Morphological comparisons of diploid and triploid hybrid grass carp, Ctenopharyngodon idella♀×Hypophthalmichthys nobilis♂

Morphological and karyological comparisons of 25–28 hybrid grass carp from each of 3 year-classes (1979–1981) indicated that diploid fish resulted during 1979 (82.1%) and 1980 (76.0%) the others being triploid. All of the fish tested from the 1981 year-class were triploid. Most triploid fish differed from diploid fish by having a faster growth rate, fewer scales in the lateral line and transverse series below the lateral line, a relatively longer gut, and fewer deformities.     

Year-class fluctuations in vendace,Coregonus albula (Linnaeus): Who's got the upper hand in intraspecific competition?

The vendace Coregonus albula (L.) populations in the lakes Mjøsa and Osensjøen exhibited fluctuating year-class strength. In Mjøsa, a strong year-class emerged every third year, except for the four year period between the strong year-classes 1969 and 1973. The difference between the strong and weak year-classes decreased from the 1960s, through the 1970s to the 1980s. The Mjøsa vendace matured sexually at age 2 +, and more than ten sexually mature age-groups were present in the population. Growth ceased at maturation, and asymptotic length was 23.6 cm. In Osensjøen, one strong year-class (1969) dominated the population during the period 1976–1987. The Osensjøen vendace matured sexually at age 3, and more than 15 sexually mature age-groups were present in the population. Growth ceased at maturation, and asymptotic length was 28.4 cm. In both lakes, vendace fed on crustacean zooplankton in the epilimnion throughout summer and autumn. Our data indicate that regular year-class oscillations occur as a result of the juvenile survival being negatively correlated to the number of adults.     

Population structure,growth and fecundity of perch (Perca fluviatilis L.) in an acidified river system in Southern Norway

Population structure, density and reproductive potential of perch (Perca fluviatilis L.) from 4 acidified lakes in a river system in Southern Norway were investigated. The upper 3 lakes were most affected by acidification and the number of perch caught per unit effort indicate three-fold increase in density from the upper Gjerstadvann to the lower Brøbørvann. Low density of perch in the upper lakes is explained by; 1) Abnormal mortality occurring in episodes due to acidification, probably caused by aluminium toxicity at pH = 5.2–5.4. 2). Acid water (pH ≤ 5.0) during spawning and development of eggs and larvae, resulting in recruitment failure. Juvenile mortality of perch may depend on the density of the parental stock. In the 3 upper lakes, the density of adult perch probably was too low to produce significant juvenile mortality, and in these lakes the recruitment probably depended more on the May water quality. Improved water quality increases egg hatching and survival and also benefits zooplankton production, the main food for perch during the first summer. Low population density has reduced competition for food and thus improved growth of perch in the 3 upper lakes compared to the lower lake. The perch in Brøbørvann mature at higher age and have lower individual fecundity than perch in Gjerstadvann, an effect of both slower growth and lower length specific fecundity in Brøbørvann. In Gjerstadvann, the perch therefore have a higher reproductive potential relative to the stock density, and may, when the water quality is good enough, give rise to strong year-classes and more frequent year-class fluctuations than perch in the less acid Brøbørvann.     

High resolution bomb dating for testing the accuracy of age interpretations for a short-lived pelagic fish, the Atlantic herring

Although stock assessments of Atlantic herring (Clupea harengus harengus) in the northwest Atlantic have relied on tens of thousands of annual age determinations each year for more than 20 years, recent analyses have suggested that there may have been systemic ageing error. Tracking of dominant year-classes and otolith exchanges confirmed the presence of substantial ageing bias among some readers, although these approaches could not be used to identify an accurate set of ages (if any). We applied bomb radiocarbon in a high resolution dating approach targeted at the 1962 year-class to assess ageing accuracy by multiple age readers and laboratories. Although bomb radiocarbon age validation studies are typically restricted to long-lived species, the availability of archived otoliths through the 1960s and 1970s made herring an ideal candidate for an approach targeted at a single year-class, and allowed the extent of any ageing error to be quantified. The results clearly demonstrated that current age reading practices under-aged fish >6 yr of age by up to 45%, although younger fish were aged accurately. Age underestimation was due to incorrect annulus interpretation rather than non-annual otolith growth. By focusing on the period of most rapid radiocarbon increase (1962), the margin of uncertainty around the targeted bomb radiocarbon ages was reduced to 0.66 yr. This study represents the first time the bomb dating method has reached sub-annual accuracy, which makes it well suited to the age validation of short lived fish species. The use of the targeted approach has considerable promise for improving the accuracy of other bomb radiocarbon studies without the problematic assumptions associated with curve estimation and environmental effects.     

Formation of year-class strength in the bream population in the shallow eutrophic Lake Tjeukemeer

Formation of year-class strength of bream Abramis brama , the most abundant fish in shallow eutrophic Lake Tjeukemeer, was studied during a 14-year period. Although the size of the spawning stock of bream was rather stable during this period, analysis of length—frequency distributions indicates that it comprised only a few year-classes. The strength of a year-class was determined at a fork length of about 11 cm, at the end of the second summer. Mortality rates of fish larger than 11 cm were low (<50% year⁻¹). As the number of fish at the end of the second summer was not correlated to the number of 0+ in the previous year, mortality rates during the second summer (ranging from 75.6 to 97.7% year⁻¹) determine the final year-class strength. These mortality rates were positively correlated to the year-class strength of 2- to 4-year-old pikeperch. The abundance of this predator, therefore, is likely to be the most important factor controlling the year-class strength of bream.     

A note on the age composition, growth rate and food of perch Perca fluviatilis (L.) in four eutrophic lakes, England

The perch populations of all lakes sampled are characterised by high growth rates (l± 320–410 mm) and year-class instability. The concurrence of four year-classes (1970,1973,1975and 1976) in three of the lakes is good evidence for climatic control of recruitment. Poor recruitment in Rostherne Mere during 1973 is attributed to the effects of cannibalism. An increase in the growth rate of young fish (0+) in all lakes during 1975 and 1976 is noted. The relatively poor growth of perch in Rostherne Mere is a consequence of the low specific growth rate there during the first year of life. An outbreak of the 'perch-disease' killed most of the adult perch present in 1975 in Rostherne Mere.     

Seasonal occurrence, topographical distribution and transmission of Gyrodactylus callanatis (Monogenea) infecting juvenile Atlantic cod in the Oslo Fjord, Norway

The seasonal occurrence and topographi0cal distribution of Gyrodactylus callanatis on three year-classes of juvenile Atlantic cod Gadus morhua , has been studied in the Oslo Fjord, Norway over a 2-year period. Both prevalence and abundance increased in spring and peaked in summer on the 1992 year-class and decreased in autumn. The highest intensities recorded in the study were observed on the 1992 year-class in June, with some fish harbouring more than 500 worms each. On the 1993 year-class, prevalence increased throughout the year, until it reached 100% in November; abundance, however, did not differ significantly between monthly samples, but intensities on individual fish were highest in June. The 0 + cod are probably infected when they immigrate to the nursery grounds in autumn and mingle with infected 1 + cod. About 74% of the worms infected the gill arches, oral cavity and pharynx, while 26% infected the body, head and fins. There was a significantly higher percentage of worms on the latter sites at intensities of less than 20 and more than 100 worms per fish. This is possibly related to the mode of transmission in G. callariatis . The initial infection sites are presumably the fins and head, and the most important route of transmission of G. callanatis is believed to be via the substrate. Other routes of transmission are also discussed.     

The fish community structure in Tjeukemeer in relation to fishery and habitat utilization

The fish community in Tjeukemeer was monitored from 1971 to 1988 by trawling, and during the summer of 1988 the distribution of the more abundant species was determined in relation to size and location. Bream, pikeperch and smelt were the most dominant fishes, whereas roach, white bream, perch and ruffe comprised <10% of the total catches. After the termination in 1977 of the intensive gillnet fishery both pikeperch >50 cm and bream >30 cm increased in biomass, but roach >15 cm and perch >15 cm virtually disappeared and pikeperch <50 cm substantially decreased. Only smelt, ruffe, white bream and bream <30 cm hardly changed in biomass. Bream, pikeperch, perch and smelt were restricted to the open water zone, but roach <15 cm and 0 + bream were confined to the littoral zone. White bream and ruffe did not show a distinct habitat preference. Because the recruitment of smelt is largely dependent on immigration from the IJsselmeer, a feed-back between the smelt and pikeperch population is lacking. Because the carrying capacity of the pikeperch population is mainly determined by smelt, the other fish <15 cm are very vulnerable to predation when the smelt population is consumed, before a new year-class of smelt is recruited, or when the smelt fails to recruit. The vulnerability to predation of the different species and their feeding habits are discussed in relation to their distribution.     

CONSEQUENCES OF EMERGENCE PHENOLOGY FOR REPRODUCTIVE SUCCESS IN RANUNCULUS ADONEUS (RANUNCULACEAE)

This study explores the effects of emergence time and reproductive phenology on seed number, seed size, and seedling survival in a population of the alpine buttercup, Ranunculus adoneus. Phenology in this snow bowl population is structured by snow depth. Plants in late melting interior portions of the bowl emerged and flowered 3 to 4 wk after those in early melting zones at the bowl perimeter during the summers of 1988 and 1989. Flowering time differences of buttercups across the bowl were consistent from one year to the next. In 1988, late flowering plants tended to set fewer seeds than early flowering ones; in 1989 no decrease in seed number accompanied flowering date. Path analysis showed that equal fecundity in early and late emerging portions of the bowl population during 1989 resulted from balancing spatial and temporal constraints on seed production. Spatial aspects of habitat quality improved toward the interior of the bowl, but temporal regimes deteriorated in these late melting sites. In both 1988 and 1989 seed size declined with delays in flowering. Path analysis of 1989 data showed that because of reduced time for seed growth, plants in late melting portions of the bowl set smaller seeds than those in earlier melting zones. Differences in seed size due to parental phenology are likely to influence fitness in snow buttercups. Under natural conditions, seedlings from large seeds (>;0.65 mg) have sixfold higher survival than do those from smaller seeds (<;0.65 mg). We conclude that seedling recruitment may be infrequent in late-melting portions of the snow bowl due to delayed parental phenology.     

Glochidia of the freshwater mussel,Anodonta anatina, affecting the anadromous European smelt (Osmerus eperlanus) from the Eider estuary, Germany

During a 2-year survey on external disease conditions of fish between January 1988 and December 1989, 0.2% of 40011 smelt, Osmerus eperlanus , from 30 stations in the German Wadden Sea and estuaries of tributary rivers, were infected with glochidia of the freshwater mussel Anodonta anatina . Seasonality was marked, with glochidia only being observed in March and April 1989, when prevalences were 37% and 15% respectively, in fish 10–20cm long at the most heavily infected site. A marked increase in prevalence in fish 15cm in length and longer, suggested that only temporarily resident spawning fish (as opposed to resident juveniles) were infected. Infection was almost exclusively restricted to the Eider estuary, where prevalence decreased with increasing salinity. It is concluded that the glochidia carried by smelt returning from their freshwater spawning sites in the Eider River to the sea would have died, thus representing a loss to the 1989 cohort of A. anatina from the Eider. The potential importance of the smelt– A. anatina relationship as indicator for the detection of (a) spawning mussel populations and of (b) possible ecological effects of climate changes is noted.     

Male reproductive strategy and reserve allocation in sand smelt from brackish lagoons of southern France

In male sand smelt Atherina boyeri , from three coastal brackish lagoons of southern France, the duration of the reproductive season differed according to fish size: larger fish exhibited an extended reproductive season from February to August while smaller males reproduced only from April to June-July. Smaller and larger fish showed differences in their seasonal pattern of reserves. Through the cold season, energy content was exhausted for over-winter maintenance and reproductive needs, and was restored just after fish ceased reproducing. In smaller fish, however, reserves were more rapidly restored than in larger fish, and exhibited a decrease in autumn. This result suggested that small fish invested their energy into growth and winter survival rather than reproduction while older fish invested in reproduction. These results are discussed in terms of life history Strategies.     

Reproductive regulation in the female Japanese minnow, Pseudorasbora parva (Cyprinidae)

The Japanese minnow, Pseudorasbora parva, is a multiple spawner that lays eggs repeatedly during the spawning season. In laboratory aquaria, 19 of 63 females did not spawn, whereas 44 other females laid 167 to 6285 eggs in 1 to 14 mating sequences. Neither fish density nor sex ratio (number of males per female) affected fecundity, but the increase in fish density significantly decreased the growth rate of females. The increase in male length significantly increased fecundity in aquaria which contained only a single male, whereas it decreased female growth rate. These relationships were not evident in aquaria housing three or ten males. Since larger males were more dominant and had larger testes than smaller males, the correlation between fecundity and male size strongly suggests that individual females regulated fecundity to increase their reproductive success.     

Temporal change in the genetic structure between and within cohorts of a marine fish,Diplodus sargus,induced by a large variance in individual reproductive success

Temporal changes at 16 allozyme loci in the Diplodus sargus population of Banyuls-sur-Mer (Mediterranean Sea, France) were monitored. Temporal genetic variation within a single population was examined over two temporal scales: (i) among three year-classes sampled at the same age, and (ii) within a single year-class sampled three times over a two-year period. We observed a significant change in the genotypic structure within the same cohort during the first two years following settlement and before recruitment into the adult population. In addition, comparison of year-classes showed that cohorts differed significantly one year after settlement, whereas they became similar later on before recruitment into the adult population. The observed changes in the genetic structure within and between year-classes may be the result of complex selective processes or genetic drift. Linkage disequilibrium and genetic relatedness data suggest that these changes are due to large variation in reproductive success, followed by homogenization through adult movement. Overall, these results demonstrated a rapid genetic change within a population.     

Predation by longfin smelt ( Spirinchus thaleichthys ) on the mysid Neomysis mercedis in Lake Washington

1. Longfin smelt and mysid samples collected in Lake Washington from 1988 to 1992 were examined, and historic data on smelt and mysid abundance were reviewed to assess the impacts of smelt predation on Neomysis mercedis. The feeding habits of smelt were also examined for differences between odd and even years, and seasonal and ontogenetic variations in smelt diet were described. 2. A reciprocal relationship in the abundance of 1+ smelt and Neomysis suggested that smelt regulate mysid abundance in the lake. 3. The proportion of smelt diet contributed by mysids was higher for less abundant year classes of smelt. 4. These results indicate that intraspecific competition for scarce mysids occurs in the even-year classes of smelt.     

Monitoring juvenile stocks of flatfish in the Wadden Sea and the coastal areas of the southeastern North Sea

Results are presented of an on-going monitoring program, started in 1970, of the demersal fish population in the Wadden Sea, Scheldt estuary, and along the continental coast between the Belgian-French border and Esbjerg. Particular attention is given to long-term trends in overall abundance and annual variations in spatial distribution of 0- and 1-group plaice and sole in relation to year-class strength and to variations in growth rate. Recruitment level in both plaice and sole appeared to be higher in the 1980s than in the 1970s. The most important nursery areas for plaice are the German Bight and the Wadden Sea, and the relative contributions of these areas to the total plaice stock in different years are relatively stable. In contrast, the most important nurseries for sole are along the continental coast, and the contribution of different parts of these nurseries vary considerably from year to year. Abundant year-classes of sole can originate from northern as well as from southern parts of the nurseries. Relations between survey results and year-class estimates from Virtual Population Analysis (VPA) for plaice are better compared to sole. No trends have been observed in mean length at age 0 and 1 during the years the surveys have been carried out. It is concluded that the quality of the continental coastal area and the Wadden Sea as nursery area for plaice and sole has not declined during the period of investigation. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988)     

Patterns of reproduction in two populations of pumpkinseed sunfish,Lepomis gibbosus,with differing food resources

Synopsis The influence of available food resources on growth and patterns of reproduction was studied in two populations of pumpkinseed sunfish in adjacent lakes in southeastern Ontario. Ideally, mature individuals should be long-lived, attain a large size, and maintain a high reproductive effort. However, rarely are environmental conditions ideal and compromises are necessary. In the present study, there were smaller quantities of preferred prey items (gastropods), and total benthos available in one lake. Pumpkinseed in this population ate less in general, and were smaller in body size in the older age classes. These fish matured at a younger age and a smaller size than individuals in the other lake studied, and in populations in this geographic area reported in the literature. In addition, the relationships of both fecundity and ovary weight to total weight were different between lakes. Mean ovary weight, ripe egg size and lifetime fecundity were less in the population of small-bodies fish. Variation in patterns of reproduction between populations suggests that fish may be able to fine-tune responses to local environmental conditions.     

池沼公鱼移殖放流试验研究

作者于1984—1986年进行了池沼公鱼移殖放流试验研究,移入水库的池沼公鱼已能自然产卵繁殖,形成了经济群体。本文论及了移殖放流的理论依据、移殖放流技术、移殖放流效果、移殖池沼公鱼的生态效益的分析和池沼公鱼的资源管理。     

Abundance and distribution of Neomysis mercedis and a major predator, longfin smelt (Spirinchus thaleichthys) in Lake Washington

Seasonal variations in the horizontal and depth distributions of Neomysis mercedis and longfin smelt (Spirinchus thaleichthys) were examined using night-time mid-water trawl and Bongo net samples collected in Lake Washington from July 1989 to February 1992. Mysid density varied spatially, seasonally and yearly. For example, during summer, and fall (odd years), mysid abundance was highest in the northern, and lowest in the southern sections of the lake, except in December 1991 when they were uniformly distributed. In fall (November 1990), mysid density was highest in the central basin of the lake. Furthermore, in winter of even years, highest mysid density occurred in the southern region of the lake, but in the central region in winter (February) of odd year. Longfin smelt horizontal distribution also varied seasonally. For example, density of the 1988 YC smelt (1+) was highest in the northern area of Lake Washington in summer but highest in the southern area in fall. During winter, distribution seemed random. The abundance of the 1990 YC smelt (YoY) was also highest in the northern section of the lake in summer, but highest in the southern section in fall; density remained high in the southern section in winter. But, by late spring when they were more than one year old, the distribution had changed such that highest abundance occurred in the northern and mid-section of the lake. By winter when they were about two years old and about to begin spawning, density had become highest again in the southern section. These suggest extensive movement of mysids and smelt from one area to another, perhaps driven by wind-induced water currents in the lake. Depth distribution patterns of mysids and smelt are discussed. Smelt were captured mainly in the shallow strata (8 m) of the lake during all seasons except during winter when they predominated at 50 m. Mysids were also mainly caught in the shallow strata of the lake during all seasons, although a significant proportion occurred at greater depths (> 30 m). The abundance of both species was positively correlated in spring and summer but negatively correlated in fall. A poor correlation was observed in winter. Negative correlation in fall was primarily due to the occurrence of mysids and smelt in different areas of the lake whereas poor correlation in winter was particularly due to their occurrence at different depths. Because of considerable overlap in the distribution of both species in the lake, mysids face a high risk of predation by smelt. This piece of information is consistent with the hypothesis that smelt control mysid abundance in Lake Washington. This revised version was published online in July 2006 with corrections to the Cover Date.     

The role of temperature and cannibalism in interannual recruitment variation of bass in British waters

Between-year variation in bass Dicentrarchus labrax year-class strength in southern British waters is investigated. Mean spring-summer seawater temperature in the year of birth was significantly positively correlated with both the level of summer recruitment of the 0 + group to the estuarine nurseries and subsequent recruitment of III + fish to the adult population. Spectral analysis of the temperature-compensated time series showed that a statistically significant proportion of the variation not attributable to temperature was periodic at 0–33 cycles year⁻¹. Therefore, a simple, three-parameter model, combining a linear relationship between temperature and abundance and a second-order autoregressive model can be used to describe and predict variation in relative adult YCS. Bass remain for their first 3 years within their estuarine nursery areas. As I + fish were observed in Southampton water to cannibalize the 0 + group, it is suggested that strong year-classes suppress recruitment for the next 2 years even if the temperature is suitable to promote a strong year-class. Cyclic variation in recruitment caused by intraspecific interactions, particularly cannibalism, may be a feature of other marine fish that use estuaries as nursery areas.     

Analysis of the effect of transplantation on morphometric and meristic characters in lake populations of the rainbow smelt,Osmerus mordax (Mitchill)

Synopsis Results of experimental transfer of rainbow smelt into lakes reclaimed by rotenone around 1960 in Maine were originally interpreted to cast doubt on the previously widely accepted hypothesis that there were two hereditarily different forms of rainbow smelt, one large and one small. Study of more recent data from some of the transplanted populations and reanalysis of the original data suggests different conclusions. The initial effect of introductions into a reclaimed lake may be accelerated and/or more prolonged growth which exceeds even interlake differences. This initial phase, however, is followed by a second phase, when the population reaches equilibrium and these effects subside. Data from phase two of the Little Concord-Shagg and Cold Stream-Coleback Lake transfers showed that the growth characteristics of the transplanted populations returned to those of the parental populations. Large differences in growth patterns were thus found only in the initial phase of the introduction. Meristic characters were little affected by transplanting.Analysis of large specimens derived from a postulated second unofficial introduction into Coleback Lake showed that they also differed significantly, having both higher gill raker and vertebral counts than the smaller smelt. This was of interest as smelt vertebral and gill raker counts usually are inversely related; hence we do not equate these for the moment with the large form of smelt known elsewhere.It is concluded that the initial interpretation of transfer experiments be delayed until conditions approaching equilibrium can be expected to exist. Further, our analysis of more recent lake transfer data has shown nothing to refute the hypothesis that there are at least two hereditarily different forms of smelt.