Marine migrations in anadromous brown trout (Salmo trutta). Fjord residency as a possible alternative in the continuum of migration to the open sea

Partial migration is a common phenomenon in many fish species. Trout (Salmo trutta) is a partially migratory species where some part of the population migrate to the marine environment, while another remains in freshwater. In the years 2008 and 2009, a total of 159 wild sea trout smolts were tagged with acoustic and PIT‐tags in the river Villestrup (Denmark) to study the initial postsmolt marine behaviour within a fjord system. We found that the strategies of the sea migrants vary: some stay in the fjord, while others migrate to the sea, suggesting that partial migration occurs even in the marine environments. Overall, a total of 53% of the tagged smolts migrated from the fjord to the sea, and 47% stayed (or potentially died) in the fjord. The ratios of fjord‐resident versus sea‐migrating postsmolts were consistent at the study times, and no differences between the early and late migration periods of the smolts were observed. The individual's size or body condition at the time of tagging did not affect survival or the migratory decisions in the fjord. High overall initial survival (74%) was found 30 days after the fjord entry. We suggest that within a continuum of migration to sea, there is a migratory decision point when sea trout postsmolts encounter a fjord system. At this point, postsmolts will assess the possibility of migration versus the alternative of fjord residency.     

Did lack of spawners cause the collapse of the European eel,Anguilla anguilla?

Since the 1980s, a 90% decline in recruitment of European eel, Anguilla anguilla (L.), has occurred across most of Europe. Whether the continental stock has equally declined is uncertain. This study compiles available landings statistics since the beginning of the 20th century and identifies trends over time and space, using a statistical model that takes varying levels of reporting into account. Landings in the pre‐1940s reached over 40 000 tonnes yr^?1, declined during World War II, rose to a peak of 40 000 tonnes yr^?1 in the 1960s (coincident with a peak in re‐stocking) and dropped to an all time low of <20 000 tonnes yr^?1 in the 1990s. The decline in recruitment since the early 1980s was preceded by a decline in landings two or more decades earlier, indicating a decline of the continental stock. Considering the continental stock and the spawning stock must have declined in parallel, insufficient spawning stock biomass might have caused the recruitment collapse currently observed.     

Forecasting fish stock dynamics under climate change: Baltic herring (Clupea harengus) as a case study

Climate change and anthropogenic disturbances may affect marine populations and ecosystems through multiple pathways. In this study we present a framework in which we integrate existing models and knowledge on basic regulatory processes to investigate the potential impact of future scenarios of fisheries exploitation and climate change on the temporal dynamics of the central Baltic herring stock. Alternative scenarios of increasing sea surface temperature and decreasing salinity of the Baltic Sea from a global climate model were combined with two alternative fishing scenarios, and their direct and ecosystem‐mediated effects (i.e., through predation by cod and competition with sprat) on the herring population were evaluated for the period 2010–2050. Gradual increase in temperature has a positive impact on the long‐term productivity of the herring stock, but it has the potential to enhance the recovery of the herring stock only in combination with sustainable fisheries management (i.e., F_msy). Conversely, projections of herring spawning stock biomass (SSB) were generally low under elevated fishing mortality levels (F_high), comparable with those experienced by the stock during the 1990s. Under the combined effects of long‐term warming and high fishing mortality uncertainty in herring SSB projections was higher and increasing for the duration of the forecasts, suggesting a synergistic effect of fishery exploitation and climate forcing on fish populations dynamics. Our study shows that simulations of long‐term fish dynamics can be an informative tool to derive expectations of the potential long‐term impact of alternative future scenarios of exploitation and climate change.     

Wind‐induced variability in coastal larval retention areas: a case study on Western Baltic spring‐spawning herring

The investigation of larval dispersal and retention, their variability and dependence on wind conditions, has become a major topic in fisheries research owing to potential effects on stock recruitment and stock structuring. The present study quantifies the wind‐induced variability of larval retention of herring in a highly productive coastal lagoon of the Western Baltic Sea. This lagoon, the Greifswalder Bodden, represents the main spawning area of Western Baltic Spring‐Spawning Herring, a stock that has recently undergone a continuous decline in recruitment. The study tests whether this decline was related to changes in larval retention, more precisely to changes in wind conditions, the main forcing of the lagoon's circulation. To answer this, a model approach was applied. Larvae were tracked as Lagrangian drifters under constant and variable wind conditions, examining the main drift patterns and reconstructing the incidents during the period of recruitment decline. For the latter, weekly cohorts of virtual larvae were released in the lagoon over the entire spawning period (April–June; >16 weeks). The fraction of retained larvae per cohort was related to observed larval abundances. On this basis, a new retention index was defined to evaluate the annual larval retention. The results presented cannot explain the observed recruitment decline but characterize the lagoon as an important larval retention area by virtue of unsteady wind conditions that prevent a steady outflow of larvae.     

Transport of larvae and juvenile fish into central and northern Norwegian waters

The distribution of juvenile fish in Norwegian waters in spring and summer provides evidence that some of these are transported into Norwegian coastal waters from spawning grounds outside the ecosystems which comprise the central and northern Norwegian coast, the Barents Sea and the Spitsbergen area. The relevant species are capelin (Mallotus villosus Müller), haddock (Melanogrammus aeglefinus L.), herring (Clupea hareri' gus L.) and saithe (Pollachius virens L.). A substantial part of the saithe and haddock stock along the central and northern Norwegian coast is considered to be recruited from spawning grounds in the northern North Sea. A contribution of these species from the spawning grounds around the Faeroe Islands and west of Scotland may also occur in some years. Herring from the northern North Sea are found scattered off the Norwegian coast north to 70^oN. It seems likely that pelagic species such as capelin and herring are transported by the prevailing currents from Iceland.     

The recruitment process of the Barents Sea capelin (Mallotus villosus) stock, 2001–2003

Oceanographic and predation processes are important modulators of fish larvae survival and mortality. This study addresses the hypothesis that immature Norwegian spring‐spawning herring (Clupea harengus), when abundant in the Barents Sea, determine the capelin reproduction success through consumption of Barents Sea capelin (Mallotus villosus) larvae. Combining a hydrodynamic model and particle‐tracking individual‐based model, a realistic spatio‐temporal overlap between capelin larvae and predatory immature herring was modelled for the summer seasons of 2001–2003. Capelin larvae originating from western spawning grounds became widely dispersed during the summer season, whereas those originating from eastern spawning grounds experienced a rapid drift into the southeastern Barents Sea. Herring caused a 3% mortality of the capelin larvae population in 2001 and a 16% mortality in 2003, but the effect of predation from herring on capelin larvae was negligible in 2002. Despite a strong capelin larvae cohort and a virtual absence of predatory herring, the recruitment from the capelin 2002 year class was relatively poor from a long‐term perspective. We show that the choice of capelin spawning grounds has a major impact on the subsequent capelin larvae drift patterns, constituting an important modulator of the capelin larvae survival. Variation in drift patterns during the summer season is likely to expose the capelin larvae to a wide range of hazards, including predation from young cod, sandeel and other predators. Such alternative predators might thus have contributed to the poor capelin recruitment during 2001–2003, leading to the collapse of the capelin stock in the subsequent years.     

Spawning of herring: day or night,today or tomorrow?

Diel variations in schooling patterns and spatial dynamics during spawning were studied in Norwegian spring-spawning herring (Clupea harengus) off south-western Norway by acoustic surveying, diel cycle experiments and school tracking by sonar, and bottom gillnet sampling. Herring formed horizontally extensive, loosely packed demersal layers shortly after darkness. At night, the fish disappeared in the acoustic dead zone, but lifted off the bottom early in the following mornings. At daytime the herring reorganised into dense pelagic schools. The evening descent to the spawning habitat was considered as part of a precautionary strategy towards visual predators, as the bottom is a high-risk zone for archetypal pelagic fish like herring. Large numbers of gadoids, which are potential herring predators, were present in the area. Herring not ready to spawn dominated the bottom samples in 4 out of 5 days, suggesting that pre-spawning herring followed the descent of ripe herring. The herring spawning layers shifted in a south-easterly direction from day to day in diel spawning waves.     

Dependence of recruitment on parent stock of the spiny lobster,Panulirus argus,in Florida

Despite management practices to achieve sustainability, commercial landings for Florida spiny lobster (Panulirus argus) have experienced a drastic decline (57%) since 2000. This is cause for concern not only for economic reasons, but for issues of sustainability. An annual index of P. argus post‐larval (puerulus) abundance, estimated with a generalized linear model with significant mean sea‐level effects, shows a 36% decrease in annual puerulus supply since 1988. In addition, local Florida spawning stock biomass estimated from an age‐structured sequential population analysis decreased 57% since 1988. Puerulus abundance follows a highly correlated (R = 0.76) trend with a 12‐month delayed spawning stock abundance, which supports the contention that the Florida spawning population is a significant contributing factor to post‐larval recruitment in Florida. Residuals about the puerulus on spawning stock abundance function follow closely an interannual North Atlantic Oscillation Index signal. This residual effect is thought of as a secondary regional population effect on Florida puerulus recruitment. The Florida spiny lobster stock is exploited with no fishing mortality controls due to the Pan Caribbean recruitment concept adopted in Florida spiny lobster management. Therefore, the potential of recruitment overfishing exists if fishing mortality controls to protect local spawning stock abundance, such as catch quotas, are not introduced.     

Environmental variability controls recruitment but with different drivers among spawning components in Gulf of St. Lawrence herring stocks

The factors affecting herring recruitment are still poorly understood, complicating the prediction of stock dynamics and the choice of operational management strategies. We investigated effects of intrinsic (SSB) and extrinsic factors (physical and biological environments, including competition and predation) on recruitment of the spring and fall spawning components of each of the two herring stocks occurring in the Gulf of the St. Lawrence between 1971 and 2014. Effects of potential explanatory factors on recruit (age 2) abundance were tested using Generalized Additive Models. Model fit was significantly improved by incorporating both physical and biological environmental variability, but effects of herring SSB and predation were not significant. Indices of zooplankton abundance and phenology explained more variance in recruitment than physical indices. Our results emphasize the dominance of bottom‐up processes over SSB in the regulation of herring recruitment. Environmental variability did not seem to act uniformly on the recruitment of either stock or their respective spawning components. A long‐term trend of decreasing recruitment in spring spawners was associated with a long‐term decline in abundance of cold water copepods. In fall spawners, optimal recruitment was dependent on warmer environmental conditions combined with an adequate supply (species composition and phenology) of zooplankton. These results provide the first empirical evidence that spring and fall spawning herring are adapted to contrasting environmental conditions and shed light on the potential mechanisms linking herring recruitment to key zooplankton community characteristics and phenology. Management strategies can be improved by incorporating this new knowledge on environmental drivers of herring recruitment.     

The highs and lows of herring: A meta‐analysis of patterns and factors in herring collapse and recovery

Pacific and Atlantic herring populations (genus Clupea) commonly experience episodic collapse and recovery. Recovery time durations are of great importance for the sustainability of fisheries and ecosystems. We collated information from 64 herring populations to characterize herring fluctuations and determine the time scales at low biomass and at high and low recruitment, and use generalized linear models and Random Survival Forests to identify the most important bottom‐up, top‐down and intrinsic factors influencing recovery times. Compared to non‐forage fish taxa, herring decline to lower minima, recover to higher maxima and show larger changes in biomass, implying herring are more prone to booms and busts than non‐forage fish species. Large year classes are more common in herring, but occur infrequently and are uncorrelated among regionally grouped stocks, implying local drivers of high recruitment. Management differs between Pacific and Atlantic herring fisheries, where at similarly low biomass, Pacific fisheries tend to be closed while Atlantic fisheries remain open. This difference had no apparent effect on herring recovery times, which averaged 11 years, although most stocks with longer recovery periods had not yet recovered at the end of the observation period. Biomass recovery is best explained by median recruitment and variability in sea surface height anomalies and sea surface temperatures—higher variability leads to shorter recovery times. In addition, the duration of recruitment failure is closely linked with low biomass. While recovery times rely on the nature of the relationship between spawning biomass and recruitment, they are still largely governed by complex and uncertain processes.     

Variation in abundance of Norwegian spring-spawning herring (Clupea harengus, Clupeidae) throughout the 20th century and the influence of climatic fluctuations

A long-term (1907–98) virtual population analysis (VPA) was made for Norwegian spring-spawning herring (NSSH), which is a huge pelagic fish stock in the north-east Atlantic. It shows that this herring stock has had large fluctuations during the last century; these fluctuations have mainly been determined by variations in the temperature of the inflowing water masses to the region. The spawning stock biomass (SSB) increased from a rather low level in the early years of this century and reached a high level of around 14 million tons by 1930. The spawning stock biomass then decreased to a level of around 10 million tons by 1940, but increased again to a record high level of 16 million tons by 1945. The stock then started to decrease and during the next 20-year period fell to a level of less than 50 000 tons by the late 1960s. Through the 1970s and 1980s, the stock slowly recovered and after the recruitment of strong year classes in 1983 and 1990–1992 the stock recovered to a spawning stock biomass of about 10 million tons. The long-term fluctuation in spawning stock biomass is caused by variations in the survival of recruits. It is found that the long-term changes in spawning stock abundance are highly correlated with the long-term variations in the mean annual temperature of the inflowing Atlantic water masses (through the Kola section) into the north-east Atlantic region. The recruitment is positively correlated with the average temperature in the Kola section in the winter months, January–April, which indicates that environmental factors govern the large-scale fluctuations in production for this herring stock.     

Prespawning migratory behaviour and spawning success of sea-ranched Atlantic salmon, Salmo salar L., in the River Gudenaa, Denmark

The migratory behaviour of sea-ranched Atlantic salmon, Salmo salar L., was analysed by radio-telemetry in the River Gudenaa, Denmark. The main objectives were to: (1) estimate mortality of returning adults through the fjord; (2) observe rate of progression and migratory pattern in the fjord and river; and (3) record whether spawning occurs in the river. Forty-two returning salmon (19 males and 23 females of total body length from 60–97 cm) reared and released as smolts, were caught and equipped with external radio transmitters in the outer estuary of the River Gudenaa in 1994 and 1995. Of the tagged salmon, 18 (43%) were caught in the estuary, four (10%) were not recorded after release and 20 (47%) entered the river. The mean rate of progression through the fjord was 7.6 km d−1 (range 1.4–18.2) in 1994 and 5.4 km  d−1 (range 1.6–17.1) in 1995. Eleven salmon were alive at the onset of the spawning period. Eight were retrieved dead from the river during or after the spawning period; four with empty gonads assumed to be successful spawners, and four with intact gonads. In 1994, unsuccessful spawners (found dead with intact gonads) entered the river earlier and had a longer total migration distance in the river compared to successful spawners. This suggests that spawning success of sea-ranched salmon is associated with time of river entry and river migration length.     

Variation in reproductive potential and influence on Icelandic herring recruitment

Explaining recruitment variation in fish is essential for successful fishery management and is consequently under constant review, with an increasing focus on how maternal factors, relative to environmental influences, operate at the level of individual female spawners and extend from the spawning stock through to recruitment. We estimate total egg production (E) in Icelandic summer‐spawning herring (Clupea harengus) from 1963 through 1999 by using sequential population analyses (SPA) and their estimates of stock biomass and recruitment, various size and maturity metrics, and individual fecundity estimates that rely on total length and the condition of the spawners. Generalized linear models indicate that maternal effects are of significance in explaining SPA‐based recruitment‐at‐age‐3 (R). The best model explained 64% of the variation in R and incorporates E constrained to the repeat spawners (40%), the NAO winter index (18%) and ocean temperature (6%). The latter two represent the winter and spring periods subsequent to year‐class formation. Recruit spawner contributions to E were of no significance in explaining variation in R despite the fact that they could contribute as much as 55% of E when their contribution to E was consistently underestimated by a factor of ～ 2, based only on their contribution to spawning stock biomass. We conclude that the spawning potential of the repeat spawners should replace total spawning stock biomass for determining recruitment potential in stock assessment. In addition to the incorporation of oceanographic factors, this would provided a more cautious and risk‐adverse approach.     

A long-term (1667–1860) perspective on impacts of fishing and environmental variability on fisheries for herring,eel, and whitefish in the Limfjord,Denmark

This investigation reconstructs the development of major fisheries for herring, eel and whitefish in the Limfjord estuary, Denmark ca. 1667–1860, and then evaluates how their long-term dynamics have been influenced by some key fishery and environmental developments. The commercially most important fishery was for herring, Clupea harengus, which spawned in the Limfjord. This fishery underwent large changes in these centuries. High landings occurred in two periods in the early 18th and again in the early 19th centuries, when 4000–8000 metric tonnes were caught annually. In 1830, the fishery collapsed and landings were <1000 tonnes until the 1910s. Even during the 20th century using modern fishing techniques, the herring fishery never exceeded 7000 tonnes. The collapse was most likely due to unsustainable fishing practices (direct impacts on adults, juveniles, larvae and eggs). The second most important fishery of the Limfjord was the eel fishery. Eel, Anguilla anguilla, seems to have fled the fjord after a winter storm in 1825 broke the narrow Agger Tange isthmus which used to separate the Limfjord from the North Sea, and permanently increased the salinity in the western part of the Limfjord from 8 psu to 33 psu. The, so-called pulse seine fishery for eel declined rapidly following the salt water intrusion, and the population needed at least one to two generations to even partly recover. One possible technological reason for the recovery of the fishery was the 1848 invention of a new type of gear in the Limfjord which today is known as the Danish seine. However, the commercial eel fishery in Limfjord ceased by the 1980s coincident with the overall decline in European eel populations. The third fishery analyzed is the fishery for common whitefish, Coregonus lavaretus. A local population formed the basis of a substantial seasonal fishery, but the whitefish did not survive the salinity obstacle presented by the salt water intrusion in 1825. This study documents both the effects of fishing and environmental variability on collapses of different Limfjord fish species.     

Tests of larval retention in a tidally energetic environment reveal the complexity of the spatial structure in herring populations

Dispersion during the larval phase is of central importance in the dynamics of marine fish and invertebrate populations. Rapid transport or dispersion of larvae may contribute to connectivity and mixing, whereas spatial persistence (retention) is hypothesized to favour stock complexity and local subpopulations. Larval retention, while rarely quantified, may be defined in species with protracted spawning by the spatial co‐occurrence of larvae of different sizes or ages. The spatial distributions of larval Atlantic herring (Clupea harengus) were examined from 22 annual autumn surveys (1975–1998) and 9 spring surveys (1975–1984) from the Bay of Fundy, a region with large tides and residual flow. Larvae of all sizes (3–27 mm in length, from hatch to nearly 4 months post‐hatch) were observed each year in two major aggregations; one off southwestern Nova Scotia, and the other in the mid‐inner Bay of Fundy off the northwestern shore of Nova Scotia. Two similar aggregations were evident over 5 months later from 9 spring surveys (1975–1984), despite the residual flow that would have swept the larvae from the region within 1 month. Larval retention was apparent from overlapping centres of mass of different size (=age) classes of larvae, and tested using a size diversity index, based on the co‐occurrence of 1‐mm‐size categories, derived from protracted spawning of several weeks. Geospatial ‘hot spots’ (G_i* statistic) of four size (age) classes were evident at specific stations in the 50–100 m bathymetric zone and not elsewhere. These metrics provide quantitative measures of retention that may be applied to many ichthyoplankton data sets. One of the three main spawning areas collapsed during the study period after a period of intense fishing and failed to rebuild, but there was no substantial change in the location of larval hotspots in subsequent years. While larval retention does not directly relate to each spawning location, larval retention in the Bay of Fundy contributes to the complex ‘metapopulation’ structure of herring stocks in the western Atlantic.     

Multiple spawning events promote increased larval dispersal of a predatory fish in a western boundary current

Transport of larvae by ocean currents is an important dispersal mechanism for many species. The timing and location of spawning can have a large influence on settlement location. Shifts in the known spawning habitat of fish, whether due to climate or the discovery of new spawning stock, can influence the distribution of juveniles and our understanding of connectivity. The globally distributed species; Pomatomus saltatrix, is one such example where a previously unrecognised summer spawning event and a more southern latitudinal extent was recently reported for the southwest Pacific population. Although restrictions are in place to protect the traditional spawning event, the importance of the newly recognised summer spawning event is uncertain. Here, we investigate larval dispersal of P. saltatrix using particle tracking simulations to identify the contributions of the different spawning events to settlement. By modelling dispersal of larvae released in northern and mid‐latitude regions over the Austral spring and summer, we show that the newly recognised mid‐latitude summer spawning event contributes over 50% of the larvae reaching southern latitudes. This is due to a reduced (1–2 days) pelagic larval duration (associated with temperature), resulting in reduced larval mortality, and the seasonal (summer) strengthening of the East Australian Current (EAC) transporting particles ~50 km further south. These findings demonstrate that in dynamic boundary current systems such as the EAC, the final settlement location of larvae that are transported by ocean currents can vary considerably depending on the timing and location of spawning and that multiple spawning events are important for maximum dispersal.     

Spatiotemporal distribution of Atlantic cod (Gadus morhua L.) with intact and blocked olfactory sense during the spawning season in a Norwegian fjord with intensive salmon farming

In several areas along the Norwegian coast local fishermen claim that wild migrating Atlantic cod (Gadus morhua L.) have changed their spawning migratory behaviour following establishment of salmon farms, and no longer enter their natural spawning grounds in the fjords. This has created a heated debate in Norway and, in some areas, passed down a moratorium on establishing new salmon farms. Research has so far not been able to show a causal connection, but it has been suggested that water-soluble odorants from high density salmon stocks in commercial fish farms might be responsible for the avoidance of areas with fish farming and abandonment of nearby traditional spawning grounds. The aim of this study was to test this hypothesis by studying the spatiotemporal distribution of wild migrating cod tagged with acoustic transmitters using an array of automatic listening stations in Øksfjord, a Norwegian fjord with intensive fish farming and traditional spawning grounds for cod. The tagged cod were released either in the outer parts of Øksfjord, i.e. in the area they were caught, or in the inner part of the fjord close to their traditional spawning areas. The olfactory sense was physically blocked for 50% of the fish. Only one cod released in the outer part of the fjord visited the traditional spawning grounds. The majority of the cod that were released in the inner part of the fjord remained in this area for approximately three weeks before they left the fjord. There were no differences in spatiotemporal distribution between cod with intact and cod with blocked olfactory sense. Thus, the results provided no evidence that migrating wild cod avoid areas with fish farming as a response to odorants, but the results do not per se contradict the fishermen's observation that coastal cod have changed their migratory pattern in this fjord.     

A biophysical model to assess the trade‐off between larval recruitment and catch in southern Australia's largest prawn fishery

Data from stock assessment surveys, published research and climate sensors were linked to model the interaction between fishing, physical‐oceanographic processes and spatial patterns of larval settlement for western king prawn [Penaeus (Melicertus) latisulcatus]. This information was used to evaluate the trade‐off between larval recruitment and catch during fishing periods that demand high prices but coincide with spawning. Total rates of larval settlement were maximized when tidal currents and atmospheric physical‐forcing components were coupled with simulations of larval swimming behaviour under average gulf temperatures. Average gulf temperatures sustained longer larval durations and increased larval settlement rates by over 12% compared with warmer gulf conditions simulated under a scenario of global warming. Reproductive data coupled with outputs from the biophysical model identified consistent inter‐annual patterns in the areas contributing to larval settlement success. Areas located in the north‐east, and central‐west of the fishery, consistently contributed to over 40% of all larvae reaching a settlement in each year. Harvest sensitivity analyses indicated that changes in the spatial patterns of pre‐Christmas fishing could lead to improvements in overall rates of the larval settlement while maintaining or improving the levels of catch. Future studies to refine the model inputs relating to physical processes, larval behaviour and mortality rates for P. latisulcatus coupled with surveys of juvenile prawn abundance to ground truth the modelled predictions, would allow stock recruitment relationships to be more closely examined and inform adaptive management of the fishery in the future.     

Distribution, density and abundance of the western Baltic herring (Clupea harengus) in the Sound (ICES Subdivision 23) in relation to hydrographical features

Biomass and duration of the over-wintering period of the Rügen spring spawning herring stock (RHS) in the Sound (ICES Subdivison 23) were investigated as well as possible hydrographical factors affecting relative distribution and triggering southwards migration towards the spawning grounds. Monitoring was performed during 27 surveys over a 6-year period (1993–1998). Abundance of 45–165 000 t in August–February, 5–60 000 t in March–May, and <2000 t in June–July was found. This indicates a longer over-wintering period than hitherto assumed. The year classes 1988, 1991 and 1994 were relatively strong occurring in the Sound from age 2 (winter-rings) and abundance in autumn 1993 and 1996–1997 was relatively high. Relative year class strengths are in accordance with variations in larval indices at the spawning grounds. Decreasing abundance in late spring in the Sound was concordant with observed peaks in commercial landings and (subsequent) peak larval indices at the spawning grounds. Larger size groups seem to migrate southwards in spring before smaller herring size groups. Peak herring densities occur in the northern Sound in autumn and they do not concentrate near the southern Drogden threshold in spring before southwards spawning migration. Highest densities were found from 8 to 22 m below sea surface, and just below or in the halocline in areas with stratified water column. Indications of inflows to the Baltic triggering southwards migration can be observed at some occasions, however, these results are not conclusive because of relatively few biomass estimates in 1996–1998. The identified patterns in herring occurrence agree with some conclusions on migration routes for RHS from previously reported tagging studies, but the present investigations indicate prolonged over-wintering in the Sound. This report quantifies the abundance and distribution of herring in the Sound over the year on basis of repeated investigations, and investigates possible influence of hydrographical factors on distribution and migration.     

The influence of life history dynamics and environment on the determination of year class strength in North Sea herring (Clupea harengus L.)

The inter-annual variability in year class strength (1976–2000) of North Sea herring (Clupea harengus) was investigated using Paulik diagrams based on survey data and Virtual Population Analysis. The herring life cycle was split into five stages: spawning stock biomass (SSB), egg production, larvae, fish with 0 winter rings on the otolith (0-wr), 1-wr and 2-wr. Surveys were used as indices and Paulik analysis revealed relationships between stages. In 80% of the years, year class strength reflected SSB. Poorer than expected year classes were determined during the larva to 0-wr phase, whilst stronger than expected year classes were apparently determined during the 0-wr to 1-wr stage. There was no clear relationship between survival of young stages of herring and the abundance of Calanus finmarchicus but the year class strength of 0-wr and 1-wr had a negative relationship to bottom water temperature. Lower sea water temperatures in the North Sea are associated with higher Calanus abundance. The analysis shows that the strength of aberrant year classes of North Sea herring is determined between the pelagic larval and the juvenile stages.