Distribution and abundance of Norwegian spring-spawning herring during the spawning season in 2023

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Distribution and abundance of Norwegian spring-spawning herring during the spawning season in 2024

During the period 13-26th of February 2023 the spawning grounds of Norwegian spring-spawning herring from Møre (62º15 ˊ N) to Tromsøflaket (71 º N) were covered acoustically by the commercial fishing vessels MS Eros and MS Vendla. The estimated biomass was about 15 % lower, and the estimated total number was about 5 % higher this year compared to last year’s survey. The uncertainty of the estimates in 2024 was much lower than last year. The surveyed population was dominated by the 2016 year class; 50 % in numbers and 57 % in biomass. Most of the spawning stock was found west of Lofoten and Vesterålen this year, more northerly distributed than in earlier years. More young herring was observed in the north than normal on this survey which is a positive sign in terms of future recruitment to the spawning stock. The estimates of abundance by age group from the survey in 2024 are recommended to be used in this year’s ICES stock assessment of Norwegian spring-spawning herring.Distribution and abundance of Norwegian spring-spawning herring during the spawning season in 2024publishedVersio

Distribution and abundance of Norwegian spring-spawning herring during the spawning season in 2022

Toktnummer 2022821 og 2022822Distribution and abundance of Norwegian spring-spawning herring during the spawning season in 2022During the period 14-27th of February 2022 the spawning grounds of Norwegian spring-spawning herring from Møre (62º15ˊN) to Troms (71ºN) were covered acoustically by the commercial vessels MS Eros and MS Vendla. The estimated biomass was about 18 % lower, and the estimated total number was about 29 % lower this year compared to the last year’s survey. The uncertainty of the estimates in 2022 was approximately equal to last year. The surveyed population of NSS herring was dominated by the 2016 year class; 52 % in numbers and 46 % in biomass. The 2016 year class was reduced by 37 % in numbers from last year’s survey. Most of the spawning stock was found outside Lofoten and Vesterålen this year, further north and more concentrated than usual. The observed maturity indicates a bit later spawning compared to last year and like last year a more northern spawning than normal. As usual, the herring in the southern part of the spawning area were older than those found in the northern part. The estimates of relative abundance from the survey in 2022 are recommended to be used in this year’s ICES stock assessment of Norwegian spring-spawning herring.publishedVersio

Development stage distribution as a proxy for feeding success and growth for first feeding Norwegian spring spawning herring larvae

The estimation of growth rates in young herring larvae (Clupea harengus) in the field can be difficult because the primary increments in the otoliths may not be discernible or formed at a daily level. Likewise, the estimation of mortality rates of fish larvae in the field is very difficult to achieve, especially in a rigorous quantitative manner. In this study, the authors suggest the use of a stage‐based proxy of feeding success, growth and potential survival or mortality risk of field‐caught larvae. The stage‐based proxy is derived based on observations from previous laboratory studies where larvae successfully completing start‐feeding on external food sources will advance through the early development stages, whereas those that do not (unsuccessful larvae) remain and accumulate in the development stage preceding first feeding. The relative occurrence of larvae in the early development stages is therefore expected to reflect feeding conditions of the larvae, with higher ratios of unsuccessful larvae indicative of poor feeding success and higher mortality risk. Using field data on Norwegian spring spawning herring, the authors document that the relative occurrence of larvae in the late non‐feeding stage is significantly higher at lower average zooplankton concentrations, in line with the predictions of the authors that this novel approach of using a stage‐based proxy could be a useful indication of feeding success, growth and mortality in the field. Further, there was a significant interaction effect with ambient temperature, with the ratio being higher at low zooplankton concentrations at higher temperatures. This study also suggests that these findings are not population specific as the same accumulation of non‐feeding larvae in the late non‐feeding stage was observed in laboratory‐reared larvae of both autumn and spring spawning herring populations.publishedVersio

Drift indices confirm that rapid larval displacement is essential for recruitment success in high-latitude oceans

Larval drift is a key process for successful fish recruitment. We used Norwegian spring-spawning herring (Clupea harengus) as model species to investigate the relationship between larval drift and recruitment. Larval drift indices were derived from simulations based on survey observations between 1993 and 2016. We show that forward simulated larval drift indices have an important positive relation to recruitment success. The relationship demonstrates elevated recruitment when larvae relocate rapidly northwards toward the Barents Sea. Negative or low larval drift indices coincide with only weak recruitment emphasizing limited survival in years with enhanced larval retention. Hence, with this work we combine drift model outcomes refined with survey data indicating that more extensive larval drift is an important component in population dynamics for high-latitude small pelagic fishes. However, larval displacement alone represents only one among many controlling factors but may offer possible predictions of the probability of higher or lower recruitment in the short term. The applicability of the drift indices is adaptable in all world oceans and all marine organisms that occupy planktonic life stages exposed to dynamic ocean currents. The study demonstrates how larval drift indices help to identify larval transport or retention to be crucial for population replenishment.publishedVersio

Environmental influences on Norwegian spring-spawning herring (<i>Clupea harengus</i> L.) larvae reveal recent constraints in recruitment success

The lack of any abundant recruiting year class of Norwegian spring-spawning (NSS) herring between 2005 and 2015 contributed to an approximate reduction of 40% in the spawning-stock biomass since 2009, i.e. from 7 to 4 million tonnes. Warming of the North Atlantic is suggested to contribute to this reduction in recruitment. In the past, a warm phase induced by a positive Atlantic Multidecadal Oscillation (AMO) in the North Atlantic was positively correlated to the NSS herring stock size. Recent unprecedented ocean warming in the Norwegian Sea ecosystem, besides elevated temperatures due to a positive AMO, seems to be outside optimal environmental conditions for early life history stages of NSS herring. We analysed 28 years of survey data using generalized additive models to reconstruct environmental conditions for drifting yolksac and preflexion stage larvae. Our results indicate that strong recruitment years were more likely when the larvae occurred simultaneously with a negative AMO during positive temperature anomalies. The transition from yolksac stages towards preflexion stages occurred while there was a slow increase in water temperature during the larval drift. Weak recruitment years generally occurred when larvae experienced elevated temperatures during the life stage transition under a positive AMO. These results augment evidence that the historical positive relationship between AMO and stock dynamics is reversed between 1988 and 2015. Albeit not implying any specific mechanistic biological interactions, we can assume that the unprecedented warming has modified the ecosystem drivers that negatively affect drifting larvae. Since 2016, the North Atlantic is shifting into a negative AMO phase, possibly resulting in the 10-year recruitment suppression of NSS herring ending soon.publishedVersio

Predicting density-dependent somatic growth in Norwegian spring-spawning herring

Density-dependent growth, which might influence the effects of fisheries on a population, is often ignored when management strategies are evaluated, mainly due to a lack of appropriate models readily available to be implemented. To improve on this, we investigated if somatic growth in Norwegian spring-spawning herring (Clupea harengus) depends on cohort density using a formulation of the von Bertalanffy growth function on cohorts from 1921 to 2014 and found a significant negative correlation between estimated asymptotic length and density. This clearly indicates density-dependent effects on growth, and we propose a model that can be used to predict the size-at-age of Norwegian spring-spawning herring as a function of herring density (the abundance of two successive cohorts) in short-term predictions of catch advice, and in Management strategy evaluations, including estimation of their reference points such as FMSY.publishedVersio

Spatial Distribution of Different Age Groups of Herring in Norwegian Sea, May 1996–2020

The commercially important Norwegian spring spawning herring is characterized by its extensive annual migrations and, on a decadal timescale, large shifts in migration patterns. These changes are not well understood, but have previously been linked to temperature, food availability, and size and age composition of the stock. Acoustic and trawl data from the International Ecosystem Surveys in the Nordic Seas, carried out annually in May since 1996, were used to analyze the spatial distribution of herring in the period 1996–2020. The dataset was disaggregated into age classes, and information about where the different age classes feed in May was derived. The analysis of herring feeding patterns in May confirms that the youngest age classes are generally found close to the Norwegian shelf, whereas the older age classes display larger variations in where they are distributed. During the period 1996–1998, the oldest age classes were found in the central and western Norwegian Sea. During the period 1999–2004, the whole stock migrated north after spawning, leaving the regions in the southern Norwegian Sea void of herring. Since 2005 the oldest herring has again congregated in the south-western Norwegian Sea, in the frontal zone between the cooler East Icelandic water and the warmer Atlantic water. There was a significant positive relationship both between stock size and distribution area and between stock size and density. Moreover, it is likely that the strong year classes 1991/1992 and 1998/1999, which were relatively old when the respective changes in migration patterns occurred, were important contributors to the changes observed in 1999 and 2005, respectively.publishedVersio

Asynchronized spawning responses of small pelaigic fish to a short-term enviromental change

We provide substantial evidence on how short-term changes in environmental conditions activate and deactivate spawning activities in small pelagic fishes. An ichthyoplankton survey was conducted along the southernmost part of the Canary Current upwelling ecosystem in May 2013, covering the area twice within 20 d. This period coincided with a strong environmental change from a cold productive upwelling regime to a warmer and less productive upwelling relaxation event. This change triggered a shift in spawning activity from European anchovy Engraulis encrasicolus to round/flat sardinella Sardinella spp. We used zero-altered negative binomial regression models with a generalized additive structure based on integrated nested Laplace approximations to link early larval distribution patterns to the 2 different regimes. The models confirmed 2 species-specific temperature spawning windows, suggesting a spawning pause of anchovy during upwelling relaxation while simultaneously activating spawning in sardinella. Observing immediate spawning responses to the 2 environmental regimes underlines the assumption that windows of spawning opportunity are the main drivers of small pelagic fish fluctuations in upwelling regions. The duration of a specific environmental condition can, therefore, increase or decrease the chances for reproductive success. The observations of this study may explain why certain small pelagic fish species can dominate over others during a particular period and might also apply to other upwelling regions of the world oceans where upwelling and relaxation events alternate.publishedVersio

Fangstprøvelotteriet 2020 – erfaringer og resultat

Etter at vi startet opp med fangstprøvelotteriet i 2018 har vi sett en gradvis og betydelig forbedring i prøvetakingen fra de kommersielle pelagiske fiskeriene. Vi ser likevel et stort forbedringspotensial framover siden det per i dag kun er ca. 60% av de bestilte prøvene som faktisk kommer frem til HI og dermed kan inngå i bestandsvurderingene. Dette skyldes blant annet at ikke alle fiskerne som mottar prøvebestilling gjennomfører prøvetakingen. I tillegg tror vi at noen prøver blir glemt igjen både om bord og på mottaksanleggene som skal sende prøvene videre til HI. Det er derfor viktig å fortsette arbeidet med å informere næringsaktørene om fangstprøvelotteriet og forbedre systemet for tilbakemelding om resultatene fra prøvene. Fangstprøvelotteriet gir de viktigste grunnlagsdataene for kvoteanbefalingene for pelagisk fisk, og en forbedring av prøvetakingen vil derfor først og fremst komme fiskerinæringen til gode.Fangstprøvelotteriet 2020 – erfaringer og resultatpublishedVersio