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.     

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.     

The ichthyoplankton of a seasonally closed estuary in temperate Australia. Does an extended period of opening influence species composition?

Fish eggs and larvae were collected monthly between September 1987 and April 1989 from sites throughout the main basin and within the saline regions of the two main tributary rivers of Wilson Inlet, a seasonally closed estuary in south-western Australia. Of the eggs, 43.7% belonged to Engraulis australis (Shaw) and the rest to unidentified teleosts. The larval fish assemblage comprised 17 families represented by 25 species. The Gobiidae contained the highest number of species (four) and contributed approximately 57% of all larvae caught. Pseudogobius olorum (Sauvage) and E. australis were the most abundant species, contributing 43.9 and 27.9% to the total larval catch, respectively. The larvae of species which breed within Wilson Inlet dominated the assemblage, both in terms of number of species (64%) and contribution to total catch (99.9%). The numbers of the eight marine species and one freshwater species represented in the ichthyoplankton were very low. Classification and multi-dimensional scaling ordination showed that the composition of the larval fish fauna at the various sites during a period when the estuary remained open to the sea (December 1988-April 1989) was similar to that of the corresponding sites during the same period in the previous year when the estuary had become closed (December 1987-April 1988). This can be attributed to the spatial distribution, time of occurrence and abundance of estuarine-spawned larvae being similar in the two periods and to the rarity of marine-spawned larvae, even in the spring and summer of 1988/1989 when the estuary was open for the whole time when most marine teleosts spawn in south-western Australia. The low occurrence of marine-spawned larvae in Wilson Inlet reflects the fact that tidal water movement within the basin of the system is so small that it is unable to facilitate the transport and dispersion of larvae. The ichthyoplankton of Wilson Inlet resembles that of other poorly-flushed estuaries in that it is low in species richness and dominated by estuarine-spawned larvae.     

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.     

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)     

The presence of anadromous twaite shad (Alosa fallax) in the Ebro River (western Mediterranean,Spain): an indicator of the population's recovery?

Although the Ebro River is one of the largest Mediterranean rivers, the presence of anadromous fish species was near to complete extirpation since the second half of the 20th century. However, many mature twaite shads (Alosa fallax) were captured at the lower reaches of this river (Xerta's weir site, 63 river km) in spring 2005, suggesting a natural recovery of this population. In this study, the first biological data on twaite shad from the Ebro River are presented, and their significance for the species conservation and recovery are discussed. The historical reasons for the decline of this species are reviewed, and data supports the proposal for urgent construction of a proper fish passage for anadromous species at the Xerta's weir. This action would allow fish access to a further 60 km of river upstream of the weir, representing a 200% increase in the amount of freshwater habitat and potential spawning areas. Improved access together with restrictive fishery regulations will assist with renewed interest in the fishery and conservation of the species.     

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)     

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.     

Phytoplankton growth rates in the freshwater tidal reaches of the Schelde estuary (Belgium) estimated using a simple light-limited primary production model

During the course of 1996, phytoplankton was monitored in the turbid, freshwater tidal reaches of the Schelde estuary. Using a simple light-limited primary production model, phytoplankton growth rates were estimated to evaluate whether phytoplankton could attain net positive growth rates and whether growth rates were high enough for a bloom to develop. Two phytoplankton blooms were observed in the freshwater tidal reaches. The first bloom occurred in March and was mainly situated in the most upstream reaches of the freshwater tidal zone, suggesting that it was imported from the tributary river Schelde. The second bloom occurred in July and August. This summer bloom was situated more downstream in the freshwater tidal reaches and appeared to have developed within the estuary. A comparison between phytoplankton growth rates estimated using a simple primary production model and flushing rate of the water indicated that no net increase in phytoplankton biomass was possible in March while phytoplankton could theoretically increase its biomass by 20% per day during summer. Chlorophyllaconcentrations at all times decreased strongly at salinities between 5–10 psu. This decline was ascribed to a combination of salinity stress and light limitation. Phytoplankton biomass and estimated annual net production were much higher in the freshwater tidal zone compared to the brackish reaches of the estuary (salinity > 10 psu) despite mixing depth to euphotic depth ratios being similar. Possible reasons for this high production include high nutrient concentrations, low zooplankton grazing pressure and import of phytoplankton blooms from the tributary rivers.     

Growth and movements of the twaite shad, Alosa fallax (Lacépède), in the Severn Estuary

Between July 1974 and April 1977, appreciable numbers of 0 group twaite shad, Alosa fallax , have been collected from the cooling water intake screens of the Nuclear Power Stations at Oldbury-upon-Severn and Berkeley on the Severn Estuary and at Hinkley Point on the Bristol Channel, England. Young of the year first appeared at Oldbury in July at a size of approximately 32 mm standard length, and grew to approximately 61 mm by October. The downstream migration in the late summer and autumn appeared to be closely correlated with water temperature since movement reached a peak soon after the temperature in the Estuary had declined below 19° C, and ceased altogether below 9° C. Thereafter, there was virtually no movement of shad until temperatures rose sharply above 7° C in the spring. Maturing adult twaite shad were captured in late April and May as they moved into freshwater to spawn, probably in mid-June. Males were recorded slightly earlier during the migration and were generally younger than females. Furthermore, mean lengths and weights for individuals belonging to the different age classes indicated that males were also smaller. Logarithmic relationships between length and weight of young and adult A. fallax and for the variation with size in the number of rakers on the first gill arch are presented.     

美国鲥鱼对我国淡水生态系统的潜在入侵风险

近年来美国鲥鱼已被引入到我国的上海和广东等地进行水产养殖。本实验报道了在纯淡水人工养殖条件下美国鲥鱼的繁殖和生长情况。人工养殖鲥鱼的生长速度接近美国北卡罗莱纳州沿岸野生鲥鱼的生长速度,而高于特拉华河野生鲥鱼的生长速度;人工养殖的2龄鲥鱼既可以在养殖池中自然产卵,也可以通过注射激素人工诱导产卵,其平均受精率、孵化率和仔鱼17天内的存活率分别约15%、75%和54%。这一结果表明,尽管鲥鱼是在海水中进行繁殖的洄游性鱼类,但在生长环境适宜的淡水条件下同样可以正常生长,并达到性成熟和繁育后代,有可能在适宜的淡水水体中形成自我维持的种群。结合本实验结果和美国鲥鱼的其他生物学特性,本文认为美国鲥鱼在我国天然水体生态系统中具有潜在的入侵性。     

Ichthyoplankton transport in relation to floodplain width and inundation and tributary creek discharge in the lower Savannah River of Georgia and South Carolina

We report the results of a 3-year study of ichthyoplankton in the lower Savannah River and its coastal plain tributaries. Sampling was weekly from February through July in 1983, 1984 and 1985. Ichthyoplankton transport was used as an estimate of ichthyoplankton production. Ichthyoplankton transport in the river, for both total and most common taxa, was highest in 1983 and lowest in 1985. Ichthyoplankton transport into the river from tributary streams was also highest in 1983 and lowest in 1985. Ichthyoplankton transported from these tributaries sometimes comprised a significant percent of the larval transport at the next river station downstream from the tributary mouth. The highest larval transport occurred when the spring flood pulse was most elevated for the longest time, and larval transport at particular river stations or creeks was correlated with floodplain width in 1983 and 1984, years when flood pulses were high enough so that the adjacent floodplain was inundated during or following spawning. In 1985 the flood pulse was brief, inundation levels were low, and inundation occurred in February before most fish had spawned. Except for American shad and sunfishes, larval transport for all common taxa was greatly reduced in 1985 compared to 1983.     

Effects of temperature gradients resulting from reservoir discharge on Dorosoma cepedianum spawning in the Savannah River

Synopsis The timing and spatial pattern of gizzard shad, Dorosoma cepedianum, spawning were markedly affected by a temperature gradient caused by the release of hypolimnetic water from an upstream reservoir into the middle reaches of the Savannah River. During 1983 and 1984, a distinct thermal gradient occurred with the warmest temperatures at the downstream end of the 257 km study reach and the coolest temperatures at the upstream end. The occurrence of gizzard shad larvae indicated that spawning began at the sample stations farthest downstream and progressed upstream, peaking at each sample station when it warmed to approximately 19° C. The estimated difference in date of peak density of larvae between the upstream and downstream ends of the gradient was 23 days in 1983 and 35 days in 1984. Because of this pattern, densities of larvae were highest at the downstream end of the study area early in the spawning season and highest at the upstream end of the study area late in the spawning season.Photoperiod and daylength appeared unrelated to the patterns under study. Far-reaching effects of anthropogenic temperature changes on fish spawning and the distribution of larvae are probably common in rivers that receive hypolimnetic discharge.     

Spatial and temporal variation of the ichthyoplankton in a subtropical river in Brazil

Studies that assess reproduction dynamics and ichthyoplankton distributions are scarce for the upper Uruguay River, especially in environments such as tributary mouths. Therefore, this study aimed to evaluate: (i) ichthyoplankton composition; (ii) spatial and temporal variation in ichthyoplankton abundance; and (iii) relationships between environmental variables and the abundance of ichthyoplankton during one annual cycle in this region. Monthly samples were collected from September 2001 to August 2002 in 48 h cycles at 6 h intervals between each sampling. Samples of eggs and larvae were collected from three of the main tributaries of the region (Ligeiro, Palomas and Chapecó rivers) and from three stretches of the Uruguay River near the confluence of these tributaries. Surface samples were collected with a 0.5 mm mesh cylindro-conical net. In general, reproductive seasonality was well-defined between October and February. It was most intense from November to January, when the photoperiod reached its highest values, flow was decreased, and the water temperature was increased. Based on egg and larval distributions, we found that spawning occurred mainly in the Ligeiro and Chapecó tributaries and in the Uruguay/Chapecó section. In contrast, fish spawning in the sites downstream of dams was more restricted. Finally, a difference was observed between the egg and larval distributions of the main river and its tributaries: the greatest reproductive activity in the tributaries occurred during periods of high flow and increased water temperature, while in the main river, more eggs and larvae were observed when the flow decreased and the water temperature increased.     

Reproductive biology and larval development of the terapontid Amniataba caudavittata, including comparisons with the reproductive strategies of other estuarine teleosts in temperate Western Australia

Samples collected monthly from the Swan Estuary between March 1978 and May 1979, together with environmental data for 1977 to 1980, have been used to elucidate various aspects of the reproductive biology of Amniataba caudavittata in this estuary. The gonads of A. caudavittala started to develop rapidly in the spring, when day length, water temperatures and salinities were increasing markedly. Spawning, which occurred mainly towards the top end of the upper estuary, was initiated in November, when water temperatures and salinities in that region were c. 24° C and 9‰, and it peaked in December/January when they were c. IT C and 17%○, Maturity is attained by at least the majority offish at the end of the second and each subsequent year of life. Although some of the larger 1-year-old fish attained maturity, this occurred in only one of the two tributary rivers, possibly reflecting differences in the salinity regimes in these rivers. Fecundities ranged from 50 000 in a 150-mm fish to 705 000 in a 254-mm fish, with a mean of 310 000. The mature, unfertilized eggs are small and spherical and have a diameter of 560 μm. The larvae are pelagic and characterized by an elongate body, which becomes moderately deep and laterally compressed during development, a short to moderate, tightly-coiled gut, a distinct gap between the anus and the origin of the anal fin and 25 or 26 myomeres. The development of fins and settlement of A. caudavittata larvae occurs at a smaller size than in the larvae of other terapontid species previously described. The success of A. caudavittata in the Swan Estuary can be attributed in part to its production of very large numbers of eggs at a time when, due to low fresh water discharge and a smait tidal range, conditions in the estuary are relatively stable. Comparisons between the biology of A. caudavittata and that of other abundant teleosts that spawn in south-western Australian estuaries show that these species exhibit a wide range of reproductive strategies.     

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.     

Contrasting effects of dams with and without reservoirs on the population density of an amphidromous goby in southwestern Japan

Ichthyological Research - Dam construction affects the distribution and abundance of freshwater fishes, particularly for diadromous species. Amphidromous fishes spawn in rivers, and hatched larvae...     

The New Zealand common smelt: biology and ecology

The common smelt is one of the most widespread indigenous freshwater fishes in New Zealand. One other member of the family Retropinnidae, Stokellia anisodon (Stokell), is present but is confined to a small region of the South Island. There are many diadromous as well as river and lake resident populations, the latter, sometimes a result of introductions to serve as forage fish for trout. Diadromous smelt spawn during austral autumn–winter on sand bars of lower riverine reaches. Larval stages inhabit coastal marine waters, and the postlarvae to immature stages re‐enter rivers and some lowland lakes. Diadromous smelt are distinguished from lowland lake resident forms by high vertebral but low gill raker numbers and larger size and from those present in some isolated waters, by high vertebral numbers alone. Lake or reservoir resident smelt usually spawn in austral spring–summer on sandy shallows at stream mouths or along shorelines. Verified smelt ages (otolith analyses) indicate that in some populations most smelt mature and spawn after c. 1 year. Adult smelt feed on a spectrum of primarily invertebrate animals ranging from small zooplankters to insects and occasionally small fishes. Smelt are a major prey for both brown trout and rainbow trout. Adult smelt are a minor food for the Maori people. As postlarvae they are a component of a few 'whitebait' fisheries. Most smelt populations are increasingly affected by environmental changes induced by human activities. Although many studies have examined problems affecting smelt, further effort is required, along with more basic research.     

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.