Spatial variability in the growth of invasive European barbel Barbus barbus in the River Severn basin, revealed using anglers as citizen scientists

Life history trait analyses of non-native fishes help identify how novel populations respond to different habitat typologies. Here, using electric fishing and anglers as citizen scientists, scales were collected from the invasive barbel Barbus barbus population from four reaches of the River Severn and Teme, western England. Angler samples were biased towards larger fish, with the smallest fish captured being 410 mm, whereas electric fishing sampled fish down to 60 mm. Scale ageing revealed fish present to over 20 years old in both rivers. Juvenile growth rates were similar across all reaches. Lengths at the last annulus and Linfinity of the von Bertalanffy growth model revealed, however, that fish grew to significantly larger body sizes in a relatively deep and highly impounded reach of the River Severn. Anglers thus supplemented the scale collection and although samples remained limited in number, they provided considerable insights into the spatial demographics of this invasive B. barbus population

Predicting the ecological impacts of an alien invader: experimental approaches reveal the trophic consequences of competition

1. Ecological theory on the trophic impacts of invasive fauna on native competitors is equivocal. While increased inter-specific competition can result in coexisting species having constricted and diverged trophic niches, the competing species might instead increase their niche sizes to maintain energy intakes. Empirical experiments can test invasion theory on competitive interactions and niche sizes across different spatial scales and complexity. 2. The consequences of increased inter-specific competition from a model alien fish Leuciscus idus were tested on two taxonomically and trophically similar native fishes, Squalius cephalus and Barbus barbus. Competitive interactions were tested in tank aquaria using comparative functional responses (CFRs) and cohabitation trials. The consequences of these competitive interactions for the trophic niche sizes and positions of the fishes were tested in pond mesocosms. 3. CFRs revealed that compared to B. barbus, L. idus had significantly higher attack and consumption rates; cohabitation trials revealed B. barbus growth rates were depressed in sympatry with L. idus. For L. idus and S. cephalus, differences in their functional response parameters and growth rates were not significant. 4. Pond mesocosms used stable isotope metrics to quantify shifts in the trophic niche sizes of the fishes between allopatry and sympatry using a substitutive experimental design. Isotopic niches were smaller and more divergent in sympatric paired species than predicted by their allopatric treatments, suggesting trophic impacts from inter-specific competition. However, an all-species sympatric treatment revealed similar niche sizes with allopatry. This maintenance of niche sizes in the presence of all species potentially resulted from the buffering of direct competitive effects of the species-pairs by indirect effects. 5. Experimental predictions from tank aquaria assisted the interpretation of the constricted and diverged trophic niches detected in the paired-species sympatric treatments of the pond mesocosms. However, the all-species sympatric treatment of this experiment revealed greater complexity in the outcomes of the competitive interactions within and between the species. These results have important implications for understanding how alien species integrate into food webs and influence the trophic relationships between native species

Variability in population traits of a sentinel iberian fish in a highly modified mediterranean-type river

Human pressures on water resources have been suggested as a driver of biological traits that induce changes in native fish populations. This study highlighted the interplay between environmental stress factors, mostly related to flow regulation, and the longitudinal river gradient in biological traits such as the growth, size structure and somatic condition of a sentinel fish, Luciobarbus sclateri. We found an increase in size-related metrics and somatic condition at population levels associated with downstream reaches, although fragmentation and habitat alteration, flow regime alteration and the abundance of non-native fish were also significantly involved in their variability. Age-related parameters and growth were only explained by flow regime alterations and the abundance of non-native fish species. The high plasticity observed in L. sclateri population traits suggests that this is a key factor in the species adaptability to resist in a strongly altered Mediterranean river basin. However, the interplay of multiple stressors plays an important role in fish population dynamics and could induce complex responses that may be essential for long-term monitoring in sentinel species

Behavioural thermoregulation in cold-water freshwater fish: Innate resilience to climate warming?

Behavioural thermoregulation enables ectotherms to access habitats providing condi-tions within their temperature optima, especially in periods of extreme thermal condi-tions, through adjustments to their behaviours that provide a “whole- body” response to temperature changes. Although freshwater fish have been detected as moving in response to temperature changes to access habitats that provide their thermal optima, there is a lack of integrative studies synthesising the extent to which this is driven by behaviour across different species and spatial scales. A quantitative global synthesis of behavioural thermoregulation in freshwater fish revealed that across 77 studies, behavioural thermoregulatory movements by fish were detected both vertically and horizontally, and from warm to cool waters and, occasionally, the converse. When fish moved from warm to cooler habitats, the extent of the temperature difference between these habitats decreased with increasing latitude, with juvenile and non- migratory fishes tolerating greater temperature differences than adult and anadro-mous individuals. With most studies focused on assessing movements of cold-water salmonids during summer periods, there remains an outstanding need for work on cli-matically vulnerable, non-salmonid fishes to understand how these innate thermoreg-ulatory behaviours could facilitate population persistence in warming conditions

Parasite infection but not chronic microplastic exposure reduces the feeding rate in a freshwater fish.

Microplastics (plastics <5 mm) are an environmental contaminant that can negatively impact the behaviour and physiology of aquatic biota. Although parasite infection can also alter the behaviour and physiology of their hosts, few studies have investigated how microplastic and parasite exposure interact to affect hosts. Accordingly, an interaction experiment tested how exposure to environmentally relevant microplastic concentrations and the trophically transmitted parasite Pomphorhynchus tereticollis affected the parasite load, condition metrics and feeding rate of the freshwater fish final host chub Squalius cephalus. Microplastic exposure was predicted to increase infection susceptibility, resulting in increased parasite loads, whereas parasite and microplastic exposure were expected to synergistically and negatively impact condition indices and feeding rates. Following chronic (≈170 day) dietary microplastic exposure, fish were exposed to a given number of gammarids (4/8/12/16/20), with half of the fish presented with parasite infected individuals, before a comparative functional response experiment tested differences in feeding rates on different live prey densities. Contrary to predictions, dietary microplastic exposure did not affect parasite abundance at different levels of parasite exposure, specific growth rate was the only condition index that was lower for exposed but unexposed fish, with no single or interactive effects of microplastic exposure detected. However, parasite infected fish had significantly lower feeding rates than unexposed fish in the functional response experiment, with exposed but unexposed fish also showing an intermediate decrease in feeding rates. Thus, the effects of parasitism on individuals were considerably stronger than microplastic exposure, with no evidence of interactive effects. Impacts of environmentally relevant microplastic levels might thus be relatively minor versus other stressors, with their interactive effects difficult to predict based on their single effects

Edad y crecimiento de Gobio lozanoi Doadrio & Madeira, 2004 (Cypriniformes: Cyprinidae) en sectores fluviales de la cuenca del río Segura (SE península ibérica)

El trabajo estudia la estructura de edad, longitud retrocalculada y tasa de crecimiento en poblaciones de Gobio lozanoi a lo largo de los principales ejes fluviales de la cuenca del río Segura (ríos Se- gura y Mundo). Se analizaron estos parámetros con individuos capturados en otoño de 2009 y en 19 localidades, así como su re- lación con variables ambientales (altitud, estado ecológico y con- ductividad). Las poblaciones reflejaron seis clases de edad (0+ a 5+) con escasas diferencias entre sexos y dominancia de edades intermedias (2+ y 3+). La estructura de edades mostró variaciones significativas en el gradiente con tendencia a presentar menor nú- mero de clases en localidades a menor altitud. La tasa de creci- miento se relacionó negativamente con la longevidad y mostró di- ferencias entre sectores fluviales. La tasa de crecimiento en la fase de transición a la madurez mostró tendencia a disminuir con el gra- diente, pero también conforme decrece el estado ecológico y au- menta la conductividad

Variability in the summer movements, habitat use and thermal biology of two fish species in a temperate river

The ability of fish to cope with warm water temperatures in summer depends on factors including their thermal traits and the ability of individuals to access cool-water refugia. Knowledge is highly limited on the in situ responses of many fishes to elevated summer temperatures, including whether they express behavioural thermoregulation. The responses of two riverine species to summer water temperatures were tested here using the movement metrics, spatial habitat use and body temperatures of individual European barbel Barbus barbus (‘barbel’) and common bream Abramis brama (‘bream’) versus river temperatures. Acoustic biotelemetry was applied in the lower River Severn basin, western Britain, in summer 2021 (barbel) and 2022 (bream), where individuals could move across > 150 km of river, including a tributary of cooler water. Across all individuals, bream occupied 37 km of river length (mainstem only), with low inter-individual variability in their spatial habitat use, movements and body temperatures. In contrast, barbel occupied 62 km of river (main river/tributary), with relatively high inter-individual variability in spatial habitat use, movements and body temperatures, with higher variation in body temperatures as river temperatures increased (maximum mean daily temperature difference between individuals on the same day: 4.2 °C). Although warmer individuals generally moved more, their activity was greatest at relatively low temperatures and higher flows, and neither species revealed any evidence of behavioural thermoregulation during elevated temperatures. Enabling phenotypically diverse fish populations to express their natural behaviours and thermal preferences in summer water temperatures thus requires maintaining their free-ranging in thermally heterogenous habitats

Plasticity in Reproductive Traits, Condition and Energy Allocation of the Non-Native Pyrenean Gudgeon Gobio lozanoi in a Highly Regulated Mediterranean River Basin

The invasion success of non-native fish, such as Pyrenean gudgeon Gobio lozanoi in several Iberian rivers, is often explained by the expression of its life history traits. This study provides the first insights into the reproductive traits, fish condition, and energy allocation (protein and lipid contents of tissues) of this species, along a longitudinal gradient in one of the most regulated river basins in the Iberian Peninsula, the Segura river. Larger sizes of first maturity, higher fecundity and larger oocytes were found in fluvial sectors with the most natural flow regimes, characterised by a low base flow with high flow peaks in spring and autumn. A delay in the reproductive period, lower fish condition and no differences in sex-ratio were observed in fluvial sectors with a high increase in base flow and notable inversion in the seasonal pattern of flow regime. Lipid contents in the liver and gonads were stable during the reproductive cycle and decreases in muscle were noted, whereas ovarian and liver proteins increased. In relation to energy allocation for G. lozanoi, an intermediate energy strategy was observed between income and capital breeding. Our results support the hypothesis that the high plasticity of G. lozanoi population traits plays a significant role in its success in a highly regulated Mediterranean river basin. Understanding the mechanisms by which flow regulation shapes fish populations in Mediterranean type-rivers could inform management actions

Plasticity in Reproductive Traits, Condition and Energy Allocation of the Non-Native Pyrenean Gudgeon Gobio lozanoi in a Highly Regulated Mediterranean River Basin

The invasion success of non-native fish, such as Pyrenean gudgeon Gobio lozanoi in several Iberian rivers, is often explained by the expression of its life history traits. This study provides the first insights into the reproductive traits, fish condition, and energy allocation (protein and lipid contents of tissues) of this species, along a longitudinal gradient in one of the most regulated river basins in the Iberian Peninsula, the Segura river. Larger sizes of first maturity, higher fecundity and larger oocytes were found in fluvial sectors with the most natural flow regimes, characterised by a low base flow with high flow peaks in spring and autumn. A delay in the reproductive period, lower fish condition and no differences in sex-ratio were observed in fluvial sectors with a high increase in base flow and notable inversion in the seasonal pattern of flow regime. Lipid contents in the liver and gonads were stable during the reproductive cycle and decreases in muscle were noted, whereas ovarian and liver proteins increased. In relation to energy allocation for G. lozanoi, an intermediate energy strategy was observed between income and capital breeding. Our results support the hypothesis that the high plasticity of G. lozanoi population traits plays a significant role in its success in a highly regulated Mediterranean river basin. Understanding the mechanisms by which flow regulation shapes fish populations in Mediterranean type-rivers could inform management actions

Influences of angler subsidies on the trophic ecology of European barbel Barbus barbus

European barbel Barbus barbus is a recreationally important riverine fish that is widely introduced outside of its natural range. Contemporary angling practices for B. barbus involve the use of baits based on marine fishmeal (MF). MF is isotopically distinct from freshwater prey via highly enriched δ13C and thus its dietary influence on B. barbus can be tested via differences in fractionation factors (Δ13C). Correspondingly, stable isotope data from 11 riverine B. barbus populations tested how their trophic ecology varied across populations according to MF from angling. Δ13C of fish with macroinvertebrate prey resources varied within and between populations (range 0.90 to 10.13 ‰) and indicated that, within populations, up to 71 % of B. barbus had relatively high dietary contributions of MF. These contributions were significantly and positively related to fish length, with MF influences increasingly apparent as fish length increased. Population isotopic niche sizes increased as the dietary influence of MF in that population increased. These results indicated that whilst MF from angling can act as a strong trophic subsidy, its influence varies spatially and with fish length, with its use as a food resource by B. barbus generally involving dietary specializations of larger-bodied individuals