Relationships between otolith microstructure, otolith growth, somatic growth and ontogenetic transitions in two cohorts of windowpane

Otolith increments in larval and juvenile windowpane Scophthalmus aquosus can provide an estimate of daily age for spring-spawned individuals held under summer conditions. Otolith increments for spring- and autumn-spawned individuals occurred at intervals >1 day under winter conditions. A significant decrease in the slope of the linear relationship between otolith size and somatic size at 40 mm L_S coincided with significant habitat, morphological and behavioural transitions. In smaller, field-captured windowpane belonging to spring- and autumn-spawned cohorts, the formation of accessory growth centres coincided with a transitional settlement period and the completion of eye migration (c. 8–20 mm L_S). Back-calculated growth rate estimates for spring-spawned windowpane were significantly faster than those for autumn-spawned windowpane and these differences could produce differential rates of survival for the two cohorts during the first year of life.     

The vertical and transverse distribution of larval herring in the River Blackwater estuary, Essex

The vertical and transverse distributions of spring-spawned larval herring were studied in the River Blackwater estuary, Essex. Sampling was undertaken during May when larval densities within the estuary were at a maximum for the year. At this time, larval lengths were in the range 7–16 mm. No clear diel pattern in the vertical distribution was found, although larval distribution at night was more even.
The tides were found to influence the vertical distribution. Surface waters (< 1.5 m in depth) were avoided during both flood and ebb tides, but used during slack water. Larval densities at ebb were higher near the sea bed than during the flood tide. It was concluded that this behaviour will tend to transport larvae into and retain them within the estuary.     

The distribution of 0-group flatfish in relation to abiotic factors on the tidal flats in the brackish Dollard (Ems Estuary, Wadden Sea)

The distribution of 0-group flatfish was investigated in 1992 in the Dollard (Ems–Dollard estuary, Wadden Sea). 0-Group plaice, flounder and sole were not evenly distributed over the sampled locations. The spatial distribution pattern of 0-group flatfish in the Dollard changed during the investigation period. In the first week of sole presence, when the mean length of sole was 24–30 mm, salinity correlated significantly with sole density. The distribution of juvenile sole larger than 40 mm total length was affected by the elevation of the location: 0-group sole was restricted to the sampled site with the lowest elevation. The distribution of 0-group plaice was related to sediment: no juvenile plaice were caught at locations with more than 10% mud fraction in the sediment. The distribution of 0-group flounder was also correlated with sediment. Later in the year, salinity correlated negatively with the distribution of 0-group flounder. The influence of sediment composition is probably indirect and linked to the abundance of preferred food items, such as Corophium volutator . Abiotic conditions were suitable to 0-group plaice, flounder and sole.     

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)     

Aspects of the reproductive biology of the sprat, Sprattus sprattus (L.) in inshore waters of the west coast of Scotland

Aspects of the reproductive biology of the sprat in inshore waters of the west coast of Scotland were investigated from August 1970 to July 1972. Spawning in sprat lasts for a period of five to six months, starting in February-March. Minimum size of maturity is 88–90 mm in both sexes but males tend to mature earlier in the season. Sprat shed their eggs in 7–10 batches. There is generally a higher proportion of females in the population. The fecundity ranges from 8700 to 46,600 in fish between 95 and 146 mm in length. It is significantly correlated to weight, length and age.     

Ecological significance of 0-group fish in the Barents Sea ecosystem

Investigations into the 0-group fish in the Barents Sea have been carried out since 1965, with the goal of estimating the abundance of 0-group fish. 0-group abundance indices have been used in the assessment of the recruitment level and in recruitment variability studies. However, the ecological importance of the 0-group fish in the Barents Sea has been less studied. Although 0-group capelin, herring, cod and haddock are widely distributed in the Barents Sea, the central area seems to be the most important, accounting for approximately 50–80% of the annual biomass. The total biomass of the four most abundant 0-group fish species can be up to 3.3 million tonnes, with an average of 1.3 million tonnes (1993–2009). Wide distribution and high biomass of pelagically distributed 0-group fish make these fishes an important element in the energy transport between different trophic levels and different geographical areas, having a critical impact on the entire Barents Sea ecosystem. In recent years, capelin have shown a pronounced northward shift in biomass distribution, and several successive strong year classes occurred during warm temperature conditions. Cod biomasses were unexpectedly low during warm years and were positively correlated with spawning stock biomass, while the correlation with temperature was not significant. Haddock and herring show, as expected, increasing biomass with increased temperature when the spawning stock is at a sufficiently high level.     

Feeding, Ichthyophonus sp. infection, distribution and growth history of Norwegian spring-spawning herring in summer

After its collapse in the late 1960s, 1991 was the first year of systematic herring investigations after the Norwegian Sea stock returned to the traditional summer-feeding areas. Of 11 prey groups three dominated the diet; Calanus finmarchicus, Parathemisto spp. and Sebastes spp. There was a high prevalence of Ichthyophonus sp. at stations in the central and western parts of the Norwegian Sea where the cpue was low. This suggests that infected individuals were unable to return from the feeding to the wintering area of the Norwegian fjords. Highest cpue were recorded between 68–73° N and 5–15° E. Age groups 2, 3 and 8 were most frequently caught and age group 8 (the 1983 year class) dominated the samples. Herring occurred in dense concentrations where C. finmarchicus dominated diet. 0-group fish dominated diet at stations along the coast from 66–73° N and out to c . 300 km off the coast. Mature herring overlapped in distribution with 0-group fish and Sebastes spp. contributed largely to herring diet in this area. The first rich year-class after the collapse, the 1983 year class, had a faster growth than rich year classes from past herring periods, but slower than poor ones.     

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.     

Diet of post-smolt and one-sea-winter Atlantic salmon in the Bothnian Sea, Northern Baltic

Until July, post-smolt salmon Salmo salar ( n =337; 129–375 mm L _T, mean 225 mm) in the Bothnian Sea relied largely on surface fauna (mainly terrestrial insects). From August onwards, fish was the principal food type. The smallest piscivorous post-smolts were <200 mm, but the main shift to piscivory occurred at sizes of 240–320 mm. Piscivory was promoted by a large smolt size. Almost all one-sea-winter (1-SW) salmon ( n =316; 278–524 mm, mean 397 mm) were piscivorous. Over 70% of the post-smolt and 96% of the 1-SW salmon with identifiable fish species in their stomachs had preyed on herring Clupea harengus. Other fish prey included the ten-spined Pungitius pungitius and three-spined sticklebacks Gasterosteusaculeatus but no sprats Sprattus sprattus. The results support earlier observations of a close relationship between recruitment of herring and production of salmon in the Bothnian Sea, and of the crucial role of smolt size in determining the ability of feeding salmon for utilizing the food resources of the area.     

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.     

Discrimination of Baltic herring populations with respect to Anisakis simplex larvae infection

Samples of herring Clupea harengus in the Baltic were collected during the spawning season in western, middle and eastern waters off the Polish coast. Two samples were taken per area, several morphometric characters were measured and some meristic characters (vertebrae and gill rakers) were counted. Infection with Anisakis simplex larvae, total length, age and maturity stage were recorded. Univariate and multivariate statistical tests were used to compare the samples, discriminant analysis was applied to separate herring populations and the fraction of herring populations in each area was estimated. The results obtained indicated that infected fish represented a mixed group of western and central Baltic herring. In the first samples collected from each area, the share of the western population decreased from 40% in the western to 18% in the middle and 13% in the eastern coasts. The share of western herring was higher in the second samples collected in the same area and decreased from 91% in the western coast to 14 and 32% in the middle and eastern ones, respectively.     

Local and large-scale climatic variables as predictors of the breeding numbers of endangered Lesser Black-backed Gulls on the Norwegian Coast

In the 1970s and 1980s, the nominate subspecies of the Lesser Black-backed Gull (Larus fuscus fuscus) showed a dramatic drop in breeding numbers on the Norwegian Coast, and in 2000, the population in some colonies was only 10–20% of the population in 1980. This decline has been attributed to the collapse in the stock of Norwegian spring spawning herring (Clupea harengus). In this study, we examined whether local climate (sea and air temperatures), winter NAO (North Atlantic Oscilliation), and the year-class strength and size of 0-group herring could predict the relative changes in breeding numbers between years, mainly after the population collapse. Breeding birds were counted in 19 of the years between 1980 and 2007 in an archipelago on the coast of Helgeland, northern Norway. The best model predicting changes in breeding numbers for the period between 1980 and 2005 (for which data on 0-group herring was available) included mean local air temperature in winter (January–March) and winter NAO, explaining 57% of the variation between years, while the other factors had little effect. When also adding the years 2006–2007 (no herring data), the best model included only mean air temperature in winter, explaining 41% of the variation. In conclusion, the high positive correlation between breeding numbers and climatic factors probably resulted from a higher availability of important fish prey after mild winters, for which 0-group herring presently may only account for a limited proportion. However, this prey might have been of much more importance prior to the population decline.     

Evidence for Distinct Coastal and Offshore Communities of Bottlenose Dolphins in the North East Atlantic

Bottlenose dolphin stock structure in the northeast Atlantic remains poorly understood. However, fine scale photo-id data have shown that populations can comprise multiple overlapping social communities. These social communities form structural elements of bottlenose dolphin (Tursiops truncatus) populations, reflecting specific ecological and behavioural adaptations to local habitats. We investigated the social structure of bottlenose dolphins in the waters of northwest Ireland and present evidence for distinct inshore and offshore social communities. Individuals of the inshore community had a coastal distribution restricted to waters within 3 km from shore. These animals exhibited a cohesive, fission-fusion social organisation, with repeated resightings within the research area, within a larger coastal home range. The offshore community comprised one or more distinct groups, found significantly further offshore (>4 km) than the inshore animals. In addition, dorsal fin scarring patterns differed significantly between inshore and offshore communities with individuals of the offshore community having more distinctly marked dorsal fins. Specifically, almost half of the individuals in the offshore community (48%) had characteristic stereotyped damage to the tip of the dorsal fin, rarely recorded in the inshore community (7%). We propose that this characteristic is likely due to interactions with pelagic fisheries. Social segregation and scarring differences found here indicate that the distinct communities are likely to be spatially and behaviourally segregated. Together with recent genetic evidence of distinct offshore and coastal population structures, this provides evidence for bottlenose dolphin inshore/offshore community differentiation in the northeast Atlantic. We recommend that social communities should be considered as fundamental units for the management and conservation of bottlenose dolphins and their habitat specialisations.     

Larval distribution, abundance, and stock structure of British Columbia herring

Pacific herring Clupea pallasi spawn in shallow and intertidal areas. Larval distributions and abundance of three major and several minor stocks were examined to determine if larvae mix among different spawning locations. The surveys were short, intensive, and occurred mainly within the first 30 days of larval life. Each major stock had a discrete larval distribution with continuous larval distributions within stock boundaries. Some overlap of distributions occurred among smaller stocks. Most larvae were found in inshore waters but there were no obvious oceanographic factors, such as fronts or eddies, to explain all the distributions. Estuarine circulation in inlets could promote larval retention in some areas. The results support the present geographic definitions for local herring stocks. The results are generally consistent with the concept of discrete larval distribution, including retention areas, as the basis for herring Stock Structure.     

Growth and population studies on Tilapia galilaea in Lake Kinneret

SUMMARY. Between 1967 and 1977, catches of Tilapia galilaea in Lake Kinneret fell from an annual average of about 200 tonnes to a low of 70 tonnes in 1974, and then returned to their former level. Total yearly mortality ( Z ) was estimated at 1.84 between 1970 and 1976. Assuming a natural mortality ( M ) of 0.94, total stock size averaged 300–700 tonnes in that period, while recruitment averaged < 0.7–3 million per year. Three broods hatched in years of relatively high water levels (1970, 1971 and 1972) yielded especially low recruitment.
The Von Bertalanffy growth coefficient ( K ) was estimated at 0.67, and final length (L∞) at 25–36cm total length. First year's growth, 23–152mm, was closely related to final length, which appeared to be affected by genetic variation in the population. Yearly fluctuations in size distribution were attributed to changes in the proportions of fast- and slow-growers, as well as in the abundance of year-classes. Recruitment to the fishery, at 17–20cm, occurred between ages l+and3+.
The highest growth rates for age-1 T, galilaea , 1–2% mean weight per day, occurred in April and May, the Peridinium season. However, only a small proportion of age-1 fish reached marketable size by July, when their food supply began to diminish. During the autumn months, there was hardly any length increment, and all T. galilaea became lean, especially the age-0 group. It is surmised that recruitment of stocked fish could be increased by introduction in December and by use of fingerlings which are potentially able to grow to marketable size in less than two seasons.     

Latitudinal Gradient in Otolith Shape among Local Populations of Atlantic Herring (Clupea harengus L.) in Norway

Otolith shape analysis of Atlantic herring (Clupea harengus) in Norwegian waters shows significant differentiation among fjords and a latitudinal gradient along the coast where neighbouring populations are more similar to each other than to those sampled at larger distances. The otolith shape was obtained using quantitative shape analysis, the outlines were transformed with Wavelet and analysed with multivariate methods. The observed morphological differences are likely to reflect environmental differences but indicate low dispersal among the local herring populations. Otolith shape variation suggests also limited exchange between the local populations and their oceanic counterparts, which could be due to differences in spawning behaviour. Herring from the most northerly location (69°N) in Balsfjord, which is genetically more similar to Pacific herring (Clupea pallasii), differed in otolith shape from all the other populations. Our results suggest that the semi-enclosed systems, where the local populations live and breed, are efficient barriers for dispersal. Otolith shape can thus serve as a marker to identify the origin of herring along the coast of Norway.     

Observations on the biology of the perch, Perca fluviatilis, in the River Stour, Dorset

SUMMARY. Opercular bones from 323 perch from the River Stour were used for age and back-calculated growth determinations. Annuli were formed during May at the beginning of the growth period. Growth was minimal from October to April. Female perch grew faster than males, and the growth rates of both sexes were higher than those observed in most other European waters. Spawning occurred during late April and early May; male gonads began development in August and had attained their maximum weight in September, but the ovaries developed gradually from August until April. Immature perch had an annual cycle of condition with a maximum in June-July and a minimum in December-January. The condition of mature males and females was affected by the gonad cycle. The fecundity of Stour perch is expressed by the formula: log egg number = 2.40 log length (mm) - 1.34.
Approximately 25% of males were mature at age I and all were mature at age II, whereas most females did not spawn until age III. Ephemeroptera nymphs and minnow fry constituted the bulk of the diet of 0-group perch; Ephemeroptera nymphs, minnow fry and Corixidae were the most numerous items in I group perch, whereas older perch contained Corixidae and a wider range of fish prey species, although minnows were the most numerous of these.     

Development, implementation, and evaluation of an international muskellunge management strategy for the upper St Lawrence River

The muskellunge, Esox masquinongy, fishery in the St Lawrence River is believed to have declined significantly from historical levels and reached critically low levels during the 1970s. Over-exploitation caused by liberal angling regulations, and loss and alteration of critical spawning and nursery habitat probably contributed to this decline. In 1980, a St Lawrence River Muskellunge Management Work Group comprising resource managers and several advisors, including E.J. Crossman, to whom this symposium is dedicated, was created to address research and management needs. A trophy muskellunge management strategy was implemented including more restrictive harvest regulations, public education promoting “catch and release”, and protection of spawning and nursery habitats. Age and growth information obtained from cleithra analysis indicated the need for increased size limits to adequately protect spawning stocks. Research efforts have developed a biological information base and monitoring tools to guide management decisions and evaluate responses. Over 100 spawning and nursery locations have been identified in US and Canadian waters leading to improved protection of critical habitats. An angler diary program shows a decline in the number of fish being harvested and a local muskellunge release award program implemented in 1987 has logged over 1000 releases of fish at least 44″ in length. Adult muskellunge monitoring in eleven spawning areas revealed an increase in mean total length of over 63 mm (>2.5 inches) after the regulation changes. Monitoring of age-0 muskellunge by use of seining surveys (1997–2005) indicates consistent reproductive success with the potential for several strong year-classes. Improvements in the muskellunge population and fishery are attributed to the progressive management action and a united community response.     

Interannual and regional variations in body length, weight and energy content of age-0 Pacific herring from Prince William Sound, Alaska

Age-0 Pacific herring were surveyed in October of 4 years in a large northern Gulf of Alaska estuary, to determine the range of variations in length, weight and whole body energy content (WBEC). These parameters reflect their preparedness for surviving their first winter's fast. During the surveys there were distinct regional and interannual variations in all three parameters for individual groups of herring in Prince William Sound. Likewise, with each collection there was typically a large range of size and WBEC values. The average standard length was (±S.D.) 80±13 mm (range=40–118), the mean whole body wet weight was 5·7±3·0 g (range=0·7–29·2) and the average WBEC of all age-0 herring captured, regardless of year or site (n=1471), was 5·4± 1·0 kJ g⁻¹ wet weight (range=2·4–9·4). The large range of WBEC and size indicates that age-0 herring at different capture sites were not all equally prepared for surviving their first winter.     

Diel variation in density, length composition, and feeding activity of juvenile alewife, Alosa pseudoharengus Wilson, and blueback herring, A. aestivalis Mitchill, at near-surface depths in a hydroelectric dam impoundment

Catches ofjuvenile blueback herring in near-surface (0 to 1.5 m depth) waters, although greater at night than during the day, did not exhibit a strict negative phototropism because catches increased significantly with increasing illumination level during the forenoon and decreased with decreasing illumination during the afternoon. Juvenile alewife were absent from near-surface waters during daylight. Feeding activity by blueback herring increased between dawn and dusk and ceased or slowed overnight but was not strictly positively phototropic. Low power (1-β) of statistical tests due to high sampling variability and low (in hindsight) sample sizes precluded conclusions about the absence of die1 and spatial differences in the pattern of density, length composition, and of feeding activity of juvenile Alosa in those cases where the null hypothesis could not be rejected. Acceptable statistical power would require substantially increased (perhaps impractically so) sample sizes aided by an increase in a (probability of a type I error) from 0.05 to 0.1.