The biology of the twaite shad, Alosa fallax fallax (Lacépède), in the Severn Estuary

The estuarine biology of the twaite shad was studied in the Severn Estuary. Adults enter the estuary at the start of the freshwater phase of their spawning migration between April and June. Peak immigration generally occurs in May and is associated with temperatures in the range 10.6–12.3°C. The mean (± s.d. ) instantaneous mortality rate for the mature population was 0.53±0.18. The effect of additional mortality on the spawning population was modelled assuming constant recruitment and no density-dependent effects.
Juvenile twaite shad are present in the estuary from July until they emigrate seaward during the autumn. A portion of these fish re-enter the estuary the following April–May and remain until late summer/early autumn before once more migrating seaward.
The 0 + age group feed mainly on harpacticoid and calanoid copepods and mysids, the relative preponderance of these in the diet being apparently related to tidal conditions. The possible implications of the proposed tidal power barrage in the Severn Estuary on the twaite shad population are discussed in relation to movement, diet and additional mortality of the mature population.     

Electrophoretic identity between allis shad, Alosa alosa (L.), and twaite shad, A.fallax (Lacépède)

Morphological characteristics as well as electrophoretic polymorphism have been analysed in eight samples of allis shad, Alosa alosa (L.) and twaite shad, Alosa fallax (Lacepede), collected in the Loire basin and the Gironde-Dordogne system. The morphological characteristics showed that allis and twaite shad were present in these samples. Moreover, specimens with intermediate characteristics were found in the Loire and assumed to be hybrids between the two forms. By contrast, the two species were monomorphic and electrophoretically indistingishable at the 22 loci analysed. Therefore, it cannot be excluded that these two forms correspond to a single species.     

The diet of juvenile and adult twaite shad Alosa fallax fallax (lacépède) from the rivers Severn and Wye (Britain)

The diet of juvenile and adult twaite shad was studied at a number of freshwater and estuarine sites in the rivers Severn and Wye and in the coastal waters of Cardigan Bay (West Wales). In the Severn estuary and in the freshwater reaches of the Severn and Wye, adult twaite shad consumed little during their pre-spawning migration. In the Severn Estuary post-spawning adults were present during the summer months where they fed actively on mysids, gammarids and shrimps. Mysids dominated the diet of the adults caught in coastal waters during the autumn. The diet of one year old twaite shad was studied in the Severn Estuary. In May the one year olds from the lower estuary consumed mainly gammarids. During the summer months, this age-class moved into the inner estuary where they fed predominantly on mysids.The larvae and juveniles (age 0⁺) fed mainly on chironomid larvae and pupae and Simuliidae larvae, while in fresh water. In the estuary copepods, cladocerans and mysids dominated the diet of the juveniles. There was some suggestion of an increase in prey size during their period of residency in the estuary. The contrasting distribution of the juveniles in two river systems is discussed in relation to the effects of channelization.     

Morphometric, meristic character and electrophoretic analyses of two Irish populations of twaite shad, Alosa fallax (Lacépède)

Since 1911 a rare lacustrine form of twaite shad, Alosa fallax killarnensis Regan (1916), has been known to occur in the Killarney Lakes. During the summers of 1985 and 1986 a gill netting programme in Lough Leane for brown trout, Salmo trutta L., yielded 32 shad (1985) and 64 shad (1986) as a by-catch. These fish were deep frozen and compared with a sample of 59 marine twaite shad, taken on rod and line from St Mullin's, Co. Carlow, which had entered the River Barrow to spawn.
Morphometric, meristic and electrophoretic character analyses were carried out on the two populations. The three analyses, particularly the isoelectric focusing, confirmed a high degree of genetic similarity. The two major differences found were the dwarfed size of the Killarney shad and the increased numbers of gill rakers carried on the first branchial arch. The merits and demerits of assigning the term subspecies to the Killarney shad are discussed in the light of the available evidence produced.     

Climate change and the green energy paradox: the consequences for twaite shad Alosa fallax from the River Severn, U.K

A stock-recruitment model with a temperature component was used to estimate the effect of an increase in temperature predicted by climate change projections on population persistence and distribution of twaite shad Alosa fallax. An increase of 1 and 2° C above the current mean summer (June to August) water temperature of 17·8° C was estimated to result in a three and six-fold increase in the population, respectively. Climate change is also predicted to result in an earlier commencement to their spawning migration into fresh water. The model was expanded to investigate the effect of any additional mortality that might arise from a tidal power barrage across the Severn Estuary. Turbine mortality was separated into two components: (1) juvenile (pre-maturation) on their out migration during their first year and on their first return to the river to spawn and (2) post-maturation mortality on adults on the repeat spawning component of the population. Under current conditions, decreasing pre-maturation and post-maturation survival by 8% is estimated to result in the stock becoming extinct. It is estimated that an increase in mean summer water temperature of 1° C would mean that survival pre and post-maturation would need to be reduced by c. 10% before the stock becomes extinct. Therefore, climate change is likely to be beneficial to populations of A. fallax within U.K. rivers, increasing survival and thus, population persistence.     

Poor water quality constrains the distribution and movements of twaite shad <Emphasis Type="Italic">Alosa fallax</Emphasis><Emphasis Type="Italic">fallax</Emphasis> (Lacépède, 1803) in the watershed of river Scheldt

Worldwide, river fragmentation is primarily responsible for the decline of populations of migrating fish. In particular, anadromous fish species, which necessarily migrate to fresh water to reproduce, are endangered since many are no longer able to reach their natural spawning sites. In addition, pollution of rivers effectively prevents upstream or downstream movements and blocks access to spawning grounds. This article investigates how poor water quality interferes with the life history cycle of twaite shad Alosa fallax fallax (Lacépède, 1803), an anadromous clupeid fish, in the watershed of River Scheldt, a heavily impacted environment in West Europe. We used two models based on known ecological and environmental information to explain past and present twaite shad distribution within the watershed and to make inferences about a future population recovery and juvenile habitat value. We demonstrated that historical spawning areas satisfy water quality conditions necessary to support spawning and successful development of early life history stages of the twaite shad. However, poor water quality conditions just upstream the freshwater–saltwater boundary still act as an effective migration barrier for upstream movement. As a consequence, spawning grounds are inaccessible and the population is dominated by seasonal adults occurring in the lower estuarine part of the watershed. This article provides testable and diagnostic information to the watershed management in that it identifies habitat and water quality requirements needed to support the expected recovery of an endangered anadromous fish population. Guest editors: S. Dufour, E. Prévost, E. Rochard & P. Williot Fish and diadromy in Europe (ecology, management, conservation)     

A highly permeable species boundary between two anadromous fishes

Meristic identification, mitochondrial DNA and a suite of microsatellite markers were employed to estimate the incidence of hybridization in wild populations of anadromous Allis shad Alosa alosa and twaite shad Alosa fallax in southern Irish riverine and estuarine waters. It was shown that 16% of the fishes examined were misclassified using meristic count of gill rakers. Next, a significant proportion of fishes that were robustly assigned to a species using nuclear markers were shown to possess the mtDNA of the other. The genomes of A. alosa and A. fallax in Ireland are extensively introgressed, which suggests a complex history of hybridization between these species, which can only partially be explained by recent man-made habitat changes.     

Larval fish production in the tidal River Elbe 1985–1986

In 1985 and 1986 ichthyoplankton composition was studied during seven sampling series at 37 stations along the tidal Elbe and its tributary rivers between Hamburg harbour and the North Sea. The highest larval density was observed in May and April. No, or very few, larvae occurred from late summer to early spring. Smelt (59% on average of all samples) and twaite shad (20%) dominate in the freshwater region. Herring/sprat (17%) dominate in the estuary. All major spawning grounds are located along the southern shore. Smelt spawn further upstream 1 month earlier than twaite shad. Three percent of all larvae found were flounder. Most of them occurred upstream from the estuary in the R. Este and in other tributary rivers. It is not clear whether flounders are able to spawn in freshwater. Attention is drawn to the fact that an unknown but probably very high percentage of the ichthyoplankton is entrained by local power plants.     

Return of twaite shad <Emphasis Type="Italic">Alosa fallax</Emphasis> (Lacépède, 1803) to the Southern Baltic Sea and the transitional area between the Baltic and North Seas

The status of twaite shad was investigated within the Southern Baltic Sea (ICES subdivision 22–27) and transitional area between the Baltic and North Seas (division IIIa). The following sources of data were analysed: (i) commercial catch statistics and relevant publications, (ii) records from ichthyological museum collections, (iii) records from recent commercial and recreational fisheries, (iv) research fisheries with trawls. A total of 476 records of twaite shad including more than 16 million individuals were obtained for the time between the years 1836 and 2005. About 72.9% of all records originated from commercial catch statistics and publications, whereas 18.9% were received from ichthyological collections. Research fisheries provided 6.3%, and 1.9% of the records were obtained from recent commercial and recreational fisheries. Most records of twaite shad were estimated for subdivisions 24 (45.2%) and 26 (35.5%). From 1836 to 1959, 29.6% of the records date from the period until 1899. 70.4% of the records of twaite shad originate from the twentieth century until 1959. The mean annual catch of twaite shad between 1891 and 1959 amounted to 86,674 kg within subdivisions 24–26 of the Southern Baltic Sea. Catch data show an approximately 20-year-cyclicity of maximum yields and minimum catches, respectively. The maximum annual yield of twaite shad in subdivisions 24–26 (474,700 kg) was registered in 1940, the minimum annual yield was estimated in 1958 (10 kg). In the 1950s, the annual catches of twaite shad declined sharply. Until 1960 twaite shad catches and records originated mainly from the Pommeranian Bay/Pommeranian Coast and adjacent waters including the Szczecin Lagoon (subdivision 24), the Bay of Gdańsk, Vistula Lagoon and Vistula Spit (subdivision 26) and from the Curonian Lagoon and Curonian Spit (subdivision 26). The highest catches of twaite shad originated from the area of Curonian Lagoon/Curonian Spit from 1941 to 1960. Seasonal catches of twaite shad showed maximum values from May to July. From 1960 to 1989, only four records of A. fallax were registered in the Southern Baltic Sea which originated from German coastal waters in subdivisions 22 and 24. A total of 107 records of twaite shad was obtained in subdivisions 20–27 from 1990 to 2005. Most of these recent records originate from a twaite-shad stock in subdivisions 24, 25 and 26. There are indications which suggest a separate stock of twaite shad in subdivisions 20 and 21. Disappearance and recovery of twaite shad stocks were probably caused by the following factors: construction of barriers in rivers with spawning sites of twaite shad; habitat destruction in those rivers as consequence of gravel extraction and reengineering scheme to improve navigation and for flood defence purposes, water pollution in the lagoons of the Southern Baltic and in their tributaries, commercial fishery in the Southern Baltic and climatic variation in the Baltic Sea basin south of the latitude of 60° N. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Guest editors: S. Dufour, E. Prévost, E. Rochard & P. Williot Fish and diadromy in Europe (ecology, management, conservation)     

Weak population differentiation in northern European populations of the endangered anadromous clupeid Alosa fallax

Examination of genetic structure among different spawning populations of the endangered twaite shad Alosa fallax showed statistically significant but weak differentiation estimated using nine microsatellite loci. Results point to substantial gene flow among the majority of individual estuary/river spawning locations, and no evidence that habitat disruption has led to increased local genetic drift.     

Genetic relationships among the shads (Alosa) revealed by mitochondrial DNA analysis

We surveyed restriction site differences in mitochondrial DNA. (mtDNA) among five species of shad ( Alosa ) from North America and Europe. Allis shad, Alosa alosa and twaite shad, Alosa fallax shared two divergent genotype groups, suggesting that the two forms are either a single species, or are distinct species that have hybridized. Phenetic and cladistic analyses of the relationships among the mitochondrial genotypes defined two groups of shad, corresponding to the subgenera, Alosa and Pomolobus . The mean estimated sequence divergence between the mtDNAs of these two groups of shad was 6.5%. Taken in conjunction with fossil data, this divergence estimate suggests that the rate of mtDNA divergence between the two subgenera has been almost 10-fold lower than the 'conventional' clock calibration for mtDNA.     

The occurrence of Eubothrium fragile (Cestoda: Pseudophyllidae) in twaite shad, Alosa fallax (Lacépède) in the River Severn

The occurrence, size and maturity changes of Eubothrium fragile have been studied in postlarvae, juveniles and adult twaìte shad, Alosa fallax , from several locations in the River Severn. Parasites were only found in adult shad and not in post-larvae or juveniles. No juvenile or recently acquired cestodes were identified as such, but adults were present in shad throughout the whole period of their spawning migration. A large proportion of the parasites were gravid upon arrival in the river and, although eggs were subsequently released into fresh water, there was no loss of cestodes from the fish. It was concluded that E. fragile is a marine species, that the parasites found in adult shad in fresh water were the residue of a marine life cycle and that the eggs released in fresh water were part of the parasite's natural reproductive wastage. The distribution and biology of E. fragile were discussed and it is considered that it is typical of the marine species of the genus.     

Regional and interannual variations in spawning activity of Pacific saury Cololabis saira during northward migration in spring in the north-western Pacific

Regional and interannual variations in spawning activity of Pacific saury Cololabis saira during the northward migration in spring were investigated for 1995–1997 in the north-western Pacific. Only females with a large body size (≥270–290 mm knob length) appeared to spawn. Almost all females in this cohort were actively spawning in a region of sea surface temperature ( T _SS) >18° C, the proportion of active spawners appeared positively related to T _SS in a region of 13° C < T _SS≤ 18° C, and no females were spawning in the region of T _SS≤13° C. Although this relationship between spawning activity and T _SS was common in all years, the distribution of females in the large-size cohort among the regions varied interannually. Consequently, the proportion of active spawners among adult females showed considerable interannual variations.     

Migrational swimming speeds of the American shad, Alosa sapidissima, in the Connecticut River, Massachusetts, U.S.A.

Of 91 sonic-tagged American shad, 78 were tracked upriver to their spawning grounds. The remaining 13 tagged shad dropped back downstream over a dam or moved downstream through the adjacent canal system. Sonic-tagged shad swam upstream individually. 'Apparent' swimming speeds (the time to travel between two points) during daylight hours ranged from 11 to 93 cm s⁻¹ when water temperatures were below 20°C and from 9.8 to 64 cm s⁻¹ when water temperatures exceeded 20°C. Swimming speeds at night ranged from 8 to 53 cm s⁻¹. As the flow rate increased, shad swam faster. A major flood, producing flows reaching 300 cm s⁻¹, flushed all sonic-tagged shad away.     

Environmental factors as forces structuring the fish community of the Elbe Estuary

Smelt, flounder, eel, three-spined stickleback, twaite shad, ruffe and herring were the major species among 62 forming the fish community of the Elbe estuary. Species richness, species diversity, evenness and total fish biomass decreased in the upstream direction. Total abundance was greatest during summer. Salinity was the most significant physico-chemical factor affecting species richness and total fish biomass, whereas water temperature was the best predictor of total abundance. Marine fish species decreased with decreasing salinity, while the freshwater species roach and ide were absent at salinities > 15‰. Bream, blue bream, white bream, perch and pikeperch occurred more frequently in shallow marginal regions with lower current velocities, while herring and other marine species occurred closer to the deep central regions with high current velocities. Annual and seasonal variations of the community structure mainly reflected in population dynamics of smelt, flounder, twaite shad, three-spined stickleback and eel, especially those of 0-age smelt. Populations of smelt and ruffe and their importance in the fish community increased between 1989 and 1992, while those of twaite shad and eel decreased. At oxygen concentrations <3 mg I¹, smelt and flounder were rare, but eels were at their maximum at 3.5 mg I⁻¹. Changes of species composition and fish abundance were closely related to daytime and tidal cycle.     

Effect of springtime water temperature on the time of emergence and size of Pteronarcys californica in the Henry's Fork catchment, Idaho, U.S.A.

1. The emergence time of Pteronarcys californica in streams in the Henry's Fork catchment, Idaho, U.S.A. was negatively correlated with mean April water temperature. Emergence was in mid- to late May at sites influenced by groundwater, where April water temperature averaged 7.9 °C. Adults emerged in mid-June in streams receiving run-off from snowmelt (mean April water temperature 5.4 °C). Intermediate emergence times were observed in a regulated section of river where water temperature was influenced, on one bank, by dam release (mean April water temperature 4.5 °C) and, on the other, by a spring-fed tributary stream (mean April water temperature 6.3 °C).
2. During each of the three study years, emergence was earlier on the bank of the regulated section that was warmer during April and May. The mean body length of P. californica exuviae, collected from the warm side of the river, averaged 1.2 mm longer than those collected from the cold side.
3. We tested the effect on emergence of altering springtime water temperature by translocating P. californica in cages from one location to another during April. Individuals moved to sites with higher April water temperature emerged earlier than individuals that remained at the site from which they were collected.     

Migration behaviour of twaite shad Alosa fallax assessed by otolith Sr:Ca and Ba:Ca profiles

Individual migration behaviour during the juvenile and adult life phase of the anadromous twaite shad Alosa fallax in the Elbe estuary was examined using otolith Sr:Ca and Ba:Ca profiles. Between hatching and the end of the first year of life, juveniles showed two migration patterns. Pattern one exhibited a single downstream migration from fresh water to the sea with no return into fresh water. In contrast, pattern two showed a first migration into the sea, then a return into fresh water and, finally, a second downstream migration into marine water. This first report of migration plasticity for A. fallax points to different exposure times to estuarine threats depending on the migration strategy. In adults, high Sr:Ca and low Ba:Ca in the majority of individuals confirmed prior reports of a primarily marine habitat use. Patterns reflecting spawning migrations were rarely observed on otoliths, possibly due to the short duration of visits to fresh water.     

Movements of juvenile Dover sole, Solea solea (L.), in the Tamar Estuary, South-western England

Solea solea start to appear in the Tamar Estuary in April–May when they are 10 mm long. The majority of young sole move into the estuary in June–July. Most sole leave the estuary towards the end of their second year in October–November, when they have reached a total length of 150–200 mm. The young O-group fish remain in the main channel, whereas the I-group move with the tide onto the mud-flats.
Approximately 7000 fish were marked by freeze-branding or dye-injection. The recaptures showed that there was an up-estuary migration in May–June. Most of the recaptures were taken at the site of original capture and there was only a limited movement of fish between adjacent mud-flat areas. Average growth rate tends to be greater in sole inhabiting the upper reaches of the estuary.     

Observations sur l'activité de migration de la grande alose Alosa alosa L. en Loire (France)

We studied the migratory activity of shad (Alosa alosa Linné) in the middle part of the Loire river, using stop net catches, in 1984.The migration started in the beginning of April, instead of February, as more usual. The end of migration is not precisely known, because of adverse hydrological conditions.During migration, the shad principally used the channel with the highest current velocity. The migration was disturbed by the presence of obstacles (weirs, dams of nuclear power stations).The daily and hourly activity of migration was strongly correlated with variations in water temperature. Shads did not run up at temperatures below 11°C. The upstream limit of ditribution was situated at more than 500 km from the estuary.

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Observations sur l'activité de migration de la grande alose Alosa alosa L. en Loire (France)

     

Recruitment,growth, and vertical distribution of the endangered mud snail Cerithidea rhizophorarum A. Adams, 1855: implications for its conservation

Some fundamental natural-history traits of the endangered mud snail Cerithidea rhizophorarum were investigated for three years in an open, bare, sandy tidal flat located on the east of Shikoku Island in southwestern Japan. Our study demonstrated a highly aggregated distribution of younger snails and complicated differences in vertical migration patterns between younger and older snails that might have prevented comprehensive understanding of the precise demography of C. rhizophorarum in previous studies. From late summer, juveniles of the first-year cohorts appeared patchily at 12–16 cm lower tidal heights than snails of the oldest cohorts and reached 5.0–7.5 mm in mean shell length by the first mid-autumn. In the second year, juveniles gradually moved upward and appeared at higher tidal heights. The second-year cohorts reached 15.6–19.1 mm in mean shell length by the end of the second autumn and converged into the oldest cohorts during the third year or later. Our results suggest that C. rhizophorarum matures sexually and starts reproduction from the fourth summer at the earliest. On the other hand, older snails showed a similar seasonal migration pattern every year—downward movement in summer and upward movement in autumn—probably associated with their reproductive activity and/or a seasonal change in the maximum tidal height. For conservation of C. rhizophorarum, it is very important to preserve both the higher area as a main habitat for older snails and the lower tidal flat as the nursery ground.