Young Herring Research Articles

Sloping migration of young black-backed herring (Alosa kessleri kessleri (Grimm, 1887)) in the lower reaches of the Volga river in 2019

In the modern period, the black-backed herring is the only surviving subspecies of the Caspian Sea passing herring. The main factors determining the level of reproduction of black-backed herring in the Volga River are: the number of producers who came to spawn from the sea, the volume of runoff, temperature and hydrological regimes of the Volga River during spawning migrations of producers and the slope of its larvae and juveniles in the sea. Spawning migration of black-backed herring producers from the sea to the river has been observed since the second half of April at a water temperature of 6–8 °C. It has a massive character in May-early June. Reproduction of the black-backed herring begins when the water temperature in the river reaches 16 °C. The descent of its larvae and juveniles in the Volga River takes place during the period of the flood wave recession and the inter-ice period (June, July, August), while the volume of water discharge from the Volgograd hydroelectric power plant, the flow rate and water level in the Volga River affect the conditions of sloping migration, survival and indicators of the number of larvae — a new generation of passing herring-blackback. The highest concentrations of rolling larvae are observed in the last decade of June and the first decade of July. In the modern period , the hydrological regime of the Volga River, the volume and duration of the flood do not provide optimal conditions for the reproduction of black-backed herring, spawning of producers takes place mainly in the middle and lower spawning zones of the Volga River, which leads to a reduction in the migration path of migrating larvae, as a result, they slip at early stages of development with a low probability of survival. Observations of the dynamics of the sloping migration of juveniles allow us to assess the conditions and efficiency of natural reproduction of black-backed herring in the Volga River, the scale of replenishment of stocks of this valuable commercial fish species.

Fatty acid composition of sprat (Sprattus sprattus) and herring (Clupea harengus) in the Baltic Sea as potential prey for salmon (Salmo salar)

Sprat (Sprattus sprattus) and small herring (Clupea harengus) are the dominant prey fish of Atlantic salmon (Salmo salar) in the Baltic Sea. If the fatty acid (FA) proportions of sprat and herring differ, the dietary history of ascending salmon could be determined from their FA profiles. Therefore, we investigated the FA composition of several age groups of whole sprat and small herring, caught from the three main feeding areas of salmon in autumn and spring. Oleic acid (18:1n-9) was the most prevalent FA in sprat and characteristic of this species. In herring, palmitic acid (16:0) was the most common FA, but herring lipid was characterized by n-6 polyunsaturated FAs, and moreover, by palmitoleic acid (16:1n-7) and vaccenic acid (18:1n-7). Due to the higher lipid content of sprat, the concentrations of all other FAs, excluding these, were higher in sprat than in herring. The concentration of docosahexaenoic acid (DHA, 22:6n-3) increased with an increase in the lipid content and was consequently highest in the youngest specimens, being in young sprat almost double that of young herring, and 2.6 times higher in the sprat biomass than in that of herring. As a result of a decrease in the DHA concentration with age, the ratio thiamine/DHA increased with respect to age in both species, and was lower in sprat than in herring. It is concluded that an abundance of DHA in the diet of salmon most likely increases oxidative stress because of the susceptibility of DHA to peroxidation, and thus decreases thiamine resources of fasting, prespawning salmon. Because the FA composition of sprat and herring differs, and the relative abundancies of prey fish differ between the feeding areas of salmon, the feeding area of ascending salmon can most probably be derived by comparing their FA profiles.

Abundance and tidal behaviour of pelagic fish in the gateway to the Wadden Sea

The shallow coast of The Netherlands is an important habitat for small pelagic fish. They form one of the major links between plankton and the higher trophic levels. Predatory fish, sea mammals and birds rely on small pelagic fish as a major food source. Currently, monitoring of fish in the Dutch coastal zone mainly focuses on demersal species, using bottom trawls and fykes. Four hydro-acoustic surveys were carried out in May and October 2010/2011 in the Marsdiep area, a relatively deep tidal inlet in the western Wadden Sea, to quantify abundances of pelagic fish. The aims of this study were to (1) describe temporal and vertical variations in fish distribution and school dimensions in relation to tide, and (2) estimate biomass of pelagic fish and their proportion to total fish biomass. The biomass of pelagic fish in the Marsdiep area ranged between 23 and 411kg/ha. These were mainly sprat, but also young herring, anchovy and pilchard. The fish was scattered in small schools with volumes smaller than 5m3 and concentrated in the top 10m below the surface. There was a clear effect of tidal cycle on school volume and fish abundance, with larger densities and larger schools at high tide compared to low tide. In May, sandeel contributed substantially to the pelagic assemblage, whereas in October sandeel was absent in the trawl catches, most likely because they stayed buried in the seabed from late summer to spring. The presence of pilchard and anchovy confirmed their re-establishment in the Southern North Sea and Wadden Sea. The abundance of pelagic fish exceeded the biomass of demersal fish in the western Wadden Sea by an order of magnitude. This finding is relevant for ecosystem studies. The fact that this study suggests that small pelagics outnumber demersal species to such a large extent calls for a rethinking of the allocation of monitoring effort in the Dutch coastal zone.

Predation on early life stages is decisive for year-class strength in the Barents Sea capelin (Mallotus villosus) stock

Year-class strength of Barents Sea capelin has been monitored closely since the early 1970s and during this � 45 years period three short periods of almost total recruitment failure leading to three stock collapses have been observed. These events triggered much attention since there was a large commercial fishery for capelin, but also because of observed ecosystem effects attributed to the first of these collapse events. This attention motivated research to clarify mechanisms behind the recruitment failures, and many papers have been published regarding the causes of these events. Here, we review this literature and try to put the various investigations into context. Most of the research conducted gives evidence in favour of a hypothesis that was formulated after the first recruitment failure event in the mid-1980s that predation on capelin larvae was the main cause of recruitment failure. Most studies also support the hypothesis that young herring (Clupea harengus) was the main predator on capelin larvae, but other predators like young-of-the-year cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) probably also played a role. Investigations of the effect of predators such as haddock, red king crab (Paralithodes camtschaticus), diving birds, and capelin on the demersal capelin eggs have also been reviewed. Usually, these predators are found to consume capelin eggs, but most likely not to an extent that would affect the recruitment to a noticeable degree. It is concluded that the predation on capelin larvae is the main reason for the observed recruitment failures, although predation from the predators reviewed here can hardly be the only reason for almost total recruitment failures observed in some periods.

Spatial analysis of North Sea cod recruitment: concurrent effects of changes in spawning stock biomass, temperature and herring abundance

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 480:263-275 (2013) - DOI: https://doi.org/10.3354/meps10315 Spatial analysis of North Sea cod recruitment: concurrent effects of changes in spawning stock biomass, temperature and herring abundance Dag Ø. Hjermann1,*, Jonathan A. D. Fisher2, Tristan Rouyer1, Kenneth T. Frank3, Nils C. Stenseth1,4 1Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of Oslo, PO Box 1066 Blindern, 0316 Oslo, Norway 2Centre for Fisheries Ecosystems Research, Fisheries and Marine Institute of Memorial University of Newfoundland, PO Box 4920 St. John’s, Newfoundland A1C 5R3, Canada 3Fisheries & Oceans Canada, Bedford Institute of Oceanography, Ocean Sciences Division, Dartmouth, Nova Scotia B2Y 4A2, Canada 4Institute of Marine Research, Department of Coastal Zone Studies, Flødevigen Research Station, 4817 His, Norway *Email: d.o.hjermann@bio.uio.no ABSTRACT: The decline of the North Sea cod Gadus morhua has been attributed to both overfishing and ocean warming. However, another hypothesis is that overfishing of piscivorous cod has caused an increased abundance of formerly predatory-controlled pelagic fish including herring Clupea harengus, which in turn has suppressed the recruitment of cod through predation on the early life stages of cod. Here we analyze 40 yr of trawl survey data in order to explore how the abundance of young herring affects cod recruitment, and how cod biomass affects the abundance of herring. In both cases we also take into account the effects of spawner biomass (of cod and herring, respectively) and sea temperature. We take a novel, explicitly spatial, approach by analyzing these effects on a local (185 × 167 km) scale. Our results indicate large spatial variability in ecological mechanisms. In the German Bight, high cod recruitment is associated with low herring abundance, low temperatures and high overall cod spawner biomass. This area used to contain a large portion of the cod recruits, and there is a strong correlation between the fraction of cod recruits found in this area and overall recruitment. In this area, herring recruitment is also negatively associated with the abundance of large cod. Thus, for this part of the North Sea, our findings are consistent with a reversal of dominance between cod and herring; however, herring may affect the cod by competition rather than by predation. KEY WORDS: Interspecific interaction · Ecosystem dynamics · Cultivation effect · Predator-prey reversal · Spatial heterogeneity · Warming Full text in pdf format Supplementary material PreviousNextCite this article as: Hjermann DØ, Fisher JAD, Rouyer T, Frank KT, Stenseth NC (2013) Spatial analysis of North Sea cod recruitment: concurrent effects of changes in spawning stock biomass, temperature and herring abundance. Mar Ecol Prog Ser 480:263-275. https://doi.org/10.3354/meps10315 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 480. Online publication date: April 22, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Environmental variability and fledging body mass of Common Guillemot Uria aalge chicks

To gain a better understanding of population processes and in the light of the critically endangered status of the Common Guillemot Uria aalge in Norway, we investigate which environmental factors might affect the fitness of guillemot chicks as they depart from the nest site over a 16-year period on a colony in NE Norway. Although prey composition did not seem to influence the fledging body mass of the chicks, there were significant relationships between the yearly variations in chick body mass and abundance of two important prey species (1-group herring Clupea harengus that is an important chick food item and 0-group cod Gadus morhua that is an important adult food item), population size and the sea surface temperature around the colony. The positive influence of young herring and cod on Common Guillemot chick mass occurred during a period of warming in the Barents Sea such that future recruitment into the population will depend partly on the long-term changes in ocean climate in the region.

Survival of juvenile herring (Clupea harengas membras) after passing through a selection grid in a pontoon trap

A common problem in the Baltic Sea and Gulf of Bothnian herring trap fishery is the bycatch of undersized Baltic herring (Clupea harengus membras). By equipping a pontoon trap with a rigid grid with vertical bars, a substantial proportion of the undersized herring can escape from the gear by swimming through the grid. However, it has not been clear whether or not the young fish are injured by their passage through a grid and what their fate is. The purpose of this study was to investigate the short term mortality of herring after passage through a grid. To capture young herring, a selection grid was mounted at the entrance to the fish chamber of a pontoon trap, so that only young herring (approximately 12–18cm in length) could swim in. The trap was then closed and the herring were confined in the fish chamber for seven days before assessing survival rates. The same procedure was performed with control herring which were trapped without passing through any grid. The whole trial was repeated three times. On average about 7% of the young herring selected through a sorting grid died during the experiment. The results demonstrated that passing through a rigid grid did not affect the short term mortality of young herring during the seven days in captivity. The highest mortality (45%) was in fact observed in one of the control groups presumably in connection with extreme temperature fluctuations.

Environment: Sun‐oil mix deadly for young herring: Fish embryos surprisingly vulnerable to 2007 SF Bay spill

Environment: Sun‐oil mix deadly for young herring: Fish embryos surprisingly vulnerable to 2007 SF Bay spill

Hypotheses concerning the decline and poor recovery of Pacific herring in Prince William Sound, Alaska

This paper updates previous reviews of the 1993 stock decline of Pacific herring (Clupea pallasi) in Prince William Sound, Alaska, and focuses on hypotheses about subsequent poor recovery. Recent age structured assessment modeling with covariate analysis indicates that the population dynamics of the sound’s herring are influenced by oceanic factors, nutrition, and, most substantially, hatchery releases of juvenile pink salmon. For the 1993 decline, poor nutrition remains the most probable cause with disease a secondary response. Concerning poor recovery, we examined 16 potential factors and found three to be causal: oceanic factors, poor nutrition, and hatchery releases of juvenile pink salmon. Absences of strong year classes at both Sitka and Prince William Sound after 1993 indicate the action of large-scale ocean processes. Beyond regional-scale environmental factors, two factors specific to the sound influence the population dynamics of herring and are likely impeding recovery. First, pink salmon fry releases have increased to about 600 million annually and may disrupt feeding in young herring, which require adequate nutrition for growth and overwintering survival. Juvenile pink salmon and age-1 herring co-occur in nearshore areas of bays in late spring and summer, and available data on dietary overlap indicates potential competition between the age-1 juvenile herring and juvenile pink salmon. Field studies demonstrate that juvenile herring reduce food intake substantially in the presence of juvenile pink salmon. Second, overwintering humpback whales may consume potentially large amounts of adult herring, but further studies must confirm to what extent whale predation reduces herring biomass.

Effect of hatchery rearing and environmental factors on the survival, growth and migration of Atlantic salmon in the Baltic Sea

The endangered Baltic salmon ( Salmo salar) stock in the Simojoki river has mainly been managed by stocking hatchery-reared smolts, but parr were also used between the years 1986 and 2007 together with tagged wild smolts. The parr were semi-wild compared to hatchery-reared smolts, as they experienced the river conditions for at least a year before descent to the sea. We studied year- and environment-related changes in the life-history traits of these three groups differing in their source. The salmon originating from reared smolts had a larger smolt size compared to semi-wild or wild salmon. Wild salmon caught during the second winter were smaller than fish in the other source groups, but at greater sea ages the three groups did not differ in size. The annual survival of the wild salmon was higher than that of the reared source groups. Wild salmon were captured more often in the Main Basin, far from their home river, as opposed to semi-wild or reared salmon that remained near the home river in the Bothnian Sea. A higher proportion of wild salmon were multi-sea-winter fish on the return migration (57%) compared to reared and semi-wild salmon (32%). Low survival was associated with a low recruitment of young herring ( Clupea harengus membras) and the mean catch weight of salmon in all source groups, but the reasons for the long-term decreasing trend in survival remain unclear.

Does ecological energetics lead to trophic competition in juvenile fish? A case study from the Vistula Lagoon (southern Baltic Sea)

Production, respiration, egestion of faeces and excretion of ammonia were measured in competing early developmental stages of fish species (herring, smelt, perch and stickleback) to calculate their daily energy (food) requirements in the Vistula Lagoon, southern Baltic Sea. The highest consumption (per capita and per unit body mass) was found in perch and European smelt, and the lowest in herring. However, young herring had the highest contribution to the exploitation of food resources because of their dominance in the Lagoon. The coefficient of assimilation efficiency U−1 and coefficient of the utilization of assimilated energy for growth K2 also had the highest values in this species.

The influence of uncertainty in target strength on abundance indices based on acoustic surveys: examples of the Baltic Sea herring and sprat

Abstract Kasatkina, S. M. 2009. The influence of uncertainty in target strength on abundance indices based on acoustic surveys: examples of the Baltic Sea herring and sprat. – ICES Journal of Marine Science, 66: 1404–1409. In situ, target-strength (TS) measurements at 38 kHz and an analysis of biometric fish characteristics are presented for the Baltic Sea herring (Clupea harengus membras) and sprat (Sprattus sprattus). It is demonstrated that the application of two TS–length functions for the Baltic Sea clupeids, the first for young herring and sprat and the second for adult herring, is reasonable, and the two functions can therefore replace the well-known equation used since 1983. Parameters of the proposed relationships, accompanied by their statistical characteristics, are included in a model to obtain uncertainty in acoustically derived abundance indices. Major components of survey uncertainty, such as spatial variability, species composition, and size structure, are also included in the simulation. The proposed TS functions should permit estimates of more realistic abundance dynamics of Baltic Sea clupeids, by years and age groups, thereby providing important information for stock assessment models.

Feeding of the Norwegian spring spawning herring Clupea harengus (Linne) at the different stages of its life cycle

Abstract The aim of the research was to investigate the diet of herring at different stages of its life cycle. For that purpose feeding of 0-group and immature herring in the Barents Sea, as well as of mature fish from the Norwegian Sea, was studied. 0-Group herring was sampled in the Barents Sea in August–September 2002–2005 during the international 0-group and trawl-acoustic survey of pelagic fish, as well as during the trawl-acoustic survey of demersal fish in November–December 2003–2004. Stomach samples of immature herring (1–3 years) were collected in late May and early of June 2001 and 2005 in the south-western part of the Barents Sea during the trawl-acoustic survey for young herring. Stomach samples of mature herring were collected in the Norwegian Sea in 1996, 1998, 1999, 2001, and 2002 in the course of the international trawl-acoustic survey of pelagic fish. Feeding intensity of herring of all age groups varied considerably between years and this was probably associated with availability and accessibility of their prey. The 0-group herring was found to have the most diverse diet, including 31 different taxa. In August–September, copepods, euphausiids, Cladocera, and larvae Bivalvia were most frequent in the diet of 0-group herring, but euphausiids and Calanus finmarchicus were the main prey taken. In November–December, euphausiids and tunicates were major prey groups. It was found that C. finmarchicus in the diet of 0-group herring was replaced by larval and adult euphausiids with increasing fish length. C. finmarchicus was the principal prey of immature herring and dominated in the diet of both small and large individuals and mainly older copepodites of C. finmarchicus were taken. Larval and adult euphausiids were found in stomachs of immature herring as well, but their share was not large. The importance of different prey for mature herring in the Norwegian Sea varied depending on the feeding area and length of the herring. On the whole C. finmarchicus and 0-group fish were the most important prey for mature herring diet, but fish prey were only important in a small sampling area. Hyperiids, euphausiids, tunicates, and pteropods were less important prey, and in 2002 herring actively consumed herring fry and redfish larvae.

The dynamics of Baltic fish stocks based on a multispecies stock production model

Summary The multispecies stock-production model of Horbowy developed in 1996 was further extended to include the unexploited part of a stock. The model was then applied to simulate stock dynamics and species interactions of cod, herring, and sprat in the Baltic from 1982 to 2001. The model indicates that there have been large declines in cod and herring biomass over the past two decades and a strong increase in sprat biomass in the 1990s. Using the extended stock-production model, the relative changes in stock biomass were similar to the changes derived using the age-structured multispecies model, the multispecies virtual population analysis (MSVPA). However, the production model estimates of the average predation mortality of young cod and young sprat are much lower than those derived from MSVPA, although the estimates for young and adult herring and adult sprat are similar in both approaches. The estimates of food suitability show that the preferred food of adult cod is adult sprat and young herring, while the suitability of young sprat, young cod, and adult herring is much smaller. The simulations performed show that the multispecies production model, which is less data-demanding than age-structured MSVPA, can provide estimates of stock dynamics and species interactions that are largely consistent with those estimated by MSVPA. The quality of input data in terms of recruitment and fishing-effort indices strongly impacts the reliability of the model's results.

Migration patterns in Norwegian spring-spawning herring: why young fish swim away from the wintering area in late summer

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 247:197-210 (2003) - doi:10.3354/meps247197 Migration patterns in Norwegian spring-spawning herring: why young fish swim away from the wintering area in late summer Cecilie Kvamme1,2,*, Leif Nøttestad1, Anders Fernö1,2, Ole Arve Misund1, Are Dommasnes1, Bjørn Erik Axelsen1, Padmini Dalpadado1, Webjørn Melle1 1Institute of Marine Research, PO Box 1870 Nordnes, 5817 Bergen, Norway 2Department of Fisheries and Marine Biology, University of Bergen, PO Box 7800, 5020 Bergen, Norway *Email: cecilie.kvamme@imr.no ABSTRACT: Norwegian spring-spawning herring Clupea harengus make extensive feeding migrations in the Norwegian Sea during summer. At the end of the feeding season herring may meet conflicting demands between moving to areas with uncertain food abundance and predation risk and migrating to the wintering area at the coast. Previous studies on the distribution of herring indicate a general eastward migration in late summer, but do not reveal actual migration patterns and orientation mechanisms. About 700 schools of herring were tracked using multi-beam sonar during 2 surveys in the Norwegian Sea in July 1995 and 1996. In coastal areas off Northern Norway, migrating schools of young herring (2 to 6 yr) swam contrary to expectations westward with a mean migration speed of 0.65 m s-1. MOCNESS samples of zooplankton biomass, herring stomach samples and condition factors indicated better feeding conditions westward off the continental slope than in coastal areas, strongly suggesting that young herring migrated westward to improve feeding conditions. This migration could be released by low food abundance and be based on a large-scale genetic predictive orientation mechanism. West of the continental shelf and slope, in waters with elevated abundance of larger prey items, migration directions were variable. Schools of young herring reaching higher food supply seemed to stop their westward migration and stay in these waters to feed. A small number of older herring (>Age 6) were located even farther west near the Arctic Front where food concentrations were greatest. Predictive orientation mechanisms toward richer feeding areas may improve with age as a result of experience and learning. Erratum KEY WORDS: Herring · Migration · Sonar · Orientation mechanisms · Feeding · Swimming speed and direction Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 247. Online publication date: February 04, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

Ecological effects of dumping of dredged sediments; options for management

Dredging and dumping of dredged sediments in estuarine and coastal waters may lead to increased turbidity and enhanced sediment deposition at dump sites. This mainly affects primary production by phytoplankton, performance of visual predators (e.g. fish, birds), and growth and survival of benthic organisms. This paper combines a compilation of literature information and results of additional experimental studies on the effect of enhanced concentrations of suspended matter (SPM) on growth of bivalve molluscs, and on survival of macro- and meiozoobenthos after dumping of dredged sediments. Furthermore, it focuses on non-toxic dredged sediments only. Release of nutrients from dredged sediments did, so far, hardly influence estuarine phytoplankton production. Increased turbidity may affect dab as well as prey location by sandwich terns. Enhanced SPM-concentrations are unfavourable for young herring and smelt. Growth of filter-feeding bivalves may be impaired, especially at SPM-concentrations >250 mg/l. Estuarine nematodes can survive burial by 10 cm of dumped dredged sediment provided that its physical characteristics are similar to those of the original sediment. Sessile benthos organisms such as mussels and oysters can cope with sediment deposition of only 1–2 cm. Other macrozoobenthos can survive sediment deposition of 20–30 cm. Recovery of benthos at a dump site will occur if the interval between successive dumpings is sufficiently long. Options for management of dumping of dredged sediments are described, relating to different locations of dump sites in estuarine and coastal waters, to different seasons, and to the actual use (area and frequency) of dump sites.

Maternal, paternal and temperature effects on otolith size of young herring ( Clupea harengus L.) larvae

Investigations were carried out to clarify the timing of first check formation, and the influence of maternal, paternal and temperature effects on otolith size of young herring larvae. Three male and three female herring of the Norwegian spring-spawning-herring-stock were used as parental fish in a 3×3 factorial design, resulting in nine parental combinations. Eggs from each of the combinations were incubated at 4, 8 and 12°C. Larvae were sampled at two ontogenetic stages: newly hatched and at the end of the yolk sac (EYS) stage. In addition, some larvae were also reared with wild zooplankton and sampled 16–21 days after hatching to compare first check size with observed otolith size of unfed larvae. The first check was deposited 1–2 days after hatching and before onset of exogenous feeding at 12°C. Significant maternal and temperature effects were observed on the size of sagitta and lapillus at hatching, with smaller otoliths at higher temperatures. Larval length showed a similar temperature response as the otoliths, whereas larval dry mass showed an opposite trend. Otolith size and larval length and mass were poorly correlated, and large variability in the otolith data were evident. Temperature also influenced the measured variables at the EYS stage while the maternal influence was not as strong as at hatching. No sagittal growth between hatching and the EYS stage was observed at 4°C although larval standard length increased. Large variability limits the direct application of the first check size as a reliable tool for discrimination of units within and between fish stocks.

Effects of the presence of herring ( Clupea harengus) on the stock-recruitment relationship of Barents Sea capelin ( Mallotus villosus)

The relationship between spawning stock biomass (SSB) and number of recruits was studied for the year classes 1972–1996 of the Barents Sea capelin ( Mallotus villosus) stock. The observations did not fit a Beverton–Holt function very accurately. A considerable improvement in the fit was obtained when all years with young herring ( Clupea harengus) present in the Barents Sea was left out of the analysis. Young herring was found to have a negative influence on the capelin recruitment. An even better fit was obtained when the abundance of 1-year old and older, herring was included as a term in a modified Beverton–Holt equation, letting the amount biomass of herring affect the half-value, but not the maximum recruitment. The spawning stock-recruitment relationship found in this paper can be used directly in multispecies models of management relevance.

Consequences of prey fish community dynamics on lake trout (Salvelinus namaycush) foraging efficiency in Lake Superior

We used a size-structured model, indexed by age, that combines bioenergetics and foraging theory to evaluate the effects of prey fish community structure (species dominance, size structure, and density) on the diet and net foraging efficiency of lake trout (Salvelinus namaycush) in Lake Superior. Prey size structure was important for young lake trout but decreased in importance for older lake trout, especially with increasing prey density. The model predicted that rainbow smelt (Osmerus mordax) should dominate the diet of young lake trout due to the size-dependent capture limitations of larger prey. In contrast, lake herring (Coregonus artedi) should dominate the diet of oldest lake trout owing to a higher net energy return than rainbow smelt. Model results are consistent with age-specific diet and size-at-age of lake trout during the last 40 years. Diets of intermediate-sized lake trout do not reflect the recent resurgence of lake herring populations. Absence of a dietary switch is probably due to higher capture probability for rainbow smelt. Lake trout growth and production will likely be highest with a mixed prey species assemblage of young rainbow smelt and older lake herring.

Hydrophysical and hydrobiological processes in the Gulf of Finland in summer 1987: characterization and relationship

On the basis of repeated CTD surveys and current measurements in June–July 1987 the generation and development of wind-induced upwelling (near the northern) and downwelling (near the southern) coasts, together with frontal zones caused by coastal water offshore transport, were observed. Strong coastal currents parallel to the coast occurred, together with downwelling near the southern coast of the Gulf of Finland. The formation of a “tongue-like” water mass with a sharp thermohaline “front” (TLF) was observed to extend from the southern coast, after the wind conditions changed. Hydrochemical and hydrobiological parameters were studied in both the upwelling and TLF, exposing the changes in biological parameters at different trophic levels in the fronts of different types. Higher values of nutrients, chlorophyll a content and primary production in the upwelling frontal zone were caused by the upwelled water. The increase in primary production in the surface layer within the upwelling area was caused by nutrient transport from the deeper layers, expressed only in the higher values of nitrogen. The absence of an increase in phosphorus, the limiting factor for primary production, can be explained by the high consuming rate. In the TLF, no increase in nutrients and only a slight increase in primary production took place. The increase in abundance of young herring and zooplankton in the TLF was caused by the distribution of the hydrophysical parameters. These organisms of the higher trophic level were carried by offshore currents, together with the less saline and warmer coastal waters. The increase in abundance of the young herring near the TLF is related also to the best feeding conditions near the front. It is concluded that temperature and salinity fronts are associated with changes in biological parameters of different trophic levels, depending upon the mechanisms of formation of the front.