Host Fish Populations Research Articles

Population status and host fishes of the freshwater pearl mussel in the eastern part of the Onega Peninsula, Northern European Russia

Four populations of freshwater pearl mussel Margaritifera margaritifera were studied at lower sections of three river basins of the eastern part of the Onega Peninsula, Northern European Russia: the Bol’shaya Syarta, the Ust’-Yaren’ga and the Lopshen’ga (including Kamenny Stream). One of the populations from the Lopshen’ga River was discovered for the first time. Here we present preliminary data on the population status of M. margaritifera, its host fishes and glochidia infestation in the studied river basins. Living juvenile mussels and fishes infested by glochidia were recorded only in two waterbodies, i.e., the Bol’shaya Syarta and Ust’-Yaren’ga rivers. The pearl mussel population from the Bol’shaya Syarta River seems to represent the most prosperous among studied ones. The average density of visible mussels was small in the lower section of the river, but the proportion of young mussels was rather high (33.1 %) and intensity of glochidia infection reached 865 glochidia per fish. The highest average density was recorded in the pearl mussel population from the Kamenny Stream, Lopshen’ga River basin. However, only one juvenile mussel and none of host fishes infested by glochidia were recorded in this watercourse that may indicate the population aging and decrease of reproduction. Morphological analyses revealed significant differences between four studied populations of M. margaritifera towards the shell convexity index (SCI) and shell elongation index (SEI). In all studied basins, salmonid fishes were also infested by other parasites, mainly flatworms and their cysts. This factor may negatively impact the salmonid host fish populations that require monitoring of the current situation. The data obtained and future studies will allow to develop a system for protecting and monitoring of populations of a rare species such as the freshwater pearl mussel M. margaritifera.

The endangered freshwater pearl mussel Margaritifera margaritifera shows adaptation to a local salmonid host in Finland

Abstract The freshwater pearl mussel Margaritifera margaritifera (FPM) is an endangered unionid which has a glochidium larva that attaches to the gills of Atlantic salmon Salmo salar or brown trout S. trutta, although some FPM populations have been shown to exclusively attach to only one of these species. The origin of host fish populations may be crucial for conservation actions for this mussel species, but the relative suitability of local (sympatric) and non‐local (allopatric) salmonid populations as the hosts for FPM has been studied only rarely. We hypothesised that FPM glochidia would show adaptation to local salmonid strains and, therefore, that they would be more successful (abundant, larger) attached to sympatric than to allopatric fish. Here, we investigated the infection success (abundance and growth of encysted larvae in fish) of FPM in local versus non‐local fish by caging different strains of brown trout and Atlantic salmon in rivers where FPM populations are present. Higher abundances of glochidia in local fish were observed in three brown trout streams, and larger glochidia were found in sympatric hosts in one brown trout stream and in one salmon river. Furthermore, non‐local allopatric fish were not better hosts than local fish in any of the FPM populations tested, neither in brown trout or salmon rivers and neither in abundance nor size of larvae. Therefore, the results supported the hypothesis that glochidia show local adaptation by being more successful when attached to local fish strains. Thus, the local, sympatric fish strain should be preferred in FPM conservation programmes that involve captive breeding of juvenile mussels and introduction of host fish, but the regional assessment of local host dependency of FPM also would be important outside the current study area. The results also indicate the importance of restoration of original salmonid populations in FPM rivers to enable the natural, effective reproduction cycle of FPM in their original, sympatric hosts, and thus to promote the recovery of endangered FPM populations.

Stable isotopes reveal the importance of terrestrially derived resources for the diet of the freshwater pearl mussel (Margaritifera margaritifera)

Abstract The freshwater pearl mussel (FPM) is among the most endangered freshwater species worldwide. The few remaining populations suffer from low recruitment rates and are subject to habitat fragmentation, pollution, siltation, decline or loss of host fish populations, and climate change. Successful conservation strategies for FPM require a holistic understanding of its ecological requirements, life history, population dynamics, and habitat prerequisites. Although habitat requirements are well described, food requirements at different life stages have received less attention. Stable isotope analyses of FPM and potential food resources in three German streams were combined with mixing model analysis to quantify organic matter resources assimilated by juvenile (first year after encystment from host fish) and semi‐adult (10 years old, immature) individuals. There were only slight differences in dietary contributions between the two life stages, and terrestrial particulate organic matter and benthic organic matter contributed substantially to the diet. Tissue type was more important in explaining variation in dietary contributions than individual variation for semi‐adult FPM. The strong reliance on terrestrial resources sheds new light on the functional role of unionid mussels and the connection of streams to their riparian area. The dependence of FPM on terrestrial resources also emphasizes the need for a stronger focus on the restoration and protection of intact riparian areas, including wetlands with their specific vegetation, when planning conservation and management strategies for threatened FPM populations.

The Patterns and Puzzles of Genetic Diversity of Endangered Freshwater Mussel Unio crassus Philipsson, 1788 Populations from Vistula and Neman Drainages (Eastern Central Europe)

Mussels of the family Unionidae are important components of freshwater ecosystems. Alarmingly, the International Union for Conservation of Nature and Natural Resources Red List of Threatened Species identifies almost 200 unionid species as extinct, endangered, or threatened. Their decline is the result of human impact on freshwater habitats, and the decrease of host fish populations. The Thick Shelled River Mussel Unio crassus Philipsson, 1788 is one of the examples that has been reported to show a dramatic decline of populations. Hierarchical organization of riverine systems is supposed to reflect the genetic structure of populations inhabiting them. The main goal of this study was an assessment of the U. crassus genetic diversity in river ecosystems using hierarchical analysis. Different molecular markers, the nuclear ribosomal internal transcribed spacer ITS region, and mitochondrial DNA genes (cox1 and ndh1), were used to examine the distribution of U. crassus among-population genetic variation at multiple spatial scales (within rivers, among rivers within drainages, and between drainages of the Neman and Vistula rivers). We found high genetic structure between both drainages suggesting that in the case of the analyzed U. crassus populations we were dealing with at least two different genetic units. Only about 4% of the mtDNA variation was due to differences among populations within drainages. However, comparison of population differentiation within drainages for mtDNA also showed some genetic structure among populations within the Vistula drainage. Only one haplotype was shared among all Polish populations whereas the remainder were unique for each population despite the hydrological connection. Interestingly, some haplotypes were present in both drainages. In the case of U. crassus populations under study, the Mantel test revealed a relatively strong relationship between genetic and geographical distances. However, in detail, the pattern of genetic diversity seems to be much more complicated. Therefore, we suggest that the observed pattern of U. crassus genetic diversity distribution is shaped by both historical and current factors i.e. different routes of post glacial colonization and history of drainage systems, historical gene flow, and more recent habitat fragmentation due to anthropogenic factors.

Colonizing Islands of water on dry land—on the passive dispersal of fish eggs by birds

AbstractThe vast majority of global freshwater ecosystems are small lakes with less than 100 m2 surface area. These lakes are often unconnected to other water bodies but frequently host fish populations. Existing explanations for how fish colonize such remote habitats often involve birds as vectors transporting fish eggs as propagules. In this study, we aim to quantify the prevalence of these explanations among relevant societal groups as well as their scientific knowledge basis. We analyzed entries in online blogs and forums and surveyed the opinions of local stakeholders and decision makers using a questionnaire. To collect published scientific knowledge, we conducted a structured literature review. Our results reveal a discrepancy between commonly found beliefs and the empirical knowledge supporting those beliefs: Dispersal of fish eggs by water birds was overall the most frequent explanation online and in the questionnaire. In the scientific literature, however, we found hardly any empirical research on passive fish egg dispersal. We propose research directions for how to close this gap of knowledge and suggest that future empirical studies on passive fish egg dispersal may be inspired by existing work on passive dispersal in aquatic invertebrates. Mitigating the belief‐evidence discrepancy on fish dispersal will be essential to better understand the patterns of fish biodiversity across landscapes, to counteract its losses, and inform management strategies for invasive fish species.

When and how? Freshwater mussel recolonization in Lake Orta

Thanks to a video posted on a social network, live mussels of <em>Unio elongatulus</em>, have been recorded from Lake Orta (Italy) over one century after the last (and only) report. With its long and well documented history of pollution, Lake Orta offers the opportunity to document the post-extirpation recovery of freshwater unionid mussels. This case evidences that recovery/recolonization is possible despite a high devastation in the past, and permits to evaluate how fast recolonization may occur, in which way, and in what conditions. The answer to the '<em>how</em> <em>fast</em>' was sought by estimating the age of the larger and seemingly older individuals of the population. To address the '<em>in which way</em>' we compared the haplotypes of Lake Orta specimens of <em>Unio elongatulus</em> (the only species present) with those of surrounding populations. We concluded that, since Lake Orta lacks a direct connection with the putative source populations, colonizing mussels were almost certainly transported by fish carrying glochidia that were used for lake restocking after liming. Data from the long-term monitoring of water chemistry and sediments have allowed defining what conditions proved to be suitable for survival making possible the start of mussels recovery. But not only water and sediment quality matters for mussels recovery, which was delayed by nearly ten years after the reappearance of fish. This delay reflects the need of the whole trophic chain to be reestablished to allow the survival of the suitable and healthy host-fish populations necessary for mussels reproduction.

Sea-trout habitat fragmentation affects threatened freshwater pearl mussel

Habitat fragmentation is a major reason for the decline and extinction of migratory species. Many host populations for parasites are partly migratory, consisting of both migratory and resident individuals, and we asked the question whether these two subpopulations differ in parasite susceptibility. If so, barriers intercepting migration may change the parasite encystment rate and recruitment. We used the River Ljungan, Sweden, as study system, where eight hydroelectric power plants hinder access of sea-migrating trout to most tributaries. We compared the encystment of the threatened freshwater pearl mussel (Margaritifera margaritifera) larvae on sympatric tributary-resident brown trout and sea-migrating trout (Salmo trutta) as hosts for their parasitic larvae. We also compared eighteen tributary-resident brown trout populations with the sea-migrating trout as hosts for mussel larvae. Encystment decreased faster over time on the tributary-resident trout than on the sea-migrating trout. Encystment was therefore higher on the sea-migrating trout compared to the tributary-resident trout at the time of excystment of the larvae. Growth of encysted mussel larvae was higher on the sea-migrating trout than on the tributary-resident trout. The natural density of the tributary-resident trout populations was lower than that of the sea-migrating trout in the autumn, when the mussels infest the fish, and in early summer, just before the excystment of juvenile mussels from the fish. Estimations of total numbers of encysted mussel larvae per unit stream area were higher on the sea-migrating trout than the tributary-resident trout in the autumn and in the spring at the time of juvenile mussel excystment. The higher mussel larval growth and the higher encystment on the sea-migrating trout imply that even seemingly healthy tributary mussel populations are reduced because of the habitat fragmentation. As sea-migrating trout are threatened or extinct in many rivers, fragmentation may be a large-scale problem, not only for sea-migrating populations, but also indirectly for highly threatened mussel populations. Restoration activities should primarily focus on creating free pathways for original host fish populations instead of sustaining them by supplementary stocking.

Fish species composition and host fish density in streams of the thick‐shelled river mussel (Unio crassus) – implications for conservation

Abstract Recruitment and dispersal of freshwater mussels of the Unionoida is strongly linked to their host fishes on which the larvae develop into juvenile mussels. Host limitation has been suggested as one of the potential causes for the decline of the formerly widespread thick‐shelled river mussel (Unio crassus), although quantitative data analysing the importance of this factor are still missing. In this study, the status of host fish populations was assessed in nine U. crassus streams within the Danube drainage system situated in southern Germany. In particular, links between recruitment patterns of U. crassus populations, fish species composition and quantitative data of host fish densities in functional and non‐functional streams were established. Fish species composition and richness ranged between three and nine species with no difference in presence data between functional and non‐functional streams. Despite this high variability, functional U. crassus streams with high proportions of juvenile mussels had significantly higher (mean: 40 Ind. per 100 m2) densities of primary hosts (Phoxinus phoxinus, Squalius cephalus, Gasterosteus aculeatus) than non‐functional streams (mean: 8 Ind. per 100 m2). In contrast, no relationship was found between the numbers and densities of juvenile mussels and densities of poor hosts as well as of non‐hosts. The results of this study indicate that the recruitment status of U. crassus strongly depends on the density of primary host fish species. Host limitation is likely to play a major role in a lack of juvenile recruitment and consequently in the decline of central European U. crassus populations. Therefore, future conservation for this endangered mussel species should more thoroughly incorporate management of host fish species and consider both the requirements of mussels and their hosts in habitat restoration. Copyright © 2014 John Wiley & Sons, Ltd.

Survival and extinction of the southern populations of freshwater pearl mussel Margaritifera margaritifera in Russia (Leningradskaya and Novgorodskaya oblast)

Information on the southern populations of the freshwater pearl mussel in Russia (Novgorodskaya and Leningradskaya oblast) is very scarce and has never been mentioned in the international scientific literature. Pearl fishing used to be common in this area in the fifteenth to eighteenth centuries but collapsed afterwards. Long-term field survey undertaken in 2006–2012 revealed that eight populations survived and at least 22 became extinct. The total number of mussels found was about 50,000, with the largest population in one small river consisting of approximately 40,000 individuals. Juveniles were found in six populations. The data obtained are of interest for assessing the resilience and stability of pearl mussel populations in response to potentially negative environmental and particularly anthropogenic impacts. The results are expected to inform future management strategies for conservation of suitable habitats for successful pearl mussel reproduction. The decline in host fish populations in the rivers appears to be the most significant threat to the survival of the existing pearl mussel populations.

Host‐specificity of the endangered thick‐shelled river mussel (Unio crassus, Philipsson 1788) and implications for conservation

ABSTRACT The complex life cycle of the endangered thick‐shelled river mussel (Unio crassus, Philipsson 1788) includes an obligatory parasitic phase on a host fish. Consequently, knowledge of the interaction of U. crassus with its host species is crucial for the development of effective conservation strategies. The objective of this study was to assess systematically the host suitability of eight different fish species, including six native species which naturally co‐occur with U. crassus, as well as two non‐native species. All tested fish species were successfully infected with glochidia of U. crassus, which were present on their gills 2 days after exposure. Phoxinus phoxinus and Squalius cephalus were the most suitable hosts as indicated by both the highest total glochidial load and the highest fish‐weight‐normalized glochidial load after 16 days and 48 days. Salmo trutta, Alburnoides bipunctatus and Cottus gobio were less suitable, losing ~90% of glochidia within 16 days. Alburnus alburnus, invasive Neogobius melanostomus and introduced Oncorhynchus mykiss lost more than 98% of glochidia within 16 days, indicating they are unsuitable hosts. U. crassus larvae did not grow significantly (<15%) during their metamorphosis on suitable hosts, suggesting that the most obvious advantage of the host‐dependent phase in the U. crassus life cycle is the dispersal by fish vectors. The observed differences in the developmental speed and the timing of excystment on different suitable host species are likely to increase the chances of successful dispersal and survival in adverse environmental conditions. The sustainable conservation management of U. crassus populations is closely linked to the effective management of their host fish populations. In particular, the currently underestimated ecological functions of low‐valued fish species such as S. cephalus and P. phoxinus clearly deserve better consideration in the conservation management of U. crassus habitats and stream ecosystems. Copyright © 2011 John Wiley & Sons, Ltd.

Assessment of stable isotopic signatures as a means to track the exchange of sea lice (Lepeophtheirus salmonis) between host fish populations

The viability of stable isotopic carbon (δ13C) and nitrogen (δ15N) signatures to track the exchange of sea lice, Lepeophtheirus salmonis , between hosts was tested. It was predicted that nonfeeding sea lice would experience lowered δ13C signatures and enriched δ15N signatures owing, at least in part, to moulting during development. It was also predicted that newly settled parasitic first generation sea lice (late stage copepodids) would retain isotopic signatures indistinguishable from their natal host fish population. During this laboratory study, δ13C signatures of nonfeeding sea lice developmental stages declined over time, while the δ13C signature of chalimus III sea lice was not significantly different from fin and gill signatures of their natal hosts, thus providing an isotopic link to their natal host fish population. In contrast, δ15N analysis did not show promise as a tracer in this study, as host fish populations did not display distinct δ15N signatures. These results suggest that δ13C signatures may be applied to identify sources of sea lice epizootics and potentially quantify the exchange of sea lice between cultured and wild salmon populations.

Distribution and habitats of Unio caffer Krauss, 1848 (Bivalvia: Unionoida: Unionidae) in South Africa based on the records in the database of the National Freshwater Snail Collection

The distribution of the Unionoida is almost cosmopolitan and reaches its greatest diversity in North America with 860 currently recognized valid species. Two genera of the family Unionidae, Unio and Coelatura, comprising four species, occur in South Africa. This article focuses on the distribution and habitats of Unio caffer Krauss, 1848 based on the records in the database of the National Freshwater Snail Collection (NFSC) of South Africa. This bivalve is considered to be endemic to South Africa and although it was sporadically reported from elsewhere in South Africa, the Western Cape is the only province from which no samples are on record in the database of the NFSC. The majority of the 58 samples on record was recovered from rivers (32.8%) and dams (20.7%) and from water conditions described as perennial, clear and fresh and 22 of the samples were collected in water bodies with a predominantly sandy substratum. A temperature index calculated for this species ranked it in fi fth position of the 12 bivalve species represented in the database on account of its association with low climatic temperatures. An integrated decision-tree analysis indicated that temperature, substratum and water bodies per se were the most important factors of those investigated that played a signifi cant role in establishing the geographical distribution of this species in South Africa. Comprehensive surveys for freshwater molluscs conducted by state and local health authorities were discontinued during the eighties of the previous century and the majority of sampling sites have not been revisited since. Therefore hardly any recent data pertaining to the conservation status and species diversity of the mollusc fauna of South Africa are available. However, during relatively recent surveys conducted by the authors at three previously positive sites for U. caffer no specimens of this species could be recovered and it is also reported in literature that its range in the south-western Cape has decreased in recent years. With regard to its conservation status, the above findings seem to suggest that U. caffer should at least be considered as vulnerable – if not endangered – as reported for some related species elsewhere in the world. Although speculative, several reasons are suggested to explain the global phenomenon of decline in freshwater bivalves. These include, amongst others, construction of impoundments, introduction of alien species, wetland drainage and canalization and pollution. However, the unique lifecycle of the Unionoida could also play an important role in this respect due to the fact that their larval stages are obligatory parasites on fish. These bivalves are therefore dependent on fish for their survival and dispersal and without their host fish populations will disappear. To sustain a viable population a water body should therefore be suitable not only for the bivalves themselves but also for their host fi sh. As mentioned earlier, the majority of samples of U. caffer were recovered from dams and rivers, water body types both under pressure of over exploitation and pollution. It is therefore recommended that thorough surveys should be planned and conducted in specific areas which could be selected with the documented geographical distribution in the database of the NFSC as guideline. A comparison of the results of such surveys with the data in the database of the NFSC could make a considerable contribution towards assessing the current conservation status and diversity of the freshwater molluscs of South Africa.

The status of host fish populations and fish species richness in European freshwater pearl mussel (Margaritifera margaritifera) streams

Abstract The status of host fish populations and fish species richness was investigated at 36 sites of 20 extant freshwater pearl mussel populations, including the drainages of the Elbe, Danube, Rhine, Weser, Aulne, Kemijoki and Tuuloma in Germany, the Czech Republic, France and Finland, by carrying out comparative electrofishings. Brown trout (Salmo trutta f. fario) were found to be the available host fish for pearl mussels in all except one of the streams investigated with mean densities of 2861 ha−1 (range 0–8710 ha−1) and a mean biomass of 119 kg ha−1 (range 0–478 kg ha−1). Streams that had been frequently stocked with brown trout had higher trout biomass and densities of host fish than natural populations, but trout stocking had no positive effect in two of the streams investigated. Fish species richness ranged from 2 to 16 species per stream and showed a negative correlation with host fish biomass and host fish densities. Undisturbed oligotrophic pearl mussel headwater streams usually only yielded a low number of fish species. Habitat degradation can reduce competitiveness of specialized trout and result in an increased abundance of ubiquitous or atypical species. A link between the lack of juvenile pearl mussels and a lack of suitable host fish was only rarely observed. Functional pearl mussel populations with relatively high numbers of juveniles had significantly lower densities and biomass of host fish than pearl mussel populations without recent recruitment. This study suggests that 0+ host fish are not necessarily required to sustain functional pearl mussel populations. Low densities of host fish can be compensated by the higher glochidia carrying capacity of older host fish with limited previous contact with pearl mussel glochidia, by the long reproductive period of mussels, and by low mortality rates of juvenile mussels during their post‐parasitic phase. Copyright © 2005 John Wiley & Sons, Ltd.

Effects of parasites on fish behaviour: a review and evolutionary perspective

Fish serve as hosts to a range of parasites that are taxonomically diverse and that exhibit a wide variety of life cycle strategies. Whereas many of these parasites are passed directly between ultimate hosts, others need to navigate through a series of intermediate hosts before reaching a host in (or on) which they can attain sexual maturity. The realisation that parasites need not have evolved to minimise their impact on hosts to be successful, and in many cases may even have a requirement for their hosts to be eaten by specific predators to ensure transmission, has renewed interest in the evolutionary basis of infection-associated host behaviour. Fishes have proved popular models for the experimental examination of such hypotheses, and parasitic infections have been demonstrated to have consequences for almost every aspect of fish behaviour. Despite a scarcity of knowledge regarding the mechanistic basis of such behaviour changes in most cases, and an even lower understanding of their ecological consequences, there can be little doubt that infection-associated behaviour changes have the potential to impact severely on the ecology of infected fishes. Changes in foraging efficiency, time budget, habitat selection, competitive ability, predator-prey relationships, swimming performance and sexual behaviour and mate choice have all been associated with – and in some cases been shown to be a result of – parasite infections, and are reviewed here in some detail. Since the behavioural consequences of infections are exposed to evolutionary selection pressures in the same way as are other phenotypic traits, few behavioural changes will be evolutionarily neutral and host behaviour changes that facilitate transmission should be expected. Despite this expectation, we have found little conclusive evidence for the Parasite Increased Trophic Transmission (PITT) hypothesis in fishes, though recent studies suggest it is likely to be an important mechanism. Additionally, since the fitness consequences of the many behavioural changes described have rarely been quantified, their evolutionary and ecological significance is effectively unknown. Potential hosts may also change their behaviour in the presence of infective parasite stages, if they adopt tactics to reduce exposure risk. Such `behavioural resistance', which may take the form of habitat avoidance, prey selectivity or avoidance of infected individuals, can be viewed as behavioural change associated with the threat of being parasitised, and so is included here. Actually harbouring infections may also stimulate fishes to perform certain types of simple or complex behaviours aimed at removing parasites, such as substrate scraping or the visitation of cleaning stations, although the efficacy of the latter as a parasite removal strategy is currently subject to a good deal of debate. The effects parasites have on shoaling behaviour of host fish have attracted a good deal of attention from researchers, and we have provided a case study to summarise the current state of knowledge. Parasites have been shown to affect most of the antipredator effects of shoaling (such as vigilance, co-ordinated evasion and predator confusion) and can also impair an individual's foraging ability. It therefore seems unsurprising that, in a number of species avoidance of parasitised individuals has evolved which may explain the occurrence of parasite-assorted shoals in the field. Parasitised fish are found more often in peripheral shoal positions and show a reduced tendency for shoaling in some fish species. Given the array of host behaviours that may be changed, the fitness consequences of shoal membership for parasitised hosts and their parasites are not always easy to predict, yet an understanding of these is important before we can make predictions regarding the ecological impact of infections on host fish populations. Clearly, there remain many gaps in our knowledge regarding the effects of parasites on the behaviour of host fish. We believe that a much greater understanding of the importance of infection-associated behaviour changes in fish could be gained from high quality research in comparatively few areas. We have completed our review by highlighting the key research topics that we believe should attract new research in this field.