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.     

Diet of the twaite shad Alosa fallax (Lacépède) (Clupeidae) in the River Tagus Estuary, Portugal

The diet of Alosafallax was studied by means of the frequency of occurrence of prey items. In the Tagus Estuary, shads feed preferentially on fish, with mysids, shrimp, isopods and insects occurring as secondary prey.     

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.     

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.     

Genetic monitoring of two decades of hybridization between allis shad (Alosa alosa) and twaite shad (Alosa fallax)

Habitat alteration has been implicated in driving hybridization between the sympatric migratory shads Alosa alosa and Alosa fallax. Morphological and molecular evidence is consistent with hybridization across the overlapping range of these species, but the temporal extent of hybrid occurrence and genetic consequences for populations have not been explored. Using eight nuclear microsatellite loci and samples collected between 1989 and 2008 in the Solway Firth (UK), we genetically identified hybrids, studied temporal changes in their frequency, and explored changes in allele frequencies of parental populations. These molecular data confirmed the hybrid status of individuals identified using morphology (number of rakers on the outer gill arch), and enabled separation of hybrids from purebred individuals. Mitochondrial cytochrome-b sequencing revealed the presence of two haplogroups, each predominantly occurring in one species. Heterospecific haplotypes were found in 22.3 and 12.8% of A. alosa and A. fallax individuals, respectively, consistent with backcrossing and suggesting that hybrids are fertile. On average, microsatellite-identified hybrids comprised 12.7% of all samples, but when individuals with cytonuclear discordance were also considered introgressed on average 25.4% of individuals were of hybrid ancestry. Overall, allelic richness remained largely unchanged within species, but there were declines in the inbreeding coefficient (F _IS) of both species and episodes of significant temporal allelic frequency change. Hybrids sampled between 2004 and 2008 showed no evidence of lower fecundity relative to purebred individuals. Together, results suggest that hybridization between shad species in northern Europe is prevalent, and has been ongoing over at least two decades. The challenge is now to understand the extent to which observed patterns are linked to immigration from other populations, and the mechanisms that have prevented species collapse despite apparent hybrid fertility and longstanding introgression of neutral markers.     

Genetic polymorphism of adenosine deaminase (ADA; E.C. 3.5.4.4.) in allis shad, Alosa alosa and twaite shad, Alosa fallax

A genetic polymorphism of adenosine deaminase (ADA), is described for the first time in European shad species, Alosa alosa and Alosa fallax . Significant heterogeneity in gene frequencies was found, both between species and within species.     

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.     

Routine metabolism of juvenile spot, Leiostomus xanthums (Lacépède), as a function of temperature, salinity and weight

Routine oxygen consumption rates of juvenile spot, Leiostomus xanthums , were measured over a range of temperatures, salinities and fish weights. As predicted, Q O₂ increased with temperature and decreased with body weight. However, Q O₂ decreased with decreasing salinity and did not show the expected minimum at isosmotic concentrations. The data are best described by the relationship: log₁₀ Q O₂ (mg O₂ g⁻¹ h⁻¹) = 0.129 log_lo salinity (%0) + 1.604 log₁₀ temperature (°C)-0.1401og₁₀(g)-2.767.     

Oxygen requirements of embryos and larvae of the largemouth bass, Micropterus salmoides (Lacépède)

Starting with the second day after fertilization, largemouth bass embryos and larvae became increasingly sensitive to oxygen deficiency until the sixth day of life. On this day few survived exposure to 1 mg O₂/l for 3 h at 20°C; many were damaged at 2 mg/1, but not at 2.5. Death was by asphyxiation or by starvation resulting from an apparent inability to close the lower jaw. On the seventh day the larvae became more resistant again, possibly because opercular movement began on that day. At 25°C the effects of reduced oxygen concentration were intensified, and even a concentration of 2.5 mg/1 became lethal in 3 h. At 3 mg O₂/l and 20° the normally quiescent yolk-sac larvae became very active and swam vertically to at least 5 or 6 cm above the substrate; 4 and 5 mg/1 had this effect at 23–24°. This behaviour could result in losses by predation and displacement from the nest.     

Dynamics of prevalence of Ligula intestinalis (L.) in Catostomus commersoni (Lacépède)

Prevalence of Ligula intestinalis in Catostomus commersoni was followed in four lakes near Algonquin Provincial Park, Ontario from 1963 to 1981. Two lakes remained in the natural state, except for regular sampling, and prevalence of the parasite fluctuated from 0.3 to 3.6% and from 1.5 to 13%. Prevalence was higher in fishes from the other two lakes. Fishes in the latter two lakes were removed with ichthyotoxin during the study. Fishes and parasite subsequently recolonized the lakes. Prevalence of the parasite peaked and then declined during recovery in each of the two lakes. Prevalence of the parasite fluctuated around levels which were unique for each of the four lakes. Differences in the level of prevalences among the lakes were probably most dependent on differences in the morphology of the lake basins.     

Susceptibility of starved, juvenile smallmouth bass, Micropterus dolomieui (Lacépède) to low pH

Juvenile smallmouth bass, Micropterus dolomieui , were starved over a period of 6 months in the laboratory and their survival in waters of low pH tested. The ability of the bass to withstand the toxic effects of hydrogen ion decreased with time. The LC50 (96-h) changed from pH 3.7 to pH 4.4 over the experimental period. During the final days, the decrease in the threshold was greatly accelerated. In the wild, some overwintering bass are subjected to pulses of water as low as pH 4.7 as a result of vernal snowmelt. Bass at the end of winter are often emaciated and are especially susceptible to acid stress.     

The effect of the migration of Alosa fallax fallax (Laécpède) into fresh water, on branchial and gut parasites

The prevalence and relative density of branchial and gut parasites taken from Almafullax in the estuary of the River Severn, at the start of the freshwater phase of its spawning migration, were compared with those taken from their spawning grounds in fresh water. Mazocrues alosue showed no significant difference in prevalence between the two habitats, though their relative density did fall significantly. Because of the direct relationship between length and relative density, this difference may be explained by the estuary samples of A. fullux being of fish significantly larger than those from fresh water. Other possible causes are discussed. The prevalence and relative density of M. alosae showed a trend to increase with age of the host. Repeat spawning A. fallax had a significantly greater relative density and prevalence of M. alosue when compared to virgin fish.
Analysis of the sampled population showed that Clavellisu emarginata declined significantly in both prevalence and relative density in fresh water, and this was confined to the younger and first time spawning A. fullux. Thynnuscuris aduncum showed significant reductions in prevalence and relative density of infection in fresh water when compared to the estuarine samples.
The species Hemiurus uppendiculatus and Pomphorhynchus laevis showed no significant difference in prevalence and relative density of infection, and Pronoprymna ventrzcosa no significant difference in prevalence, between the two habitats.     

Age and growth determinations in the catfish, Chrysichthys nigrodigitatus (Lacépède) by use of the dorsal spine

Annual rings on the first dorsal spine were used to determine the age and growth of the catfish, Chrysichthys nigrodigitatus (Lacépède), in the Lagos Lagoon, Nigeria. From back-calculations, it was found that the mean total length ofthe species when the first annulus was formed was 18.5 cm, 32.6 cm for second annulus, 43.5 cm for the third annulus and 53.9 cm for the fourth annulus. It was shown that this species in the Lagos Lagoon attained four years of age and that the fastest growth in length was achieved during the first year of life. Thereafter, increase in length gradually decreased with age. The relationship between the body length and dorsal spine radius was shown to be linear and highly correlated.     

Diseases of the African estuarine catfish Chrysichthys nigrodigitatus (Lacépède) from the Cross River estuary, Nigeria

A survey based on monthly sampling of commercial catches was carried out in the Cross River estuary in 1984 and 1985 to determine the extent of parasitization and occurrence of externally visible lesions on the bagrid catfish Chrysichthys nigrodigitatus. Infections with the following parasites were encountered: Henneguya chrysichthyi (32%), Protancylodiscoides chrysichlhes (77%). Siphodera ghanensis (4.2%), Aspidogaster africanus (0.1%), Hysterothylacium larvae (64%), Nerolica orbignyi (1.5%), Ergasilus lalus (27%), and Chonopeltis brevis (0.02%). A new kind of epidermal papilloma was found and is described. On a yearly average, less than 0.5% of the catfish population was affected by either skeletal deformities or skin ulceration. Fin rot was observed from December to May, reaching a prevalence of 79% in February. Gill myxozoosis due to Hemwguya chrysichthyi is considered the most important parasitic disease of the fish, occurring throughout the year with a maximum prevalence of 76% in October. Results of the study are discussed in the light of a projected intensive cultivation of the fish in the extensive estuaries and lagoons bordering Nigeria's coastline.     

The food and feeding habits of the catfishes, Chrysichthys walkeri (Gunther), Chrysichthys filamentosus (Boulenger) and Chrysichthys nigrodigitatus (Lacépède) in theLekki Lagoon, Nigeria

A total of 1436 specimens were examined in the study of the food habits of the three Chrysichthys species in Lekki Lagoon. Certain differences were found in their food habits. Chrysichthys walkeri fed mostly on insects, Chrysichthys filamentosus on crustaceans and Chrysichthys nigrodigitatus on molluscs. It is suggested the observed diversity in feeding habits might be due to an active response to interspecific competition.     

Base and stressed ventilation rates for Leiostomus xanthurus Lacépède and Morone americana (Gmelin) exposed to strobe lights

A biomonitoring system interfaced with a microcomputer was used to monitor ventilation rates for white perch (Morone americana) and spot (Leiostomus xanthurus) under baseline and stressed conditions caused by strobe lights. Tests were conducted on light‐ and dark‐acclimated specimens. These two estuarine species have been found to exhibit avoidance behavior to strobe lights. Potential accommodation to the strobe light stimulus was explored over a 24 h period. The biomonitoring system successfully recorded ventilation rates under baseline and stressed conditions. Baseline mean ventilation rates for 0.5 h intervals ranged from 1 count per minute (cpm) to 97 cpm for light‐acclimated white perch with an overall mean for 24 h (x ) of 41 cpm. Mean stressed rates ranged from 1 to 100 cpm with an overall mean of 44 cpm. Baseline rates for dark‐acclimated white perch ranged from 1 to 79 cpm (x = 35 cpm), with stressed rates from 2 to 83 cpm (x = 30 cpm). Light‐acclimated spot had baseline ventilation rates ranging from 3 to 146 cpm (x = 42 cpm), while stressed rates ranged from 2 to 134 cpm (x = 36 cpm). Mean baseline rates for dark‐acclimated spot ranged from 1 to 94 cpm (x = 40 cpm), and stressed rates ranged from 1 to 72 cpm (x = 25 cpm). The difference in ventilation rates between base and stressed conditions (as absolute values) for light‐acclimated white perch over the 24 h experiments ranged from 0 to 43 cpm (x = 11.01 cpm). Dark‐acclimated white perch had differences ranging from 0 to 78 cpm (x = 11.13 cpm). Light‐acclimated spot had differences ranging from 0 to 101 cpm (x = 14.68 cpm). Dark‐acclimated spot had differences ranging from 0 to 70 cpm (x = 20.56 cpm). Ventilation rates varied between species and among individuals within a species. Ventilation rates were generally lower for dark‐acclimated specimens. For both species under all conditions, the base and stressed rates were significantly (P < 0.05) different during the 24 h period. However, dark‐acclimated specimens exhibited a more distinct difference than light‐acclimated specimens. The lack of accommodation to strobe light and a stronger reaction under dark conditions indicate that strobe lights continue to offer potential as behavioral guidance systems for these species.     

New observations on the rodlet cell (Rhabdospora thelohani) in the white sucker Catostomus commersoni (Lacépède): LM and EM studies

Light and electron microscopic studies of the morphological features of immature and mature rodlet cells in Catostomus commersoni are presented emphasizing the cells' association with epithelial tissues. The peripheral fibrillar layer is lacking from the apex and from the base of the cell. A cytoplasmic extension from the base may be a feeding mechanism whereby the rodlet cell obtains nutrient at the expense of adjacent cells leaving intercellular spaces often containing myelin figures. RNAase digestion studies demonstrate the presence of RNA in the electron dense rodlet core.
The structure and histochemistry of the rodlets which do not appear to disintegrate upon expulsion from the cell are compared to the cytoplasmic inclusions of both normal fish cells and protozoan parasites. The possible association of the rodlet cell with various pathological conditions is briefly reviewed and the authors conclude that it is premature to disregard the possibility that this cell could be a parasite or infective agent.     

Metabolic rates in an anadromous clupeid, the American shad (Alosa sapidissima)

To assess the energetics of migration in an anadromous fish, adult American shad (Alosa sapidissima) were swum in a large respirometer at a range of speeds (1.0–2.3 body lengths (BL) s⁻¹, 13–24 °C). Metabolic rate (M_O2) was logarithmically related to swimming speed (Bl s⁻¹; r ² = 0.41, slope = 0.23 ± 0.037) and tailbeat frequency (beats × min⁻¹; r ² = 0.52, slope = 0.003 ± 0.0003). Temperature had a significant effect on metabolic rate (r ² = 0.41) with a Q₁₀ of 2.2. Standard metabolic rate (SMR), determined directly after immobilization with the neuroblocker gallamine triethiodide, ranged from 2.2–6.2 mmolO₂ kg⁻¹ h⁻¹ and scaled with mass (W) such that SMR = 4.0 (±0.03)W^{0.695(±0.15)}. Comparison of directly determined and extrapolated SMR suggests that swimming respirometry provides a good estimate of SMR in this species, given the differences in basal activity monitored by the two methods. Overall, American shad metabolic rates (M_O2 and SMR) were intermediate between salmonids and fast-swimming perciforms, including tunas, and may be a result of evolutionary adaptation to their active pelagic, schooling life history. This study demonstrates variability in metabolic strategy among anadromous fishes that may be important to understanding the relative success of different migratory species under varying environmental conditions. Accepted: 3 March 1999     

Growth and abundance of O-group herrings, Clupea harengus L., in the Severn Estuary

Collections made from the cooling water intake screens of Power Stations in the Severn Estuary and the Bristol Channel have been used to investigate the biology of O-group herrings in this region. The abundance of herrings at Oldbury, the principal sampling site, was low in most years and in five successive seasons between July 1972 and December 1976, large numbers were caught only between the summer of 1975 and the spring of 1976. The herrings were first observed in July, when many had still not completed metamorphosis, and reached peak numbers in September and October during which period they were clearly growing. Numbers declined in December before rising dramatically in January at which time the length-frequency curves displayed a pronounced bimodality. On the basis of data from other parts of the Bristol Channel and Severn Estuary, it is suggested that this bimodality is due to the influx of some animals from other regions where the growth rate was greater. Although movement is apparently taking place at this time and the numbers decline markedly in subsequent months, the last remnants of the population do not leave the estuary until early May. Post-pelvic scute (K₂) and vertebral counts (VS) of 13.82 and 55.24 respectively, indicate that the Severn Estuary herrings are the product of a spring spawning stock, a view entirely consistent with length and field data.