River fragmentation and barrier impacts on fishes have been greatly underestimated in the upper Mekong River.

River barriers reduce river connectivity and lead to fragmentation of fish habitats, which can result in decline or even extinction of aquatic biota, including fish populations. In the Mekong basin, previous studies have mainly focused on the impacts of large dams but ignored the impacts of small-scale barriers, or drew conclusions from incomplete barrier databases, potentially leading to research biases. To test the completeness of existing databases and to evaluate the catchment-scale fragmentation level, a detailed investigation of river barriers for the whole Upper Mekong (Lancang catchment) was performed, by conducting visual interpretation of high-resolution remotely sensed images. Then, a complete catchment-scale barrier database was created for the first time. By comparing our barrier database with existing databases, this study indicates that 93.7% of river barriers were absent from the existing database, including 75% of dams and 99.5% of small barriers. Barrier density and dendritic connectivity index (DCID and DCIP) were used to measure channel fragmentation within the catchment. Overall, 50.5% of sub-catchments contained river barriers. The Middle region is the most fragmented area within the Lancang catchment, with a median [quartiles] barrier density of 5.34 [0.70-9.67] per 100 km, DCIP value of 49.50 [21.50-90.00] and DCID value of 38.50 [9.00-92.25]. Furthermore, since 2010, distribution ranges of two representative fish species Schizothorax lissolabiatus (a rheophilic cyprinid) and Bagarius yarrelli (a large catfish) have reduced by 19.2% and 32.8% respectively, probably due in part to the construction of river barriers. Our findings indicate that small-scale barriers, in particular weirs and also small dams are the main reason for habitat fragmentation in the Lancang and must be considered alongside large dams in water management and biodiversity conservation within the Mekong.

Personality, density and habitat drive the dispersal of invasive crayfish.

There is increasing evidence that personality traits may drive dispersal patterns of animals, including invasive species. We investigated, using the widespread signal crayfish Pacifastacus leniusculus as a model invasive species, whether effects of personality traits on dispersal were independent of, or affected by, other factors including population density, habitat, crayfish size, sex and limb loss, along an invasion gradient. Behavioural traits (boldness, activity, exploration, willingness to climb) of 310 individually marked signal crayfish were measured at fully-established, newly-established and invasion front sites of two upland streams. After a period at liberty, recaptured crayfish were reassessed for behavioural traits (newly-established, invasion front). Dispersal distance and direction of crayfish movement, local population density, fine-scale habitat characteristics and crayfish size, sex and limb loss were also measured. Individual crayfish exhibited consistency in behavioural traits over time which formed a behavioural syndrome. Dispersal was both positively and negatively affected by personality traits, positively by local population density and negatively by refuge availability. No effect of size, sex and limb loss was recorded. Personality played a role in promoting dispersal but population density and local habitat complexity were also important determinants. Predicting biological invasion in animals is likely to require better integration of these processes.

Fish community and abundance response to improved connectivity and more natural hydromorphology in a post-industrial subcatchment.

Barrier removal and fish pass construction are increasingly used as tools to restore river connectivity and improve habitat quality, but the effectiveness of subcatchment-scale connectivity restoration on recovery of fish communities is poorly understood. We used a before-after-downstream-upstream methodology to determine the effects of subcatchment-scale connectivity restoration on fishes in a fragmented tributary of the River Wear, Northeast England, between 2013 and 2019. Following restoration (three barriers removed, five barriers fitted with fish passes, two barriers unaltered), riffle habitat increased, fine sediment decreased, and most fish species benefitted. Total fish abundance, comprising seven native species, increased 3 years after the restoration and remained elevated to the end of the study. Mean brown trout (Salmo trutta) density increased from 20.9 ±â€¯6.3 to 33.8 ±â€¯16.8 per 100m2 from 2013 to 2019, with Young-of-Year trout increasing from 10.6 ±â€¯4.6 to 19.8 ±â€¯11.8 per 100m2. Connectivity restoration reduced the mean age of trout, suggesting a change to an increased migratory component of the population. Density of bullhead (Cottus perifretum), a species with poor dispersal ability, increased from 4.6 ±â€¯2.7 to 32.6 ±â€¯17.9 per 100m2 over 2013 to 2019. Stone loach (Barbatula barbatula), also a less mobile species but tolerant to fine sediment, decreased in abundance where barriers were removed. Atlantic salmon (Salmo salar) were absent over the study timescale, despite being common in the Wear, and despite suitable habitat and water quality in the restored subcatchment, suggesting a hysteresis effect. Our findings indicate that, where good water quality exists, restoring river connectivity and hydromorphology at a subcatchment scale is beneficial for most native resident and migratory fishes. However, the ecological benefits of connectivity restoration, especially in rivers with many barriers, may take several years to develop. We encourage well-controlled long-term studies reporting the outcomes of large-scale connectivity restoration.

The role of individual behavioral traits on fishway passage attempt behavior.

Variations in behavioral traits are widely recognized to drive animal behaviors exhibited within a population. However, information on how behavior traits influence behavior in anthropogenically modified habitats is lacking. Many habitats have become highly fragmented as a result of human processes. To mitigate this and improve habitat connectivity, wildlife passes are increasingly employed, with the aim of enabling animals to move freely between habitats. However, wildlife passes (e.g., fishways) are not always effective in achieving passage and it remains uncertain what factors play a role in an individual's likelihood of passing successfully. This study measured three behavioral traits (boldness, exploration, and activity) in juvenile brown trout (Salmo trutta; n = 78) under field conditions within a river and tested whether these behavior traits influenced both the passage success and the behaviors exhibited during upstream fishway passage attempts. Although behavioral traits were found and collapsed into two behavioral trait dimensions, behavioral traits had low repeatability and so did not contribute to a personality spectrum. Boldness was found to negatively influence the number of passage attempts carried out by an individual and to positively influence passage success, with bolder individuals carrying out fewer attempts and having an increased probability of passage success. No behavioral traits were found to be related to other passage metrics (passage success, Time until First Attempt, and Passage Duration) during the first passage. But all three behavioral traits were significantly negatively related to the changes in passage behaviors at consecutive, successful passage attempts, with bolder, more exploratory and more active individuals passing through a fishway quicker on the second passage than on the first. This study suggests that bolder and more active individuals may perform better during fishway passage attempts, particularly within rivers where multiple barriers to movement exist.

More than one million barriers fragment Europe's rivers.

Rivers support some of Earth's richest biodiversity1 and provide essential ecosystem services to society2, but they are often fragmented by barriers to free flow3. In Europe, attempts to quantify river connectivity have been hampered by the absence of a harmonized barrier database. Here we show that there are at least 1.2 million instream barriers in 36 European countries (with a mean density of 0.74 barriers per kilometre), 68 per cent of which are structures less than two metres in height that are often overlooked. Standardized walkover surveys along 2,715 kilometres of stream length for 147 rivers indicate that existing records underestimate barrier numbers by about 61 per cent. The highest barrier densities occur in the heavily modified rivers of central Europe and the lowest barrier densities occur in the most remote, sparsely populated alpine areas. Across Europe, the main predictors of barrier density are agricultural pressure, density of river-road crossings, extent of surface water and elevation. Relatively unfragmented rivers are still found in the Balkans, the Baltic states and parts of Scandinavia and southern Europe, but these require urgent protection from proposed dam developments. Our findings could inform the implementation of the EU Biodiversity Strategy, which aims to reconnect 25,000 kilometres of Europe's rivers by 2030, but achieving this will require a paradigm shift in river restoration that recognizes the widespread impacts caused by small barriers.

Are we designing fishways for diversity? Potential selection on alternative phenotypes resulting from differential passage in brown trout.

Fishways are commonly employed to improve river connectivity for fishes, but the extent to which they cater for natural phenotypic diversity has been insufficiently addressed. We measured differential upstream passage success of three wild brown trout (Salmo trutta) phenotypes (anadromous, freshwater-resident adult and parr-marked), encompassing a range of sizes and both sexes, at a Larinier superactive baffle fishway adjacent to a flow-gauging weir, using PIT telemetry (n = 160) and radio telemetry (n = 53, double tagged with PIT tags). Fish were captured and tagged downstream of the weir in the autumn pre-spawning period, 2017, in a tributary of the River Wear, England, where over 95% of tributary spawning habitat was available upstream of the weir. Of 57 trout that approached the weir-fishway complex, freshwater-resident adult and parr-marked phenotypes were less successful in passing than anadromous trout (25%, 36%, and 63% passage efficiency, respectively). Seventy-one percent of anadromous trout that passed upstream traversed the weir directly. Although the fishway facilitated upstream passage, it was poor in attracting fish of all phenotypes (overall attraction efficiency, 22.8%). A higher proportion (68.2%) of parr-marked trout that approached the weir were male and included sexually mature individuals, compared with that of freshwater-resident (37.8%) and anadromous trout (37.0%). The greater passage success of anadromous trout was likely due to their greater size and locomotory performance compared to the other phenotypes. Barriers and fishways can act as selection filters, likely the case in this study, and greater consideration needs to be given to supporting natural diversity in populations when proposing fishway designs to mitigate river connectivity problems.

Surgical implantation of radio tags in three eel species (Anguilla spp.) in South Africa.

Studies have reported poor survival of surgically tagged freshwater fishes in warm African waters. This study aimed to assess the applicability of using radio telemetry (and surgical implantation of tags) for Anguilla spp. Nineteen yellow eels (Anguilla bengalensis, A. marmorata and A. mossambica) were surgically implanted with radio tags between October 2018 and January 2019 in the Thukela River, South Africa. Most eels were alive 6 months after tagging, and recaptured eels displayed advanced or complete healing at the incision site. Therefore, this method appears suitable for African freshwater eels.

Freezer on, lights off! Environmental effects on activity rhythms of fish in the Arctic.

Polar regions are characterized by acute seasonal changes in the environment, with organisms inhabiting these regions lacking diel photoperiodic information for parts of the year. We present, to our knowledge, the first high-resolution analysis of diel and seasonal activity of free-living fishes in polar waters (74°N), subject to extreme variation in photoperiod, temperature and food availability. Using biotelemetry, we tracked two sympatric ecomorphs of lake-dwelling Arctic charr (Salvelinus alpinus n = 23) over an annual cycle. Charr activity rhythms reflected the above-surface photoperiod (including under ice), with diel rhythms of activity observed. During the dark winter solstice period, charr activity became arrhythmic and much reduced, even though estimated light levels were within those at which charr can feed. When twilight resumed, charr activity ensued as diel vertical migration, which continued throughout spring and with increasing day length, despite stable water temperatures. Diel activity rhythms ceased during the polar day, with a sharp increase in arrhythmic fish activity occurring at ice-break. Despite contrasting resource use, circannual rhythms were mirrored in the two ecomorphs, although individual variability in activity rhythms was evident. Our data support conclusions of functionally adaptive periods of arrhythmicity in polar animals, suggesting maintenance of a circannual oscillator for scheduling seasonal behavioural and developmental processes.

Adaptive management in the context of barriers in European freshwater ecosystems.

Many natural habitats have been modified to accommodate for the presence of humans and their needs. Infrastructures - such as hydroelectric dams, weirs, culverts and bridges - are now a common occurrence in streams and rivers across the world. As a result, freshwater ecosystems have been altered extensively, affecting both biological and geomorphological components of the habitats. Many fish species rely on these freshwater ecosystems to complete their lifecycles, and the presence of barriers has been shown to reduce their ability to migrate and sustain healthy populations. In the long run, barriers may have severe repercussions on population densities and dynamics of aquatic animal species. There is currently an urgent need to address these issues with adequate conservation approaches. Adaptive management provides a relevant approach to managing barriers in freshwater ecosystems as it addresses the uncertainties of dealing with natural systems, and accommodates for future unexpected events, though this approach may not be suitable in all instances. A literature search on this subject yielded virtually no output. Hence, we propose a step-by-step guide for implementing adaptive management, which could be used to manage freshwater barriers.

Evaluating the effectiveness of restoring longitudinal connectivity for stream fish communities: towards a more holistic approach.

A more holistic approach towards testing longitudinal connectivity restoration is needed in order to establish that intended ecological functions of such restoration are achieved. We illustrate the use of a multi-method scheme to evaluate the effectiveness of 'nature-like' connectivity restoration for stream fish communities in the River Deerness, NE England. Electric-fishing, capture-mark-recapture, PIT telemetry and radio-telemetry were used to measure fish community composition, dispersal, fishway efficiency and upstream migration respectively. For measuring passage and dispersal, our rationale was to evaluate a wide size range of strong swimmers (exemplified by brown trout Salmo trutta) and weak swimmers (exemplified by bullhead Cottus perifretum) in situ in the stream ecosystem. Radio-tracking of adult trout during the spawning migration showed that passage efficiency at each of five connectivity-restored sites was 81.3-100%. Unaltered (experimental control) structures on the migration route had a bottle-neck effect on upstream migration, especially during low flows. However, even during low flows, displaced PIT tagged juvenile trout (total n=153) exhibited a passage efficiency of 70.1-93.1% at two nature-like passes. In mark-recapture experiments juvenile brown trout and bullhead tagged (total n=5303) succeeded in dispersing upstream more often at most structures following obstacle modification, but not at the two control sites, based on a Laplace kernel modelling approach of observed dispersal distance and barrier traverses. Medium-term post-restoration data (2-3years) showed that the fish assemblage remained similar at five of six connectivity-restored sites and two control sites, but at one connectivity-restored headwater site previously inhabited by trout only, three native non-salmonid species colonized. We conclude that stream habitat reconnection should support free movement of a wide range of species and life stages, wherever retention of such obstacles is not needed to manage non-native invasive species. Evaluation of the effectiveness of fish community restoration in degraded streams benefits from a similarly holistic approach.

Fine-scale behavioural differences distinguish resource use by ecomorphs in a closed ecosystem.

Temporal differences in habitat use and foraging specialisms between ecomorphs represent aspects of behavioural phenotype that are poorly understood with regard to the origin and maintenance of ecological diversity. We tested the role of behaviour in resource use divergence of two Arctic charr (Salvelinus alpinus) phenotypes, a slim, putatively pelagic-dwelling morph and a robust, putatively littoral-dwelling generalist morph, over an annual cycle, using biotelemetry and stable isotopes. Pelagic morph charr exhibited significantly greater δC(13) depletion, concordant with increased zooplanktivory, than for the Littoral morph. Although three-dimensional space-use of the morphs strongly overlapped, on average, the Littoral morph used that habitat 19.3% more than the Pelagic morph. Pelagic morph fish were significantly more active, further from the lake bed and at greater depth than Littoral fish (annual means respectively, Pelagic, 0.069 BL s(-1), 8.21 m and 14.11 m; Littoral, 0.047 BL s(-1), 5.87 m and 10.47 m). Patterns of habitat use differed between ecomorphs at key times, such as during autumn and at ice break, likely related to spawning and resumption of intensive foraging respectively. Extensive space-use overlap, but fine-scale differences in habitat use between charr ecomorphs, suggests the importance of competition for generating and maintaining polymorphism, and its potential for promoting reproductive isolation and evolution in sympatry.

A moving target--incorporating knowledge of the spatial ecology of fish into the assessment and management of freshwater fish populations.

Freshwater fish move vertically and horizontally through the aquatic landscape for a variety of reasons, such as to find and exploit patchy resources or to locate essential habitats (e.g., for spawning). Inherent challenges exist with the assessment of fish populations because they are moving targets. We submit that quantifying and describing the spatial ecology of fish and their habitat is an important component of freshwater fishery assessment and management. With a growing number of tools available for studying the spatial ecology of fishes (e.g., telemetry, population genetics, hydroacoustics, otolith microchemistry, stable isotope analysis), new knowledge can now be generated and incorporated into biological assessment and fishery management. For example, knowing when, where, and how to deploy assessment gears is essential to inform, refine, or calibrate assessment protocols. Such information is also useful for quantifying or avoiding bycatch of imperiled species. Knowledge of habitat connectivity and usage can identify critically important migration corridors and habitats and can be used to improve our understanding of variables that influence spatial structuring of fish populations. Similarly, demographic processes are partly driven by the behavior of fish and mediated by environmental drivers. Information on these processes is critical to the development and application of realistic population dynamics models. Collectively, biological assessment, when informed by knowledge of spatial ecology, can provide managers with the ability to understand how and when fish and their habitats may be exposed to different threats. Naturally, this knowledge helps to better evaluate or develop strategies to protect the long-term viability of fishery production. Failure to understand the spatial ecology of fishes and to incorporate spatiotemporal data can bias population assessments and forecasts and potentially lead to ineffective or counterproductive management actions.

Allometric scaling of intraspecific space use.

Allometric scaling relationships enable exploration of animal space-use patterns, yet interspecific studies cannot address many of the underlying mechanisms. We present the first intraspecific study of home range (HR) allometry relative to energetic requirements over several orders of magnitude of body mass, using as a model the predatory fish, pike Esox lucius. Analogous with interspecific studies, we show that space use increases more rapidly with mass (exponent = 1.08) than metabolic scaling theories predict. Our results support a theory that suggests increasing HR overlap with body mass explains many of these differences in allometric scaling of HR size. We conclude that, on a population scale, HR size and energetic requirement scale allometrically, but with different exponents.

Contrasting population genetic structure among freshwater-resident and anadromous lampreys: the role of demographic history, differential dispersal and anthropogenic barriers to movement.

The tendency of many species to abandon migration remains a poorly understood aspect of evolutionary biology that may play an important role in promoting species radiation by both allopatric and sympatric mechanisms. Anadromy inherently offers an opportunity for the colonization of freshwater environments, and the shift from an anadromous to a wholly freshwater life history has occurred in many families of fishes. Freshwater-resident forms have arisen repeatedly among lampreys (within the Petromyzontidae and Mordaciidae), and there has been much debate as to whether anadromous lampreys, and their derived freshwater-resident analogues, constitute distinct species or are divergent ecotypes of polymorphic species. Samples of 543 European river lamprey Lampetra fluviatilis (mostly from anadromous populations) and freshwater European brook lamprey Lampetra planeri from across 18 sites, primarily in the British Isles, were investigated for 13 polymorphic microsatellite DNA loci, and 108 samples from six of these sites were sequenced for 829 bp of mitochondrial DNA (mtDNA). We found contrasting patterns of population structure for mtDNA and microsatellite DNA markers, such that low diversity and little structure were seen for all populations for mtDNA (consistent with a recent founder expansion event), while fine-scale structuring was evident for nuclear markers. Strong differentiation for microsatellite DNA loci was seen among freshwater-resident L. planeri populations and between L. fluviatilis and L. planeri in most cases, but little structure was evident among anadromous L. fluviatilis populations. We conclude that postglacial colonization founded multiple freshwater-resident populations with strong habitat fidelity and limited dispersal tendencies that became highly differentiated, a pattern that was likely intensified by anthropogenic barriers.

Paradoxical exploitation of protected fishes as bait for anglers: evaluating the Lamprey bait market in Europe and developing sustainable and ethical solutions.

A reoccurring conservation problem is the resolution of consumptive use of threatened wildlife and is especially difficult to defend when it occurs for recreational practices. We explored the commercial capture and supply of threatened European river lamprey (Lampetra fluviatilis) to anglers, to determine the extent of exploitation and seek opportunities for improved conservation. The trade began in 1995 from England, but by 2012 involved sale of lamprey from England, The Netherlands and Estonia, including from protected populations. Lamprey are sold frozen for the capture of predatory fish, mostly in freshwater. In the year 2011/2012 9 tonnes (>90,000 lampreys) of river lamprey were supplied, almost exclusively to British anglers. Although annual catches in the main English lamprey fishery (River Ouse) have varied widely since 1995, catch per unit effort did not decline between 2000 and 2012. Conservation actions since 2011 have included a cap on fishing licenses, catch quotas and restricted fishing seasons. Now, 86% of lamprey bait is imported to Britain. Most bait sellers interviewed would not stock lamprey if they knew they were from threatened populations; many felt their trade would not be impacted if lamprey were not stocked. This facilitates opportunities to enter into dialogue with anglers over alternative baits to threatened lamprey. The study emphasises the need to inform stakeholders about conservation species subjected to market-driven exploitation.

Role of fish as predators of mosquito larvae on the floodplain of the Gambia River.

We examined the potential of using native fish species in regulating mosquitoes in the floodplain of the Gambia River, the major source of mosquitoes in rural parts of The Gambia. Fishes and mosquito larvae were sampled along two 2.3-km-long transects, from the landward edge of the floodplain to the river from May to November 2005 to 2007. A semifield trial was used to test the predatory capacity of fish on mosquito larvae and the influence of fish chemical cues on oviposition. In the field, there was less chance of finding culicine larvae where Tilapia guineensis, the most common floodplain fish, were present; however, the presence of anophelines was not related to the presence or absence of any fish species. In semifield trials, both T. guineensis and Epiplatys spilargyreius were effective predators, removing all late-stage culicine and anopheline larvae within 1 d. Fewer culicines oviposited in sites with fish, suggesting that ovipositing culicine females avoid water with fish. In contrast, oviposition by anophelines was unaffected by fish. Our studies show that T. guineensis is a potential candidate for controlling mosquitoes in The Gambia.

Factors triggering floodplain fish emigration: Importance of fish density and food availability.

Emigration is a widespread phenomenon among fish species in seasonal habitats, but little is known about the factors which trigger this behaviour. In controlled experiments using Tilapia guineensis, a species widely occurring in the seasonal floodplains of West Africa, density of fish played a significant role in triggering fish migration, whereas a lack of food available caused an increase in exploratory behaviour but with no impact on successful emigration. The impact of fish density and subsequently interactions between individuals on emigration suggests that this may be an important causal factor of emigration in fish species exhibiting social interactions.

The effect of high pH on ion balance, nitrogen excretion and behaviour in freshwater fish from an eutrophic lake: a laboratory and field study.

Slapton Ley is a freshwater hyper-eutrophic lake of two basins connected by a narrow channel. One part of the lake experiences summer blooms of cyanobacteria and poor water quality, including elevated water pH (maximum pH recorded=10.54), the other part is shaded by reed beds, and remains clear and neutral. This study used laboratory and field physiological measurements together with radio-tracking to investigate the potential impacts of alkaline pH on the physiology and behaviour of fish from Slapton Ley. Exposure of perch (Perca fluviatilis) from Slapton Ley to pH 9.50 water in the laboratory caused an immediate inhibition of sodium uptake and ammonia excretion to 34 and 32% of control levels, respectively. Net sodium balance recovered by day 3 of exposure whereas ammonia excretion only partially recovered to 60-70% of the control value from 8 h onwards. Urea excretion did not increase as a result of high pH exposure. Fish from the alkaline part of the lake (pH 9.90) had almost three-fold greater plasma ammonia compared to fish from neutral waters, indicating a pronounced disruption of ammonia excretion in the field. There was no significant disturbance to plasma sodium, chloride or total protein in fish sampled from the alkaline water of Slapton Ley. The radio-tracking provided no evidence of adult perch and pike (Esox lucius) trying to seek refuge from the alkaline conditions, despite having access to adjacent parts of the lake with neutral pH. It seems likely that there are advantages (e.g. better foraging, less predation) of withstanding the high pH conditions that outweigh the benefit of moving into more pH neutral parts of the lake.

Changes in microcystin production by Microcystis aeruginosa exposed to phytoplanktivorous and omnivorous fish.

With direct exposure to phytoplanktivorous fish (Hypophthalmichthys molitrix), increased mass-specific microcystin production occurred in three monoclonal Microcystis aeruginosa strains (NIES 44, 88 and 99). Total mass-specific microcystin content of NIES 44 exposed to H. molitrix was over 50 times higher than controls (a mean value of 16.2 microgg(-1)-dry cell in controls versus 878.6 microgg(-1)-dry cell in treatments). Up to nine times higher microcystin levels were detected in NIES 88 exposed to H. molitrix compared to controls (a mean value of 553 in controls versus 5145 microgg(-1)-dry cell in treatments). The microcystin levels of all strains were significantly different between controls and H. molitrix treatments (P < 0.01 for NIES 44 and 88; P < 0.05 for NIES 99). The microcystin response to the omnivorous Carassius gibelio langsdorfi was weaker than that of H. molitrix, though the levels in all strains exposed to the fish were higher than in controls and a significant difference in microcystin production between controls and omnivorous fish treatments occurred for NIES 44 (a mean value of 6.9 in controls versus 41.5 microgg(-1)-dry cell in treatments; P < 0.01) and NIES 88 (a mean value of 359.8 versus 480.4 microgg(-1)-dry cell; P < 0.05). Microcystis cells were observed in the both fish faeces and gut contents, and microcystin was also detected in the body tissues (from 0.6 to 2.5 microgg(-1)-dry weight) and faeces of both fish species on the final day of experiment, although 98% of fish in three strains of Microcystis cultures had lost weight (mean +/- S.E. fish growth rate with M. aeruginosa; -0.90 +/- 0.06% per day, n = 96). This study showed that several M. aeruginosa strains increased toxin production when exposed to fish, especially phytoplanktivorous species, even though fish appeared not to feed vigorously on toxic Microcystis, and supports the hypothesis that this response is a fish-induced defence mediated by physical contact associated with feeding or by chemical cues (e.g. kairomones).