# Direct evidence of predation on endangered sea lamprey in estuarine tidal area

Julie Lucas, Nils Teichert, Marie-Caroline Husset, Jezabel Lamoureux, Emma

Robin, Eric Feunteun, Thomas Trancart

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► To cite this version:

Julie Lucas, Nils Teichert, Marie-Caroline Husset, Jezabel Lamoureux, Emma Robin, et al. Direct evidence of predation on endangered sea lamprey in estuarine tidal area. Journal for Nature Conservation, 2025, 87, pp.126998. 10.1016/j.jnc.2025.126998. hal-05375993

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Submitted on 22 Nov 2025

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# Direct evidence of predation on endangered sea lamprey in estuarine tidal area

<figure><img src="image_4.png" /></figure>

Julie Lucas<sup>a,b,c</sup>, Nils Teichert<sup>a,b</sup>, Marie-Caroline Husset<sup>a,b</sup>, Jezabel Lamoureux<sup>a,b</sup>,
Emma Robin<sup>a,b</sup>, Eric Feunteun<sup>a,b,d</sup>, Thomas Trancart<sup>a,b,* ID</sup>

## ARTICLE INFO

Keywords:

Sea lampreys

Acoustic telemetry

Conservation

Management

Endangered species

Silurus glanis

## ABSTRACT

The sea lamprey *Petromyzon marinus* is a anadromous species that has undergone a dramatic decline in many European rivers over recent decades, observed for instance as substantial decrease in the number of returning spawners (i.e., the reproductive stock), observed in most major river systems. Consequently, in Western Europe, its conservation status varies from Near Threatened (NT) in Ireland, to Least concern (LC) in Great Britain, Vulnerable (VU) in Spain, Endangered (EN) in France, and Critically Endangered (CR) in Italy, according to national IUCN assessments. This decline is attributed to multiple pressures, including habitat degradation, obstacles to migration, climate change, and overfishing. However, the potential impact of predation on this endangered species remains poorly documented, especially in estuarine areas. This study aimed to evaluate predation on sea lampreys during their upstream migration in the tidal oligohaline zone of the Loire estuary, using acoustic telemetry over two years. Fish were captured, tagged with acoustic tags (with or without digestion sensors), and released into the river. The predation rates (certified by digestion tag and estimated with classical tags) ranged from 8.8 to 20 %, suggesting that predation could represent a significant, yet overlooked, threat in estuarine zones. Based on indirect evidence and catfish fishing data, the European catfish *Silurus glanis* — an invasive and opportunistic predator increasingly abundant in the Loire system — appears to be the most likely predator responsible for these events. This study provides the first evidence suggesting sea lamprey predation by European catfish in estuarine tidal zones. It highlights the need to consider the impact of invasive species on migratory fish populations, not only in upstream sections of rivers but also from the very first stages of their freshwater migration.

## 1. Introduction

Diadromous species (species migrating between saltwater and freshwater, (McDowall, 1997)) in North Atlantic are currently facing critical declines, with many on the verge of extinction (Limburg & Waldman, 2009). Consequently, many diadromous species are now classified as vulnerable, endangered, critically endangered or locally extinct by the International Union for Conservation of Nature (IUCN, 2024; Nunn et al., 2023). These declines are now broadly acknowledged and are consistently attributed to a range of pressures, which are generally common across diadromous species (Lassalle et al., 2009;

Limburg & Waldman, 2009; Merg et al., 2020; Verhelst et al., 2021), although their intensities differ between species and area. Among these pressures are the loss of river connectivity both longitudinally and laterally due to dam construction and floodplain reclamation, habitat destruction, contamination by organic and metallic pollutants, climate change, exploitation by commercial and recreational fisheries, and predation by invasive species (Lassalle et al., 2009; Limburg & Waldman, 2009; Merg et al., 2020; Verhelst et al., 2021).

For global freshwater biodiversity around the world, the introduction of invasive species represents one of the major drivers of decline (Doherty et al., 2016; Dudgeon et al., 2006; Simberloff et al., 2013).

<footnote><sup>a</sup> UMR 7208 BOREA, Laboratoire de Biologie des Organismes et Ecosystèmes Aquatiques, MNHN, SU, CNRS, IRD, UA), Paris, France</footnote>

<footnote><sup>b</sup> MNHN, Station Marine de Dinard, Dinard, France</footnote>

<footnote><sup>c</sup> Laboratoire de Biotechnologie et Chimie Marines - EMR CNRS 6076, Centre Yves Coppens UFR SSI, Université Bretagne Sud, Campus de Tohannic - CS 60573, 7 Rue André Lstoff, 56 017 Vannes Cedex, France</footnote>

<footnote><sup>d</sup> EPHE-PSI, Centre de Géoécologie Littorale (CGEL), Dinard, France</footnote>

<footnote>* Corresponding author at: Station marine de Dinard, 38 rue de Port blanc, 35800 Dinard, France.</footnote>

<footnote>E-mail address: thomas.trancart@mnhn.fr (T. Trancart).</footnote>
Introduced species can lead to declines in native communities, specifically abundance or diversity, and may cause economic harm for fisheries. Among them, the European catfish (*Silurus glanis*, L. 1758) was introduced in France in the 1970s-1980s. Native of Eastern Europe and Western Asia, it is actually well established in the inland freshwaters of western Europe (Copp et al., 2009). With its large size up to 2.7 m, *S. glanis* is among the 20 largest freshwater fish in the world and the largest in Europe (Boulètreau & Santoul, 2017; Cucherousset et al., 2018; Stone, 2007). As a top predator, it can exert competitive pressure or significant predation impact on native species, including diadromous species (Boulètreau et al., 2018, 2020, 2021; Carol et al., 2007; Copp et al., 2009; Guillierault et al., 2017, 2019; Libois et al., 2016; Syväranta et al., 2009).

Few studies have clearly demonstrated the importance of local predation by European catfish on migratory fishes. For example, a study carried out on the Loire River suggests that catfish display a marine isotope signature indicative of predation on shad, lamprey or salmon spawners (Belhamiti & Boisneau, 2015). Guillierault et al. (2017) have found DNA of Atlantic salmon *Salmo salar* in catfish faeces in the Garonne River. Bouletreau et al. (2018) observed that the period of upstream migration of Atlantic salmon coincides with passages of *S. glanis* at Golfech dam. They reported that 35 % of the 40 migrating salmon observed at this dam were predated inside the fishway, highlighting that returning adults of Atlantic salmon represent a targeted prey for the European catfish. In the same river, 12 to 37 % spawning acts of *Alosa* spp. were interrupted by a catfish attack (Boulètreau et al., 2021).

The sea lamprey *Petromyzon marinus* is a diadromous species that has experienced an important decline in France and across Western Europe. For example, in the Loire river — one of the species' main historical rivers in France — the annual number of spawners observed at the monitoring stations in the fish pass dropped from several tens of thousands between 2007 and 2015 to fewer than 20 individuals since 2022. As for the other diadromous species, the impact of European catfish has been previously highlighted: a predation rate of 80 % was observed on marine lamprey during migration in Garonne and Dordogne River (Boulètreau et al., 2020) and in Loire River (Trancart et al., 2023).

Currently, most studies examining the impacts of European catfish on diadromous species populations have focused on the upper reaches of river basins, where these predators are often found in higher densities. However, little attention has been paid to the downstream areas, particularly the estuarine zones, where the dynamic of predator-prey interactions may differ. These areas, often characterized by complex habitats and variable environmental conditions, could play a crucial role on the life cycles of migratory species, including sea lampreys. Understanding the effects of European catfish predation on sea lampreys in these downstream areas is thus important to reveal new insights into the broader ecological impacts of this invasive predator and to manage this endangered species. The objective of this study was to assess the occurrence and importance of predation on sea lampreys during their upstream migration through an estuarine tidal area. Specifically, we aimed to (1) determine whether predation occurs in this transition area between marine and freshwater environments, (2) estimate the rate of predation, and (3) identify the most likely predator species involved. To address these questions, we conducted an acoustic telemetry monitoring program involving 180 sea lampreys tagged and released in 2022 and 2023. A subset of 60 individuals was equipped with tags with digestion sensors. This recent technology enables the detection of predation events based on digestive activities following ingestion. This ensures that the tracked fish is still the originally tagged individual and that no accidental host switching has occurred. The remaining 120 individuals were equipped with standard tags without digestion sensor, and we developed a classification tree (CART) model based on the detection patterns of the 60 sensor-tagged lampreys, which was used to infer the probability of predation for the individuals equipped with standard tag.

## 2. Material and methods

### 2.1. Study site

This study took place in the Loire estuary (Fig. 1), in a spatially restricted reach (5 km) near Nantes (47.21837 °N, -1.55362 °E) in France. The area is subject to tidal influences, with bidirectional tidal currents occurring during low water conditions (discharge < 1000 m³/ s). Located in oligohaline water, the salinity of this estuarine area is low to nil (0.1-0.6 g/l, Département de Loire-Atlantique / DREAL / pays de la Loire / EDF / SHOM / GIP Loire Estuaire). The study site is on the fishing area called "lot 13".

### 2.2. Acoustic telemetry system

The study used an acoustic telemetry system (VR2W 69 kHz, Innovasea), which was paired with acoustic tags with predation sensors. Over the course of 2022 and 2023, a total of 23 receivers were deployed each year (Fig. 1). These receivers were installed on the riverbed, secured with 600 kg anchor blocks, and were left in place throughout the entire sea lamprey migration period, from early February to early July each year. The water depth above the receivers varied between 3 and 8 m, depending on tidal range and river discharge. To assess detection performance, two standardized detection range tests were conducted each year. Each test consisted of six transects (three upstream and three downstream) paddled by canoe across the receiver array, while towing an acoustic tag identical to those implanted in fish (random delay 40-80 s), maintained at approximately 1 m above the riverbed. During downstream transects, the canoe drifted with the current; during upstream transects, it moved at ~2-3 km/h. Each test lasted about 3 h, allowing coverage of a range of water levels and generating approximately 180 tag positions per test. Detection data from all the receivers in the site were aggregated into 5-minute intervals to account for the lack of receiver time synchronization. Across all tests, the number of detections per 5-minute intervals ranged from 6 to 69. The lowest detection rates occurred at the edges of the array (up to ~400 m from the nearest receiver), while the highest rates were recorded when the tag passed through the central zone of the array, with a lot of receivers. These tests confirmed that under the tested hydrological conditions, a tagged fish could not cross the study area without being detected numerous times. Based on detection density and the spatial distribution of receivers, the system provided extensive coverage, resulting in a very high likelihood of detecting tagged individuals moving at typical lamprey speeds reported in the literature.

### 2.3. Collection and tagging of sea lampreys

Sea lampreys were captured by professional fishermen during the winters of 2022 and 2023 using drift gillnets deployed downstream from the study area. Upon capture, the lampreys were transported to the shore for tagging. The transport was carried out in a certified boat equipped with a large 1 m³ water tank, continuously aerated to ensure optimal conditions. The duration of transport was limited to a maximum of twenty minutes.

They were first anesthetized with Aquabenzocaina (50 mg/l), and then equipped with acoustic transmitters with or without digestion sensors (V7D or V7 69 kHz, Innovasea, random delay 40-80 s), which have a diameter of 9 mm, a length of 24 mm, and weigh 4 g in air. Innovasea's digestion tags uses an inert polymer covering that gets broken down by the stomach acids of a predator during digestion. The transmitter changes the data associated with its ID code when the polymer coating is broken down, allowing the determination of the time of activation (in hours) Previous study showed various time lags (from some hours to 7 days) between the predation events and the first predation signal emission (Shorgan et al., 2024; Trancart et al., 2023). This time seems to be influenced by water temperature (influencing the speed
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Location of the acoustic receivers (blue points) on the study site with distance from the release site of sea lampreys (red point). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

of the digestion process), the sizes of the prey and of the predators (Trancart et al., 2023). The tagging adhered to the recommended 2 % transmitter-to-body mass ratio (Winter, 1996). Each lamprey received an internal tag placement via a 20 mm incision on the ventral side, positioned 10 cm anterior to the anus. The incision was closed with absorbable sterile sutures (3-0 Ethicon Monocryltm, Ethicon Ltd., Livingston, UK) and disinfected with a bactericidal antiseptic (0.05 % chlorhexidine). The total duration of surgery (including incision, tag insertion and suture, under anaesthesia) was kept under 5 min for each lamprey. After the anaesthesia effects were fully dissipated and the lampreys had regained their full locomotor abilities, they were released back into the river, at fishing location. Fishing operations took place in the morning, so releases usually occurred around noon or in the early afternoon. A total of 30 sea lampreys were tagged on April 12, 2022, and 30 other individuals were tagged on March 14, 2023 with digestion tags, while 120 sea lampreys were tagged on February 24, 2022, March 3 and 24, 2022, and March 15, 2023 (n = 30 for each tagging session) with tags without digestion sensors. The total length (mm) and body weight (g) were recorded for each individual. Length of tagged lampreys ranged from 745 to 1040 mm (mean = 865 mm), and weight ranged from 727 to 1848 g (mean = 1192 g). The tagging method used in this study had been previously tested in 2020 and 2021, with no issues reported and a 100 % migration recapture rate each time (Trancart et al., 2022; Trancart et al., 2023).

## 2.4. Assessment of false activations in digestion tags

In parallel, 10 sea lampreys were captured and tagged using a surgical approach with digestion sensor-equipped transmitters inserted into their peritoneal cavity. The objective was to observe wound healing and validate the reliability of the digestion sensors under the study conditions, specifically the durability of the sensor membranes within the sea

lamprey's body. These lampreys were tagged on February 16, 2023, and kept in a 3.2 m3 tank immersed in the river near the study site until March 14, 2023. At the end of this period, the lampreys were euthanized with a progressive overdose of anaesthetic, then dissected. Sex was determined, and both external and internal wounds from the marking, as well as internal tissues, were individually examined. Additionally, the activation of the predation sensors in the acoustic transmitters was verified using a mobile acoustic receiver (Innovasea Vemco VR-100), and the condition of the sensor membranes, visible externally, was inspected for each individual.

## 2.5. European catfish presence

To evaluate the presence of European catfish in various areas of the Loire estuary, data from CESMIA — a legal reporting tool for professional fisheries landings in freshwater and estuarine environments — were analysed by calculating total monthly European catfish landings in the study area (lot 13). The study area corresponded to Lot 13, spanning 11 km in length.

Legally, fishers are required to report either the total weight or the total number of fish caught, but not necessarily both. When only one metric was provided, the second was not interpolated. This reporting structure introduces uncertainties into the dataset. As a result, the total weights and numbers presented here should be interpreted as minimum estimates, since that are the declared values and not the real landing values. Moreover, only landings are required to be declared, making it impossible to calculate the associated fishing effort. These data are therefore used for informational purposes only, to estimate the presence of catfish in the study area.

Data on European catfish fishing captures were not available for 2022, as the monitoring CESMIA database began in 2023.
## 2.6. Environmental monitoring

Water flow (in m³/s) and temperature (in °C) values were recorded at regular 1-hour intervals throughout the study period. Measurements were taken at the Nantes Malakoff monitoring station, located 2 km downstream of the study area. These data were generously provided by the GIP Loire.

## 2.7. Ethical considerations and animal welfare

All fish handling was conducted in compliance with European Union regulations on the protection of experimental animals. The research protocol received approval from the Ethics and Animal Experimentation Committee of the MNHN (CEEA - 068, APAFIS #35047-2022013021381957 v5) and the French Ministry of Research. The tagging procedures were carried out exclusively by authorized personnel.

## 2.8. Data analysis

The analyses were carried out in R 4.3.3 (R Development Core Team, 2011).

### 2.8.1. Sea lamprey predation rate with digestion tags

The predation rate was determined based on the number of sea lampreys with activated digestion tags relative to the total number of tagged individuals with digestion tags. For each activation, the date of activation was recorded. Since there is a delay between predation and sensor activation, this data provides a time range around the probable date of predation. Proportion tests (prop.test function, {stats} package) were used to evaluate difference in predation rate between the two study years (2022 vs 2023).

Using individual profiles, we then calculated two metrics aiming to discriminate non-predated and predated individual, based on their behavioural and migratory profiles. 1) *The occurrence duration* in the study site was defined as the time difference (in days) between the first and the last records on any receivers within the study area. In case of predation, the main hypothesis is that predation should result in a longer presence time within the study site, whereas live sea lamprey transits through this area very quickly. 2) *The downstream-to-upstream movement ratio* was defined using the mean individual positions calculated at 10-min intervals (Center Of Activity, (Simpfendorfer et al., 2002). This ratio ranges from 0 (indicating linear upstream movement) to ∞ (indicating linear downstream movement), with a value of 1 representing an equal proportion of downstream and upstream movements. In case of predation, the main hypothesis is that predation results in a succession of upstream and downstream movements in the study site, whereas non-predated sea lamprey transits through this area in a linear upstream movement.

The two metrics were used to identify decision rules able to discriminate non-predated and predated profiles, based on a classification tree (CART) model (rpart function, {rpart} package). This approach produces threshold values for metrics, which can thereafter be used to infer which profiles were probably predated or not. The CART model validation was performed using a bootstrap approach, in which 1000 resampled datasets were created, allowing us to estimate model performance across varying subsets of the data. For each iteration, out-of-bag data (observations not included in the bootstrap sample) were used to test the model, and confusion matrices were computed. A global confusion matrix was then generated by averaging the results across all iterations, providing robust estimates of model accuracy and identifying combinations of threshold values that minimize predation risk.

### 2.8.1.1. Predation rates of sea lampreys without digestion tags.
The thresholds identified using the CART model were applied to the profiles

of tagged sea lampreys that were not equipped with digestion sensors. For each individual, the *occurrence duration* and the *downstream-to-upstream movement ratio* were calculated using the same methodology described earlier. Individuals whose metrics fell within the thresholds associated with predation absence were classified as likely not predated, while those outside these thresholds were classified as potentially predated.

This classification assumes that individuals tagged with standard and predation-sensitive transmitters behaved similarly and were equally exposed to predation risk. This assumption is reasonable since the transmitters had identical dimensions and hydrodynamic profiles. However, fish were not released on the exact same days, and that some variation in environmental conditions and predator activity may have occurred between release dates and between the two study years. For individuals equipped with predation-sensitive tags, we intentionally targeted the central part of the migration period to ensure that they were released during representative average conditions, both in terms of environmental parameters and predator presence

Proportion tests (prop.test function, {stats} package) were used to evaluate difference in predation rate between the two study years (2022 vs 2023).

Finally, some individuals fell within the threshold for one variable but not the other. In these cases, we adopted a precautionary approach and considered the individual was still alive.

Additionally, for each tagged sea lampreys, an individual migration pattern was established by analysing the distance between detection sites and the release site over the time. Migration profiles from sea lampreys equipped with digestion sensors were used to define certified predated and certified non-predated profiles. The migration patterns of sea lampreys tagged with standard acoustic transmitters were then compared to these certified profiles to identify potential predation events.

## 3. Results

### 3.1. Assessment of false activations in digestion tags

Throughout the false activation assessment period, none of the digestion sensors were activated.

### 3.2. Confirmed predation of sea lampreys

The mean confirmed predation rate across the two years of the study was 15 % (9 out of 60 individuals, Table 1). During the first year of monitoring (2022), 6 out of the 30 tagged sea lampreys equipped with acoustic tags with predation sensors were confirmed as predated, representing a predation rate of 20 %. In the second year (2023), 3 out of 30 individuals were predated, resulting in a predation rate of 10 %. There is no difference between the two years (proportion test, p = 0.4696).

The migration profiles of predated and non-predated individuals exhibited clear behavioural difference:

- non-predated sea lampreys displayed fast and linear upstream movements, with a limited presence in the study area, typically lasting a few hours (Fig. 2-a);

- predated sea lampreys initially exhibited similar migration behaviour (as long as the primary host had not been predated).

<caption>Table 1</caption>

Summary of predation rates in sea lampreys, for individuals with digestion tags (certified predation rate) and without (probably predation rate).

<table><thead><tr><td>Year</td><td>Certified predation rate</td><td>Probable predation rate</td><td>Total predation rate</td></tr></thead><tbody><tr><td>2022</td><td>20 % (6/30)</td><td>8.8 % (8/90)</td><td>11.6 % (14/120)</td></tr><tr><td>2023</td><td>10 % (3/30)</td><td>10 % (3/30)</td><td>10 % (6/60)</td></tr><tr><td>Total</td><td>15 % (9/60)</td><td>9.2 % (11/120)</td><td>11.1 % (20/180)</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Example of migration profiles from non-predated sea lampreys (a) and predated sea lampreys (b). Note that activation time differs from predation time due to activation delay.</figcaption></figure>

However, once predation occurred, their behavior shifted abruptly, leading to prolonged presence in the study area (several days) with alternated downstream and upstream movements, likely corresponding to the acoustic tag being carried inside the predator (Fig. 2-b).

For all tagged sea lampreys, the median *occurrence duration* on the study site was 0.15 days (mean = 2.40, min = 0.01, max = 29.47 days). The median *downstream-to-upstream movement ratio* was 0 (mean = 0.19, min = 0, max = 1.4).

The assignation model developed using the classification tree (CART) method revealed that the two metrics well discriminated the predated profiles from the non-predated ones (Fig. 3).

For individuals that spent less than 2.41 days on the study site, the probability of being a predation profile was null, with 100 % of individuals in this group classified as non-predated. Conversely, individuals with a longer occurrence duration (≥2.41 days) exhibited more complex patterns based on their movement ratios. Those with a downstream/upstream movement ratio greater than 0.27 were assigned

with 100 % certainty to have been predated. This was the strongest indicator of risk of classification as predated, with the model classifying all individuals in this subgroup as predated. In contrast, those with a lower ratio (≤0.27) were classified as non-predated.

The bootstrap cross-validation generated an average accuracy of 96 %, indicating a high level of prediction correctness. The 95 % confidence interval for accuracy ranged from 0.87 to 1, suggesting that the assignment performances are highly stable and reliable across the data subsets. The estimated true positive and false positive rates from the global confusion matrix analysis were respectively 0.861 and 0.023.

### 3.3. Assigned predation of sea lampreys

The assignation model identified 11 individuals as having likely been predated (8 in 2022, 3 in 2023). Test for equality of proportion showed no difference between the two study years for certified (p = 0.47) and probable predation rate (p = 1). The predation rates from certified and
<figure><img src="image_7.png" /><figcaption>**Fig. 3.** CART model used for assigning predation profiles of sea lamprey, with the number of individuals and the probabilities of profile assignation for each split.</figcaption></figure>

assigned profiles were summarised in Table 1.

### 3.4. Catfish captures in the study area

In 2023, a total of 193 fishing sessions targeting catfish were conducted within the study area (Lot 13, 11 km long). A total of 833 individuals were caught (minimum, 11 fishing operations were without number of captured individuals), with a cumulative biomass of 11,143 kg (minimum, 43 fishing operations were without the total weight of captured individuals). The seasonal variations in catfish landings were evident, with two distinct periods of heightened capture rates: the first spanning from March to May and the second occurring between September and October (Fig. 4).

### 3.5. Environmental conditions during predations

In 2022, sea lampreys were tagged in April, when discharge rates ranged between 700 and 760 m³/s, and the average water temperature was 13.1 °C. In 2023, tagging occurred earlier, in March, with higher discharge rates of 1070 to 1160 m³/s and a lower average temperature of 10.7 °C.

### 3.6. Location of predations

A large proportion (80 %) of the predation events occurred at PK 5200, beneath the bridge piers. The remaining predation events took place downstream in the study area, around PK 1800. After predation, the predator individuals continued to frequent these same areas.

## 4. Discussion

### 4.1. Reliability of predation estimates

We are confident that the sea lampreys identified as predated were actually predated, as we used highly reliable predation sensors attached to the acoustic tags, which we specifically tested during this study and for which no false positives were observed (0 % false detection).. These sensors specifically detected digestion events, providing a robust method for confirming predation events (Halfyard et al., 2017; Schultz et al., 2017; Shorgan et al., 2024; Weinz et al., 2020). However, it is important to note that our estimates likely represent a minimum pre- dation rate, as it is possible that some predation events occurred outside the coverage area of our acoustic receivers.

For the estimation of predation on the population equipped with

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Daily catfish captures from professional fishermen on the study site (2023, lot 13).</figcaption></figure>
standard acoustic tags (without predation sensors), we are also confi-
dent in the validation of individuals classified as “predated” because the
confusion table and the accuracy, both for training data and for cross-
validation were very high. Similarly, as for individuals with digestion
sensors, it is also important to acknowledge that predation events could
have occurred outside the coverage area of our receivers. Therefore, the
true predation rate is likely higher than the calculated predation rate we
have reported.

Finally, capture-related stress may have affected the vulnerability of tagged sea lampreys to predation. Although fish were released as quickly as possible to minimize handling stress, and fishers were instructed to limit net soak time during capture operations, it is likely that some degree of stress was experienced by the individuals. However, whether this stress could significantly increase the likelihood of subsequent predation remains unknown. As we have no empirical data to assess this effect, any conclusion would be speculative. Nonetheless, this factor should be considered in future studies, especially when interpreting fast post-release predation events.

## 4.2. Identification of the predator

While we cannot formally identify the species causing the predation
events, nor confirm whether multiple predator species were involved,
we can make some informed assumptions.

Firstly, it is unlikely that avian species were responsible for the predation, because the acoustic tag would have been pulled out of the water and would no longer have been detectable by the receivers. The size of the tagged sea lampreys (averaging 1192 g) makes also the hypothesis of predation by local avifauna, such as cormorants, unlikely. Indeed, the size of the prey consumed by cormorants in literature is significantly smaller than that of our individuals (see for instance (Carpentier et al., 2009; Ovegård et al., 2017; Skov et al., 2014). The osprey *Pandion haliaetus* is the second bird species that could potentially prey on large fish. However, it is absent from the study area, and the size of the tagged lampreys exceeds the typical prey size for this species (Kalvans & Bajinskis, 2016). Additionally, if the tag were released into the water by a bird, it would have remained immobile, which was never observed in our study as all the predated tags continued to move after predation. In the case of predation by a fish, data from the literature are very scarce, but it appears that tags tend to remain in the predator's stomach for a relatively long time before being expelled into the environment (Klinard et al., 2019; Trancart et al., 2023).

Secondly, considering the size and weight of the tagged individuals (mean 86 cm for tagged sea lampreys), only a few large predatory fish species inhabiting estuarine environments could be responsible for predation events. In theory, a large northern pike (*Esox lucius*) could prey on a tagged sea lamprey. However, the steering committee, composed of professional fishers, suggested that this would require a pike at least 90–100 cm in length, which is rare in the study site. Therefore, the probability that one of our tagged sea lampreys was predated by a pike can be considered negligible.

Unlike northern pike, European catfish capable of preying on sea lampreys — individuals larger than 100–110 cm (Trancart et al., 2023) — are very common in the Loire system. Moreover, this species is already known to be an opportunistic predator (Ferreira et al., 2019; Libois et al., 2016; Vagnon et al., 2022) and to prey heavily on sea lampreys in upstream sections of river basins (Bouletreau et al., 2020; Trancart et al., 2023). Finally, local fishing data indicate a high abundance of European catfish (11 tons fished in 2023) in the study area. These three observations strongly suggest that European catfish are the most likely predator responsible for the observed predation events.

Predation of sea lampreys by European catfish at such low positions in rivers, within estuarine zones, had never been documented before: our study represents the first demonstration of this phenomenon in these areas.

4.3. Timing of predation

Siluriform predators like European catfish (*Silurus glanis*) may still have been emerging from a winter latency phase, akin to hibernation, during which their activity and appetite are reduced (*TRANCART et al., 2023*). Although earlier grey literature suggested that catfish cease feeding below 10 °C, more recent studies (e.g.*Trancart et al. 2023*) indicate that they do feed in colder conditions, albeit with reduced food intake. The present study confirms a possible predation in water around 10 °C.

4.4. *Intensity and location of predation*

While the observed predation rates might seem relatively low, it is
important to consider it the context of the study site's limited size. In
other studies, predation rates on diadromous fish have exceeded 80 %
(e.g. *Bouletreau et al., 2020; Trancart et al., 2023*), but these figures
typically reflect mortality over the entire upstream migration route. In
the present study, the 15 % predation rate concerns only a short tidal
stretch, which suggests a high local predation pressure. This estimated
predation rate should not be extrapolated to the entire Loire River.
Indeed, such extrapolation would result in an over-estimated predation
rate for the river's full length. Instead, it is more appropriate to focus on
the number of “hotspots” of predation.

In our study, predation events were clearly concentrated at a primary hotspot, located at the foot of the bridge piles (PK 5200 m), with a secondary, much weaker hotspot near a rocky outcrop. This spatial concentration of predation suggests that specific locations within the estuary are more favourable for predation. Such areas may act as natural or anthropogenic “predation bottlenecks” where the movement of migratory prey is funneled into constrained spaces, increasing their vulnerability. This phenomenon has been well documented in other systems, where predators aggregate in areas where prey density is elevated due to physical or behavioral constraints (Kennedy et al., 2016; Michel et al., 2020; Mosman et al., 2023). In our case, factors such as changes in water flow, anthropogenic structures like bridge pilings, or geomorphological features such as sandbanks or gravel pits likely contribute to the creation of favorable conditions for ambush or active hunting.

4.5. Necessity of using predation sensors tags

In the current context of invasive species in France, and more broadly
across Europe, conducting acoustic telemetry studies without predation
sensors has become increasingly risky when working in areas with a
known presence of European catfish and biological models vulnerable to
predation. While movement trajectories can indeed provide valuable
insights and may, in some cases, be sufficient to infer predation events,
the broader societal and political context surrounding certain invasive
species, particularly the European catfish, complicates the interpreta-
tion of such data. In France, for example, strong resistance remains
among certain stakeholders to acknowledge the predatory impact of this
species on migratory fish, despite growing scientific evidence. In this
context, predation-sensitive tags offer a critical layer of objectivity and
robustness, helping to secure the credibility of ecological inferences. The
results presented here were part of a larger study aimed at investigating
the migration of sea lampreys in the Loire River (Trancart et al., 2024).
Without the use of these predation-sensitive tags and this approach, we
would have likely misinterpreted the predation of diadromous species as
natural migration behaviors, erroneously attributing movements to the
species itself rather than to predator activity. Therefore, the use of
predation sensors not only improves the accuracy of predator–prey
studies in aquatic ecosystems but also plays a pivotal role in ensuring the
acceptance and reliability of scientific findings in sensitive environ-
mental contexts.

In situations where financial constraints limit the use of predation
tags, subsampling can still be used to identify patterns that allow for the estimation of a "probably predated" rate close to the actual confirmed predation. By carefully selecting a representative sample and applying the comparative analysis of migratory profiles, it is possible to generate reliable insights into predation dynamics, even without full coverage of predation-sensitive tags. This approach ensures that valuable data on predator-prey interactions can still be obtained, offering a practical solution when resources are limited.

## 5. Conclusions

This study highlights three major points. First, predation on sea lampreys, very probably by European catfish, begins as soon as they enter the estuary, and they are not confined to the upper reaches of river basins. This finding suggests that management strategies need to consider predation risks across the full length of estuaries and rivers.

Second, for scientists working possible predated species context, the use of predation-sensitive tags has become indispensable. These tags allow for a more accurate understanding of predator-prey interactions, which would otherwise be overlooked. More broadly, this study emphasizes the need to rethink tracking methodologies in ecosystems impacted by top-predators. Ensuring that these predators do not skew the results of tracking efforts is crucial for the validity of ecological studies and the effective management of vulnerable species.

Third, while financial constraints may limit the ability to mark all individuals with digestion tags, a targeted subsampling approach can still provide valuable insights, allowing for accurate estimation of probable predation rates that closely align with confirmed predation data.

## CRediT authorship contribution statement

**Julie Lucas:** Writing – review & editing, Writing – original draft, Conceptualization.
**Nils Teichert:** Writing – review & editing, Writing – original draft, Formal analysis, Conceptualization.
**Marie-Caroline Husset:** Writing – review & editing, Writing – original draft, Data curation.
**Jezabel Lamoureux:** Writing – review & editing, Writing – original draft, Data curation.
**Emma Robin:** Data curation.
**Eric Feunteun:** Writing – review & editing, Writing – original draft, Validation, Supervision, Conceptualization.
**Thomas Trancart:** Writing – review & editing, Writing – original draft, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

We gratefully acknowledge the support of Voies Navigables de France (VNF) for their assistance during field operations and valuable contributions to discussions throughout the study. We thank Valérie Simon from the Conservatoire des Espaces Naturels 44 for this great collaboration and Frederic Santoul (University of Toulouse) for relevant discussion about European catfish. We thank the Groupement d'Intérêt Public (GIP) Loire Estuaire for providing essential data and the Office Français pour la Biodiversité (OFB), LOGRAMI, and the Loire-Atlantique Federation for Fisheries and Aquatic Environment Protection for their scientific collaboration.

We are also grateful to the Direction Régionale de l'Environnement, de l'Aménagement et du Logement (DREAL) Centre-Val de Loire and other relevant authorities for facilitating the legal authorizations required for this study. Special thanks are extended to the professional

fishers for their logistical support during tagging campaigns.

This study was funded by the Pays de la Loire Regional Council and the Loire Bretagne Water Agency as part of the "Contrat pour la Loire et ses annexes" (2021-2023). We also acknowledge the logistical and administrative support of the Muséum National d'Histoire Naturelle (MNHN) and its marine station in Dinard.

## Data availability

Data will be made available on request.

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