Cod Larvae Research Articles

Age, not growth, explains larger body size of Pacific cod larvae during recent marine heatwaves

Marine heatwaves (MHWs) are often associated with physiological changes throughout biological communities but can also result in biomass declines that correspond with shifts in phenology. We examined the response of larval Pacific cod (Gadus macrocephalus) to MHWs in the Gulf of Alaska across seven years to evaluate the effects of MHWs on hatch phenology, size-at-age, and daily growth and identify potential regulatory mechanisms. Hatch dates were, on average, 19 days earlier since the onset of MHWs, shifting a mean of 15 days earlier per 1 ℃ increase. Size-at-capture was larger during & between MHWs but, contrary to expectations, larvae grew slower and were smaller in size-at-age. The larger size during & between MHWs can be entirely explained by older ages due to earlier hatching. Daily growth variation was well-explained by an interaction among age, temperature, and hatch date. Under cool conditions, early growth was fastest for the latest hatchers. However, this variation converged at warmer temperatures, due to faster growth of earlier hatchers. Stage-specific growth did not vary with temperature, remaining relatively similar from 4 to 8 ℃. Temperature-related demographic changes were more predictable based on phenological shifts rather than changes in growth, which could affect population productivity after MHWs.

The impact of advection on a Subarctic fjord food web dominated by the copepod Calanus finmarchicus

Fjord and shelf food webs are frequently supplemented by the advection of external biomass, which in high-latitude seas often comes in the form of lipid-rich copepods that can support a wide range of fish species, including Northeast Arctic cod (Gadus morhua). A seasonal match or mismatch at the lower trophic levels (phytoplankton and zooplankton) is central in determining how much energy and biomass is available for higher trophic levels (fish). Here, we quantify the inflow of the copepod Calanus finmarchicus into the Vestfjorden fjord system using high-resolution measurements of ocean currents and zooplankton (laser optical plankton counter). We evaluate a spatio-temporal match/mismatch between the phytoplankton bloom and Calanus and assess the input of advected copeods at the lower trophic level fjord and shelf food web based on an integrative approach employing stable isotope analyses (C, N), fatty acid trophic marker analyses, and biovolume spectrum analyses. Our results suggest two different sources of the Calanus population in the fjord/shelf system: one fraction overwintered locally and started ascending early to feed on the phytoplankton bloom that peaked around April 11. The other fraction had only recently (end of April) been and still was being advected from the oceanic overwintering habitats. Ca. 119 g C/s of Calanus were advected into the fjord, comparable to the biomass of Calanus advected into an Arctic fjord, and the mesozooplankton community was dominated by the copepod. The fjord food web was tightly coupled between the phytoplankton spring bloom, the local part of the Calanus population (trophic level 1.8–2.4) and cod larvae (high levels of wax esters). On the shelf, our results suggest that the impact of advected Calanus in the food web is at its starting point (low trophic level, large difference of δ13C of POM and Calanus). We highlight important factors that can contribute to the successful spawning of Northeast Arctic cod: an extended phytoplankton bloom that can support both locally and advected Calanus, which in turn can supply the essential nauplii prey for first-feeding cod larvae.

Annual, seasonal and spatial differences in the growth rate of Baltic cod larvae and early juveniles in relation to zooplankton biomass fluctuations in 2006-2014

It is an acknowledged theory that a faster growth rate, determined by various environmental drivers, may boost the survival of larval and juvenile fish. In this study we examined the differences in the growth rate of larval and juvenile cod (age 2 – 136 d; SL: 4.1 – 39.2 mm) between the years 2006 and 2014, sub-areas of the Baltic Sea (Bornholm Basin, BB; Słupsk Furrow, SF; Gdańsk Basin, GB), and seasons (spring and summer). The average growth rate for all specimens was 0.25 mm/d, with significantly lower values between 2012 and 2014 than between 2006 and 2011. A reduction in zooplankton biomass, especially the large zooplankton fraction, was observed after the 2006–2008 period, which was related to the prevailing temperature conditions in the surface layers. The reduction in zooplankton biomass was accompanied by changes in the structure of zooplankton: less T. longicornis, C. hamatus, and Pseudocalanus spp., and more Acartia spp. The results suggest that the inter-annual differences observed in zooplankton biomass and structure are likely responsible for the observed reduction in the growth rate of cod larvae between 2012 and 2014 compared to the years between 2006 and 2011. The growth rate reduction could be one of the reasons for the decline in the recruitment of eastern Baltic cod between the years 2013 and 2015, after the high recruitment years of the period 2011-2012.

Static magnetic fields reduce swimming activity of Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae

Abstract Static magnetic fields (B-fields) are introduced in marine ecosystems by anthropogenic sources such as subsea power cables. Larvae of the gadoids Atlantic haddock (Melanogrammus aeglefinus) and Atlantic cod (Gadus morhua) disperse over the continental shelf and may encounter B-fields, which could alter their behavior. We tested the behavior of 184 gadoid larvae while they were being exposed to a B-field (22–156 μT), which is in the intensity range of that produced by a high-voltage direct current cable, for a duration of 10 min in a raceway tank. We present a reanalysis of published data on 92 haddock larvae combined with newly collected data on 92 cod larvae tested under the same settings. Exposure did not affect the spatial distribution of the larvae. Both species displayed the same proportion of nonexploratory (78%) and exploratory (22%) individuals. Exposure significantly reduced the average swimming speed of the nonexploratory larvae by 32% and the average acceleration by 30%. Exploratory larvae were not affected. These results indicate that the swimming behavior of a high proportion of gadoid larvae would be reduced by weak static B-fields. Consequences of exposure on dispersal and survival at the population scale awaits further work using biophysical-coupled models.

Using hydraulics to evaluate ecological benefits, risks, and trade‐offs from engineered flooding

AbstractRegulation and water extraction have reduced river flow worldwide. In some rivers, this has led to reductions in the frequency and duration of floodplain inundation and, in turn, declining condition of flood‐dependent vegetation. One management response has been the construction and modification of infrastructure, or ‘regulators’, to engineer floodplain inundation at discharges that do not otherwise cause overbank flooding. Such actions have the potential to benefit floodplain vegetation, yet the atypical hydrodynamics of engineered floods may threaten riverine ecological processes and biota and increase the risk of adverse water quality. We investigated the influence of engineered flooding, and associated reductions to in‐channel hydraulics, on key ecological processes. Specifically, the transport and retention of larvae of an iconic fish species (Murray Cod, Maccullochella peelii), and a common microinvertebrate Trichocerca. Analysis of different flow management scenarios for floodplain inundation indicates that maintaining in‐channel velocities greater than 0.2 m s−1 produces higher Trichocerca densities, while velocities greater than 0.3 m s−1 creates suitable habitat for Murray cod larvae, thereby providing targets for avoiding detrimental effects of engineered floods on in‐channel biota. To evaluate responses to regulator operation, velocity was represented by a hydrological model using pre‐computed results from detailed hydrodynamic models. These results were used to relate upstream discharge and downstream water level to the proportion of each reach in the hydrological model meeting each velocity criterion. The model also represents changes in discharge, inundated area and a water quality parameter, dissolved oxygen. Model scenarios for engineered floodplain inundation were used to demonstrate the potential benefits, impacts and trade‐offs between the different metrics identified. The model framework enables a more holistic evaluation of infrastructure operation, extending analysis beyond discharge and inundated area to risks and benefits to key indicators of the ecosystem. This refined integrated approach to the management of regulated river systems may become critical in the future where water resources are projected to further decline under a changing climate.

Acclimatizing laboratory-reared hatchling cod (Gadus morhua) to salinity conditions in the Baltic Sea

Decades of overfishing and unsustainable management, together with habitat degradation and eutrophication, depleted the cod (Gadus morhua) stocks in the Baltic Sea. Accompanying severe oxygen deficiency and decreased salinity in their spawning grounds restricted successful spawning today to the Bornholm basin, resulting in decreased recruitment. In order for the species to recover, several different measures are required and proposed, among others restocking. We therefore investigated the possibility of producing laboratory-reared cod larvae acclimatized to the current environment in the Baltic Sea. For this, cod were reared from newly fertilized eggs to non-feeding yolk-sac larvae, testing the effect of different salinity reduction treatments during early development on mortality, hatching success, and neutral buoyancy. The results show that a sudden ambient salinity decrease after hatching has no strong effect on survival or hatching (around 60% and 95%, respectively), while it decreased neutral buoyancy of larvae from 18 to minimum 12.5 psu. Lowest buoyancy was reached in treatments with a salinity change in the early egg stage. Gradual salinity decrease starting early in the egg stage yielded to significantly increased mortality and reduced hatching success, but also lowest buoyancy of 12 psu. We showed that a decrease of ambient salinity enables the production of yolk-sac cod larvae with reduced buoyancy, which are potentially better acclimatized to survive in current environmental conditions in the Baltic Sea.

Long-Term Influence of PCB- and PBDE-Spiked Microplastic Spheres Fed through Rotifers to Atlantic Cod (Gadus morhua) Larvae.

Omnipresent microplastics (MPs) in marine ecosystems are ingested at all trophic levels and may be a vector for the transfer of persistent organic pollutants (POPs) through the food web. We fed rotifers polyethylene MPs (1-4 µm) spiked with seven congeners of polychlorinated biphenyls (PCBs) and two congeners of polybrominated diphenyl ethers (PBDEs). In turn, these rotifers were fed to cod larvae from 2-30 days post-hatching (dph), while the control groups were fed rotifers without MPs. After 30 dph, all the groups were fed the same feed without MPs. Whole-body larvae were sampled at 30 and 60 dph, and four months later the skin of 10 g juveniles was sampled. The PCBs and PBDEs concentrations were significantly higher in MP larvae compared to the control larvae at 30 dph, but the significance dissipated at 60 dph. Expression of stress-related genes in cod larvae at 30 and 60 dph showed inconclusive minor random effects. The skin of MP juveniles showed disrupted epithelial integrity, fewer club cells and downregulation of a suite of genes involved in immunity, metabolism and the development of skin. Our study showed that POPs were transferred through the food web and accumulated in the larvae, but that the level of pollutants decreased once the exposure was ceased, possibly related to growth dilution. Considering the transcriptomic and histological findings, POPs spiked to MPs and/or MPs themselves may have long-term effects in the skin barrier defense system, immune response and epithelium integrity, which may potentially reduce the robustness and overall fitness of the fish.

Short and Long-Term Effects of Early and Late Weaning on Atlantic Cod, Gadus morhua

Studies have shown negative short-term effects of early weaning (EW) in finfish larvae but information on long-term effects of EW on growth and subsequent economic loss is lacking. We evaluated the short- and long-term effects of EW and late weaning (LW) on Atlantic cod. Cod larvae were fed with enriched rotifers from 2 to 35 days post-hatch (dph) and weaning carried out from 21 to 35 dph (EW) or with enriched rotifers from 2 to 29 dph followed by enriched Artemia nauplii from 25 to 56 dph and weaning carried out from 45 to 56 dph (LW). At 190 dph, 50 fish from each tank were tagged with an electronic tag and were transferred to sea cages at 10 months old. At the end of 30 months post-hatch, the weight of the fish was recorded. Our results showed a significant short-term effect of the weaning method on the growth of Atlantic cod at 65 dph, but no significant difference at 90 and 190 dph. However, fish from LW showed a significantly higher body weight compared to fish from EW at 30 months post-hatch. A cost analysis indicated substantial benefit for commercial cod farming by using LW and we recommend using LW to gain sizable financial benefit.

Half a century of high-latitude fisheries oceanography research on the “recruitment problem” in Northeast Arctic cod (Gadus morhua)

AbstractPredicting recruitment in a reliable fashion is a great challenge within operational fisheries advice. Here, we consulted the unique but undercommunicated IMR Cod Larva Project (1975–1990), its spin-offs, placed in an international era of advancements over the last 50 years to glance into the future. Few initiatives of this kind have applied such extensive research approaches, spanning from laboratory, mesocosm, tank, and field studies to process modelling. The “critical period” concept appeared misleading, covering months rather than days of the early life history stages (ELHS) of Northeast Arctic cod. Larval feeding success was strongly modified by improved encounter rates from wind-induced turbulence. In addition, the following maternal effect studies evidenced that the dynamics of stock demography prior to spawning should be upheld to promote recruitment success. Although we now have lower-trophic level models as well as ELHS individual-based models, such models are still insufficiently reflecting the needed spatiotemporal resolution. The same problem applies to climate/circulation models. Nevertheless, this long-lasting research has significantly improved the mechanistic understanding of ELHS dynamics but also of the more predictable adult reproductive parameters. Based on a “to-list list,” we suggest research avenues that should be pursued to further improve our ability predicting recruitment strength in marine fish stocks.

Optimizing intensive culture protocols for Atlantic cod (Gadus morhua) larvae

Larval cod require live prey as food, and prey concentration (PC) and feeding frequency (FF) often affect their growth and survival. Apart from this, water exchange rates/water flow (WER/WF) and water current (WC) also affect the prey resident time in the tank and larval/early juvenile behaviour, respectively. High water current is also known to induce stress in finfish larvae, and this stress response is believed to be dependent on larval developmental stage. Thus, we conducted a study to evaluate three larval rearing protocols varying in prey concentration, feeding frequency, and water current/exchange rate. Three protocols were used: low prey concentration (PC), low feeding frequency (FF), and low water flow (protocol 1); medium PC, medium FF, and medium WF (protocol 2); and high PC, high FF, and high WF (protocol 3) (see Table 1). Larvae were sampled periodically for growth and cortisol measurements. Cortisol extraction and radioimmunoassay (RIA) were conducted using methods previously validated for cod larvae. Larvae reared using protocols 2 and 3 were significantly bigger and heavier than larvae reared using protocol 1. Rearing protocol had a significant effect on the cortisol level in larval cod. Larvae showed a developmental stage-dependent stress response. Protocol 2 had significantly higher survival than protocols 1 and 3. Our results indicate that an intermediate PC, FF, and WF (protocol 2) is suitable during cod larval rearing.

Atlantic cod (Gadus morhua) larvae are attracted by low-frequency noise simulating that of operating offshore wind farms

The number and size of offshore wind (OW) turbines is increasing rapidly. OW turbines produce continuous, low-frequency noise that could impact marine fish dispersing/migrating through the facilities. Any such impact would be relevant for larval stages, which have limited possibility to swim away from OW facilities. If directional movement of fish larvae at sea is impacted by low-frequency continuous sound is unknown. We observe the behavior of Atlantic cod larvae (N = 89) in response to low-frequency sound while they are drifting in a Norwegian fjord inside transparent drifting chambers. We transmit 100 Hz continuous sound in the fjord, in the intensity range of OW turbines’ operational noise, and measure the sound pressure and 3-D particle motion. Half of the larvae (N = 45) are exposed to low-frequency (100 Hz) continuous sound, while the other half (N = 44) are observed under the same conditions but without the sound. Exposure does not affect the routine and maximum swimming speeds or the turning behavior of the larvae. Control larvae orient to the northwest. In contrast, exposed larvae orient towards the source of low-frequency sound and particle motion. This provides a basis to assess how OW might impact dispersal in this species.

Latitudinally distinct stocks of Atlantic cod face fundamentally different biophysical challenges under on‐going climate change

AbstractThe reproductive success of marine ectotherms is especially vulnerable in warming oceans due to alterations in adult physiology, as well as embryonic and larval survival prospects. These vital responses may, however, differ considerably across the species' geographical distribution. Here we investigated the life history, focusing on reproductive ecology, of three spatially distant populations (stocks) of Atlantic cod (Gadus morhua, Gadidae) (50–80° N), in the Irish/Celtic Seas‐English Channel Complex, North and Barents Seas, under past and projected climate. First, experimental tracking of spawning behaviour evidenced that the ovulation cycle is highly distressed at ≥9.6 (±0.25)°C (Tup). This knife‐edge threshold resulted in erratic spawning frequencies, whereas vitellogenin sequestration remained unaffected, indicating endocrine rather than aerobic scope constraints. Cod in the Celtic Sea‐English Channel are, therefore, expected to show critical stock depensation over the next decades as spawning grounds warm above Tup, with Irish Sea cod subsequently at risk. Second, in the relatively cooler North Sea, the northward retraction of Calanus finmarchicus (Calanidae) and Para‐Pseudocalanus spp. (Clausocalanidae) (1958–2017) limit cod larvae feeding opportunities, particularly in the southernmost subarea. However, the contrasting increase in Calanus helgolandicus (Calanidae) does not counteract this negative effect, likely because cod larvae hatch ahead of its abundance peaks. Overfishing again comes as a twin effect. Third, in the still relatively cold Barents Sea, the sustainably harvested cod benefit from improved food conditions in the recent ice‐free polar region but at the energetic cost of lengthier and faster spawning migrations. Consequently, under climate change local stocks are stressed by different mechanistic factors of varying management severity.

Aquaculture rearing systems induce no legacy effects in Atlantic cod larvae or their rearing water bacterial communities

The microbial rearing quality influences the survival of marine larvae. Microbially matured water treatment systems (MMS) provide a more favourable rearing water microbiome than flow-through systems (FTS). It has previously been hypothesised, but not investigated, that initial rearing in MMS leaves a protective legacy effect in Atlantic cod larvae (Gadus morhua). We tested this hypothesis through a crossover 2 × 2 factorial experiment varying the rearing water treatment system (MMS vs FTS) and the microbial carrying capacity (+ /− added organic matter). At 9 days post-hatching, we switched the rearing water treatment system. By comparing switched and unswitched rearing tanks, we evaluated if legacy effects had been established in the larvae or their surrounding rearing water bacterial community. We analysed the bacterial communities with flow cytometry and 16S rRNA gene sequencing. We found no evidence that the initial rearing condition left a legacy effect in the communities by evaluating the bacterial community diversity and structure. Instead, the present rearing condition was the most important driver for differences in the rearing water microbiota. Furthermore, we found that MMS with high microbial carrying capacity appeared to seed a stable bacterial community to the rearing tanks. This finding highlights the importance of keeping a similar carrying capacity between the inlet and rearing water. Moreover, we reject the hypothesis that the initial rearing condition leaves a protective legacy effect in larvae, as the larval survival and robustness were linked to the present rearing condition. In conclusion, our results highlight the importance of maintaining a beneficial microbial rearing environment from hatching and throughout the larval rearing period.

Warming Accelerates the Onset of the Molecular Stress Response and Increases Mortality of Larval Atlantic Cod.

Temperature profoundly affects ectotherm physiology. Although differential thermal responses influence fitness, thus driving population dynamics and species distributions, our understanding of the molecular architecture underlying these responses is limited, especially during the critical larval stage. Here, using RNA-sequencing of laboratory-reared Atlantic cod (Gadus morhua) larvae of wild origin, we find changes in gene expression in thousands of transcripts consistent with a severe cellular stress response at both ambient and projected (+2°C and+4°C) temperatures. In addition, specific responses to stress, heat, and hypoxia were commonly identified in gene ontology enrichment analyses and 33 of the 44 genes comprising the minimum stress proteome of all organisms were upregulated. Earlier onset of the stress response was evident at higher temperatures; concomitant increased growth and mortality suggests a reduction in fitness. Temporal differences in gene expression levels do not correspond to differences in growing degree days, suggesting negative physiological consequences of warming beyond accelerated development. Because gene expression is costly, we infer that the upregulation of thousands of transcripts in response to warming in larval cod might act as an energetic drain. We hypothesize that the energetically costly stress response, coupled with increased growth rate at warmer temperatures, leads to faster depletion of energy reserves and increased risk of mortality in larval cod. As sea surface temperatures continue to rise over the next century, reduced fitness of Atlantic cod larvae might lead to population declines in this ecologically and socioeconomically important species. Further, our findings expand our understanding of transcriptomic responses to temperature by ectothermic vertebrate larvae beyond the critical first-feeding stage, a time when organisms begin balancing the energetic demands of growth, foraging, development, and maintenance. Linking the molecular basis of a thermal response to key fitness-related traits is fundamentally important to predicting how global warming will affect ectotherms.

Effect of diet on molecular relationships between Atlantic cod larval muscle growth dynamics, metabolism, and antioxidant defense system

We studied molecular effects (RNAseq and qPCR) of first feeding prey types (copepods or rotifers/Artemia) on skeletal muscle myogenesis and growth dynamics (proliferation, differentiation), metabolism (glycolysis, gluconeogenesis, oxidative phosphorylation), and antioxidant defense system (production/regulation of reactive oxygen species (ROS) in cod (Gadus morhua) larval skeletal muscle. Larval somatic growth rates were significantly higher in copepod fed larvae, although shifts in gene expressions related to muscle growth dynamics between hypertrophy and hyperplasia and generation and regulation of ROS mostly occurred around 5-, 10-, and 15-mm standard length (SL) for both groups. Gene expression for cell membrane proteins (such as nox1 and igf1r) peaked at 7 mm SL in all larvae, corresponding with increased ROS expressions in cod muscle during the exponential stratified hyperplasia phase from 7 mm SL. Expression for muscle differentiation (mef2a) occurred continuously (strongest from 10 mm SL). Expressions for muscle proliferation (pcna) and hydrogen peroxide (H2O2) generation (sod1 and sod2) occurred in the 5 - 15 mm SL range, peaking at 10 mm SL in all larvae. A downregulation of sod1 and sod2 in skeletal muscle from 15 mm SL indicated the first response of the defense antioxidant system. Gene expressions related to glucose metabolism (slc2A11, pfk, fpb2, ldha) was 3 - 10 times higher in copepod-fed larvae than in rotifer/Artemia-fed larvae between 7 – 10 mm (live prey period). Copepods move faster than rotifers, and cod larvae will also gradually increase their active swimming periods, due to less viscous forces. Active swimming during the strongest muscle stratified hyperplasia phase (7 – 10 mm SL) could promote a better delivery and transport across the muscle membrane and intracellular flux through glycolysis and oxidative phosphorylation and would contribute to the observed earlier and more effective glucose metabolism in the larvae fed copepods. We suggest that active swimming is an important factor promoting cod larval muscle growth, especially during the strongest muscle hyperplasia phase between 7 and 10 mm SL. The rapid movements of copepods and better nutritional composition could play important roles in stabilizing ROS levels, promoting high swimming activities and enhancing long-term muscle growth in cod.

Immune response of pacific cod larvae stimulated with PolyI:C

Pacific cod (Gadus macrocephalus), as one of the most important economic fish, was seriously infected by the nervous necrosis virus (NNV), especially in the larvae stage. PolyI:C is an effective stimulus of the antivirus system, but whether it can work in the larvae stage is still unknown. In this study, the transcriptomic profiles of 10 day-post hatching (dph) cod larvae challenged with polyI:C was analysed using next-generation sequencing technology. After assembly and annotation, a total of 77,562 unigenes were acquired and 780 differentially expressed genes (DEGs) were identified, including 508 upregulated and 272 downregulated genes. The DEGs were involved in diverse pathways including protein processing in endoplasmic reticulum, peroxisome, carbon metabolism, fatty acid metabolism, and PPAR signaling pathway. Gene expression patterns of five immune relevant genes belonging to IFN signal pathway, such as TLR3, IRF3, MDA5, IPS1 and ATG5, were detected using qPCR. The transcript levels of TLR3, IRF3, MAD5 and IPS1 in cod larvae seems very low, but these genes were upregulated significantly 48h post-challenged with polyI:C, while ATG5 was downregulated 12h after polyI:C challenged. The results indicated that IFN system of cod larvae can be induced by polyI:C, thus considering polyI:C as a potential antivirus agent for cod larvae.

Resource partitioning may limit interspecific competition among Arctic fish species during early life

Arctic cod (Boreogadus saida) strongly dominates the ichthyoplankton assemblages of High Arctic seas, hence competition with other native species seldom has been studied. Yet, interspecific competition could negatively impact the survival of early life stages of fishes in Arctic areas where higher diversity prevails. We surveyed the ichthyoplankton community of the Greenland Sea, in August–September 2017. Gadids (mostly Arctic cod, with a low number of ice cod Arctogadus glacialis) and non-gadids (bigeye sculpin Triglops nybelini and gelatinous snailfish Liparis fabricii) co-dominated age-0 fish assemblages. Here, we document their diet, prey selectivity, horizontal and vertical distributions as well as that of their prey to assess resource partitioning and the potential for interspecific competition. All fish species occupied the top 30 m of the water column, but Arctic cod occurred in highest abundances over the continental slope, whereas other species distributed almost exclusively over the continental shelf. A particle track analysis suggests that Arctic cod larvae could have hatched in the open waters of the Northeast Water Polynya, drifted with the East Greenland Current, and benefited from the high secondary production associated with these oceanographic features. The diet of gadids did not overlap significantly with the diet of non-gadids, but strong selectivity for Pseudocalanus spp. and Calanus spp. copepodites among the larvae suggests potential competition for these key prey items, although limited by size partitioning of the prey. We thus conclude that interspecific competition among early life stages of Arctic fishes is limited for now. However, changing conditions and the northward range expansion of boreal species following climate change could increase competition and, in turn, negatively affect the recruitment of Arctic ichthyoplankton.

The interactive effects of temperature and food consumption on growth of larval Arctic cod (Boreogadus saida)

Understanding larval growth, mediated by the interaction of early life traits and environmental conditions, is crucial to elucidate population dynamics. We used a bioenergetic model as an integrative tool to simulate the growth of Arctic cod (Boreogadus saida) larvae and to test the sensitivity of modeled growth to temperature and food quantity and quality. The growth was computed as the energy gained through food consumption minus the energy lost through respiration and other metabolic processes. We extended a previously published bioenergetic model to cover the full range of larval length and used a simplified feeding module. This simplification allowed us to build a predictive tool that can be applied to larval Arctic cod at a large spatial scale. Our model suggested that with subzero temperatures in the High Arctic, larvae need to increase food consumption in order to reach the observed length-at-age in late summer. The modeled growth agreed well with the field observations in the High Arctic but was 2–3 times higher than the laboratory-derived growth rate, probably due to differences in food type and selective mortality. Our study reveals important knowledge gaps in our understanding of larval cod growth in the High Arctic, including the lack of empirical estimations of daily ration and respiration for larvae under the natural habitat temperatures.

Two Decades of Match-Mismatch in Northeast Arctic Cod – Feeding Conditions and Survival

The successful recruitment of Northeast Arctic (NEA) cod is thought to depend on sufficient and suitable prey for the newly hatched larvae, in particular the nauplii stages of the lipid-rich calanoid copepod species Calanus finmarchicus. The role of spatial and temporal variations in prey availability in combination with temperature and other factors in influencing growth and survival of cod larvae is, however, incompletely understood. By combining an individual based model for NEA cod larvae at the Norwegian coast with a high-resolution ocean model and a nutrient-phytoplankton-zooplankton-detritus model providing 18 years of daily environmental conditions and prey availability we assessed larval growth and survival until they settle in their feeding habitat in the Barents Sea in early fall. We find on average a two-week delay from the peak timing of first-feeding cod larvae to the peak in prey availability. In warm years, more larvae experience food limitation than in normal years. The positive effects of high temperature on growth, survival and ultimately recruitment are nonetheless larger than the negative effects of food limitation. Food limitation mainly affects larvae spawned in southern areas or late in the spawning season as these larvae experience the highest temperatures and have the highest energy requirements. Our findings highlight the spatial and temporal differences in mechanisms that regulate growth and survival of early life stages of NEA cod and suggest that spatially resolved data may be essential for understanding match-mismatch dynamics.

Commensal and Opportunistic Bacteria Present in the Microbiota in Atlantic Cod (Gadus morhua) Larvae Differentially Alter the Hosts' Innate Immune Responses.

The roles of host-associated bacteria have gained attention lately, and we now recognise that the microbiota is essential in processes such as digestion, development of the immune system and gut function. In this study, Atlantic cod larvae were reared under germ-free, gnotobiotic and conventional conditions. Water and fish microbiota were characterised by 16S rRNA gene analyses. The cod larvae’s transcriptional responses to the different microbial conditions were analysed by a custom Agilent 44 k oligo microarray. Gut development was assessed by transmission electron microscopy (TEM). Water and fish microbiota differed significantly in the conventional treatment and were dominated by different fast-growing bacteria. Our study indicates that components of the innate immune system of cod larvae are downregulated by the presence of non-pathogenic bacteria, and thus may be turned on by default in the early larval stages. We see indications of decreased nutrient uptake in the absence of bacteria. The bacteria also influence the gut morphology, reflected in shorter microvilli with higher density in the conventional larvae than in the germ-free larvae. The fact that the microbiota alters innate immune responses and gut morphology demonstrates its important role in marine larval development.