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

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

A study of spring-and autumn-spawned herring (Clupea harengus L.) larvae in the Norwegian Coastal Current during spring 1990

An intensive sampling program for yolk-sac herring larvae and microzooplankton was carried out in the main spawning area of Norwegian spring-spawning herring during March to April 1990 (between 62^o and 63^o30'N) to estimate their hatching period and the abundance of copepod eggs and nauplii. Additional investigations were carried out in the Skagerrak area during January-March and on the Norwegian Shelf in May to study the otolith microstructure of the herring larvae. In May both autumn- and spring-spawned herring larvae were found in the samples from the Norwegian Shelf. They were easily distinguished by differences in otolith microstructure. The pattern in increment widths in the otoliths of the autumn-spawned larvae indicated that these larvae had not been transported through the Skagerrak area, but more likely were carried directly from the northern North Sea across the Norwegian Trench and into the Norwegian Coastal Current system. The calculated hatching of the spring-spawned larvae sampled in May occurred significantly later than the observed hatching over the spawning grounds. The results suggest a mismatch between the abundance of first-feeding herring larvae and their prey organisms, resulting in a higher survival of those herring larvae hatching during the latest part of the spawning period. This coincides with a general increase towards the middle of April in the abundance of prey organisms, from 1 to 4 1^_1. There were no differences in otolith microstructure among spring-spawned herring larvae sampled on the shelf in May, indicating that these larvae originated from the same cohort and were well mixed throughout the whole shelf area.     

黄海北部辽宁近岸海域鳀产卵场分布特征及其影响因素

为掌握黄海北部辽宁近岸海域鳀(Engraulis japonicus)产卵场的分布特征及其关键环境因子,基于2021年4—12月开展的产卵场综合调查获取的鳀样品及其鱼卵密度数据,运用Garrison重心分布法阐释鳀产卵洄游分布特征及其主产卵期;通过基于Tweedie分布的广义可加模型(generalized additive model, GAM)的构建,分析主产卵期内鳀卵密度与同步获取的海水表层温度(SST)、海水表层盐度(SSS)、海水表层叶绿素浓度(Chla)、浮游动物丰度(Fd)、浮游植物丰度(Fz)和深度(Depth)等6个环境因子,以及时间(月份,Month)和空间(经纬度、Lon和Lat)因子之间关系,并识别主控因子。结果显示,海域内鳀产卵期较长,由4月持续至11月,5—8月为主产卵期,其中,5—6月为产卵盛期。鳀产卵场规模和位置时空变化明显,时空因子与鳀卵密度分布呈密切非线性相关(累积偏差解释率为48.1%),(SST, SSS) (18.7%)和Depth (5%)次之。鳀产卵期适温范围较广,产卵场分布表现出高温高盐(低温低盐)增效作用和高温低盐限制作用。产卵初期(4月),鳀产卵场规模和鱼卵密度均较低,产卵重心位于海洋岛东南侧深水区;盛期(5月底—6月初)在SST主导下,鳀产卵场规模和鱼卵密度均至年内最高值,核心产卵场位于石城岛–庄河河口一带海域;此后,随着辽南沿岸水系盐度的下降,高温低盐的抑制作用使SSS因素主导产卵鱼群避开沿岸海域,鳀产卵场迁移至外海深水区,7月后位于30~50 m等深线之间;9—10月鳀繁殖活动基本结束,10月鳀卵仅零星分布于调查海域,直至12月未有鳀卵采获。研究可为黄海北部辽宁近岸海域鳀产卵场研究及鳀资源合理开发利用提供参考依据。     

The growth of larval cod and haddock in the Irish Sea: a model with temperature,prey size and turbulence forcing

We applied a physiological individual‐based model for the foraging and growth of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae, using observed temperature and prey fields data from the Irish Sea, collected during the 2006 spawning season. We used the model to estimate larval growth and survival and explore the different productivities of the cod and haddock stocks encountered in the Irish Sea. The larvae of both species showed similar responses to changes in environmental conditions (temperature, wind, prey availability, daylight hours) and better survival was predicted in the western Irish Sea, covering the spawning ground for haddock and about half of that for cod. Larval growth was predicted to be mostly prey‐limited, but exploration of stock recruitment data suggests that other factors are important to ensure successful recruitment. We suggest that the presence of a cyclonic gyre in the western Irish Sea, influencing the retention and/or dispersal of larvae from their spawning grounds, and the increasing abundance of clupeids adding predatory pressure on the eggs and larvae; both may play a key role. These two processes deserve more attention if we want to understand the mechanisms behind the recruitment of cod and haddock in the Irish Sea. For the ecosystem‐based management approach, there is a need to achieve a greater understanding of the interactions between species on the scale a fish stock is managed, and to work toward integrated fisheries management in particular when considering the effects of advection from spawning grounds and prey–predator reversal on the recovery of depleted stocks.     

Impacts of climate change on the complex life cycles of fish

To anticipate the response of fish populations to climate change, we developed a framework that integrates requirements in all life stages to assess impacts across the entire life cycle. The framework was applied on plaice (Pleuronectes platessa) and Atlantic herring (Clupea harengus) in the North Sea, Atlantic cod (Gadus morhua) in the Norwegian/Barents Seas and European anchovy (Engraulis encrasicolus) in the Bay of Biscay. In each case study, we reviewed habitats required by each life stage, habitat availability, and connectivity between habitats. We then explored how these could be altered by climate change. We documented environmental processes impacting habitat availability and connectivity, providing an integrated view at the population level and in a spatial context of potential climate impacts. A key result was that climate‐driven changes in larval dispersion seem to be the major unknown. Our summary suggested that species with specific habitat requirements for spawning (herring) or nursery grounds (plaice) display bottlenecks in their life cycle. Among the species examined, anchovy could cope best with environmental variability. Plaice was considered to be least resilient to climate‐driven changes due to its strict connectivity between spawning and nursery grounds. For plaice in the North Sea, habitat availability was expected to reduce with climate change. For North Sea herring, Norwegian cod and Biscay anchovy, climate‐driven changes were expected to have contrasting impacts depending on the life stage. Our review highlights the need to integrate physiological and behavioural processes across the life cycle to project the response of specific populations to climate change.     

A spatially explicit fitness-based model of capelin migrations the Barents Sea

The geographical distribution and production of the Barents Sea capelin (Mallotus villosus, Osmeridae) is modelled by the use of a state-variable optimization technique (dynamic programming), where the main objective of individuals always is to maximize fitness, or total expected reproduction (R_O), by selecting the most profitable habitats through time. Fitness is gained by successful reproduction (a function of size) during the spawning season on the breeding grounds off northern Norway. The environment (predators, temperature and zooplankton prey) is determined by a meteorologically forced circulation model for the year 1980, creating a spatial and seasonal fluctuation in the environment. Predation from cod is the main source of mortality, and the distribution of the cod (Gadus morhua) stock is assumed to vary with temperature. Growth is predicted from a bioenergetic model, incorporating the cost of swimming between feeding areas and spawning grounds. Field data of the capelin stock recorded during autumn cruises from 1979 is implemented at the start of the model, and then this stock is modelled through 1980 and the first months of 1981. Model predictions are compared with the observed distribution of capelin in autumn 1980. Habitat selection has consequences for the dynamics of the population and growth of individuals, demonstrating the importance of combining external (environmental) and internal (evolutionary) forcing to understand and predict the dynamics of fish populations. This study is the first application of dynamic programming to model the dynamics and ecology of horizontal fish migration, and we suggest that the method may be developed into a useful tool for the management of short-lived species.     

Availability of horse mackerel (Trachurus trachurus) in the north-eastern North Sea, predicted by the transport of Atlantic water

One of the clearest relationships between a single marine climate variable and fisheries yield is found between the transport of Atlantic water into the North Sea in the winter and catches of horse mackerel (Trachurus trachurus L.) in the northern North Sea 6 months later. A three‐dimensional numerical model, the NORWegian ECOlogical Model system (NORWECOM), has estimated this inflow during the winter from 1976 to 2000, which is strongly correlated with the Norwegian fleet's catch of horse mackerel (r² = 0.70) in the following autumn. It is hypothesized that a large influx of this relatively warm and nutrient rich water during the winter may support an early spawning of zooplankton and high biological production during the spring and summer. This might be the biological reason for a large fraction of the horse mackerel stock migrating into the North Sea during strong inflows.     

Multiple cohorts of juvenile jack mackerel Trachurus japonicus in waters along the Tsushima Warm Current

The jack mackerel, Trachurus japonicus, has a prolonged spawning season and widely spread spawning grounds. The population in the coastal waters of Japan seems to be composed of several cohorts spawned seasonally from different waters. To understand its population structure along the Tsushima Warm Current, we analysed hatchdates and growth histories of fish from Kunda Bay, the southern, central and northern East China Sea (ECS), the southern Sea of Japan, and Maizuru Bay. Seven cohorts were detected from fish collected between June 2005 and June 2006 in Kunda Bay. Comparing hatchdate distributions and growth trajectories of the seven cohorts with those of the other five regional samples, we did not find that cohorts collected in Kunda Bay originated in the southern ECS. Therefore, these coastal waters of Japan appear to be significant spawning grounds for juvenile jack mackerel.     

Distribution of jack mackerel (Trachurus japonicus) larvae and juveniles in the East China Sea, with special reference to the larval transport by the Kuroshio Current

Horizontal distribution patterns of jack mackerel (Trachurus japonicus) larvae and juveniles were investigated in the East China Sea between 4 February and 30 April 2001. A total of 1549 larvae and juveniles were collected by bongo and neuston nets at 357 stations. The larvae were concentrated in the frontal area between the Kuroshio Current and shelf waters in the upstream region of the Kuroshio. The abundance of small larvae (<3 mm notochord length) was highest in the southern East China Sea (SECS) south of 28°N, suggesting that the principal spawning ground is formed in the SECS from late winter to spring. Jack mackerel also spawned in the northern and central East China Sea (NECS and CECS, respectively), as some small larvae were also collected in these areas. In the SECS, the abundance of small larvae was highest in February and gradually decreased from March to April. The habitat temperature of small larvae in the SECS and CECS (20–26°C) was higher than that in the NECS (15–21°C), suggesting higher growth rates in the SECS and CECS than in the NECS. The juveniles (10‐ to 30‐mm standard length) became abundant in the NECS off the west coast of Kyushu Island and CECS in April and were collected in association with scyphozoans typical of the Kuroshio waters. However, juveniles were rarely collected in the SECS, where the small larvae were concentrated. Considering the current systems in the study area, a large number of the eggs and larvae spawned and hatched in the SECS would be transported northeastward by the Kuroshio and its branches into the jack mackerels’ nursery grounds, such as the shallow waters off the west coast of Kyushu and the Pacific coast of southern Japan.     

Viral haemorrhagic septicaemia virus in marine fish and its implications for fish farming--a review

Viral haemorrhagic septicaemia virus (VHSV) has, in recent decades, been isolated from an increasing number of free-living marine fish species. So far, it has been isolated from at least 48 fish species from the northern hemisphere, including North America, Asia and Europe, and fifteen different species including herring, sprat, cod, Norway pout and flatfish from northern European waters. The high number of VHSV isolations from the Baltic Sea, Kattegat, Skagerrak, the North Sea and waters around Scotland indicate that the virus is endemic in these waters. The VHSV isolates originating from wild marine fish show no to low pathogenicity to rainbow trout and Atlantic salmon, although several are pathogenic for turbot. Marine VHSV isolates are so far serologically indistinguishable from freshwater isolates. Genotyping based on VHSV G- and N-genes reveals four groups indicating the geographical origin of the isolates, with one group representing traditional European freshwater isolates and isolates of north European marine origin, a second group of marine isolates from the Baltic Sea, a third group of isolates from the North Sea, and a group representing North American isolates. Examples of possible transfer of virus from free-living marine fish to farmed fish are discussed, as are measures to prevent introduction of VHSV from the marine environment to aquaculture.     

Transport and survival processes of eggs and larvae of jack mackerel <Emphasis Type="Italic">Trachurus japonicus</Emphasis> in the East China Sea

ABSTRACT:   Recent surveys showed substantial aggregation of larvae of jack mackerel in the southern East China Sea, indicating intensive spawning grounds near Taiwan. A numerical model was applied to investigate transport and survival processes of eggs and larvae of jack mackerel from the spawning area to the nurseries. The results show that: (i) the distributions of larvae simulated by the model agreed well with those obtained by field survey; (ii) the stock of jack mackerel in the Sea of Japan is composed of both groups from north of Taiwan and from the western coast of Kyushu. It takes more than two months for the former to reach the Sea of Japan, while it is within 40 days for the latter; and (iii) large proportions of the eggs and larvae spawned off the north of Taiwan are transported rapidly to the Pacific side of Kyushu by the Kuroshio Current, and the rest slowly to the east or north-east along the continental slope in the East China Sea. In contrast to the larval flux, survivors are more abundant in the northern East China Sea than in the Pacific Ocean, indicating that survival in the northern East China Sea would determine the jack mackerel stock in Japan.     

东、黄海大黄鱼种群划分与地理隔离分析

以往研究认为,大黄鱼(Larimichthys crocea)可划分为3个种群,其中在东黄海,浙江的岱衢洋大黄鱼和福建的官井洋大黄鱼被认为分属于两个不同的种群。本研究根据中国大陆10多个主要渔业公司1971 1982年间的大黄鱼捕捞统计资料,从地理隔离、数量动态和海洋水文方面,重新审视了东黄海大黄鱼种群划分问题,为大黄鱼种群划分和大黄鱼资源兴衰的研究提供科学依据。研究表明:东海北部外海和东海南部近海是大黄鱼主要的两个越冬场,其中闽东温台水域的大黄鱼产量在东海南部近海产量占主导地位。东海南部近海大黄鱼地理分布表明,从温台渔场到闽东渔场大黄鱼的越冬场在空间分布上具有连续性,而官井洋大黄鱼正是闽东渔场大黄鱼的主体部分。由此可以认为:官井洋所在的闽东渔场的大黄鱼和东黄海大黄鱼同属于东黄海大黄鱼种群。这一结论通过3个旁证得到印证:其一,大黄鱼标志放流结果显示,1959年4月21日在连江县北茭洋东32 m深的地方(26°21′5″N、119°50′E)重捕到浙江水产实验所于1958年5月20日在岱衢洋寨子山东偏北大黄鱼产卵场放流的1尾雄性大黄鱼,因此,闽东渔场的大黄鱼和岱衢洋是相互混栖的同一群体;其二,东海沿岸流和台湾暖流终年影响着闽浙近海,难以形成大黄鱼种群隔离、种群分化所需要的海洋学条件;其三,官井洋大黄鱼春夏之交产卵,与岱衢洋和猫头洋大黄鱼相似,而与粤东和粤西大黄鱼在9-12月产卵完全不同。本研究旨在为大黄鱼资源的研究提供依据。     

Spawning of North Sea fishes linked to hydrographic features

Spawning of fishes takes place across a wide area of the North Sea. However, more intense spawning is seen in restricted areas, indicating that such areas present favorable conditions. To update information on fish spawning in the North Sea and analyze potential linkages to hydrographic characteristics, an internationally coordinated survey was conducted in the winter/spring of 2004. Oblique hauls for fish eggs and larvae and vertical profiles of temperature and salinity were carried out at 393 stations across the entire North Sea. The hydrography was strongly influenced by the interfacing of water masses of different salinity, and frontal zones were seen along all coastal areas and off the Dogger and Fisher Banks. Total abundances of eggs and larvae, including fish species such as cod, haddock, plaice, long rough dab and sandeel, peaked in the vicinity of the frontal areas. Hence our findings indicate that the main spawning locations of fish are linked to recurrent hydrographic features such as salinity fronts. Such a linkage may provide survival advantages, as the fronts present favorable feeding conditions, and the related physical processes may confine egg and larval dispersal and transport them towards suitable nursery habitats.     

基于鱼卵仔鱼数据的东海中南部日本鲭产卵场分析

利用2016年4—5月在东海中南部海域的调查数据,对该海域日本鲭(Scomber japonicus)鱼卵仔鱼的分布特征进行了研究,并对产卵场的划分展开讨论及提出保护措施和管理建议。结果显示:4月,日本鲭鱼卵仔鱼主要出现在台湾海峡和东海南部海域,出现率均大于25%,但仔鱼在东海南部外海的出现率和丰度均较低,分别为5.00%和0.01个·100m-3。5月,日本鲭鱼卵仔鱼在台湾海峡的出现率和丰度均显著降低,分别下降至5.88%和0.22个·100m^-3;东海南部外海日本鲭鱼卵仔鱼的出现率和丰度则均显著升高,分别升至50%和36.48个·100m^-3,其中鱼卵集中出现在南部外海,出现率和丰度分别达到了40%和36.46个·100m^-3;东海中部外海从5月开始出现日本鲭鱼卵和仔鱼。结合历史资料,可以判断东海中南部日本鲭产卵场主要有台湾海峡、东海中南部近海、东海南部外海、东海中部外海等4个。日本鲭鱼卵丰度与表层水温呈显著相关关系(R=0.37,P<0.05),与表层盐度无显著相关关系(P>0.05);仔鱼丰度与表层水温及表层盐度均无显著相关关系(P>0.05)。结果表明,水温对日本鲭的产卵活动影响较大,随着水温的逐步升高,性腺发育速度加快,开始产卵的亲体数量也大量增加,这从鱼卵仔鱼的数量大量增加得以体现;而盐度变化对日本鲭的产卵活动影响相对较小。针对日本鲭不同的产卵场,建议制定差异化的繁殖亲体和幼鱼保护措施。     

南海北部浮游动物的景观格局分析

依据1997～2002年开展的国家海洋勘测专项＂南海专属经济区和大陆架渔业资源及其栖息环境调查（HY126-02）＂所取得的浮游动物调查数据,利用地理信息系统（geographic information system,GIS）空间局部插值法构建景观分析模型,对南海北部浮游动物景观格局进行了分析。结果表明,南海北部浮游动物生物量高值区在沿岸海域呈斑块状分布,其景观格局的季节变化与季风气候和沿岸流转换一致,且与南海北部几个重要上升流区和传统渔场的渔情渔汛关系密切。     

Management of transboundary and straddling fish stocks in the Northeast Atlantic in view of climate‐induced shifts in spatial distribution

The introduction of 200 n.m. exclusive economic zones (EEZs) in the late 1970s required increased collaboration among neighbouring coastal states to manage transboundary and straddling fish stocks. The established agreements ranged from bilateral to multilateral, including high‐seas components, as appropriate. However, the 1982 United Nations Convention on the Law of the Sea does not specify how quotas of stocks crossing EEZs should be allocated, nor was it written for topical scenarios, such as climate change with poleward distribution shifts that differ across species. The productive Northeast Atlantic is a hot spot for such shifts, implying that scientific knowledge about zonal distribution is crucial in quota negotiations. This diverges from earlier, although still valid, agreements that were predominately based on political decisions or historical distribution of catches. The bilateral allocations for Barents Sea and North Sea cod remain robust after 40 years, but the management situation for widely distributed stocks, as Northeast Atlantic mackerel and Norwegian spring‐spawning herring, appears challenging, with no recent overall agreements. Contrarily, quotas of Northern hake are, so far, unilaterally set by the EU despite the stock's expansion beyond EU waters into the northern North Sea. Negotiations following the introduction of EEZs were undertaken at the end of the last cooler Atlantic Multidecadal Oscillation (AMO) period, that is, with stock distributions generally in a southerly mode. Hence, today's lack of management consensus for several widely distributed fish stocks typically relates to more northerly distributions attributed to the global anthropogenic signal accelerating the spatial effect of the current warmer AMO.     

Selective haddock (Melanogrammus aeglefinus) trawling: Avoiding cod (Gadus morhua) bycatch

The critical condition of the North Sea cod stocks has resulted in restrictions on not only cod, but also haddock and other species that are caught together with cod. Thus full exploitation of the haddock stock is unachievable unless cod can be excluded from the haddock catch. We designed a selective trawl based on the behavioral differences between haddock and cod as they enter a trawl, i.e., cod stay close to the seabed whereas haddock rise above it. The trawl's fishing line is raised ～60 cm above the seabed to allow cod to escape beneath the trawl while haddock are retained. To collect the escapees, three sampling bags were attached beneath the raised fishing line. The selective haddock trawl reduced the total catch of cod by 55% during the day and 82% at night, and 99% of the marketable haddock was caught during the day and 89% at night. Cod escape rates were highly length dependent: smaller cod escaped the trawl in greater numbers than did larger individuals. Whiting, saithe, lemon sole, and plaice were included in the analysis.     

The development of the northern European fishery for north Atlantic bluefin tuna Thunnus thynnus during 1900–1950

North Atlantic bluefin tuna, Thunnus thynnus, used to migrate to northern European waters (Norwegian Sea, North Sea, Skagerrak, Kattegat, and Øresund) where it supported important commercial and sportfisheries. The species disappeared from the region in the early 1960s and the species is now still extremely rare. The factors which led to the development of the fishery and its subsequent decline remain unclear and poorly documented. This investigation documents the development of the fishery in terms of landings, effort, and gears with focus on the time period from 1900 to 1950 when landings were increasing. The species was frequently sighted while fishermen were targeting other species (herring, mackerel) and occasionally was caught as bycatch with these and other species. Information from scientifically trained observers demonstrate that tuna schools were common in the North Sea for 2–3 months during the summers of 1923–1931. As fishermen realized that the species had market value, new catch methods were developed and employed. These included harpoon-rifle, improved hook and line methods, and hydraulically operated purse seines. Landings rose sharply as did the number of vessels and the capacity of processing facilities for bluefin tuna. Bluefin tuna in this area were generally medium-large (>50 kg whole weight). The most important countries which participated in bluefin tuna fisheries in this period were Norway, Denmark and Sweden, but bluefin tuna were also exploited by France, Germany, The Netherlands and the United Kingdom. Similarly sportfishing increased in popularity in some of these countries and attracted many foreign participants. The increase in landings between 1900 and 1950 was driven particularly by an increase in fishing effort and technology. We found no evidence that the increase was due to a temperature-related shift in habitat into the region. Our results demonstrate that the species was an important part of the ecosystem at least back to the early 1900s and that commercial and recreational fisheries were well established in northern European waters before official ICCAT records.     

Linking spatial pattern of bottom fish assemblages with water masses in the North Sea

Understanding the links between large scale spatial structuring of fish assemblages and shaping factors is essential to develop comprehensive ecosystem-based fisheries management. In this study, we investigated spatial patterns of bottom fish assemblages in the North Sea in relation to prevailing water masses in the region. We based our analysis on catch data from the German Small-Scale Bottom Trawl Survey conducted between 1987 and 2005 and used both ordination techniques and Mantel tests. Spatial variability of bottom fish assemblages was larger than inter-annual variability. Five significantly different bottom fish assemblages were associated with the following prevailing hydrographical regimes: i) the English Channel, ii) Continental Coastal, iii) central North Sea, iv) northern North Sea, and v) northern Atlantic water masses. Associations were generated by gradients in relative proportions of abundant species such as grey gurnard ( Eutrigla gurnardus ), dab ( Limanda limanda ), whiting ( Merlangius merlangus ), haddock ( Melanogrammus aeglefinus ) and Norway pout ( Trisopterus esmarki ). Taking into account large scale spatial structuring of catch data Mantel tests confirmed significant correlation between the fish assemblages and hydrographical variables. In summary, our results strongly support the hypotheses that hydrographical features such as water masses, fronts, and residual currents could shape bottom fish associations in the North Sea. Spatial demarcations of bottom fish assemblages indicated by this study can be used to support ecosystem-based fisheries management strategies.     

Does larval mortality influence population dynamics? An analysis of North Sea herring (Clupea harengus) time series

The daily mortality rates of North Sea herring early‐stage larvae are found to vary over decades. Larval abundance data were used with a spatio‐temporal oceanographic model to reconstruct temperature histories of the observed larvae. The histories were used in conjunction with a temperature‐based growth model to estimate larval age. Mean daily mortality rates were then estimated for the four spawning components (Downs, Banks, Buchan and Orkney/Shetland) using the vertical life table approach, which considers instantaneous abundances across all ages rather than following distinct cohorts. All spawning components, but especially Downs (in the south), exhibited a steady rise in mortality associated with increasing population size. In addition, the three northern components shared a distinct trend in mortality that was significantly correlated with ambient water temperatures experienced by the larvae during the respective time periods after hatching. This trend was also significantly negatively correlated with the residuals of the whole stock‐recruitment relationship. These findings were generally robust to assumptions about growth and hatch length of larvae. The compensatory increase in productivity in the late 1980s and poor recruitment since 2000 coincide with changes in the mortality of larvae younger than 30 days post hatch and covary with larval density and temperature. Thus we suggest that the mortality of early‐stage larvae does impact on the population dynamics in North Sea herring in its current productivity regime, implying a critical period in the determination of year class strength.