Eroded Swimmeret Syndrome: Update of the Current Knowledge

Eroded swimmeret syndrome (ESS) was first described in 2014 from Swedish signal crayfish (Pacifastacus leniusculus (Dana)), and later also from Finland, with gross symptoms and disease agent candidates identified and described by 2015. The ESS was first discovered affecting alien signal crayfish in Fennoscandia. The ESS is caused by a multiple infection involving Aphanomyces astaci (Schikora) and Fusarium species complex (SC). The ESS symptoms include first melanised spots in swimmerets, then partial swimmeret erosion and finally loss of a swimmeret. There could be a total loss of all swimmerets in the most severe cases. Both females and males can be affected by the ESS. In females, the ESS lowers reproductive success while in males the ESS often causes erosion of the gonopods and thus possible partial failure in mating. The ESS is more frequent among mature females that have reproduced once compared to immature females or those that are mature but have not yet reproduced. The proportion of females with ESS has ranged from 10 to 50% among Lake Saimaa signal crayfish in Finland and in a wider survey from Sweden the range was from 0 to 38%. Among Lake Saimaa male signal crayfish, the ESS proportion has been less than 10%, while it was only 0.6% in the Swedish data. The ESS has also been observed among alien signal crayfish in Switzerland. There are recent observations of ESS affecting narrow-clawed crayfish, Pontastacus leptodactylus (Eschscholtz), in Croatia and Romania (i.e., among native European crayfish stocks). Here, we summarise current knowledge about the ESS and speculate on a few potentially crucial impacts of this syndrome.

Sperm motility and offspring pre- and post-hatching survival in hybridization crosses among a landlocked and two anadromous Atlantic salmon populations: implications for conservation

The landlocked salmon (Salmo salar m. sebago) endemic to Lake Saimaa, Finland, is critically endangered and severely threatened by low genetic diversity and inbreeding. To explore the possibility of increasing the genetic diversity of threatened salmon populations by controlled hybridization (genetic rescue), we studied sperm motility and offspring pre- and post-hatching survival in hybridization crosses of landlocked salmon with two geographically close anadromous salmon populations (Rivers Neva and Tornio) relative to the pure-bred populations. While some degree of gametic incompatibility between landlocked and Tornio salmon cannot be ruled out, there were no indications of outbreeding depression in survival traits in these first-generation hybridizations. Instead, pre-hatching survival of landlocked salmon eggs fertilized with Neva salmon sperm and post-hatching survival of anadromous salmon eggs fertilized with landlocked salmon sperm were higher than in pure-bred landlocked salmon. These differences might imply genetic rescue effects (hybrid vigor), although there were also strong maternal effects involved. Our results on early viability point to the possibility of applying genetic rescue to the landlocked salmon population by hybridization with an anadromous population.

Spawning season movements of transported landlocked Atlantic salmon in a newly restored river habitat

Certain spawning areas of the critically endangered Lake Saimaa landlocked Atlantic salmon (Salmo salar m. sebago) have been recently restored by excavator- and helicopter-scattered gravel, but the success of applied methodologies has remained open. Here, we monitored the spawning-related movements and redds occurrence of transported semi-wild and hatchery-reared salmon in the restored River Ala-Koitajoki, Eastern Finland using radiotelemetry and snorkelling. Nearly all fish accepted the newly-restored spawning habitats in the river that maintains a low flow rate (only 4 m3 s-1). The movements of females released up to two weeks before spawning time were restricted mainly to the closest rapids from the release site, and most of the movements were directed downstream. Semi-wild and hatchery background fish exhibited similar mobility, which was more variable among males. A majority of redds were found in restored areas, and both helicopter- and excavator-scattered gravels were accepted as spawning substrates. Our results indicate that natural reproduction of the studied landlocked salmon population can be successfully returned to its original spawning river by restoration of habitats and transfer of spawners.

Host records and geographical distribution of Corynosoma magdaleni, C. semerme and C. strumosum (Acanthocephala: Polymorphidae)

A literature survey was conducted to investigate the host and geographical distribution patterns of three Corynosoma species (Acanthocephala: Polymorphidae), viz. C. magdaleni, C. semerme and C. strumosum. All three species appear to be restricted to the Northern Hemisphere. Occurrence records of C. magdaleni are limited to the Northern Atlantic coasts, while C. semerme has a circumpolar distribution. The geographical range of Corynosoma strumosum encompasses the distributions of the other two species, but also extends into warmer southern regions. Some Corynosoma populations are living with their definitive hosts in very isolated locations, such as in the brackish Baltic Sea or different freshwater lakes (e.g. Lake Saimaa). All three species have a heteroxenous life cycle, comprising a peracaridan intermediate host, a fish paratenic host and a mammalian definitive host. Occasionally, an acanthocephalan may enter an accidental host, from which it is unable to complete its life cycle. The host records reported here are categorised by type, i.e. intermediate, paratenic, definitive or accidental. While most of the definitive hosts are shared amongst the three Corynosoma species, C. strumosum showed the broadest range of paratenic hosts, which reflects its more extensive geographical distribution. One aim of this study and extensive literature summary is to guide future sampling efforts and therewith contribute to throw more light on the on-going species and morphotype discussion for this interesting parasite species.

Early winter foraging success, swimming performance, and morphology of juvenile landlocked Atlantic salmon reared under semi-wild and hatchery conditions

Several Finnish populations of salmonids have been maintained exclusively by stocking hatchery-reared fish for several generations, and it is crucial to know whether domestication has affected fitness-related traits and to assess how the developmental environment influences fish phenotypes. Here, we focused on Lake Saimaa landlocked salmon (Salmo salar m. sebago) and studied the role of the early rearing environment in trait formation by comparing juveniles (fingerlings) from three backgrounds: (i) semi-wild (stocked as alevins), (ii) standard hatchery, and (iii) enriched hatchery conditions We compared juvenile morphology, swimming performance, and capability of feeding on natural prey in early winter. We found no differences between standard and enriched hatchery conditions. Semi-wild fish consumed more prey items, but wet mass of stomach contents did not differ among backgrounds. Swimming endurance was comparable among backgrounds. Semi-wild fish had slenderer body shape and longer fins than hatchery-reared fish. The lack of performance differences between hatchery juveniles and their semi-wild conspecifics is possibly due to reduced phenotypic plasticity caused by extremely low levels of genetic diversity in this population.

Chronic crayfish plague infection and eroded swimmeret syndrome in Lake Saimaa (Finland) signal crayfish

We present data and results from a 9-year survey (2009 – 2017) of the Lake Saimaa signal crayfish population in Finland. This population has a history of chronic infection with Aphanomyces astaci. It has now been discovered that female signal crayfish from Lake Saimaa suffer from various stages of eroded swimmeret syndrome (ESS), and male crayfish also show symptoms of ESS-like trauma (i.e., eroded swimmerets and gonopods). Our data demonstrates the prevalence of A. astaci infection, with gross symptoms prevailing throughout the duration of the entire 2009 – 2017 survey, and that prevalence of ESS among female signal crayfish is correlated with the prevalence of A. astaci infection in the population. The data shows that an increasing proportion of female crayfish suffer from ESS, and have regenerated swimmerets, however, our observations indicate that partially regenerated swimmerets do not fully function during egg hatching. Based on data and observations during the survey, we speculate that low production levels, as indicated by the low catch per unit effort (CPUE), within the Lake Saimaa signal crayfish population could be related to both high A. astaci infection levels and ESS prevalence among females. The former could be causing increasing mortality among adult crayfish and the latter could be lowering Lake Saimaa signal crayfish reproductive output.

Signal crayfish in Lake Saimaa could be maladapted to the local conditions due to Aphanomyces astaci infection: A seven-year study

We conducted a seven-year survey (years 2009 - 2015) of the Lake Saimaa signal crayfish population. Lake Saimaa is the largest single waterbody in Finland, with a productive fishery and crayfishery. The signal crayfish were introduced to Lake Saimaa in mid-1990’s and a commercial fishery was initiated in the mid-2000s. At first, there was a small proportion of noble crayfish among the catch, but after 2007, an acute crayfish plague epidemic eradicated them, and the signal crayfish stock started showing frequent gross symptoms of chronic crayfish plague infection (e.g., melanised lesions, eroded uropods and pleopods, lost appendages with melanised stumps). This stock now shows gross symptoms of the infection at a rate of 45% to 79% of the annual trap catch, in addition to showing signs of eroded swimmeret symdrome (ESS) at a rate of 2.8 to 15.4%. The CPUE has remained rather low, between one and three crayfish throughout the survey, while the proportion of the commercial grade catch has been between 35% and 68% of the total catch. The signal crayfish populations in Lake Saimaa are still rather fragmented, and production is low. It appears that the Lake Saimaa signal crayfish population has developed slowly and is producing less than expected.