Climate change and condition of herring (Clupea harengus) explain long-term trends in extent of skipped reproduction

It is commonly assumed that iteroparous fish, once mature, normally reproduce in all consecutive seasons. Recent work has suggested, however, that in Norwegian spring-spawning herring—a population that undertakes extensive spawning migrations—almost one in two adults may skip their second spawning migration. Why should herring not return to spawn the year after first spawning, but instead wait an extra year? For herring, participation in distant, energetically costly, and risky spawning migrations will only pay off in terms of fitness if individuals are sufficiently large, and in sufficient condition, to both successfully migrate and spawn. Changes in the environment and individual condition should therefore affect the likelihood of skipped spawning. This paper describes long-term changes in the extent to which the second reproductive season is skipped in this herring population. These are shown to be linked to the size and condition of herring as first-time spawners, and to climatic factors possibly related to food availability. The findings corroborate the hypothesis that skipped reproduction results from trade-offs between current and future reproduction, growth and survival.     

Differential utilization of energy during wintering and spawning migration in Norwegian spring-spawning herring

The energy utilization in non-feeding Norwegian spring-spawning herring Clupea harengus was examined during periods of wintering in a fjord system in northern Norway and southward spawning migration (800 km distance) to the main spawning area off western Norway. At the onset of wintering the gonadosomatic index (IG) was higher and the amount of mesenteric fat was lower in male than in female herring, whereas the subsequent increase in IG and decrease in mesenteric fat occurred at a higher rate in females than in males. However, no differences were recorded between sexes in fat content (%) of whole fish at any time of the year. The absolute energy loss increased, whereas the relative energy loss decreased, with fish length. Furthermore, the absolute and relative length specific energy losses were two to three and three to four times higher, respectively, during spawning migration than during wintering. The distinct energy loss was attributed mainly to fat metabolism, whereas the storage protein only was transferred from the soma to the gonads and remained relatively constant in whole herring throughout the wintering and spawning migration. © 1999 The Fisheries Society of the British Isles     

State-dependent spawning migration in Norwegian spring-spawning herring

Norwegian spring-spawning herring Clupea harengus winter in fjords of northern Norway, whereas the subsequent spawning occurs at various locations along the coast with a main bulk off the south-western coast. The distance of the southward spawning migration tends to increase with the length and condition of the fish. The costs and benefits of the southward migration were modelled in terms of fitness (number of surviving offspring). The model assumes that larvae have increasing growth and survival rates further south as they pass through warmer water during the northward drift in the coastal current. In agreement with the observed spawning distribution, optimal spawning grounds were predicted off the southwestern coast and farther south with increasing fish length and condition. The present study suggests that homing is not a successful strategy for these herring, and instead the selection of spawning grounds depends on individual internal state (length, condition), the cost of migration and the probability of larval survival.     

Spatially structured interactions between a migratory pelagic predator, the Norwegian spring-spawning herring Clupea harengus L., and its zooplankton prey

Spring-spawning herring Clupea harengus was patchily distributed over large parts of the Norwegian Sea in May 1995–2005, during the early phase of the annual feeding migration. Overall, herring tended to be found in areas with intermediate biomasses of zooplankton prey, intermediate water temperatures and relatively high salinities. Herring had more food in their stomachs in areas of relatively low water temperature and high herring abundance. Hydrographical conditions revealed that herring was feeding mainly within Atlantic water masses, and more intensely in western and northern regions of the Norwegian Sea. Zooplankton biomass was patchily distributed, and was generally higher towards the western parts of the Norwegian Sea. Here, zooplankton biomass in year _{i +1} was also negatively associated with herring spawning stock biomass in year _i , while there was no evidence for such an association in the eastern region; indicating that herring may have a geographically structured 'top–down' effect on the recruitment of its zooplankton prey. The fact that herring was not typically associated with the areas containing the greatest zooplankton biomasses may reflect that the fish had not yet reached the most profitable feeding grounds or alternatively that herring was depleting zooplankton biomass.     

Scale analysis suggests frequent skipping of the second reproductive season in Atlantic herring

A common assumption in fish studies is that iteroparous fishes, once mature, normally reproduce in all consecutive seasons. An analysis of scales from Norwegian spring-spawning herring collected between 1935 and 1973 revealed a strong under-representation (47% of expected) of second-time spawners on the spawning grounds. This reduction is not satisfactorily explained by errors in scale-reading, suggesting that the second reproductive season is frequently skipped. Skipping a season may relate to trade-offs between growth, current and future reproduction, and survival, which are likely to be particularly strong for young adult herring.     

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

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

Ovarian dynamics and fecundity regulation in blueback herring,Alosa aestivalis,from the Connecticut River,US

We analyzed ovarian dynamics of anadromous blueback herring, Alosa aestivalis, in Connecticut River with the principal aim of exploring oocyte recruitment and how it shapes the fecundity pattern. We examined the oocyte release strategy and analyzed spawning cyclicity by linking oocyte growth to the degeneration of postovulatory follicles. Females were accordingly classified as pre-spawners, early and late active spawners, and oocyte recruitment intensity was compared among the different spawning phases. Oocyte recruitment occurred continuously and in parallel with spawning activity, a pattern which is diagnostic of indeterminate fecundity. However, both fecundity and oocyte recruitment intensity progressively decreased (tapered) throughout spawning, until the ovary was depleted of vitellogenic oocytes. There was no massive atresia of vitellogenic oocytes at the end of the spawning season, which is atypical of indeterminate spawners. We propose that tapering in oocyte recruitment and fecundity is an adaptation to the high energetic expenditure of the upstream spawning migration.     

Effects of fish size on spawning time in Norwegian spring-spawning herring

During October-January in a northern Norwegian fjord system and in February-March at the main western Norway spawning grounds, an index I _G of gonad weight of spring-spawning herring Clupea harengus in percentage of expected gonad weight at full maturity for a given total length L _T, tended to increase with L _T in the range 27–31 cm as the proportion of recruit spawners decreased. Insignificant differences in I _G were found between L _T groups in the range 32–37 cm (repeat spawners). This is contrary to that suggested in other studies and signifies that generally the stock spawns in only two waves, repeat spawners first and the recruit spawners second. I _G also increased with somatic and total condition factor, which signifies that spawning time may be influenced by summer feeding conditions.     

Recruitment prediction for Pacific herring (Clupea pallasi) on the west coast of Vancouver Island, Canada

The accurate prediction of recruitment to the fishery is a very important tool within the management structure of any fish stock being exploited. In the case of the Pacific herring, Clupea pallasi, fishery in Canada, a forecast of the abundance of each herring stock is particularly important for formulating an annual catch quota. The sustainable management of the fishery and the resource is based in part on accurate recruitment forecasting because Pacific herring are short-lived and so the recruitment contributes a significant part of the total spawning run targeted by the fishery each year. Several factors are believed be important in determining the success of recruitment besides spawners biomass. Since herrings are “r” strategists, conditions related to the egg, the planktonic, or even the juvenile stage might determine the future level of recruitment. Recently a formula that defines conditions for a semi-quantitative level of recruitment forecast was elaborated using genetic algorithms and current study attempts to improve on this model. Using salinity in two quarterly periods during the planktonic and pre-recruit stages, temperature and spawning biomass for the west coast of Vancouver Island stock, classification rules that define recruitment in 3 different levels (low, medium and high) were developed with a genetic algorithm, setting low and high boundaries for each condition. A 75% success in classifying recruitment was obtained. The model was shown to be particularly effective at predicting when the recruitment would be low, which could be important from the perspective of the Precautionary Approach and the sustainable management of this stock.     

Characteristics of the Norwegian Coastal Current during Years with High Recruitment of Norwegian Spring Spawning Herring (Clupea harengus L.)

Norwegian Spring Spawning herring (NSSH) Clupea harengus L. spawn on coastal banks along the west coast of Norway. The larvae are generally transported northward in the Norwegian Coastal Current (NCC) with many individuals utilizing nursery grounds in the Barents Sea. The recruitment to this stock is highly variable with a few years having exceptionally good recruitment. The principal causes of recruitment variability of this herring population have been elusive. Here we undertake an event analysis using data between 1948 and 2010 to gain insight into the physical conditions in the NCC that coincide with years of high recruitment. In contrast to a typical year when northerly upwelling winds are prominent during spring, the years with high recruitment coincide with predominantly southwesterly winds and weak upwelling in spring and summer, which lead to an enhanced northward coastal current during the larval drift period. Also in most peak recruitment years, low-salinity anomalies are observed to propagate northward during the spring and summer. It is suggested that consistent southwesterly (downwelling) winds and propagating low-salinity anomalies, both leading to an enhanced northward transport of larvae, are important factors for elevated recruitment. At the same time, these conditions stabilize the coastal waters, possibly leading to enhanced production and improved feeding potential along the drift route to Barents Sea. Further studies on the drivers of early life history mortality can now be undertaken with a better understanding of the physical conditions that prevail during years when elevated recruitment occurs in this herring stock.     

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

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

Social transmission of behavioural traditions in a coral reef fish

Traditional behaviours involve the non-genetic transmission of social information across age classes or generations. French grunts (Haemulon flavolineatum) exhibit social traditions of daytime schooling sites and twilight migration routes. Individuals transplanted to new schooling sites and allowed to follow residents at the new sites used the new migration routes and returned to the new sites in the absence of resident fish. Control fish with no opportunity to learn showed no such directionality or return. This is the first demonstration of apparent pre-cultural behaviour in free-living fish. Our observations suggest additional classes of behaviour and taxonomic groups in which pre-cultural activities are likely to have evolved.     

Coastal zone use and migratory behaviour of the southern population of Mugil liza in Brazil

We analysed the ratios Sr:Ca and Ba:Ca in the otoliths of 55 adults of the southern population of Mugil liza in Brazil (Paraná to Rio Grande do Sul) to investigate its coastal zone use and migratory behaviour. All individual M. liza analysed had Sr:Ca and Ba:Ca values indicating that their birth was in the marine environment, which is consistent with the southern population migration to spawn in the ocean,. Juveniles exhibited at least three coastal use and recruitment strategies (contingents): the majority (89%) of M. liza juveniles migrated toward brackish water. They entered the estuary before completing the first year of life (64%) or after (25%) their first year of life. The remaining 11% did not appear to enter brackish or freshwater water as a nursery or at any point in their life cycle. Some adults returned to the estuary after spawning in the ocean but others (of both sexes) never returned to the estuary after spawning, remaining in the marine environment. The pattern of juvenile habitat use in the Brazilian southern population of M. liza seems to be recurrent throughout the extent of its distribution as a consequence of the reproductive spawning aggregation behaviour, which mixes all contingents (with marine or estuarine preferences).     

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

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

An analysis of allozyme variation in herring <Emphasis Type="Italic">Clupea pallasii</Emphasis> from the White and Barents Seas

An analysis of genetic variation is made according to four allozyme loci of reproductive (spawning) groups of oligovertebrate herring Clupea pallasii collected in various bays of the White Sea and the south-eastern part of the Barents Sea in 1995–2002. The temporal stability of genetic characteristics during several years is shown. The analysis of genetic variation revealed a significant difference between herring from the south-eastern part of the Barents Sea and spring-spawning and summer-spawning herring from inner bays of the White Sea. The analysis of geographic variation of genetic characteristics of spawning aggregations revealed the change in frequencies of alleles of loci LDH-2* and MDH-4* from the north-east to the south-west along the coast.     

Feeding ecology of mackerel and dietary overlap with herring in Icelandic waters

The Northeast Atlantic mackerel (Scomber scombrus) has been extending its summer feeding distribution north and west, including around Iceland, since around 2006. The objective of this study is to quantify the weight gain and total food consumption of mackerel and to evaluate the food competition between mackerel and herring (Clupea harengus) feeding in the marine ecosystem around Iceland during the summers 2009–2011. Mackerel feeding in Icelandic waters gained around 43% on average in weight during these summers. Based on swept-area abundance estimates of mackerel from an international survey in 2011 and available estimates of food conversion efficiency in mackerel, the weight gain in Icelandic waters in 2011 corresponded to a total consumption of around 3.4 (2.4?4.5) million tonnes. Overall, 98% of 2314 mackerel, 91% of 398 Icelandic summer-spawning herring and 96% of 424 Norwegian spring-spawning herring stomachs, collected over the summers 2009?2011, contained food. The mean weight of the stomach content of mackerel was higher than for herring in all the years. While the stomach content weight of mackerel was generally highest in southeastern and southwestern areas, it was highest for herring in western, eastern and northern areas. Analysis of stomach contents showed that Copepoda were the most important food of mackerel in most areas, while Copepoda and Euphausiacea were the most important food items for herring. Fish prey contributed a higher proportion in stomachs of mackerel than herring, although relatively low for both species.     

Feeding Ecology of Northeast Atlantic Mackerel,Norwegian Spring-Spawning Herring and Blue Whiting in the Norwegian Sea

The Norwegian spring-spawning (NSS) herring (Clupea harengus), blue whiting (Micromesistius poutassou) and Northeast Atlantic (NEA) mackerel (Scomber scombrus) are extremely abundant pelagic planktivores that feed in the Norwegian Sea (NS) during spring and summer. This study investigated the feeding ecology and diet composition of these commercially important fish stocks on the basis of biological data, including an extensive set of stomach samples in combination with hydrographical data, zooplankton samples and acoustic abundance data from 12 stock monitoring surveys carried out in 2005–2010. Mackerel were absent during the spring, but had generally high feeding overlap with herring in the summer, with a diet mainly based on calanoid copepods, especially Calanus finmarchicus, as well as a similar diet width. Stomach fullness in herring diminished from spring to summer and feeding incidence was lower than that of mackerel in summer. However, stomach fullness did not differ between the two species, indicating that herring maintain an equally efficient pattern of feeding as mackerel in summer, but on a diet that is less dominated by copepods and is more reliant on larger prey. Blue whiting tended to have a low dietary overlap with mackerel and herring, with larger prey such as euphausiids and amphipods dominating, and stomach fullness and feeding incidence increasing with length. For all the species, feeding incidence increased with decreasing temperature, and for mackerel so did stomach fullness, indicating that feeding activity is highest in areas associated with colder water masses. Significant annual effects on diet composition and feeding-related variables suggested that the three species are able to adapt to different food and environmental conditions. These annual effects are likely to have an important impact on the predation pressure on different plankton groups and the carrying capacity of individual systems, and emphasise the importance of regular monitoring of pelagic fish diets.     

Experimental Evidence of Threat-Sensitive Collective Avoidance Responses in a Large Wild-Caught Herring School

Aggregation is commonly thought to improve animals'' security. Within aquatic ecosystems, group-living prey can learn about immediate threats using cues perceived directly from predators, or from collective behaviours, for example, by reacting to the escape behaviours of companions. Combining cues from different modalities may improve the accuracy of prey antipredatory decisions. In this study, we explored the sensory modalities that mediate collective antipredatory responses of herring (Clupea harengus) when in a large school (approximately 60 000 individuals). By conducting a simulated predator encounter experiment in a semi-controlled environment (a sea cage), we tested the hypothesis that the collective responses of herring are threat-sensitive. We investigated whether cues from potential threats obtained visually or from the perception of water displacement, used independently or in an additive way, affected the strength of the collective avoidance reactions. We modified the sensory nature of the simulated threat by exposing the herring to 4 predator models differing in shape and transparency. The collective vertical avoidance response was observed and quantified using active acoustics. The combination of sensory cues elicited the strongest avoidance reactions, suggesting that collective antipredator responses in herring are mediated by the sensory modalities involved during threat detection in an additive fashion. Thus, this study provides evidence for magnitude-graded threat responses in a large school of wild-caught herring which is consistent with the “threat-sensitive hypothesis”.     

Meta-ecosystems and biological energy transport from ocean to coast: the ecological importance of herring migration

Ecosystems are not closed, but receive resource subsidies from other ecosystems. Energy, material and organisms are moved between systems by physical vectors, but migrating animals also transport resources between systems. We report on large scale energy transport from ocean to coast by a migrating fish population, the Norwegian spring-spawning (NSS) herring Clupea harengus. We observe a rapid body mass increase during parts of the annual, oceanic feeding migration and we use a bioenergetics model to quantify energy consumption. The model predicts strong seasonal variation in food consumption with a marked peak in late May to July. The copepod Calanus finmarchicus is the most important prey and 23×10⁶ tonnes (wet weight) of C. finmarchicus is consumed annually. The annual consumption-biomass ratio is 5.2. During the feeding migration 17% of consumed energy is converted to body mass. The biomass transported to the coast and left as reproductive output is estimated from gonad weight and is about 1.3×10⁶ tonnes for the current population. This transport is to our knowledge the world’s largest flux of energy caused by a single population. We demonstrate marked temporal variation in transport during the last century and discuss the effects of NSS herring in the ocean, as a major consumer, and at the coast, where eggs and larvae are important for coastal predators. In particular, we suggest that the rapid decline of lobster Homarus gammarus landings in Western Norway during the 1960s was related to the collapse of NSS herring. We also discuss spatial variation in energy transport caused by changed migration patterns. Both climate and fisheries probably triggered historical changes in the migration patterns of NSS herring. New migration routes emerge at the level of individuals, which in turn determines where resources are gathered and delivered, and therefore, how meta-ecosystems function.     

Movement,site fidelity and connectivity in a top marine predator,the killer whale

Movement, site fidelity and connectivity have important consequences for the evolution of population structure and therefore the conservation and management of a species. In this study photographs of naturally marked killer whales collected from sites across the Northeast Atlantic are used to estimate fidelity to sampling locations and movement between locations, expressed as transition probabilities, pt, using maximum likelihood methods. High transition probabilities suggest there is high inter-annual site fidelity to all locations, and large-scale movement between the spawning and wintering grounds of both Norwegian and Iceland stocks of Atlantic herring. There was no evidence of movement between the Norwegian herring grounds and Icelandic herring grounds, or between the mackerel fishing grounds and the herring fishing grounds. Thus the movement of predictable and abundant prey resources can lead to intrinsic isolation in this species We also find movement between the Northern Isles, Scotland and East Iceland. An association network indicates that killer whales predating seals around the Northern Isles, Scotland are linked to the community of killer whales that follow the Icelandic summer-spawning herring. This adds support to existing evidence of a broad niche width in some populations.