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.     

Temperature determines growth rates of larval round herring Etrumeus teres in the Pacific coastal waters off southern Japan

Seasonal variation in daily growth rates in the early and middle larval stages of round herring Etrumeus teres were largely determined by the sea temperatures experienced by hatch-date cohorts in the Pacific coastal waters off southern Japan. Round herring larvae were collected by purse seining in the coastal waters of central Tosa Bay. A total of 451 larvae were aged by reading daily rings in otoliths. Individuals within a range of 2–5 hatch dates were grouped as hatch-date cohorts. We selected 16 cohorts that hatched during September 2000 and March 2002 and calculated mean widths of otolith growth increments for each cohort during the first feeding stage (W _FF, increments 1–5) and the maximum increment width in the middle larval stage (W _MAX). Seasonal variation in mean W _FF and W _MAX among the 16 cohorts was largely (80–90 %) explained by the sea temperature in the bay. These results indicate that temperature was a predominant determinant of larval growth rates; other environmental factors, such as food availability, did not substantially affect growth rates of round herring larvae in coastal waters along the subtropical Kuroshio Current off southern Japan.     

Seasonal changes in otolith increment width trajectories and the effect of temperature on the daily growth rate of young sardines

We studied the otolith microstructure and growth of sardine, Sardina pilchardus, in the North Aegean Sea (eastern Mediterranean Sea), using samples of larvae and juveniles that had hatched in winter (November–January) and winter–spring (February–May), respectively. The juveniles had developed during an extended period coinciding with marked pelagic ecosystem changes (from winter, mixed conditions to summer, stratified waters). To examine the relationship between environmental changes and the observed variability in their otolith increment–width trajectories (width‐at‐age), we summarized the shape of trajectories with a four‐parameter set estimated from a growth model fit to each width trajectory. The individual parameter sets were then related to the potential oceanographic conditions that fish experienced during their development, derived from a hydrodynamic–biogeochemical model (POM‐ERSEM), implemented in the sampling area. Substantial seasonal effects were demonstrated on the otolith microstructure (platykurtic versus leptokurtic trajectories in winter‐mixed versus summer‐stratified conditions), which were related to the progressive sea surface warming. In a subsequent step, in order to study the effect of oceanographic conditions on larval and juvenile daily growth rates, a GAM (Generalized Additive Model) analysis of otolith increment widths was carried out, using model‐derived oceanographic parameters and taking into account the ‘inherent otolith growth’, expressed by the explanatory variables ‘previous increment width’ and ‘Age’. Results showed a strong and positive, linear effect of temperature on the growth rate of winter‐caught larvae, whereas in juveniles, which had developed within a wide range of temperatures, an optimum temperature for growth was observed at around 24°C.     

Variability of larval Baltic sprat (Sprattus sprattus L.) otolith growth: a modeling approach combining spatially and temporally resolved biotic and abiotic environmental key variables

Plankton sampling was conducted in the Baltic to obtain sprat larvae. Their individual drift patterns were back‐calculated using a hydrodynamic model. The modelled positions along the individual drift trajectories were subsequently used to provide insight into the environmental conditions experienced by the larvae. Autocorrelation analysis revealed that successive otolith increment widths of individual larvae were not independent. Otolith increment width was then modelled using two different generalized additive model (GAM) analyses (with and without autocorrelation), using environmental variables determined for each modelled individual larval position as explanatory variables. The results indicate that otolith growth was not only influenced by the density of potential prey but was controlled by a number of simultaneously acting environmental factors. The final model, not considering autocorrelation, explained more than 80% of the variance of otolith growth, with larval age as a factor variable showing the strongest significant impact on otolith growth. Otolith growth was further explained by statistically significant ambient environmental factors such as temperature, bottom depth, prey density and turbulence. The GAM analysis, taking autocorrelation into account, explained almost 98% of the variability, with the previous otolith increment showing the strongest significant effect. Larval age as well as ambient temperature and prey abundance also had a significant effect. An alternative approach applied individual‐based model (IBM) simulations on larval drift, feeding, growth and survival starting as exogenously feeding larvae at the back‐calculated positions. The IBM results revealed optimal growth conditions for more than 97% of the larvae, with a tendency for our IBM to slightly overestimate larval growth.     

Modelling the influence of light, turbulence and ontogeny on ingestion rates in larval cod and herring

Based on existing models of fish vision and turbulence-mediated ingestion rates, we develop a model of ingestion rates in larval fish that combines several physical properties of the environment (turbulence, irradiance, light attenuation) and visual characteristics of predators and prey. The model of visual range was calibrated with observed estimates in larval herring ( Clupea harengus ) and cod ( Gadus morhua ). The improved visual ability with length of larvae was predicted to be the most sensitive part of the model. Both turbulence and light had strong impacts on the ingestion rate of larval fish. The optimal level of turbulence increased with larval length. Also, due to the exponential decay of light with depth, it was predicted that larvae will have higher ingestion rates near the surface, even at high wind velocities. It is also proposed that larvae (particularly the smallest larvae) should concentrate on larger prey in turbulent environments. We suggest that biophysical models of larval growth and survival in field situations should include these factors to account for environmental effects on growth, survival and recruitment processes in the early life stages.     

RNA–DNA ratio of herring and sand lance larvae from Port Moller, Alaska: Comparison with prey concentration and temperature

A key assumption of hypotheses that link the production of prey for larval fish with year-class strength of fish is that larval growth and condition is food-limited. We tested this assumption by comparing whole-body RNA-DNA ratios of individual Pacific herring, Clupea pallasi , larvae and Pacific sand lance, Ammodytes hex-apterus , larvae from Port Moller, a subarctic Alaskan estuary, with prey concentration and temperature. RNA-DNA ratios were correlated with larval length, but not with prey concentration or temperature. Ratios were not significantly different between a warm, well-mixed station with low prey concentrations and a colder, stratified station with higher prey concentrations. Using RNA-DNA ratios, we classified as starving 11 to 23% of first-feeding (< 13 mm long) herring larvae and 45% of first-feeding (< 7 mm long) sand lance larvae. However, starvation could not have been caused by low concentrations of prey because micro-zooplankton prey concentrations were high enough (16 to 84 prey L^_l) to support relatively high rates of growth. Therefore, starving larvae were either abnormal or they were stIII learning to forage. We conclude that the magnitude of starvation among first-feeding herring and sand lance larvae, and, therefore, the total contribution of starvation to year-class strength, is dependent not only on prey concentration, but on the proportion of a population of larvae that can feed effectively.     

大黄鱼仔稚鱼不同发育阶段矢耳石形态发育和微结构特征

对人工培育大黄鱼(Larimichthys crocea)的生长发育与矢耳石形态及微结构特征进行研究,结果表明:(1)大黄鱼矢耳石上的轮纹是每日形成的,第1日轮在孵化后第2天形成,与其初次摄食相对应。(2)大黄鱼卵黄囊期和前弯曲期仔鱼的耳石形态为圆形,进入弯曲期耳石长轴迅速伸长,在后弯曲期耳石形态变为椭圆形。进入稚鱼期,矢耳石开始形成次生核。随后次生核数量逐渐增加,在孵化后47～78 d的个体中,次生核数量稳定在5～7个,耳石近似盾形。(3)根据耳石日轮宽度推算的大黄鱼稚鱼在其仔鱼期生长率(b)与第1个次生核的形成时间(tsp1)之间存在明显的线性关系,表明生长较快的个体形成次生核的时间较早,进入稚鱼期所需的时间更短。以上结论表明,大黄鱼矢耳石可以反演其早期生活史阶段的生长发育特征。     

Modelling the dispersal of herring and hake larvae in the Strait of Georgia for the period 2007–2009

The Strait of Georgia (SoG), between Vancouver Island and mainland British Columbia, is a larval rearing ground for both hake and herring stocks, which are commercially important. Year‐to‐year variability in larval retention within the strait is examined by simulating drift tracks of larvae for these species using an ocean circulation model and a particle‐tracking model. Larvae with different vertical swimming behaviors were tracked in the springs of 2007, 2008, and 2009. Since herring larvae mostly stay near the surface, their distribution is heavily influenced by the wind. Strong winds to the north soon after the hatching period tend to wash herring larvae out of SoG and winds to the south help retain herring larvae inside the Strait. In 2007, the model indicates a massive wind‐driven export of herring larvae which may have led to the observed failure of herring production. In contrast, hake larvae reside deeper in the water column (50–200 m). Their distribution is less sensitive to surface forcing but is shaped by a deep gyre with cross‐strait currents. This study also suggests that the northern and southern SoG are weakly connected for herring larvae dispersal, which makes both regions potentially important to recruitment.     

Early Larval Growth of Pacific Halibut Hippoglossus stenolepis

Growth of Pacific halibut Hippoglossus stenolepis larvae was studied in the laboratory during 1989 and 1991. Larvae increased in length from 6.3 mm at hatching to 9.9 mm 20 d post-hatch. The average daily length increment was 0.17 mm. Dry weight of the larvae increased from an average of 210 μg at hatching to 570 μ g on day 20, providing a specific growth rate of 4.99. During the same period, mean yolk sac weight decreased from 1,390 μ g to 646 μ g, resulting in a yolk to body conversion efficiency of 48.5%. At hatching, the larval body made up only 13% of total dry weight. On day 20, the larval body made up 46.9% of the total weight. Larvae started feeding at a length of 12 mm after about 90% of their yolk sac had been absorbed.     

Otolith development and daily increment formation in laboratory-reared larval and juvenile black-spot tuskfish <Emphasis Type="Italic">Choerodon schoenleinii</Emphasis>

Microstructures of lapilli were examined for reared larvae and juveniles of black-spot tuskfish Choerodon schoenleinii. Lapilli of larvae at 1 day after hatching have one diffuse and obscure ring with an otolith radius of 4.3 ± 0.50 μm (mean ± SD, N = 8). The slope and intercept of the regression between the number of days after hatching and increment counts did not differ significantly from one and zero, respectively, indicating that lapillus increments were formed on a daily basis after hatching. There was an ontogenetic shift in the relative values of somatic and otolith growth, which corresponded to the transition from pelagic larvae to settlement stage. Simultaneously, the daily increment width reached the maximum value. These findings suggest that age at maximum value of increment width can be used as an indicator of the planktonic larval duration while settlement mark is not found. Since ontogenetic shift in the relationship between otolith radius and body size was observed, back-calculation of somatic growth in black-spot tuskfish using the otolith radius during the early life stages should be analyzed with caution, and the method requires further validation.     

Annual variation in otolith increment widths of walleye pollock (Gadus chalcogrammus) larvae in Funka Bay,Hokkaido, Japan

To clarify relationships between year‐class strength and larval growth of walleye pollock (Gadus chalcogrammus), and oceanographic conditions in the Pacific stock off Hokkaido and Tohoku, Japan, we undertook conductivity/temperature/depth (CTD) observations and investigated larval densities, larval otolith increment widths and larval prey densities (of copepod nauplii) of the 2008, 2009, 2010 and 2011 yr classes in Funka Bay. Oyashio Coastal Water (OCW) flowed into the bay in late February in 2008, 2010 and 2011, and the mean water temperatures decreased to 1.9–3.1 °C in March. OCW was not observed in 2009, and it was warm in late February (≥3.4 °C). Increment widths of lapillar otoliths during the yolk‐sac stage were wide in 2009 and 2011, medium in 2010 and narrow in 2008. Increment widths during the first‐feeding stage tended to become wider as the hatch month progressed, and the annual variation during the first‐feeding stage was larger than that of the yolk‐sac stage. The densities of the primary food for the larvae were high in 2008 when larval increment widths were narrowest, so the effect of prey abundance on larval growth appeared to be small. The ranking of the larval abundance in March was nearly coincident with that of the increment width during the larval stage. We, therefore, suggest that the larval growth rate is associated with the mortality rate and that the growth–mortality hypothesis may be applicable to walleye pollock in Funka Bay. Feeding success under warm water conditions may be an important factor that contributes towards high growth rates.     

Larval growth of turbot, Scophthalmus maximus (L.) produced with fresh and cryopreserved sperm

The present paper assesses the fertilization and hatching rates, as well as the growth, of larvae obtained from four artificial fertilizations (AF) using fresh and cryopreserved sperm of the turbot Scophthalmus maximus (L.). Larvae growth in both sperm groups, measured in terms of length and weight at culture days 0, 7, 14 and 31, are compared, as well as their growth rates. The two groups' fertilization and hatching rates were not significantly different. Likewise, no significant differences in length and wet weight of 7‐ and 14‐day‐old larvae were found using fresh and cryopreserved sperm; however, significant differences were found in 31‐day‐old larvae, which were more attributable to the variability inherent in larval turbot culture, and to variability in the reproductive specimens used in our study, than to the type of sperm employed. These results indicate that the type of sperm used in artificial fertilization, i.e. fresh or cryopreserved, is not a determining factor, either for fertilization and hatching, or for subsequent larval development. Our results also confirm once again the high quality of cryopreserved turbot sperm, and its usefulness in commercial hatcheries.     

A spatially explicit study of prey–predator interactions in larval fish: assessing the influence of food and predator abundance on larval growth and survival

We apply a coupled biophysical model to reconstruct the environmental history of larval radiated shanny in Conception Bay, Newfoundland. Data on the larvae, their prey and predators were collected during a 2‐week period. Our goal was to determine whether environmentally explicit information could be used to infer the characteristics of individual larvae that are most likely to survive. Backward drift reconstruction was used to assess the influence of variations in the feeding environment on changes in the growth rates of individual larvae. Forward drift projections were used to assess the impact of predators on mortality rates as well as the cumulative density distribution of growth rates in the population of larvae in different areas of the bay. There was relatively little influence of current feeding conditions on increment widths. Patterns of selective mortality indicate that fast‐growing individuals suffered higher mortality rates, suggesting they were growing into a predator's prey field. However, the mortality rates appeared to increase with decreasing predator abundance, based on the drift reconstructions. The relationship of growth and mortality with environmental conditions suggests that short‐term, small‐scale variations in environmental history may be difficult to describe accurately in this relatively small system (～1000 km²).     

Using patch studies to link mesoscale patterns of feeding and growth in larval fish to environmental variability

We present results from a series of three patch studies designed to examine links between environmental variability and mesoscale patterns of feeding and growth of larval radiated shanny ( Ulvaria subbifurcata ). We examine the effects of variability in temperature, turbulence and prey concentration on both the mean (i.e. population level) and the variance (i.e. individual level) of larval feeding and growth rates among the three bays. Although both gut fullness and growth rates differ significantly between bays, our results show only weak environmental influences. When larvae are pooled across bays (i.e. treated as independent observations), environmental factors generally explain <4% of the variability in gut fullness. When treated as daily mean residuals, however, temperature accounts for 41% of the variability in mean gut fullness, while both temperature and prey concentration also explain significant portions of the variance in gut fullness (38 and 43%, respectively). Between-bay differences in larval growth rates are consistent with patterns of temperature variation but not with patterns of prey availability. Studies relying on tracking a single patch of larvae typically suffer from having too few observations to detect significant relationships between feeding or growth and environmental variables. By following three patches we collected a larger number of observations. However, as we encountered only a limited range of environmental conditions it remains difficult to adequately assess the role of environmental factors. In part, this problem stems from the inability of fisheries oceanographers to track the recent environmental history of individual larvae on the same fine scales currently employed to collect biological data (e.g. guts and otoliths) on individuals.     

Effects of delayed first feeding on larval growth,survival and development of the sea cucumber Apostichopus japonicus (Holothuroidea)

The time of first feeding is an important factor for establishing successful exogenous feeding in aquaculture hatcheries as it is crucial for larval survival and growth. The effects of delayed first feeding by the sea cucumber Apostichopus japonicus larvae were investigated. Groups of larvae were fed immediately after completion of the digestive tract (fed control) or starved for 24, 48, 72, 108 and 120 h post fertilization. The growth rates of A. japonicus larvae were the same as those of the control group when first feeding was delayed for less than 72 h. When the period of starvation was beyond 72 h, growth rates were reduced. The fed control group had the highest percentage of late auricularia larvae (70.0%) on day 11, followed by the groups starved for 24, 48, and 72 h, respectively, which had 35.00%, 43.48% and 41.67% of late auricularia larvae. When larvae were starved for more than 72 h, the percentage of late auricularia larvae never exceeded 12.0%. Most larvae halted in development and were deformed before reaching metamorphosis when starved for 108 and 120 h. Prolonged exposure to starvation (more than 48 h) resulted in a significant decrease in percentage of larvae reaching settlement. The point‐of‐no‐return (PNR) of A. japonicus larvae was calculated 110.4 h. Ratios of stomach length to total length, stomach width to total width were more sensitive to starvation than other larval morphometrics. To avoid mortality and obtain adequate growth and development, A. japonicus larvae in commercial culture should be fed within 48 h of the completion of the digestive tract.     

Feeding rhythm and growth of the tongue sole, Cynoglossus semilaevis Günther, during its early life stages

The feeding rhythm and growth characteristics of the early life stages of the tongue sole, Cynoglossus semilaevis Günther, were studied. Larvae began to prey on rotifers about 2–3 days after hatching at 23°C. About 12 days after hatching, larvae had grown to 8–9 mm in total length and were able to prey on Artemia larvae. As the larvae grew, they showed an increasing feeding capacity and a distinct feeding rhythm. Feeding intensity for day‐6 larvae was highest at 12:00 and 18:00 hours, about 2–3 h after the maximum feeding incidence. The highest levels of feeding intensity for day‐16 larvae occurred between 09:00 and 18:00 hours. By day 26, when the larvae had metamorphosed, feeding capacity had again increased considerably and, in contrast to the earlier stages, feeding intensity peaked at 18:00 and 24:00 hours concurrently with feeding incidence. Thus, tongue sole were found to have different feeding rhythms in the pre‐ and post‐metamorphosis stages, with the highest feeding activity in the daytime during the larval planktonic stage, and at night during the juvenile benthic stage. The estimated maximum daily feeding rates were 65%, 40% and 11% of body weight on days 6, 16 (larvae) and 26 (juveniles) respectively. Size variation increased markedly with development.     

依耳石显微结构判断安氏新银鱼的早期生活史

安氏新银鱼的矢耳石呈不规则卵圆形。60尾样品(体长37~52 mm)耳石长半径y与体长x呈线形关系,y=260.335+2.6234x。光镜下观察了耳石制片的显微结构。耳石中心圆形的核平均直径(25.17±2.40)μm(SD,后同)。核中心原基平均直径(7.80±2.22)μm。核周围为同心环纹,即日轮。耳石上日轮数73~101,78.3%的样品分布于83~97日轮范围内。前10个日轮平均间距最窄,为1.76μm,之后日轮间距逐渐增宽,60~70日轮平均间距最宽(2.70μm),而后日轮间距又变窄。依日轮间距推算的体长生长,前10日龄平均日增长0.34 mm,以后生长加快,60~70日龄平均日增长最快,为0.52 mm。依据采样日期,日轮数和胚胎发育期推断,样品鱼的产卵期为4月中旬至5月中旬,出生日期为4月下旬至5月下旬。从最初2~3个日轮间距最宽,4~10日轮间距较窄判断,其卵黄营养期为3~4 d,混合营养期为6~7 d。部分样品在48~72日轮处有过渡轮,应是由近海(盐度27~30)到河口(盐度6~18)盐度急剧变化诱导形成的,表明幼鱼有到河口摄食洄游的习性。     

Responses in growth rate of larval northern anchovy (Engraulis mordax) to anomalous upwelling in the northern California Current

We examined variability in growth rate during the larval stage of northern anchovy (Engraulis mordax) in response to physical and biological environmental factors in 2005 and 2006. The onset of spring upwelling was anomalously delayed by 2–3 months until mid‐July in 2005; in contrast, spring upwelling in 2006 began as a normal year in the northern California Current. Larval and early juvenile E. mordax were collected in August, September, and October off the coast of Oregon and Washington. Hatch dates ranged from May to September, with peaks in June and August in 2005 and a peak in July in 2006, based on the number of otolith daily increments. Back‐calculated body length‐at‐age in the June 2005 hatch cohort was significantly smaller than in the August 2005 cohort, which had comparable growth to the July 2006 cohort. Standardized otolith daily increment widths as a proxy for seasonal variability in somatic growth rates in 2005 were negative until late July and then changed to positive with intensification of upwelling. The standardized increment width was a positive function of biomass of chlorophyll a concentration, and neritic cold‐water and oceanic subarctic copepod species sampled biweekly off Newport, Oregon. Our results suggest that delayed upwelling in 2005 resulted in low food availability and, consequently, reduced E. mordax larval growth rate in early summer, but once upwelling began in July, high food availability enhanced larval growth rate to that typical of a normal upwelling year (e.g., 2006) in the northern California Current.     

Coupling ecosystem and individual‐based models to simulate the influence of environmental variability on potential growth and survival of larval sprat (Sprattus sprattus L.) in the North Sea

To investigate the impact of changing environmental conditions in the North Sea on the distribution and survival of early life stages of a marine fish species, we employed a suite of coupled model components: (i) an Eulerian coupled hydrodynamic/ecosystem (Nutrients, Phyto‐, Zooplankton, Detritus) model to provide both 3‐D fields of hydrographical properties, and spatially and temporally variable prey fields; (ii) a Lagrangian transport model to simulate temporal changes in cohort distribution; and (iii) an individual‐based model (IBM) to depict foraging, growth and survival of fish early life stages. In this application, the IBM was parameterized for sprat (Sprattus sprattus L.) and included non‐feeding (egg and yolk‐sac larval) stages as well as foraging and growth subroutines for feeding (post‐yolk sac) larvae. Sensitivity analyses indicated that the angle of visual acuity, assimilation efficiency and the maximum food consumption rate were the most critical intrinsic model parameters. As an example, we applied this model system for 1990 in the North Sea. Results included not only information concerning the interplay of temperature and prey availability on larval fish survival and growth but also information on mechanisms underlying larval fish aggregation within frontal zones. The good agreement between modelled and in situ estimates of sprat distribution and growth rates in the German Bight suggested that interconnecting these different models provided an expedient tool to scrutinize basic processes in fish population dynamics.     

Climate‐induced environmental conditions influencing interannual variability of Mediterranean bluefin (Thunnus thynnus) larval growth

Daily growth variability of bluefin (Thunnus thynnus) larvae sampled in their Balearic Sea spawning grounds during the 2003–2005 spawning seasons was examined. Multi‐factorial ANOVA was applied to study the effects of environmental variables, such as temperature at 10 m depth (T10), microzooplankton dry weight (MDW) and protein/dry weight ratio (PROT/MDW) on larval growth. The 2003 bluefin tuna (BFT) larval cohort showed the fastest growth, recognizable from enhanced otolith and somatic mass increment compared to the 2004–2005 larval cohorts. The 2003 BFT larvae showed greater recent growth than the 2004–2005 BFT cohorts, which decreased in the last stages of development. Growth differences between the 2004 and 2005 larval cohorts were not significant. The environmental conditions between 2003 and 2004–2005 were highly contrasting as a result of the 2003 warming anomaly. Somatic and otolith growth rates (OGR) were significantly related to T10 and MDW, as well as to the PROT/MDW ratios. Nonetheless, the effect of T10 on OGR depended on the relative high (H) or low (L) levels of MDW and PROT/DW. Higher OGR was observed when T10 was high, MDW was low and PROT/DW was high. This environmental scenario conditions were met during 2003, which recorded the highest surface temperature and low planktonic biomass. Somatic growth, expressed as larval DW growth increase (DWGR), showed three‐factor significant interactions with T10*MDW*PROT/MDW, in which the two‐way interactions of MDW*PROT/MDW showed differences in the function of T10 levels.