistics used for streamflow classification referred to as the “Magnificent Seven” (MAG7, Archfield et al., 2013). A principal components analysis was performed on the resulting output of annual flow metrics for each station to produce summary variables describing flow conditions (Jacquemin et al., 2015). The influence of flow regime was evaluated on growth rate using a repeated-measures mixed-model ANCOVA using the RGI or relative recruitment values as the dependent variable and using the individuals as a subject effect.

 Results

Annual flow regimes during 2010–2022 in Makiki Stream tended to be markedly different from those in Mānoa and Pālolo Streams despite considerable interannual variability in their annual flow regimes and despite flows being generally greater overall in Mānoa Stream (Figure 3). Annual flow regimes in the three streams tended to fall within a similar range of conditions (Figure 4). Five principal components accounted for approximately 66% of the variability in annual flow regime across years and streams. The first principal component (eigenvalue: 28.4, proportion of variance explained: 0.28) primarily separated Makiki Stream from Mānoa and Pālolo Streams (Figure 4) and was primarily defined by minimum daily flow, median daily flow, and the mean minimum flow for February and April–July (Table 1). The remaining four principal components collectively explained an additional 38% of the variance and encompassed variables related to flow magnitudes, and to a lesser extent, frequency and duration. None of the hydroecological indices related to the timing of flow events meaningfully contributed to the principal components. However, the minimum flow for February indicates the lower flow during a wet month, and the minimum flow between April to July indicates the low flow during the dry season.

Annual hydrographs illustrating the 95% confidence interval around the mean daily discharge in Makiki [(A) USGS gage 16238000], Mānoa [(B) USGS gage 16241600], and Pālolo Streams [(C) USGS gage 16247100] in the Ala Wai Watershed on O’ahu, Hawai’i during 2010–2022. All stream gage data taken from U.S. Geological Survey (2022).

Three vertically stacked line graphs labeled A, B, and C, representing Makiki, Manoa, and Palolo, respectively, display mean daily discharge in cubic meters per second versus day of year. Graph A uses red, graph B uses blue, and graph C uses yellow lines, with graph B showing consistently higher and more variable discharge than A and C. All graphs have a y-axis from zero to three and an x-axis from zero to three hundred sixty.

Biplots of the first five principal components (PC) calculated from hydroecological statistics of stream flow describing the annual flow regime in Makiki, Mānoa, and Pālolo Streams in the Ala Wai Watershed on O’ahu, Hawai’i during 2010-2022 (A: PC2 vs PC1; B: PC3 vs PC1; C: PC4 vs PC1; D: PC5 vs PC1). The hydroecological statistics for each principal component can be found in Table 1.

Four-panel scientific graphic showing principal component analysis with scatter plots labeled A, B, C, and D; each plot displays red, blue, and yellow dots representing Makiki, Manoa, and Palolo, respectively, with labeled axes indicating different hydrological metrics related to flow variability, base flows, pulse duration, and seasonal changes.

Eigenvalues of the first five principal components calculated from hydroecological statistics of stream flow describing the annual flow regime in Makiki, Mānoa, and Pālolo Streams in the Ala Wai Watershed on O’ahu, Hawai’i during 2010–2022.

Flow regime component Variable PC 1 PC 2 PC 3 PC 4 PC 5
​ Eigenvector 28.4 18.0 10.4 5.7 4.4
Proportion of variance explained 0.28 0.18 0.10 0.06 0.04
Magnificent Amplitude 0.03 −0.01 −0.05 0.23 −0.20
Seven Autoregressive lag-one correlation coefficient 0.03 0.03 0.00 0.17 0.20
Phase 0.01 0.03 −0.02 0.13 −0.15
Duration Annual minimum daily flow 0.16 0.06 0.09 0.01 −0.02
Annual minimum daily flow/Median of entire record 0.07 −0.12 0.21 0.04 0.05
Low (75%) exceedance flows 0.04 −0.12 0.22 −0.05 0.07
Low (90%) exceedance flows 0.06 −0.13 0.21 −0.01 0.06
Low flow pulse duration −0.05 0.01 −0.02 0.18 −0.01
Frequency Low flow pulse count 0.07 −0.02 0.06 −0.21 0.01
Magnitude Mean monthly flow - January 0.08 0.06 0.10 −0.10 −0.17
(Mean) Mean monthly flow - February 0.11 0.09 0.01 −0.15 −0.18
Mean monthly flow - March 0.07 0.07 0.10 −0.16 0.10
Mean monthly flow - July 0.12 0.12 −0.02 0.07 0.15
Mean monthly flow - August 0.10 0.12 −0.01 0.17 0.11
Mean monthly flow - September 0.12 0.10 −0.01 0.12 −0.17
C.V. of monthly flows - February −0.01 0.05 −0.11 −0.26 −0.03
C.V. of monthly flows - March −0.05 0.04 −0.03 −0.22 0.16
C.V. of monthly flows - June −0.01 0.06 −0.08 0.11 0.18
C.V. of monthly flows - November −0.01 0.05 −0.05 −0.12 0.21
C.V. of monthly flows - December −0.06 0.09 0.07 −0.12 0.17
Variability across monthly flows (max - min/median) −0.11 0.09 0.19 0.09 −0.06
Variability across monthly flows (Q90 - Q10/median) −0.11 0.10 0.18 0.09 −0.06
Median of daily mean flows 0.16 0.11 0.03 0.01 −0.02
Skewness in monthly flows −0.10 0.10 0.18 0.11 −0.07
Range in daily flows, ratio of Q10 to Q90 −0.08 0.14 −0.17 0.01 −0.01
Range in daily flows, ratio of Q20 to Q80 −0.07 0.14 −0.18 0.01 −0.03
Range in daily flows, ratio of Q25 to Q75 −0.06 0.15 −0.18 0.02 −0.04
Coefficient of daily variation −0.12 0.18 0.01 −0.02 0.01
Skewness of daily flows −0.11 0.08 0.17 −0.06 0.08
Kurtosis of monthly flows −0.11 0.05 0.18 −0.06 0.08
Magnitude Mean maximum flow - January 0.04 0.06 0.12 −0.10 −0.15
(Maximum) Mean maximum flow - February 0.09 0.09 −0.01 −0.18 −0.16
Mean maximum flow - March 0.05 0.07 0.09 −0.17 0.15
Mean maximum flow - July 0.08 0.10 −0.03 0.19 0.13
Mean maximum flow - October 0.09 0.10 −0.01 0.03 −0.20
C.V. of maximum flows −0.10 0.05 0.17 0.03 −0.08
High flow index, 10% exceedance −0.08 0.17 −0.03 0.01 0.00
High flow index, 25% exceedance −0.06 0.17 −0.11 −0.03 −0.03
High flow volume, mean volume of flows > 3X median −0.11 0.09 0.17 0.07 −0.03
High flow volume, mean volume of flows > 7X median −0.12 0.09 0.18 0.06 0.00
High peak flow −0.12 0.09 0.16 0.03 −0.02
Magnitude Mean minimum flow - January 0.14 0.05 0.06 −0.02 −0.19
(Minimum) Mean minimum flow - February 0.15 0.06 0.05 −0.06 −0.17
Mean minimum flow - April 0.15 0.08 0.06 −0.09 0.03
Mean minimum flow - May 0.16 0.09 0.05 −0.06 0.04
Mean minimum flow - June 0.16 0.09 0.04 0.02 0.02
Mean minimum flow - July 0.15 0.10 0.03 0.04 0.09
Mean minimum flow - September 0.13 0.10 0.01 0.15 0.07
Mean minimum flow - November 0.13 0.08 0.01 0.16 −0.09
Mean minimum flow - December 0.15 0.08 0.05 0.02 −0.16
Annual minimum flow 0.07 −0.12 0.21 0.04 0.05
Low flow index, ratio of annual minimum to median 0.09 −0.17 0.11 0.04 0.03
Median of annual minimum flows 0.07 −0.12 0.21 0.04 0.05
Base flow, based on 7 day moving average 0.10 −0.16 0.11 0.02 0.03
Base flow, ratio of minimum annual flow to mean annual flow 0.09 −0.17 0.11 0.04 0.03
Base flow, based on 5 day minimum flows 0.08 −0.18 0.02 0.08 −0.08
Phase 0.01 0.03 −0.02 0.13 −0.15

Bold values highlight coefficients ≥ |0.15|.

A total of 234 Suckermouth Catfish were captured from Mānoa, Makiki, and Pālolo Streams during 15 sampling events conducted during 2022–2023 (Table 2). The overall mean (±SE) TL of the collected fish was 195 ± 4 mm (range: 60–359 mm) and the overall mean weight was 97 ± 6 g (range: 2–440 g). The samples of Suckermouth Catfish captured from Pālolo consisted of larger, heavier, and older individuals than the samples from Makiki and Mānoa (F 2,118 ≥ 44.90, P ≤ 0.01; Table 2). Similarly, individuals captured from Makiki were larger, heavier, and older than those from Mānoa (F 2,118 ≥ 44.90, P ≤ 0.01; Table 2). The length-weight relationships of Suckermouth Catfish in the Ala Wai Watershed exhibited a similar pattern of variability, with individuals being captured from Mānoa Stream weighing less than individuals of the same length captured from Makiki or Palolo Streams (F 8,231 = 2.25, P < 0.01; Figure 5 and Table 2).

Mean (±SE) and range of total length (TL), weight (W), age and length-weight relationship parameters estimates (a, b) of Suckermouth Catfish Hypostomous c.f. watwata captured from Makiki, Mānoa, and Pālolo Streams in the Ala Wai Watershed on O’ahu, Hawai’i during 2022–2023.

Stream n Mean (±SE) TL (mm) Range TL (mm) Mean (±SE) W (g) Range W (g) a (±SE) b (±SE)
Makiki 47 209 ± 9 137–339 112 ± 13 5–440 0.000005 ± 0.000002 3.12 ± 0.08
Mānoa 102 138 ± 5 76–335 31 ± 3 2–140 0.000011 ± 0.000009 2.95 ± 0.16
Pālolo 85 249 ± 5 154–359 162 ± 9 30–435 0.000014 ± 0.000004 2.93 ± 0.05
Overall 234 195 ± 4 60–359 97 ± 6 2–440 0.000007 ± 0.000002 3.05 ± 0.04

The length-weight relationships were fitted to the function: W = a   · L b where a and b are species and population-specific constants.

Length-weight relationship of Suckermouth Catfish Hypostomus c.f. watwata captured from Makiki, Mānoa, and Pālolo Streams in the Ala Wai Watershed on O’ahu, Hawai’i during 2022–2023. The presented curves use the function: with parameter estimates (±SE) differing between Makiki (a = 0.000005 ± 0.000002, b = 3.12 ± 0.08), Mānoa (a = 0.000011 ± 0.000009, b = 2.95 ± 0.16), and Pālolo (a = 0.000014 ± 0.000004, b = 2.93 ± 0.05) Streams (F 8,231 = 2.25; P < 0.01).

Scatter plot with three curved lines compares weight in grams on the y-axis to total length in millimeters on the x-axis, using red for Makiki, blue for Manoa, and yellow for Palolo. Data points cluster along the axes, and all lines show an upward exponential trend.

We were only able to successfully estimate the age for 137 of the 234 individuals captured due to a combination of the extremely small size of the Suckermouth Catfish otoliths and the difficulty associated with getting reliable age estimates from lapilli or the sagitta of ostariophysan fishes (Michaletz et al., 2009; Long and Stewart, 2010). Individuals sampled from the Ala Wai Watershed ranged from 0–16 years old (Figure 6) with a mean (±SE) age of 4.8 ± 0.4 years and a median age of 5 years. While the age-frequency distribution of Suckermouth Catfish from the Ala Wai Watershed exhibited distinct peaks, these peaks were not apparent in the length-frequency distribution (Figure 5).

Length-frequency [n = 234; (A)] and age-frequency [n = 137; (B)] distributions of Suckermouth Catfish Hypostomus c.f. watwata captured from Makiki, Mānoa, and Pālolo Streams in the Ala Wai Watershed on O’ahu, Hawai’i during 2022–2023.

Two side-by-side stacked bar charts compare proportions of three groups: Makiki (red), Manoa (blue), and Palolo (yellow). Panel A displays total length in millimeters, and Panel B shows age in years. All categories are labeled, with a maximum proportion around zero point three.

Suckermouth Catfish grew quickly during their first year, reaching a mean (±SE) of 91 ± 1 mm TL, but growth slowed to 20–30 mm TL per year in the second and third year and then remained <17 mm TL per year in subsequent years (Table 3). The VBGF fitted to the back-calculated length at age data yielded parameter estimates (±SE) of L ∞ = 342.2 ± 10.3 mm TL, k = 0.102 ± 0.008 years-1, and t 0 = −2.222 ± 0.248 years (Figure 7). Mean back-calculated TL at age varied between the three streams (F 2,533 = 5.44, P < 0.01; Figure 7). Individuals captured from Pālolo were on average about 8% larger at any given age than individuals from Mānoa (t 533 = −3.11, P = 0.01). Individuals from Makiki tended to exhibit more variability in their back-calculated length at age than the other two streams and therefore were statistically similar to the other two streams (|t 533| ≤ 2.20, P ≥ 0.07).

Mean (±SE) back calculated total length (TL) and growth by age and cohort of Suckermouth Catfish Hypostomus c.f. watwata captured from Makiki, Mānoa, and Pālolo Streams in the Ala Wai Watershed on O’ahu, Hawai’i during 2022–2023.

Age class Cohort n Mean back-calculated TL (mm) at age
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
XVI 2006 3 94 ± 13 123 ± 12 143 ± 10 161 ± 9 176 ± 9 188 ± 11 198 ± 12 207 ± 13 218 ± 11 231 ± 12 242 ± 8 253 ± 7 266 ± 5 273 ± 6 283 ± 5 292 ± 4
XV 2007 1 93 122 136 152 161 179 193 203 217 234 250 269 277 297 307 ​
XIV 2008 1 80 104 121 138 154 168 179 187 197 205 213 226 238 246 ​ ​
XIII 2009 0 — — — — — — — — — — — — — ​ ​ ​
XII 2010 3 103 ± 16 139 ± 15 158 ± 14 177 ± 16 194 ± 17 206 ± 16 223 ± 15 239 ± 16 250 ± 18 262 ± 19 280 ± 23 280 ± 23 ​ ​ ​ ​
XI 2011 2 104 ± 10 129 ± 14 147 ± 19 163 ± 18 180 ± 18 194 ± 19 205 ± 20 221 ± 19 232 ± 16 242 ± 16 251 ± 15 ​ ​ ​ ​ ​
X 2012 7 96 ± 5 130 ± 7 154 ± 6 171 ± 6 189 ± 6 206 ± 7 222 ± 8 234 ± 10 247 ± 11 256 ± 12 ​ ​ ​ ​ ​ ​
IX 2013 8 93 ± 7 122 ± 7 145 ± 7 164 ± 8 180 ± 8 192 ± 8 205 ± 8 217 ± 9 231 ± 10 ​ ​ ​ ​ ​ ​ ​
VIII 2014 13 93 ± 4 124 ± 5 150 ± 6 168 ± 7 183 ± 7 199 ± 8 210 ± 8 221 ± 9 ​ ​ ​ ​ ​ ​ ​ ​
VII 2015 15 90 ± 4 121 ± 4 144 ± 5 162 ± 5 179 ± 5 191 ± 5 203 ± 6 ​ ​ ​ ​ ​ ​ ​ ​ ​
VI 2016 11 93 ± 6 119 ± 6 138 ± 4 155 ± 4 168 ± 4 179 ± 5 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
V 2017 6 84 ± 8 118 ± 5 139 ± 4 154 ± 3 167± 3 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
IV 2018 2 105 ± 6 128 ± 5 141 ± 3 160 ± 1 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
III 2019 6 92 ± 4 121 ± 4 140 ± 5 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
II 2020 8 88 ± 4 122 ± 2 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
I 2021 28 87 ± 2 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Mean TL (mm) at age 91 ± 1 123 ± 2 145 ± 2 162 ± 2 178 ± 2 193 ± 3 208 ± 3 222 ± 4 234 ± 6 247 ± 7 250 ± 9 262 ± 10 263 ± 7 272 ± 9 289 ± 7 292 ± 4
Mean growth (mm) 91 ± 1 31 ± 1 21 ± 1 17 ± 1 15 ± 1 14 ± 1 12 ± 1 12 ± 1 12 ± 1 11 ± 1 10 ± 1 12 ± 1 12 ± 1 10 ± 2 10 ± 1 ​

Age-observed length and age-back calculated length relationships of Suckermouth Catfish Hypostomus c.f. watwata captured from Makiki, Mānoa, and Pālolo Streams in the Ala Wai Watershed on O’ahu, Hawai’i during 2022–2023. The presented curve represents a Von Bertalanffy growth function fitted to the mean back-calculated length at age data and is described as follows: T L a g e = 342.2 · 1 − e − 0.10 a g e + 2.22 .

Scatter plot showing total length in millimeters on the y-axis and age in years on the x-axis, comparing back-calculated length at age (gray dots) and observed length at age (open circles) with a fitted Von Bertalanffy growth curve.

The relationship between the annual RGI of Suckermouth Catfish and annual flow conditions varied by stream (Figure 8; Tables 1, 4). The RGI of individuals residing in Makiki was higher during years with shorter-duration low flow pulses, higher rise rates, and higher flows during September and October, while years having a greater number of low flow pulses and higher maximum flows and flow variability in February resulted in lower RGI (Figure 8; Tables 1, 4). While individuals from Mānoa tended to exhibit negative RGI regardless of annual flow conditions, RGI was higher in years with higher low flow exceedance values and generally higher flows. Individuals in Mānoa exhibited lower RGI during years with greater variability in daily flows (Figure 8; Tables 1, 4). Suckermouth Catfish inhabiting Pālolo grew faster during years with higher flows during the winter, spring and summer months and exhibited lower RGI in years with a greater range in daily flows (Figure 8; Tables 1, 4).

Relationship between the mean standardized growth by year of Suckermouth Catfish Hypostomus c.f. watwata and interannual variation in annual flow regime as assessed by the first five principal components (PC) calculated from hydroecological statistics of stream flow describing the annual flow regime in Makiki, Mānoa, and Pālolo Streams in the Ala Wai Watershed on O’ahu, Hawai’i during 2010-2022 (A: PC1; B: PC2; C: PC3; D: PC4; E: PC5). The eigenvalues of the hydroecological statistics for each principal component can be found in Table 1. Error bars represent standard error.

Five scatter plots labeled A to E display standardized growth versus principal components, one through five, respectively, summarizing interannual streamflow variability for three locations: Makiki (red), Manoa (yellow), and Palolo (blue). Data points show variability with error bars, and each axis is annotated with hydrological factors influencing the principal component, such as increased median flows and base flows or increased flow variability and low flow pulse duration. A color-coded legend indicates which color corresponds to each location.

Results of a repeated-measures mixed effects model evaluating the influence of stream and interannual variation in annual flow regime as assessed by the first five principal components calculated from hydroecological statistics of stream flow describing annual flow regime on the standardized growth of Suckermouth Catfish Hypostomus c.f. watwata in Makiki, Mānoa, and Pālolo Streams in the Ala Wai Watershed on O’ahu, Hawai’i during 2010–2022.

Factor Parameter estimate (SE) F-value (df1, df2) t-value (df) P
Intercept 0.032 (0.022) – 1.43 (109) 0.16
Stream – 3.67 (2,428) – 0.03
Makiki −0.073 (0.072) – −1.01 (428) 0.31
Mānoa −0.098 (0.037) – −2.66 (428) 0.01
Pālolo 0.000 – – –
PC1(Stream) – 3.02 (3,428) – 0.03
Makiki −0.028 (0.064) – −0.44 (428) 0.66
Mānoa 0.012 (0.030) – 0.40 (428) 0.69
Pālolo 0.031 (0.011) – 2.95 (428) <0.01
PC2(Stream) – 1.40 (3,428) – 0.24
Makiki −0.009 (0.033) – −0.28 (428) 0.78
Mānoa −0.013 (0.013) – −0.97 (428) 0.33
Pālolo −0.012 (0.007) – −1.78 (428) 0.08
PC3(Stream) – 3.07 (3,428) – 0.03
Makiki −0.016 (0.011) – −1.46 (428) 0.15
Mānoa −0.040 (0.021) – −1.91 (428) 0.06
Pālolo 0.015 (0.008) – 1.85 (428) 0.07
PC4(Stream) – 2.26 (3,428) – 0.08
Makiki 0.012 (0.005) – 2.50 (428) 0.01
Mānoa −0.001 (0.004) – −0.21 (428) 0.83
Pālolo 0.001 (0.002) – 0.68 (428) 0.50
PC5(Stream) – 2.85 (3,428) – 0.04
Makiki 0.011 (0.006) – 1.88 (428) 0.06
Mānoa −0.001 (0.003) – −0.30 (428) 0.77
Pālolo −0.004 (0.002) – −2.22 (428) 0.03

Suckermouth Catfish in the Ala Wai Watershed on O’ahu exhibited an overall total instantaneous mortality rate (Z ± SE) of 0.14 ± 0.04 years-1 and their relative year class strength seemed to vary on a 3–5 years cycle during 2006–2021 (Figure 9). The top performing model only included PC5 indicating that relative year class strength was higher in years with greater mean and maximum monthly flows during winter and a smaller range of daily flows (F 1,11 = 4.67, P = 0.05, R 2 = 0.30). Our sample size was insufficient to statistically evaluate whether annual mortality rates or relative year class strength varied between Makiki, Mānoa, and Pālolo Streams.

Relative year-class strength as estimated from the descending leg of the catch curve of Suckermouth Catfish Hypostomus c.f. watwata captured from Makiki, Mānoa, and Pālolo Streams in the Ala Wai Watershed on O’ahu, Hawai’i during 2022–2023. Positive values indicate a year-class that was stronger than predicted from age-frequency data while negative values indicate weaker than predicted year classes.

Bar chart showing relative year-class strength from 2005 to 2021, with values fluctuating above and below zero. The lowest point occurs around 2018, while 2021 shows the highest positive value. Discussion

This study represents one of the first efforts to characterize the demographics and flow ecology of a loricariid population, either introduced or within its native range. We were unable to find any age and growth information for any Hypostomus species in the literature, and thus cannot say whether the patterns observed in our study streams are typical for an introduced population or how they might compare to the species in its native range. An introduced population of another loricariid species, Vermiculated Sailfin Catfish Pterygoplichthys disjunctivus, in a Florida spring system reached a similar size (100 mm; Gibbs et al., 2013) in their first year as Suckermouth Catfish in the Ala Wai Watershed and also exhibited a consistent, low growth rate thereafter. However, Suckmouth Catfish in the Ala Wai Watershed seem to be capable of reaching greater maximum age than Vermiculated Sailfin Catfish in Florida (approx. 5–6 years; Gibbs et al., 2013) or introduced populations of Armored Sailfin Catfish P. pardalis in Kerala, India (approx. 5–6 years; Raj et al., 2019). It is important to state that there are currently no age validation studies available in the literature for any Hypostomus spp. While this lack of validation should warrant consideration in the conclusions drawn from the age estimates generated in this study, it is also important to note that there have been numerous studies validating annuli formation in other catfishes, both in temperate (Spurgeon et al., 2015) and tropical systems (de Soares et al., 2020), and that none of them have noted anything other than a pattern of yearly annulus formation.

Suckermouth Catfish generally exhibited higher growth rates during years with greater stability in flows and tended to grow more slowly during years with extreme high or low flows (Figure 8). This general pattern has been observed across a wide range of taxa, including centrarchids (Sammons and Maceina, 2009; Grabowski et al., 2019; Groeschel-Taylor et al., 2020), Channel Catfish Ictalurus punctatus (Spurgeon and Pegg, 2017), Flathead Chub Platygobio gracilis (Haworth and Bestgen, 2016). However, the observed relationship between interannual variability in the growth of Suckermouth Catfish in the Ala Wai Watershed and the annual flow conditions experienced by the fish was not as strong as expected. Despite being a demersal/benthic species associated with relatively low current-velocity habitats in the lower, tidally influenced reaches of rivers (Weber et al., 2012), Suckermouth Catfish growth does not seem to be negatively impacted with high flow pulses in the Ala Wai Watershed, to the extent that growth was positively correlated with flows. Furthermore, the species is primarily herbivorous (Weber et al., 2012) and thus would not necessarily exhibit faster growth due to an increase in insect drift or access to floodplain habitats associated with higher flows, as seen with centrarchids (Sammons and Maceina, 2009; Grabowski et al., 2019; Groeschel et al., 2020). In contrast, the stable moderate flows that tend to produce conditions conducive to maximizing phytobenthos biomass (Lowe, 1979; Law, 2011), particularly if augmented with inputs of terrestrial nutrients associated with a low frequency of mild to moderate high flow pulses (Buffam et al., 2001; Dalzell et al., 2005), would potentially represent ideal conditions for Suckermouth Catfish growth. However, baseflows of R2R systems in Hawai’i, particularly those on O’ahu, are predicted to decrease in response to climate change, with the frequency and magnitude of high flow events increasing (Leta et al., 2018; Clilverd et al., 2019). Our results suggest that Suckermouth Catfish growth, and to a lesser extent, recruitment, may decrease in response to the climate-driven changes in streamflow in Hawaiian R2R systems.

Similar to growth, Suckermouth Catfish recruitment seemed to be higher in years with stable flows and higher flows during the winter months. There is very little known about the early life history of loricariid catfishes, so it is difficult to speculate mechanisms by which flow conditions may influence the survival of young of year individuals. However, like many loricariid species, Suckermouth Catfish excavate burrows in the streambank into which the eggs are deposited and guarded by the male (Hoover et al., 2004; Nico et al., 2009; Gibbs et al., 2013). Years with higher flows would make more streambank habitat available to spawning individuals and stable flows would help ensure that the burrows remained submerged for the incubation period, which might allow the population to produce strong year classes. It is not known whether Suckermouth Catfish spawn year-round in the Ala Wai Watershed or whether they have a discrete spawning season. Spawning seasonality, or a lack thereof, seems to be highly variable amongst loricariid species and locations. However, the results of the current study suggest that the species may spawn in the winter or wet season, i.e., November–April, in Hawai’i. It is also possible that some of the variability in year class strength observed in the Ala Wai Watershed is attributable to the eradication efforts of the Paʻēpaʻē o Waikolu program. The program started collecting Suckermouth Catfish in 2017 and averaged 134.2 individuals per trip, with individuals within the size range of young of year individuals making upwards of half the catch, and 35 trips per year with a large proportion occurring from late fall to early spring. In 2023, the program averaged 40.0 Suckermouth Catfish per trip with 37 trips per year. While the efforts of Paʻēpaʻē o Waikolu match up with a period of reduced year-class strength, it is perhaps most interesting that the year during which collection efforts were completely halted due to the global COVID-19 pandemic, 2021, saw the strongest year class since the inception of the program.

For a species originating from slow moving, coastal river systems, Suckermouth Catfish have proven to be remarkably adaptable to flow conditions in the R2R systems in the Ala Wai Watershed. However, Suckermouth Catfish growth and recruitment seems to be sensitive to elements of the R2R flow regime that are most likely to change, such as flow variability, high flow events and baseflows, with changing precipitation patterns predicted by climate projections (Clilverd et al., 2019). Suckermouth Catfish may find future conditions in R2R systems less hospitable, which may help to limit their population size and range within and across watersheds. Further, our results suggest that Suckermouth Catfish may be a poor model species to examine flow ecology in R2R systems due to their limited sensitivity to flow conditions and small otoliths that are challenging to work with. Future efforts attempting to build on this work may find stronger flow-growth relationships with other regularly encountered invasive species, such as Smallmouth Bass Micropterus dolomieu.

 Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

 Ethics statement

The animal study was approved by University of Hawaii Institutional Animal Care and Use Committee. The study was conducted in accordance with the local legislation and institutional requirements.

 Author contributions

TG: Project administration, Writing – original draft, Data curation, Supervision, Formal Analysis, Methodology, Visualization, Investigation, Conceptualization, Writing – review and editing, Funding acquisition, Resources. YT: Investigation, Conceptualization, Writing – review and editing, Visualization, Formal Analysis, Funding acquisition. DB: Investigation, Writing – review and editing, Data curation, Project administration, Methodology. CY: Methodology, Investigation, Writing – review and editing, Resources. JeF: Conceptualization, Funding acquisition, Writing – review and editing. JB: Funding acquisition, Conceptualization, Writing – review and editing. JaF: Writing – review and editing, Funding acquisition, Conceptualization.

 Acknowledgements

We thank A. Charuk, J. Hijii, and D. Wallace, as well as the students of Gus Webling Elementary (Fourth grade), Hōkūlani Elementary (Fifth grade), Hau’ula Elementary (Fourth Grade), ‘Iolani School (Fourth/fifth grade), Kamehameha Elementary (First Grade), Nānākuli elementary (Fourth Grade), Saint Louis School (Eighth Grade), Waialua High School, and the University of Hawai’i at Mānoa NREM 662 for their assistance in collecting fishes. A. Larson and D. McSwain assisted in the preparation of the otoliths for analysis. J. Long and H. Mauro-Koike provided comments and suggestions that greatly improved the quality of this manuscript.

 Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

 Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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Edited by: Katelyn Lawson, Auburn University, United States

Reviewed by: Richard Blob, Clemson University, United States

Josh Perkin, Texas A and M University, United States