Abstract: Numerically small but statistically significant genetic differentiation has been found in many marine fish species despite very large census population sizes and absence of obvious barriers to migrating individuals. Analyses of morphological traits have previously identified local spawning groups of herring (Clupea harengus L.) in the environmentally heterogeneous Baltic Sea, whereas allozyme markers have not revealed differentiation. We analysed variation at nine microsatellite loci in 24 samples of spring-spawning herring collected at 11 spawning locations throughout the Baltic Sea. Significant temporal differentiation was observed at two locations, which we ascribe to sympatrically spawning but genetically divergent 'spawning waves'. Significant differentiation was also present on a geographical scale, though pairwise F(ST) values were generally low, not exceeding 0.027. Partial Mantel tests showed no isolation by geographical distance, but significant associations were observed between genetic differentiation and environmental parameters (salinity and surface temperature) (0.001 < P < or = 0.099), though these outcomes were driven mainly by populations in the southwestern Baltic Sea, which also exhibits the steepest environmental gradients. Application of a novel method for detecting barriers to gene flow by combining geographical coordinates and genetic differentiation allowed us to identify two zones of lowered gene flow. These zones were concordant with the separation of the Baltic Sea into major basins, with environmental gradients and with differences in migration behaviour. We suggest that similar use of landscape genetics approaches may increase the understanding of the biological significance of genetic differentiation in other marine fishes.

Abstract: Oceanographic conditions in the brackish central Baltic Sea are strongly linked to atmospheric forcing and the unusual period of persistently strong westerlies that, since the late 1980s, have resulted in an increase in average water temperatures and decreasing salinity. These changes in temperature and salinity resulted in a change in the dominance of the mesozooplankton community from Pseudocalanus sp. to Temora longicornis and Acartia spp. Similar to the copepod community, the central Baltic fish community shifted from cod (Gadus morhua), dominant during the 1980s, to sprat (Sprattus sprattus), dominant during the 1990s. Further, the commercially important pelagic fish species herring (Clupea harengus) and sprat exhibited reductions in growth. Using Principal Component and Correlation Analyses we investigated the temporal variability in the importance of the food supply as well as competition on condition of central Baltic pelagic fish species. Our results indicate that herring condition results from a combined effect of changes in the food environment and increased competition with sprat, while sprat condition appeared to be primarily determined by intra-specific competition.

Abstract: Jellyfish (Scyphozoa) prey on and consume many of the same food items as do larvae of herring Clupea harengus and could therefore have a detrimental impact on larval survival. A reduc- tion in the spawning stock biomass of herring may release jellyfish from competition for prey with herring and exacerbate any impact by jellyfish on herring survival. Both jellyfish abundance and the spawning success of herring fluctuate from year to year and increase under similar environmental conditions. The abundance of medusae (diameter = 1 to 47 cm, June to August) at herring spawning areas in the North Sea was correlated positively with the abundance of recently hatched herring lar- vae ( 0.60, p < 0.05). Management of North Sea herring may benefit from the inclusion of

Abstract: Stable 13C and 15N isotope analyses of scale, bone, and muscle tissues were used to investigate diet and trophic position of North Atlantic bluefin tuna (Thunnus thynnus Linnaeus) during residency in the northwest Atlantic Ocean off the northeast coast of the United States. Adult bluefin tuna scales collected from fish between June and October 2001 were significantly enriched in 13C compared to both muscle and bone across all months, while muscle was significantly enriched in 15N compared to either bone or scale throughout the same period. In muscle tissue, there was evidence of a shift over the summer from prey with δ13C values (−17‰ to −18‰) that were characteristic of silver hake (Merluccius bilinearis) to species with δ13C values of −20‰ to −21‰ that were similar to Atlantic herring (Clupea harengus) and sandlance (Ammodytes americanus). Depletion of 15N values in adult scales and bone compared to muscle tissue may be explained by bone and scale samples representing juvenile or life-long feeding habits, isotopic routing, or isotopic differences in amino acid composition of the three tissue types. Adult bluefin tuna were estimated to be feeding at a trophic position similar to pelagic sharks in the northwest Atlantic Ocean, while the trophic positions of yellowfin tuna (Thunnus albacares), albacore tuna (Thunnus alalunga), and juvenile bluefin tuna were indicative of a diet of up to a full trophic position below adult bluefin tuna. The close relationship between the juvenile bluefin δ15N values and those of suspension feeders suggests that nektonic crustaceans or zooplankton may contribute significantly to the diet of bluefin tuna, a food source previously overlooked for this species in the northwest Atlantic Ocean.

Abstract: 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.

Abstract: The Atlantic herring Clupea harengus has played a pivotal role in the formulation of ideas relating to population structuring in marine fishes, yet considerable uncertainty remains as to the extent to which phenotypic and genetic differentiation coincide in such a highly mobile species. In this study, we examined genetic population structure across the major herring spawning aggrega- tions in the North Sea and adjacent waters over 2 years, 2002 and 2003. We analysed 1660 spawning individuals across 9 microsatellite loci. Data were analysed using several approaches, taking into account the effect of location, year-class and sex, as well as pooling all individuals together, making no assumption as to the number of populations present in the data set. The results suggest the pres- ence of a genetically homogeneous unit off Northern Scotland, and a temporally stable pattern of iso- lation by distance determined predominantly by the divergence of the English Channel samples and, in 2003, by the Norwegian spring spawners. Our data suggest that the current view of North Sea herring as a unit-stock might be adequate, but confirm the considerable degree of demographic independence of the herring populations in the English Channel. Despite major recent population collapses, genetic data indicated no evidence of bottlenecks affecting the genetic diversity of extant North Sea herring populations. Finally, despite evidence of weak population structuring, we discuss the risks of underestimating population differentiation in marine fish of large population sizes, and with reference to herring population history and dynamics, we attempt to reconcile the existing theories on herring population structure.

Abstract: Energetic demands are high for Steller sea lions Eumetopias jubatus during spring, when females are pregnant and lactating and males are preparing for extended fasts on breeding territories. Therefore, we predicted that the distribution of sea lions in SE Alaska in spring would be influenced by the distribution of spring spawning aggregations of high-energy prey species (Pacific herring Clupea pallasii and eulachon Thaleichthys pacificus). The spatial distribution of sea lions during spring reflected the distribution of spawning eulachon in northern Southeast Alaska, particularly in Lynn Canal and along the Yakutat forelands. Haulouts with peak numbers of sea lions in spring were located significantly closer to eulachon spawning sites than haulouts that peaked at other times of year. Some haulouts were occupied only during the eulachon spawning period. The maximum number of sea lions at haulouts in spring was inversely correlated with the distance to the closest eulachon aggregation and was positively associated with the number of eulachon within 20 km. Aerial surveys conducted every 7 to 10 d during March through May in 2002 and 2003 revealed large numbers of sea lions in the water at herring spawning sites in 2002 and 2003; however, there were no significant relationships between the number of herring spawning sites and number of sea lions (except at distances >60 km). The number of sea lions was greater at herring spawning sites in 2003, corresponding to higher herring biomass. Seasonally aggregated, high-energy prey species influence the seasonal distribution of sea lions and may be critical to their reproductive success.

Abstract: Conventional 'and electronic tags were used to investigate social segregation, distribution, movements and migrations of salmon sharks Lamna ditropis in Prince William Sound, Alaska. Sixteen salmon sharks were tagged with satellite transmitters and 246 with conventional tags following capture, and were then released in Prince William Sound during summer 1999 to 2001. Most salmon sharks sexed during the study were female (95%), suggesting a high degree of sexual segregation in the region. Salmon sharks congregated at adult Pacific salmon Oncorhynchus spp. migration routes and in bays near Pacific salmon spawning grounds in Prince William Sound during July and August. Adult Pacific salmon were the principal prey in 51 salmon shark stomachs collected during summer months in Prince William Sound, but the fish appeared to be opportunistic predators and consumed sablefish Anoplopoma fimbria, gadids, Pacific herring Clupea pailasi. rockfish Sebastes spp. and squid (Teuthoidea) even when adult Pacific salmon were locally abundant. As Pacific salmon migrations declined in late summer, the salmon sharks dispersed; some continued to forage in Prince William Sound and the Gulf of Alaska into autumn and winter months, while others rapidly moved south-east thousands of kilometres toward the west coasts of Canada and the U.S. Three movement modes are proposed to explain the movement patterns observed in the Gulf of Alaska and eastern North Pacific Ocean: 'focal foraging' movements, 'foraging dispersals' and 'direct migrations'. Patterns of salmon shark movement are possibly explained by spatio-temporal changes in prey quality and density, an energetic trade-off between prey availability and water temperature, intra-specific competition for food and reproductive success. Transmissions from the electronic tags also provided data on depth and water temperatures experienced by the salmon sharks. The fish ranged from the surface to a depth of 668 m, encountered water temperatures from 4.0 to 16.8° C and generally spent the most time above 40 m depth and between 6 and 14° C (60 and 73%. respectively).

Abstract: The lipid content in herring from research vessel surveys and commercial landing was measured by chloroform/methanol/water extraction and correlated to Fatmeter-, NIR and NMR-measurements A broad variation in lipid content was found within catches, and herring size and maturity status could not be used to sort herring according to lipid content The results showed the Fatmeter, NIR and NMR to have different suitability and applicability Fatmeter readings were influenced by gonad maturity in whole herring and could therefore only be used in a primary sorting of fillets High correlation was found between NIR and solvent extraction, but the prediction results were influence by measuring point Measurements performed by NIR and Fatmeter on meat side of fillets could not predict the lipid content in herring fillets The average lipid content in a fillet or whole herring could be predicted from single NIR spectra if measurements were performed in the middle section of the sample A high correlation was also found with the NMR measurements on mince The NIR technique showed highest potentiality as a production line measurement for sorting whole herring or fillets into more homogenous batches, and thus allowing more optimal utilization of the resources

Abstract: In a Cord in northern Norway, both immature and maturing individuals (170-500 mm fork length) of anadromous Arctic charr Salvelinus atpimis and anadromous brown trout (sea trout) Salmo trutta were captured up to 5000 m offshore. Both species had a high feeding intensity and an almost exclusive piscivorous diet, with herring Clupea harengus as the dominant prey. an apparently opportunistic response to a high density of small-sized herring in the pelagic zone.

Abstract: The consumption of estuarine copepods by juvenile herring and sprat during estuarine residency was estimated using fish biomass data and daily rations calculated from two models of feeding in fish: a bioenergetic model and a gastric evacuation model. The bioenergetic model predicted daily rations that were, on average, three times higher than those estimated by a model based on field records of stomach contents. The biomass of herring and sprat in the estuary was negatively correlated with the daily ration suggesting that the clupeid fish populations were resource-limited. Copepod production decreased towards the winter and peaked in spring and summer. The relative importance of predation changed seasonally in function of the migration pattern of herring and sprat. In the spring and the summer, in situ production of␣copepod biomass was higher than the in situ consumption by fish. During the fall and the winter, consumption exceeded production. This suggests that top–down control exerted by marine pelagic fish may be an important force structuring estuarine copepod populations.

Abstract: Recent studies have shown that some clupeid fishes, including shad and menhaden, can detect ultrasound (sound with frequencies higher than 20 kHz) and actively avoid it. However, other clupeids, including sardines and anchovies, do not detect ultrasound. The hearing abilities of herring are of particular interest because of their commercial importance, our reliance on acoustics to monitor their populations and behavioural evidence of responses to high-frequency sound by some clupeid species. We measured the hearing sensitivity of Pacific herring ( Clupea pallasii ) using the auditory brainstem response and found that they were unable to detect ultrasonic signals at received levels up to 185 dB re 1 μPa. Herring had hearing thresholds at lower frequencies (100–5000 Hz) that were typical of other non-ultrasound‐detecting clupeids. This lower‐frequency hearing sensitivity could explain the results of several earlier studies showing responses to broadband sounds.

Abstract: The ability of otoliths to withstand digestion in a seal stomach appears to differ between fish taxa with the otoliths of gadoids being particularly resistant to gastric erosion. Few studies have examined the effect of otolith size on the rate of otolith dissolution. We exposed 50 otoliths (total sagittal length: 1.4-7.0 mm) from capelin (Mallotus villosus, n = 20), herring (Clupea harengus, n = 10), and polar cod (Boreogadus saida, n = 20) as well as nine whole capelin (total body length = 100-120 mm) to digestive solutions assumed to mimic the gastric environment of Arctic seals in vitro (pH: ∼2, temperature: ∼37°C, pepsin concentrations: 0-750-μg ml −1 solution). Pepsin did not affect the dissolution rates of sagittae but resulted in a complete digestion of whole capelin within 10 h. Irrespective of species examined, sagittae fitted the same pattern of dissolution rate with the smaller sagittae being dissolved faster than did the larger ones. We suggest that scaling controls sagittal dissolution rates (DR, μm min −1 ), and that the sagittae from the three forage fish combined dissolve according to sagittal length (SL, mm) as shown by the equation: DR Common = 27.348e −0.120 SL (n = 46, r 2 = 0.688, p < 0.001). Given this relationship, the numerical importance of prey taxa with the smaller otoliths - i.e. also the small-sized gadoids (polar cod) - is likely to be underestimated in feeding studies with Arctic seals.

Abstract: Recent studies have shown that some clupeid fishes, including shad and menhaden, can detect ultrasound (sound with frequencies higher than 20 kHz) and actively avoid it. However, other clupeids, including sardines and anchovies, do not detect ultrasound. The hearing abilities of herring are of particular interest because of their commercial importance, our reliance on acoustics to monitor their populations and behavioural evidence of responses to high-frequency sound by some clupeid species. We measured the hearing sensitivity of Pacific herring ( Clupea pallasii ) using the auditory brainstem response and found that they were unable to detect ultrasonic signals at received levels up to 185 dB re 1 μPa. Herring had hearing thresholds at lower frequencies (100–5000 Hz) that were typical of other non-ultrasound‐detecting clupeids. This lower‐frequency hearing sensitivity could explain the results of several earlier studies showing responses to broadband sounds.

Abstract: Herring (Clupea harengus) enter and remain within North Sea estuaries during well-defined periods of their early life history. The costs and benefits of the migrations between offshore spawning grounds and upper, low-salinity zones of estuarine nurseries are identified using a dynamic state-variable model, in which the fitness of an individual is maximized by selecting the most profitable habitat. Spatio-temporal gradients in temperature, turbidity, food availability and predation risk simulate the environment. We modeled predation as a function of temperature, the optical properties of the ambient water, the time allocation of feeding and the abundance of whiting (Merlangius merlangus). Growth and metabolic costs were assessed using a bioenergetic model. Model runs using real input data for the Scheldt estuary (Belgium, The Netherlands) and the southern North Sea show that estuarine residence results in fitter individuals through a considerable increase in survival probability of age-0 fish. Young herring pay for their migration into safer estuarine water by foregoing growth opportunities at sea. We suggest that temperature and, in particular, the time lag between estuarine and seawater temperatures, acts as a basic cue for herring to navigate in the heterogeneous space between the offshore spawning grounds at sea and the oligohaline nursery zone in estuaries.

Abstract: In the present study, we question whether the mixing of autumn spawning herring with Norwegian spring spawning herring (Clupea harengus L.) in feeding and wintering areas is a result of year class twinning, that is, that they hatched in spring as products of the spring spawning stock, but ended up spawning in autumn. The otolith microstructure of the summer feeding autumn spawners (July 2001) was similar to the wintering autumn spawners (January 2002), stable and low in comparison with the sympatric spring spawners caught in the same two seasons. Hence, the otolith microstructure of the analysed larval herring seemed representative of their own spawning season. Data from 1982 to 2003 demonstrated that the fraction of autumn spawners by year class has been quite low at 1–14%, but relatively high in some of the weak year classes, indicating a different recruitment success between the spawning groups. Length at age and somatic weight at length did not differ between the spawning groups. In comparison, the neighbour North Sea autumn spawning herring were significantly smaller at the same age.

Abstract: The hydro-acoustic target strength (TS) of herring (Clupea harengus L.) was estimated from the catch composition of 19 pelagic-trawl hauls and from simultaneous recordings with a split-beam, 38 kHz echosounder. The data were collected in September 2000 during a Bothnian Sea survey in the northern Baltic Sea. The dependence of TS (in dB) on fish length (L, cm) was modelled with the equation TS = a log 10 L + b. The fitted model was TS = 16.8 log 10 L − 60.0. With a predefined slope of 20 the TS vs. log-fish length relationship was TS = 20 log 10 L − 63.9. The analyses suggested that TS was higher in the Bothnian Sea than is assumed in most studies in the Baltic Sea. Applying the revised TS-length dependence considerably enhanced the agreement between the biomass estimates from hydroacoustics and those from a catche-at-age analysis (VPA).

Abstract: Skaret, G., Axelsen, B. E., Nottestad, L., Ferno¨, A., and Johannessen, A. 2005. Thebehaviour of spawning herring in relation to a survey vessel. e ICES Journal of MarineScience, 62: 1061e1064.Vessel avoidance of spawning herring (Clupea harengus L.) was studied off the coast ofsouthwestern Norway in April 2000. In eight repeated night-time passages a demersal layerof herring was recorded acoustically by a small stationary reference vessel (96 GRT), whilea survey vessel (710 GRT) passed at short ranges (8e40 m). No avoidance attributable tothe survey vessel was observed. We interpret vessel avoidance as a response to a perceivedthreat and herring are known to exhibit strong avoidance reactions to survey vessels duringwintering and the spawning migration. At the spawning site, the high priority given toreproductive activities seems to overrule the avoidance responses to a passing surveyvessel.

Abstract: Indices of energy reserves may represent interesting parameters that can be used as bioindicators in environmental studies. The goal of this study was to identify a water–energy model that could predict energy reserves in winter flounder Pseudopleuronectes americanus. Winter flounder kept in captivity and fed different food types (either capelin Mallotus villosus or Atlantic herring Clupea harengus, amphipods Anonyx sarsi, and wet pellets) for 2, 5, and 14 months and wild fish captured in May, July, and October were used to show a large range in energy content. High levels of correlation were observed between water and energy contents in fish carcasses (r 2 = 0.82) and muscle (r 2 = 0.75). However, the biochemical composition of the liver remained relatively constant, despite changes in the hepatosomatic index. The condition factor (somatic weight/length3) was associated with energy reserves (i.e., water contents), but the coefficients of determination were smaller (0.18 < r 2 < 0.34). We found t...

Abstract: The fatty acid composition and contents of fat and fat-soluble vitamins of three salted products prepared from Icelandic herring were analyzed. The effects of storage on the products over their shelf life, 6 or 12 months, were investigated. The average oil content of salted, gutted herring and salted fillets in vacuum remained constant, 17 and 12% of wet weight, respectively. In the pickled product the oil content decreased during the 12 months of storage from 13 to 12%. The composition of the products was typical for herring, the most abundant fatty acids being oleic (18:1n−9), palmitic (16:0), cetoleic (22:1n−11), and gadoleic (20:1n−9) acids. Monounsaturated acids constituted clearly the main group with a proportion of >50% of all fatty acids. Eicosapentaenoic acid (EPA, 20:5n−3) and docosahexaenoic acid (DHA, 22:6n−3) comprised together >12% of all fatty acids. During storage, some hydrolysis of triacylglycerol (TAG) occurred, causing a slight reduction in practically all esterified fatty acids. In no...

Abstract: To determine the effects of diet mixing on digestive performance, harbour seals (Phoca vitulina L., 1758) were offered either pure diets of Pacific herring (Clupea pallasii Valenciennes, 1847) or w...

Abstract: Marginal otolith increment width analysis was performed on field-collected larval and juvenile spring-spawned herring Clupea harengus that experienced variable feeding conditions and high temperatures that were above the optimum for growth. Although drastic zooplankton biomass reduction had a significant effect on increment width, a delay of a few days in the otolith response was observed. More importantly, a very clear, positive temperature effect on marginal increment width was demonstrated in fish characterized by temperature independent somatic growth. These results indicate that under natural conditions it may be impossible to distinguish increment width changes related to variation in feeding conditions from changes caused by temperature fluctuations. Therefore, it was concluded that marginal otolith increment width analysis could not be used as a recent growth index (IG) for herring larvae and juveniles exposed to drastic temperature fluctuations. The implication of these results is significant not only for the use of marginal increments as a recent growth index, but also if growth rate backcalculation is to be used as a research method.

Abstract: A Statistical Resampling Method to Calculate Biomagnification Factors Exemplified with Organochlorine Data from Herring (Clupea harengus) Muscle and Guillemot (Uria aalge) Egg from the Baltic Sea

Abstract: Distributional and dietary responses of wintering Steller’s Eiders (Polysticta stelleri) to spring spawning of Baltic Herring (Clupea harengus) were studied along the Lithuanian coast of the Baltic Sea. Herring spawn is patchy, but is abundant and energy-rich when present. The objective of this study was to determine whether Steller’s Eiders modified their foraging sites and food habits to take advantage of spawn, or whether they were inflexible foragers as suggested by earlier studies. Steller’s Eiders altered their habitat use during herring spawn, moving to habitats where fish spawning occurred. Also, diet analysis demonstrated that herring eggs became an important food when available. Although the importance of herring spawn for Steller’s Eiders remains speculative, this study indicates that spawning sites could be important as a source of nutrients and energy for subsequent migration or reproduction, and should receive conservation consideration.

Abstract: Among larvae from populations of Pacific herring Clupea pallasii in Washington State, those from Cherry Point have consistently demonstrated abnormalities indicative of distress, including low weights and lengths at hatch, increased prevalences of skeletal abnormalities, and shorter survival times in food deprivation studies. The biomass of adult, prespawn Pacific herring at Cherry Point declined from 13,606 metric tons in 1973 to a record low 733 metric tons in 2000. However, correlation of larval abnormalities with adult recruitment was weak, indicating that the larval abnormalities did not directly cause the decline. Larval abnormalities originated primarily from factors independent of conditions at the spawning location because they were not reproduced by incubation of foreign zygotes along the Cherry Point shoreline but were reproduced after the development of indigenous zygotes in controlled laboratory conditions. Although the precise cause of the abnormalities was not determined, recent zo...

Abstract: Inter-annual variation in recruitment (R) of many fish stocks is generally large (e.g. two orders of magnitude). One of many competing hypotheses proposed to explain this variation addresses how maternal factors, operating at the stock and individual level of female spawners, can propagate to R through variation in female number, size, age, and condition, egg size, fecundity, spawning experience and spawning time. In this thesis I address the influence of these maternal factors on recruitment in a comparative manner using Icelandic summer- (ISS) and Scotia-Fundy summer- and autumn-spawning (SFS) herring (Clupea harengus). The results show that spawning of ISS herring consists of one primary and contracted spawning wave although recruit spawners spawn, on average, 17 days later than repeat spawners. In contrast, SFS herring spawning appears to consist of several indistinct and protracted spawning waves. A twofold range in egg weight was observed for both stocks where 20% of the variation could be explained by spawner size. Total egg production (E) in each stock, estimated annually over several decades, using established fecundity relations that incorporated total length and body condition of spawners was not affected by atresia. When constrained to the repeat spawners, Generalized Linear Models showed E best explained R at age-3 for ISS herring. In contrast, Generalized Additive Models showed neither stock-related variables nor environmental variables were of significance in explaining variation in R at age-3 for SFS herring. However, in relation to the latter, an observed relationship between E (constrained to repeat spawners) and a larval abundance index, suggests that maternal effects on recruitment potential realized at the larval stage are not realized in R. Estimates of E derived from recruit spawners did not help explain variation in the larval abundance index for SFS herring nor in R for ISS herring. These findings appear to be related to the smaller egg size (both stocks) and the later spawning (ISS herring) of recruit spawners. It is concluded that maternal factors influence recruitment potential in herring and help provide better estimates of reproductive potential than those provided by spawning stock biomass.