Distribution patterns of Baltic herring larvae, Clupea harengus L., in the coastal waters off Helsinki, Finland

Changes in the spatial distribution of Baltic herring larvaethrough time were investigated in the waters off Helsinki, thenorthern Baltic Sea. The average larval densities were comparableto the relatively low levels (mean around 2 larvae m^–2)found in other studies in the Gulf of Finland. In shallow baysvirtually no herring larvae were found in early June. In lateJune and early July the densities of larvae longer than 10 mmincreased to 1–4 larvae m^–2 in shallow areas (depth<2 m), i.e. to levels higher than those recorded in deeperwaters. In combination with data on the size distributions ofthe larvae, these observations suggest that a significant proportionof the herring larvae are found in shallow water after the yolksac stage. We conclude that shallow coastal waters are potentiallyimportant nursery areas for herring larvae and that this featureof herring larval biology should be taken into account in planningsampling programs for larval herring.     

Ecology of larval herring in relation to the oceanography of the Gulf of Maine

The dynamics of larval herring in the north-eastern Gulf ofMaine are reviewed with reference to the important physicaland biological oceanographic processes of the region. Particularattention is given to the apparently conflicting hypothesesof larval drift from the tidally well-mixed spawning areas tonursery areas, and larval retention in spawning areas for aperiod of several months. Both processes have been reportedfor the eastern Maine-Grand Manan spawning area of the Gulfof Maine, but the relative importance of each to larval survivalthrough the winter and recruitment to the juvenile stage isnot clear. Both transport and retention are interpreted in lightof oceanographic processes that might impart variability, especiallybetween years, in the proportion of larvae transported awayversus that retained. Results of recent survey cruises in theGulf of Maine show both hypotheses to have merit. It is suggestedthat (i) interannual differences in slope water intrusions intothe Gulf of Maine as they affect the residual circulation, (ii)the lunar periodicity in the intensity of tidal mixing in relationto hatching times, and (iii) the potential for variable spawninglocations relative to the tidal fronts, may affect the distributionsof herring larvae immediately after hatching in the fall, andmay control the proportion of larvae that are advected awaywith the residual currents versus that retained in the vicinityof spawning. It is also suggested that those processes thataffect larval distributions and survival in the fall are importantin determining the overwintering distributions of larvae inthe Gulf of Maine, where the subtle influences of variable foodregimes and water temperatures could potentially exact largeinterannual differences in winter survival and recruitment tothe juvenile stage.     

Larval retention and dynamics of the prawns Palaemon longirostris H. Milne Edwards and Crangon crangon Linnaeus (Decapoda,Caridea) in the Mira estuary,Portugal

Summary

Palaemon longirostris and Crangon crangon larval and post-larval stages were collected intensively in two 24-h cycles during neap and spring tidal periods in a fixed station located in the mid-Mira estuary (southwest Portugal). In each case, on the previous day, horizontal distribution of larval stages was studied in a series of 20 stations from the mouth of the estuary to near freshwater. Horizontal distribution of the discrete instars of both species were similar, suggesting larval retention. Results from the 24-h cycles indicate a semilunar cycle of larval release activity, and an initial displacement of the larvae from the parental stock, as newly-hatched larvae concentrate in the surface layer of the water column on post-crepuscular ebbing tides. The sequence of larval stages tends to be progressively more dependent of the bottom layers, and the whole larval and post-larval development is accomplished within the estuarine boundaries. Larval release cycles, coupled with duration of development and progressive change in vertical position, induce mutual exclusion of different aged larvae, which may minimise cannibalistic behaviour and competition between larvae with different capabilities.     

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.     

Temperature-driven changes in early life-history stages influence the Gulf of Riga spring spawning herring (<Emphasis Type="Italic">Clupea harengus m</Emphasis>.) recruitment abundance

Processes occurring during early life-history stages influence the year-class abundance of marine fish. We found that the abundance of 1-year-old spring spawning herring is statistically significantly determined by the number of post-flexion herring larvae in the Gulf of Riga (Baltic Sea). The abundance of consecutive developmental stages of larvae: yolk-sac, pre-flexion, flexion and post-flexion strongly correlated with each other, indicating that factors which already influence the yolk-sac stage are important in determining the abundance of post-flexion herring larvae. Winter air temperature before spawning determined the timing of maximum abundance of pre-flexion herring larvae, but not their main prey: copepod nauplii, implying that different mechanisms governing major preconditions for the formation of year-class strength. The abundance of post-flexion larvae displayed a potential dome-shaped relationship with sea surface temperature experienced after hatching. We suggest that increased summer temperatures, which exceed the physiological optimum negatively, affect the survival of post-flexion herring larvae. Overall, future climate warming poses an additional risk to larval herring survival and this may lead to a reduction in those herring stock which rely on recruitment from shallow coastal areas.     

Distribution of Anisakis larvae, identified by genetic markers, and their use for stock characterization of demersal and pelagic fish from European waters: an update

In the present paper, recent results obtained on the use of different distributions observed in larval species of Anisakis, genetically identified by means of allozyme markers, for stock characterization of demersal (Merluccius merluccius), small (Trachurus trachurus) and large pelagic (Xiphias gladius) finfish species in European waters, are reviewed and discussed. Several species of Anisakis were identified in the three fish hosts: A. simplex (s.s.), A. physeteris, A. typica, A. ziphidarum, A. pegreffii, A. brevispiculata and A. paggiae. Canonical discriminant analysis performed on all the samples of the three fish species collected in areas comprising their geographical range, according to the different species of Anisakis identified, showed distinct fish populations in European waters. In all the three fish hosts, the pattern of distribution of Anisakis larvae allowed discrimination of Mediterranean stocks from Atlantic stocks. In the case of swordfish, the possible existence of a southern Atlantic stock separated from a northern one is also suggested. Congruence and discordance with the population genetic data inferred from allozyme markers on the same samples of the three fish species are also discussed.     

The effects of thin layers on the vertical distribution of larval Pacific herring, Clupea pallasi

Temporal and spatial heterogeneity of resources is likely to have dramatic effects on the behaviors of zooplankton and ichthyoplankton, which in turn are likely to have strong effects on ecological dynamics such as predation, growth, and mating. The objective of this study was to determine whether vertically thin layers of extreme prey concentration affect the vertical distribution of larval Pacific herring (Clupea pallasi). We employed 2-m tall experimental tanks equipped with video cameras that scanned the vertical extent of the tanks to investigate the effects of thin layers on the vertical distribution of 5- and 10-day-old herring larvae. Three treatments were established: (1) a thin layer of prey (rotifers, Brachionus plicatilis) through density (salinity) stratification, (2) homogeneous vertical distribution of both prey and density, and (3) density (salinity) stratification, but with a homogeneous distribution of prey. We found that in all treatments the majority of larval herring were at the surface, near the light, despite the absence of a peak in rotifer abundance at this depth in some instances. However, there were also clear effects of the thin layers—secondary subsurface peaks in herring abundance occurred at the mid-depths in the stratified tanks, in and around the thin layers. In addition, our results provide some evidence that thin layers specifically, rather than prey patches generally, influence the vertical distribution of larval herring, i.e., larvae may use the physical properties of thin layers to locate and distribute themselves, instead of reacting solely to the prey patches. Thus thin layers can affect directly the vertical distribution of larval herring, and perhaps indirectly their horizontal distribution, as herring larvae live in environments (e.g., estuaries) where advective transport is also often vertically heterogeneous.     

Comparative analysis of alternative statistical models for differentiation of herring stocks based on meristic characters

A basic stock assessment problem is the mixing and separation of herring populations in their specific areas. Within the transition zone between the North and the Baltic seas (Skagerrak, Kattegat, The Sound) a mixing of two herring populations ( Clupea harengus L.) temporarily takes place. One major component stems from the Baltic Sea (spring spawner), and the other from the North Sea (autumn spawner); each stock exhibits different meristic characteristics. In order to separate the two herring populations, 'pure' learning samples of meristic characters were created in 1995 as differential variables. Mainly gained during two research surveys at two geographically remote areas where a mixing of herring components was relatively unlikely, these learning samples were considered as representative for one or the other herring population. This paper compares the current (ICES) separation approach for herring stock assessment with two alternative methods wherein vertebra counts (vc) are used as meristic characters. The two proposed methods are (a) an inverted variance weighted linear model, and (b) a separation rule based on a quadratic discriminant analysis. The paper summarizes and discusses the results of predicting fractions of the two herring components derived from all three separation models. The underlying example data set stems from a 1991–97 routine Swedish survey in the transition zone. In comparison, the quadratic discriminant analysis separation model was identified as superior to the two other methods. Furthermore, the model suggests a higher degree of mixing of the two herring stocks in the transition area than was previously thought; inter-annual changes in the geographical distribution of the two populations are suggested as being less variable than previously assumed.     

Larval growth and drift pattern and the separation of herring spawning groups in the North Sea*

Data from two internationally co-ordinated survey programmes are used in an attempt to display the events in the early life that took place during the ‘recovery period’ of the North Sea herring Stock, 1979–1986. During the period, an increasing importance of the spawning grounds off Buchan and off Yorkshire is evident from distributions of newly hatched herring larvae. The dramatic changes in relative importance of spawning grounds are traced in the data on both late larvae (6 months old) and juveniles (18 months old). The onset of extended spawning off Buchan and off Yorkshire was followed by increase in distinct groups of late larvae of large mean length, and by enlargement of a group of medium-sized juveniles. It is hypothesized that the identified groups are interconnected. Thus, larvae spawned off Buchan and off Yorkshire are found to have growth rates superior to those attained by larvae from west of Scotland and around the Orkney/Shetland Isles, whereas the relationship is reversed in the juveniles. In the investigated period, larval drift routes exhibited the same trends from year to year, the drift of the northerly spawned larvae being the most variable. Thus, changes in the relative contributions from spawning grounds apparently influence the overall spatial distribution as well as size composition of larval and juvenile North Sea herring. The findings indicate that groups of larvae retain, to a large extent, separate distributions until metamorphosis, and point to larval drift pattern as a determinant of spawning group distinctness.     

Survival, activity and feeding ability of Baltic cod (Gadus morhua) yolk-sac larvae at different salinities

Due to unfavourable conditions (declines in salinity and water oxygen content) in the spawning areas, there has been a considerable decrease in the Baltic cod stock since the beginning of the 1980s, and consequently a decrease in catches. In order to examine the feasibility of introducing yolk-sac larvae in areas of low salinity to improve the stock, laboratory experiments were performed on the effects of salinity on the survival, level of activity and feeding ability of larvae. Yolk-sac larvae from spawning cod caught off northern Gotland, Sweden, were exposed to four different salinities: 10 and 15%○ (salinities of the main spawning areas); and 5 and 7%○ (salinities in the Bothnian Sea and the Baltic proper respectively).
The survival of yolk-sac larvae was high at all salinities, even though there was an indication of higher mortality at low salinities in less viable larval groups. No differences were found in swimming speed or feeding ability at the four salinities, but a significant difference in vertical distribution was recorded. There were significant differences in survival, vertical distribution and feeding ability among larval groups, which indicates that larval quality or viability is of greater importance for larval survival than salinity, in the range of 5–15%○.     

Towards better Understanding of Successful Management of the Gulf of Riga Herring Stock

The Baltic herring is one of the key elements of the pelagic ecosystem of the Baltic Sea, being of the most important commercial species of the sea. The structure and dynamics of herring populations (stocks) have been assessed and managed on international level since 1970s. Since 1990, all local herring populations have been combined and assessed as one stock (Central Baltic Herring in Sub-divisions 25–29 and 32). However, the continuously decreasing trend in stock biomass throughout almost 30-years period of observations indicates the failure of the stock management implementation of the scientific advice. The separate assessments of different stocks and results of hydro-acoustic surveys of different sub-units (herring in Sub-divisions 25–27, Sub-divisions 28, 29 and 32 and the Gulf of Riga herring) have revealed rather different situation within combined assessment unit: e.g. fishing mortality of the Gulf of Riga herring has decreased while a sharp increase in mortality is observed in the herring stocks of the North-eastern Baltic (Sub-divisions 28, 29 and 32). At the same time stock abundance and biomass of the Gulf of Riga herring have increased to record high in early 2000s allowing also higher catches. It is concluded that only separate analytical assessment of local populations combined with regular acoustical surveys and following different protection measures have proved to be a successful way in managing the herring stocks. Joint assessment and management of several populations with different structure and dynamics as one combined stock do not allow revealing the real situation and trends in its parts.     

Invasion in tidal zones on complex coastlines: modelling larvae of the non‐native Manila clam,Ruditapes philippinarum,in the UK

Aim To evaluate whether natural larval transport and behaviour alone can explain the pattern of invasion and establishment of the non‐indigenous Manila clam, Ruditapes philippinarum ( Adams & Reeve, 1850 ), and its spread beyond the point of introduction in the UK. Location The study is focused on Poole Harbour, south England, the point of introduction of the Manila clam in the UK. Methods We use fine‐resolution hydrodynamic models coupled with a water salinity model and an individual behaviour model of Manila clam larvae. The model was informed by experimental studies on the vertical response of larvae to salinity and field studies of the species in its natural and new environments. Results Variations in the behavioural response of larvae to salinity in the model considerably affected the retention of clam larvae within the harbour. High levels of predicted larval retention occurred in two of five zones in the harbour when the salinity target was set at 17 practical salinity units. Persistently high densities of adult clams and recruits are accurately predicted in these regions. Main conclusions Even within a relatively small region such as Poole Harbour, there is both localized retention of larvae or ‘closed’ areas and areas that are considerably more ‘open’ and potentially connected. The behavioural response of larvae to salinity significantly affected the degree of retention and ‘openness’ of the harbour to this species. Although, through natural transport, larvae could theoretically reach the next available habitat within the duration of their pelagic stage our study indicates that areas of sufficiently reduced salinity may be necessary for sufficient retention, recruitment and establishment of new adult populations in estuaries. High resolution hydrodynamic models, coupled with larval behaviour, can accurately simulate and predict biological invasion along complex coastlines and contribute to risk assessment of the introduction of non‐indigenous species for aquaculture and spatial management of marine protection.     

Biodiversity and exploitation of the main fish stocks in the Norwegian - Barents Sea ecosystem

Juvenile herring and capelin are the main stocks of plankton feeders in the Barents Sea, the cod is the dominant predator. Warm climate favours recruitment of herring and cod, but large stocks of juvenile herring hamper survival of the capelin fry. Since the early 1970s, the herring stock has been grossly overexploited, which could have led to an imbalance in the state of the predatorprey relationships in the Barents Sea. In the 1970s and early 80s, however, cod could feed on capelin which had excellent growth and recruitment conditions when the herring was absent. The consequences of the reduced herring stock were triggered in the mid 1980s, when excellent recruitment conditions for herring and cod occurred. Three abundant year classes of cod were recruited, but the herring stock was not sufficiently large to take full advantage of the favourable recruitment conditions. Given the lack of juvenile herring and a reduced capelin stock, the rapidly growing cod stock grazed down all other available prey species in the area, including its own progeny, and starved cod, seabirds and seals have in later years appeared on the north Norwegian coast. The capelin fishery collapsed, and the traditional coastal cod fisheries have been struck by the most serious crisis on record.     

Spatial and temporal variations in parasite prevalence and infracommunity structure in herring (Clupea harengus L.) caught to the west of the British Isles and in the North and Baltic Seas: implications for fisheries science

Herring Clupea harengus L. viscera were examined for endoparasitic infections as part of a multidisciplinary stock identification project (WESTHER, EU Contract no. Q5RS-2002-01 056) which applied a range of stock discrimination techniques to the same individual fishes to obtain comparable results for multivariate analysis. Spawning and non-spawning adults, and juvenile herring were caught, over 3 years, by commercial and research vessels from numerous locations to the west of the UK and Ireland, along with control samples of spawning fish from the eastern Baltic Sea, and juveniles from sites in the eastern and western North Sea, and the north of Norway. The metacercariae of two renicolid digeneans (Cercaria pythionike and Cercaria doricha), one larval nematode (Anisakis simplex s.s.) and one larval cestode (Lacistorhynchus tenuis) were selected as tag species. Results were compared with those from herring collected between 1973 and 1982, which suggested remarkable stability in the parasite fauna of herring in the study area. These species were used to compare the parasite infracommunities of spawning herring. A significant variation in infracommunity structure was observed between different spawning grounds. These results suggest that the parasite fauna of herring are spatially variable but remain temporally stable in both the short and long term. Significant differences in prevalence and abundance of infections and comparisons of parasite infracommunity enabled the separation of putative herring stocks west of the British Isles. Distinctive patterns of parasite infection in two different spawning groups off the north coast of Scotland suggest that this area is occupied by two spawning populations, one recruiting from the west of Scotland, the other from outside this area, and most likely from the eastern North Sea. The distribution patterns of L. tenuis, C. doricha and C. pythionike suggest the potential for fish that spawn in three distinct International Council for the Exploration of the Seas (ICES) management units to be present in mixed aggregations found over the Malin Shelf, with significant implications for management in this area.     

Morphological and meristic differences among spawning aggregations of north-east Atlantic herring, Clupea harengus L.

Univariate and multivariate analyses were performed on seven anatomical characters of Atlantic herring, Clupea harengus , sampled from seven discrete spawning grounds to the west of the British Isles and from spawning sites in the Baltic and Thames Estuary. Analysis of mean similarities, Mahalanobis D², percentage overlap and projections from prinicipal co-ordinate analysis demonstrated that a high degree of anatomical similarity exists among the herring spawning communities to the west of the British Isles. Although local aggregations had distinct centres of abundance they appeared to have large overlapping distributions. However, the Celtic Sea sample was found to be distinctly different from adjacent and neighbouring stocks. The samples of coastal herring from the Thames Estuary and the Baltic Sea showed little overlap with other samples.     

Feeding and growth of larval herring,Clupea harengus,in relation to density of copepod nauplii

Synopsis Feeding and growth rates of 1–3 wk old herring larvae from four different stocks were compared in laboratory experiments (8°C). For most of the larval groups, feeding rate was saturated at nauplii (Acartia tonsa, nauplii stages 3–5) densities over 301^–1 (5 g d.w. 1^–1). Specific growth rate increased asymptotically with nauplii density, and reached about 6% d^–1 at densities over 120 nauplii 1^–1. The growth rates attained in the laboratory were similar to field measured growth rates of similarly aged herring larvae at comparable food densities. Since food particles were homogenously distributed in the laboratory tanks, patches of dense plankton concentrations are, thus, apparently not necessary for larval growth and survival in the sea. Growth efficiency differed between larval groups, with large sized larvae being the most efficient in transforming ingested matter into growth. The difference probably relates to different sizes rather than to the different geographical origins of the larvae.     

Stock-Specific Growth Rates of Lake Herring, Coregonus artedi, in Western Lake Superior

We constructed environmental growth histories, termed biochronologies, for lake herring, Coregonus artedi, from four areas of western Lake Superior using scale samples from historical collections. We created chronologies using a linear growth model (LGM) to describe individual somatic growth as the sum of both intrinsic and environmental factors. We built a master chronology using all possible scale samples age five or younger, then examined subsets of the data according to collection site (Bayfield Wisconsin, Black Bay Ontario, French River Minnesota, and Thunder Bay Ontario). The LGM best fit the site-specific data, indicating statistically different stock-specific growth rates. These differences were primarily due to environmental growth, as age effects were similar across sites. We show that the LGM is a useful tool for identifying lake herring stocks in western Lake Superior, a situation where genetic stock identification techniques have been unsuccessful. Our findings demonstrate that the stocks at these four sites are growing at different rates and therefore require different management strategies according to the unit stock concept. Further refinement of these chronologies and coupling with abundance data may allow managers to determine the degree of stock rehabilitation.     

On the distribution of larvae and localization of spawning stocks of White Sea herring <Emphasis Type="Italic">Clupea pallasii marisalbi</Emphasis>

On the basis of ichthyoplankton surveys made in June 2004–2005 and 2007, June–July 2010, and July 2011 in these bays and beyond them (in open waters of the White Sea Basin and adjacent areas of the Gorlo) larvae of White Sea herring were absent. Principal aggregations of larvae are found in the Kandalaksha Bay in June 2004–2005 and 2007. In the Onega Bay and in the Dvina Bay surveyed in June 2007 abundance of larvae was ratter low and in June–July 2010 and July 2011 in these bays and beyond them (in open waters of the White Sea Basin and adjacent areas of the Gorlo) larvae of White Sea herring were absent. Within the Kandalasksha Bay, from year to year, there were two disconnected aggregations of larvae. The space between them was situated in the open part of the bay along the transect of the Chupa Estuary and the Umba Estuary. One of the aggregations of larvae occupied the tail of the bay, and the second aggregation occupied the ante-mouth and mouth areas of the Chupa Estuary. It is supposed that these aggregations result from spawning of two independent spawning groups of the White Sea herring spawning in isolated regions of the Kandalaksha Bay. Presence of the bulk of larvae of the White Sea herring within the limits of the Kandakaksha Bay and their almost complete absence at the boundary of the bay with the White Sea Basin and at the boundaries between the Onega Bay and the Dvina Bay and the Basin support the hypothesis on the absence of an exchange with larvae between stocks of the White Sea herring spawning in large bays of the White Sea. The larvae are retained within shallow waters of the Kandalaksha Bay by the system of two-layer water circulation in the areas of spawning of herring in bays and gulfs of the estuarine type. Their drift outside of the Onega Bay and the Dvina Bay may be delimited by frontal divides at their boundaries with the Basin.     

Incidence and within field dispersion pattern of pod borer,Helicoverpa armigera (Lepidoptera: Noctuidae) in chickpea in Ethiopia

Podborer is a major pest of chickpea in Ethiopia. Field surveys were conducted in 2013/2014 and 2014/2015 in central and northwestern Ethiopia to assess the prevalence and within field distribution of the pest. Although podborer was prevalent throughout the surveyed areas, there was a significant difference in larval density (0.10–3.75 larvae per m²) among zones and districts within zones. This variation was attributed to cropping history, cropping patterns, chickpea-crop adjacency, and weather (rainfall and temperature) conditions before and during the season as well as control measures applied. The optimum sample size required for precision ranged from 98, when podborer density was 10/m², to 1045, when podborer density was ≤1/m². Within field distribution of the larvae fitted a negative binomial distribution indicating that the larvae had a clumped/aggregated dispersion pattern. These results will enable chickpea stakeholders to develop and apply appropriate integrated crop management techniques for the control of podborer.     

Vertical migrations of herring,Clupea harengus,larvae in relation to light and prey distribution

Synopsis The influence of light and prey abundance on the vertical distribution of herring larvae was evaluated by three investigations made under calm weather conditions in the North Sea off the Scottish coast. The investigations took place at different time after hatching and the vertical distributions of three size groups of larvae (mean sizes 8,15 and 19 mm) were related to time of day and the vertical distribution of copepods. No migratory behaviour of copepods was observed but their vertical distribution differed between investigations. In the investigation on intermediate sized larvae, copepod density peaked at the pycnocline (40 m). Larvae concentrated at this depth at noon. At dawn and dusk larvae migrated towards the surface and the vertical distributions fluctuated semidielly. In the two other investigations, copepods were homogeneously distributed in the water column and after migration towards the surface at dawn larvae stayed in the upper water column during the day. The observations suggest that the daytime vertical distribution of larvae in calm weather is mainly determined by feeding conditions: the larvae move to depths were light is sufficient for feeding, and refinement within that zone is made according to a compromise between optimal light conditions for feeding and optimal prey densities.