Cod Larvae Research Articles

Consequences of differences among readers in age estimations of Baltic cod larvae and early juveniles for growth rate and hatch date analysis. Is more experience always better?

We evaluated the precision (i.e., reproducibility of estimations) and accuracy (i.e., reflection of true age) of ageing larval and early juvenile Baltic cod, Gadus morhua, (4.1–33 mm SL) from sagittae and lapilli by inexperienced and experienced readers and an expert. For all three readers, lapilli provided higher precision and accuracy of age estimates than did sagittae. Moreover, with the used method of unpolished otoliths analysis, lapilli allowed ageing juveniles larger (up to 33 mm SL) than did sagittae (up to approximately 18 mm SL). With regard to the level of experience of the reader ageing larval fish, the highest precision in ageing was achieved by the expert and experienced reader, and the lowest by the inexperienced reader. Interestingly, reader experience was not reflected in the accuracy of estimates as it was higher with the inexperienced reader than with the experienced one. Differences in ageing among readers had significant consequences for growth rate analysis (slope of regression line fitted to SL-at-age data). The results obtained covered a wide range of values, from 0.11 mm day−1 to 0.45 mm day−1 for sagittae and from 0.21 mm day−1 to 0.68 mm day−1 for lapilli. The consequences of ageing error for back-calculated hatching dates were statistically significant as well. It may be therefore recommended to use lapilli for cod larvae and early juveniles ageing. Moreover, it is recommended not to make a quick assumption that ageing done by more experienced reader must provide more accurate results that better reflect true age, even if precision of the increment counts is for the more experienced reader high.

The basal food sources for Murray cod (Maccullochella peelii) in wetland mesocosms

The use of freshwater for human consumption and agriculture has resulted in many wetland ecosystems being some of the most seriously impacted ecosystems in the world. In attempts to rehabilitate wetland ecosystems environmental flows are used to restore parts of the hydrological regime altered by human water use. The use of environmental water intends to improve ecosystem health, but frequently aims to have specific outcomes for populations of higher-order consumers such as iconic fish and bird species. To date, research and monitoring has mainly focused on understanding ‘flow-ecology’ relationships, without investigating the mechanisms underlying them. We sought to understand the importance of different basal food sources to the growth of the endangered Murray cod in temporary wetland systems using fatty acid biomarkers. We flooded replicate mesocosms with two different wetland soils to produce sufficient zooplankton prey to sustain and grow Murray cod larvae for approximately 2 weeks. The fatty acid profiles of Murray cod and percentages of different biomarkers were compared at the start and finish of the experiment and our results suggest that the most important basal food source is green algae. However, the biomarkers of diatoms, cyanobacteria and bacteria also increased and differed between wetlands with different hydrological regimes. It is unclear if our results can be extended to other wetland systems and we encourage further research both into the relationship between length of wetland flooding and invertebrate densities in other systems. We also encourage research into the mechanistic pathways in which green algae carbon is transferred through food webs to higher order consumers in wetland systems to help generalise our results to other wetlands and support the management of wetlands through the timing and duration of flooding from environmental water.

Transcriptome profiling reveals exposure to predicted end-of-century ocean acidification as a stealth stressor for Atlantic cod larvae

Ocean acidification (OA), a direct consequence of increasing atmospheric CO2 concentration dissolving in ocean waters, is impacting many fish species. Little is known about the molecular mechanisms underlying the observed physiological impacts in fish. We used RNAseq to characterize the transcriptome of 3 different larval stages of Atlantic cod (Gadus morhua) exposed to simulated OA at levels (1179 µatm CO2) representing end-of-century predictions compared to controls (503 µatm CO2), which were shown to induce tissue damage and elevated mortality in G. morhua. Only few genes were differentially expressed in 6 and 13 days-post-hatching (dph) (3 and 16 genes, respectively), during a period when maximal mortality as a response to elevated pCO2 occurred. At 36 dph, 1413 genes were differentially expressed, most likely caused by developmental asynchrony between the treatment groups, with individuals under OA growing faster. A target gene analysis revealed only few genes of the universal and well-defined cellular stress response to be differentially expressed. We thus suggest that predicted ocean acidification levels constitute a “stealth stress” for early Atlantic cod larvae, with a rapid breakdown of cellular homeostasis leading to organismal death that was missed even with an 8-fold replication implemented in this study.

Polar cod egg and larval drift patterns in the Svalbard archipelago

Spawning of polar cod (Boreogadus saida) in the vicinity of the Svalbard archipelago has not been directly observed. From the distribution pattern of polar cod 0-group observed during annual monitoring of the Barents Sea, it has, however, been inferred that spawning occurs in Svalbard waters most years. We wanted to investigate the possibility of back-tracking the larvae from these observed distribution areas to the spawning areas from which they originated and applied a coupled ocean–sea ice and particle tracking model to simulate the drift of particles released at suggested spawning sites. The model was run for 1 year (December 2006–September 2007), and the results were compared to observations of polar cod larvae in the autumns of 2007 and 2004–2010. The particles released in the western fjords were mostly retained in the fjords. For the rest of the suggested spawning grounds, the released particles drifted mostly clockwise around the archipelago. Model runs mainly indicated a drift pattern with end points that qualitatively match the main features of the August–September distribution of the polar cod 0-group observed. We conclude that the suggested spawning sites on the southern, northern and eastern sides of Svalbard could have caused a distribution of 0-group polar cod similar to that observed during August–September 2007. From the environmental factors experienced during drift of eggs and larvae and assumptions about habitat suitability for survival and growth, we conclude that spawning in the Svalbard area probably occurs on the southern and eastern sides and later than the area in the southeastern Barents Sea.

Effects of PUFA-enriched Artemia on the early growth and fatty acid composition of Murray cod larvae

The Murray cod (Maccullochella peelii peelii), an iconic Australian freshwater fish, is subject to a small but growing aquaculture sector in Australia. Optimal larviculture techniques for this species have yet to be established, particularly with regard to the nutritional quality of starter diets. As such, optimisation of current Murray cod hatchery practices and feeding regimes is essential to maximize the production of high quality larvae and expand commercial aquaculture prospects. The aim of this investigation was to assess the effects of Artemia nauplii enriched with high levels of long-chain polyunsaturated fatty acids (LC-PUFA; ENR treatment), on the growth and fatty acid composition of larval Murray cod. Using untreated Artemia nauplii as a control treatment (CTL), larvae were fed either treatment to apparent satiation twice daily in triplicate groups of 700 fish with initial mean weights of 0.012 ± 0.06 g for 21 days at 19.85 °C. Survival was not significantly different between treatments (P > .05). Final mean weight and length, as well as specific growth rate, were significantly higher for ENR compared to CTL. Enriched larvae also contained significantly higher total lipid contents and across all three of the major fatty acid groups. Of the individual fatty acids, increases in ARA (20:4n-6) and DHA (22:6n-3) were recorded in the ENR fish, while CTL fish contained high concentrations of 18:4n-3, 20:4n-3 and 22:5n-3, which are intermediate fatty acids along the n-3 conversion pathway, suggesting desaturation and elongation toward EPA (20:5n-3) and DHA. Results demonstrate that enriched Artemia nauplii are vastly superior to untreated Artemia as a starter diet for optimal growth and fatty acid composition of Murray cod larvae. Thus, despite the clear ability of Murray cod larvae to undergo elongation and desaturation of ALA (18:3n-3), the present study demonstrates that this is not sufficient to sustain growth or LC-PUFA tissue levels akin to those possible through high dietary LC-PUFA intake.

Complement component C4-like protein in Atlantic cod (Gadus morhua L.) - Detection in ontogeny and identification of post-translational deimination in serum and extracellular vesicles

The complement system is a critical part of teleost immune defences, with complement component C4 forming part of the classical and lectin pathways. Cod C4-like protein was isolated from plasma, specific antibodies generated and C4-like protein was assessed in cod sera, mucus and in extracellular vesicles (EVs) from serum and mucus. Higher levels of C4-like protein were detected in serum- than mucus-derived EVs. Post-translational deimination, caused by conversion of arginine into citrulline, can affect protein structure and function. Here we detected deiminated forms of C4-like protein in cod serum and at lower levels in mucus. C4-like protein was also found in deiminated form at low levels in EVs from both serum and mucus. C4-like protein was assessed by immunohistochemistry in cod larvae and detected in a range of organs including in liver, kidney, gut, muscle, skin and mucus, as well as in neuronal tissues of the brain, spinal cord and eye. This abundance of C4-like protein during early development may indicate roles in tissue remodelling, in addition to immune defences. The presence of deiminated C4-like protein in serum and mucosa, as well as in EVs, may suggest C4 protein moonlighting via post-translational deimination.

Elevated CO2 alters behavior, growth, and lipid composition of Pacific cod larvae

High-latitude seas, which support a number of commercially important fisheries, are predicted to be most immediately impacted by ongoing ocean acidification (OA). Elevated CO2 levels have been shown to induce a range of impacts on the physiology and behavior of marine fish larvae. However, these responses have yet to be characterized for most fishery species, including Pacific cod (Gadus macrocephalus). Based on laboratory experiments, we present a multi-faceted analysis of the sensitivity of Pacific cod larvae to elevated CO2. Fish behavior in a horizontal light gradient was used to evaluate the sensitivity of behavioral phototaxis in 4–5 week old cod larvae. Fish at elevated CO2 levels (∼1500 and 2250 μatm) exhibited a stronger phototaxis (moved more quickly to regions of higher light levels) than fish at ambient CO2 levels (∼600 μatm). In an independent experiment, we examined the effects of elevated CO2 levels on growth of larval Pacific cod over the first 5 weeks of life under two different feeding treatments. Fish exposed to elevated CO2 levels (∼1700 μatm) were smaller and had lower lipid levels at 2 weeks of age than fish at low (ambient) CO2 levels (∼500 μatm). However, by 5 weeks of age, this effect had reversed: fish reared at elevated CO2 levels were slightly (but not significantly) larger and had higher total lipid levels and storage lipids than fish reared at low CO2. Fatty acid composition differed significantly between fish reared at high and low CO2 levels (p < 0.01) after 2 weeks of feeding, but this effect diminished by week 5. Effects of CO2 on FA composition of the larvae differed between the two diets, an effect possibly related more to dietary equilibrium and differential lipid class storage than a fundamental effect of CO2 on fatty acid metabolism. These experiments point to a stage-specific sensitivity of Pacific cod to the effects of OA. Further understanding of these effects will be required to predict the impacts on production of Pacific cod fisheries.

Tolerable ranges of fluid shear for early life-stage fishes: implications for safe fish passage at hydropower and irrigation infrastructure

Egg and larval fish drifting downstream are likely to encounter river infrastructure leading to mortality. Elevated fluid shear is one likely cause. To confirm this and determine tolerable strain rates resulting from fluid shear, egg and larvae of three Australian species were exposed to a high-velocity, submerged jet in a laboratory flume. Mortality was modelled over a broad range of strain rates, allowing critical thresholds to be estimated. Eggs were very susceptible to mortality at low strain rates and 100% of golden and silver perch died once strain rate exceeded 629 and 148s–1 respectively. Larvae were less vulnerable than eggs, but mortality increased at higher strain rates and at younger ages. Most ages of larvae will be protected if strain rate does not exceed 600s–1, although a lower guideline of less than 400s–1 may be needed in areas where very early stage Murray cod larvae drift. Golden perch and silver perch were not susceptible to shear once maturity reached ~25 days post-hatch (nearing juvenile metamorphosis). The thresholds described here will prove useful when refining design and operational guidelines for hydropower and irrigation infrastructure to improve fish survival.

Nekotorye svedeniya o poimkakh lichinok tikhookeanskoj treski Gadus macrocephalus u zapadnogo poberezh'ya Kamchatki v 2015 g

Nekotorye svedeniya o poimkakh lichinok tikhookeanskoj treski Gadus macrocephalus u zapadnogo poberezh'ya Kamchatki v 2015 g

Modeling the toxicity of dissolved crude oil exposures to characterize the sensitivity of cod (Gadus morhua) larvae and role of individual and unresolved hydrocarbons

Toxicity of weathered oil was investigated using Atlantic cod (Gadus morhua) larvae. A novel exposure system was applied to differentiate effects associated with dissolved and droplet oil with and without dispersant. After a 4-day exposure and subsequent 4-day recovery period, survival and growth were determined. Analytical data characterizing test oil composition included polyaromatic hydrocarbons (PAH) based on GC/MS and unresolved hydrocarbon classes obtained by two-dimensional chromatography coupled with flame ionization detection was used as input to an oil solubility model to calculate toxic units (TUs) of dissolved PAHs and whole oil, respectively. Critical target lipid body burdens derived from modeling characterizing the sensitivity of effect endpoints investigated were consistent across treatments and within the range previously reported for pelagic species. Individually measured PAHs captured only 3–11% of the TUs associated with the whole oil highlighting the limitations of traditional total PAH exposure metrics for expressing oil toxicity data.

Effects of different incubation and start-feeding temperature regimes on growth, survival, and histomorphology of cod larvae

Mean lengths of newly hatched cod larvae from egg incubation at 4 °C (Low-L) and 9.5 °C (High-H) were similar, and only minor differences were observed in larval histomorphology. However, growth performance of larvae reared at 4 °C (T1-LL) and 9.5 °C (T2-LH) from the 4 °C egg incubation and 4 °C (T3-HL) and 9.5 °C (T4-HH) from the 9.5 °C incubation group were different during start-feeding and metamorphosis. Incubation and larval rearing temperature affected developmental rate and survival. T1-LL and T2-LH larvae were larger than T4-HH larvae at sampling stage 4 (early larvae), but the differences disappeared thereafter. Larvae from T3-HL did not survive beyond stage 8 (late larvae/start metamorphosis), and survival of T1-LL larvae at the end of the experiment was very low. Larvae from T2-LH were significantly larger than larvae from T4-HH at stage 12 (end metamorphosis). Comparative studies of the histomorphology of vital organs did not reveal temperature-related differences between sampled larvae/early juveniles. Characteristic traits in the histomorphology of cod larvae at the selected developmental stages are presented. Our results show that egg incubation and subsequent larval rearing temperature affected the growth performance and survival but did not have any significant effects on the organ development and histomorphology.

Post-translational protein deimination in cod (Gadus morhua L.) ontogeny novel roles in tissue remodelling and mucosal immune defences?

Peptidylarginine deiminases (PADs) are calcium dependent enzymes with physiological and pathophysiological roles conserved throughout phylogeny. PADs promote post-translational deimination of protein arginine to citrulline, altering the structure and function of target proteins. Deiminated proteins were detected in the early developmental stages of cod from 11 days post fertilisation to 70 days post hatching. Deiminated proteins were present in mucosal surfaces and in liver, pancreas, spleen, gut, muscle, brain and eye during early cod larval development. Deiminated protein targets identified in skin mucosa included nuclear histones; cytoskeletal proteins such as tubulin and beta-actin; metabolic and immune related proteins such as galectin, mannan-binding lectin, toll-like receptor, kininogen, Beta2-microglobulin, aldehyde dehydrogenase, bloodthirsty and preproapolipoprotein A-I. Deiminated histone H3, a marker for anti-pathogenic neutrophil extracellular traps, was particularly elevated in mucosal tissues in immunostimulated cod larvae. PAD-mediated protein deimination may facilitate protein moonlighting, allowing the same protein to exhibit a range of biological functions, in tissue remodelling and mucosal immune defences in teleost ontogeny.

Effects of temperature and food availability on the survival and growth of larval Arctic cod (Boreogadus saida) and walleye pollock (Gadus chalcogrammus)

Abstract Arctic cod (Boreogadus saida) is an ecologically significant species that is uniquely adapted to occupy ice edges, but warming and loss of sea ice are hypothesized to favour more facultative gadids, such as walleye pollock (Gadus chalcogrammus). To test this hypothesis, we experimentally measured the growth and survival of Arctic cod and walleye pollock at two larval stages across a range of temperature and food conditions in the laboratory. Results indicated early and late-stage Arctic cod larvae have a competitive growth and survival advantage over walleye pollock at low temperatures. However, these advantages are lost under warmer, food-productive conditions where walleye pollock larvae survived and experienced accelerated growth rates. Growth models developed from this study emphasize the need to account for both species- and stage-specific differences in the thermal response of closely related marine fish larvae. More broadly, these new vital rate data provide a mechanistic framework to forecast spatial-temporal shifts of gadids at the Arctic-boreal interface resulting from climatic warming and altered productivity regimes.

Exploring the Effect of Phage Therapy in Preventing Vibrio anguillarum Infections in Cod and Turbot Larvae.

The aquaculture industry is suffering from losses associated with bacterial infections by opportunistic pathogens. Vibrio anguillarum is one of the most important pathogens, causing vibriosis in fish and shellfish cultures leading to high mortalities and economic losses. Bacterial resistance to antibiotics and inefficient vaccination at the larval stage of fish emphasizes the need for novel approaches, and phage therapy for controlling Vibrio pathogens has gained interest in the past few years. In this study, we examined the potential of the broad-host-range phage KVP40 to control four different V. anguillarum strains in Atlantic cod (Gadus morhua L.) and turbot (Scophthalmus maximus L.) larvae. We examined larval mortality and abundance of bacteria and phages. Phage KVP40 was able to reduce and/or delay the mortality of the cod and turbot larvae challenged with V. anguillarum. However, growth of other pathogenic bacteria naturally occurring on the fish eggs prior to our experiment caused mortality of the larvae in the unchallenged control groups. Interestingly, the broad-spectrum phage KVP40 was able to reduce mortality in these groups, compared to the nonchallenge control groups not treated with phage KVP40, demonstrating that the phage could also reduce mortality imposed by the background population of pathogens. Overall, phage-mediated reduction in mortality of cod and turbot larvae in experimental challenge assays with V. anguillarum pathogens suggested that application of broad-host-range phages can reduce Vibrio-induced mortality in turbot and cod larvae, emphasizing that phage therapy is a promising alternative to traditional treatment of vibriosis in marine aquaculture.

Effect of TDA-producing Phaeobacter inhibens on the fish pathogen Vibrio anguillarum in non-axenic algae and copepod systems.

SummaryThe expanding aquaculture industry plays an important role in feeding the growing human population and with the expansion, sustainable bacterial disease control, such as probiotics, becomes increasingly important. Tropodithietic acid (TDA)‐producing Phaeobacter spp. can protect live feed, for example rotifers and Artemia as well as larvae of turbot and cod against pathogenic vibrios. Here, we show that the emerging live feed, copepods, is unaffected by colonization of the fish pathogen Vibrio anguillarum, making them potential infection vectors. However, TDA‐producing Phaeobacter inhibens was able to significantly inhibit V. anguillarum in non‐axenic cultures of copepod Acartia tonsa and the copepod feed Rhodomonas salina. Vibrio grew to 106 CFU ml−1 and 107 CFU ml−1 in copepod and R. salina cultures, respectively. However, vibrio counts remained at the inoculum level (104 CFU ml−1) when P. inhibens was also added. We further developed a semi‐strain‐specific qPCR for V. anguillarum to detect and quantify the pathogen in non‐axenic systems. In conclusion, P. inhibens efficiently inhibits the fish larval pathogen V. anguillarum in the emerging live feed, copepods, supporting its use as a probiotic in aquaculture. Furthermore, qPCR provides an effective method for detecting vibrio pathogens in complex non‐axenic live feed systems.

Rearing Water Treatment Induces Microbial Selection Influencing the Microbiota and Pathogen Associated Transcripts of Cod (Gadus morhua) Larvae.

We have previously shown that K-selection and microbial stability in the rearing water increases survival and growth of Atlantic cod (Gadus morhua) larvae, and that recirculating aquaculture systems (RAS) are compatible with this. Here, we have assessed how water treatment influenced the larval microbiota and host responses at the gene expression level. Cod larvae were reared with two different rearing water systems: a RAS and a flow-through system (FTS). The water microbiota was examined using a 16S rDNA PCR/DGGE strategy. RNA extracted from larvae at 8, 13, and 17 days post hatching was used for microbiota and microarray gene expression analysis. Bacterial cDNA was synthesized and used for 16S rRNA amplicon 454 pyrosequencing of larval microbiota. Both water and larval microbiota differed significantly between the systems, and the larval microbiota appeared to become more dissimilar between systems with time. In total 4 phyla were identified for all larvae: Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. The most profound difference in larval microbiota was a high abundance of Arcobacter (Epsilonproteobacteria) in FTS larvae (34 ± 9% of total reads). Arcobacter includes several species that are known pathogens for humans and animals. Cod larval transcriptome responses were investigated using an oligonucleotide gene expression microarray covering approximately 24,000 genes. Interestingly, FTS larvae transcriptional profiles revealed an overrepresentation of upregulated transcripts associated with responses to pathogens and infections, such as c1ql3-like, pglyrp-2-like and zg16, compared to RAS larvae. In conclusion, distinct water treatment systems induced differences in the larval microbiota. FTS larvae showed up-regulation of transcripts associated with responses to microbial stress. These results are consistent with the hypothesis that RAS promotes K-selection and microbial stability by maintaining a microbial load close to the carrying capacity of the system, and ensuring long retention times for both bacteria and water in the system.

Comparative effects of temperature on rates of development and survival of eggs and yolk-sac larvae of Arctic cod (Boreogadus saida) and walleye pollock (Gadus chalcogrammus)

AbstractChanges in Arctic fish assemblages resulting from climate change will likely be determined by the differential thermal response of key species during their early life history. In this study, we incubated multiple batches of eggs and larvae of two ecologically important gadids co-occurring at the Pacific–Arctic interface, Arctic cod (Boreogadus saida) and walleye pollock (Gadus chalcogrammus). Fertilized egg batches (n = 11 Arctic cod; n = 6 walleye pollock) were collected in the late winter/early spring from laboratory broodstock held under simulated seasonal environmental conditions. Image and lipid analyses indicated that Arctic cod eggs and larvae were ∼25–35% larger than walleye pollock and had nearly 3–6× more energetic reserves. Two batches of eggs from each species were incubated in replicated containers (n = 3/batch/temperature) at −0.4, 1.2, 2.5, 3.8, 5.0, 9.0, and 12.0°C for Arctic cod and −0.8, 0.3, 2.2, 4.5, 9.0, and 12.0°C for walleye pollock. Both species had very similar low thermal tolerance, but Arctic cod were much more sensitive to higher thermal stress in terms of hatch success and size-at-hatch. For example, Arctic cod hatch success declined precipitously at temperatures above 3.5°C yet remained above 50% in walleye pollock at 9°C. Arctic cod also had significantly longer development times, such that embryos could survive for ∼4 months at temperatures &amp;lt;0°C from the time of spawning to first-feeding. Collectively, these results indicate Arctic cod have a much smaller thermal window for survival, but can survive for longer periods in the absence of food than walleye pollock at cold temperatures. These temperature-dependent rates will be useful in the development of population forecasts and biophysical transport models for these species in the northern Bering, Chukchi, and Beaufort seas.

Correlations of age and growth rate with microbiota composition in Atlantic cod (Gadus morhua) larvae

Little information is available on the link between host development (growth rate and ontogeny) and the composition of the microbiota in fish larvae. This study was carried out to examine potential correlations of microbiota composition with age and growth rate of Atlantic cod larvae. Small and large cod larvae of the same age, representing slow and fast growing individuals, were sampled 10 times during a period of 42 days post hatching (dph), and the composition of the larval microbiota was investigated using a PCR/DGGE (Denaturing Gradient Gel Electrophoresis) strategy. We found significant differences in the intestinal microbiota of small and large larvae of the same age for 4 of the 10 age stages studied. We further found that the variation in the composition of the larval microbiota was more strongly correlated to age than to growth rate for larvae up to 28 dph, whereas for the older larvae growth rate and age was equally correlated to the composition of the microbiota. These results indicate that larval development may structure the microbiota through a change in selection pressure due to host-microbe and microbe-microbe interactions, and that the composition of the microbiota may influence larval development through improved energy gain.

Some Thoughts on Estimating Change to Arctic Cod Populations from Hypothetical Oil Spills in the Eastern Alaska Beaufort Sea

We describe a fecundity-hindcast model that incorporates Arctic cod (Boreogadus saida) acute toxicity data, field studies of Arctic cod larval distribution and abundance, natural mortality estimates for Arctic cod eggs and larvae, and an oil spill fate model in Alaska Beaufort Sea. Three orders of magnitude of spill events (1000, 10 000, and 100 000 tons) were evaluated for both physically and chemically dispersed oil. Using worst-case assumptions in our model, a 100 000 ton spill of crude oil treated with dispersants resulted in 266 million m3 of water that exceeded our acute toxicity threshold, compared to a volume of 71 million m3 for a 100 000 ton spill not treated with dispersants, and resulted in exposure of about 2 million Arctic cod larvae remaining from an initial 87 million eggs. This represents the reproductive output of about 7300 adult females. Adult Arctic cod populations in the Alaska Beaufort number in the tens to hundreds of millions. The results show that even with an order of magnitude variation in exposure, the effect of dispersing a large oil spill on the regional cod population is expected to be insignificant (∼0.7%). The recent hiatus in Arctic oil and gas development affords an opportunity to acquire additional data to further strengthen this conclusion.

Changes in spatial and temporal variability of prey affect functional connectivity of larval and juvenile cod

Abstract Changes in structural connectivity as it can affect functional connectivity, the biological and behavioural responses of an organism, has been examined here over 2 contrasting years when the spatial distribution of larval and juvenile cod and their prey shifted from the flank to the crest on Georges Bank. New data on the gut contents of pelagic juvenile cod are compared with known prey distributions, potential growth and how climate warming can change connectivity in this region. Centropages spp. was the most important prey for pelagic juveniles, especially in June 1987 when they had high abundance on the crest and were dominant in the cod stomachs. In June 1986, copepod abundance was low where the juveniles were distributed along the flank. The potential growth of juvenile cod was greater in June 1987, consistent with the higher abundance of Centropages spp., and with higher recruitment survival, compared with June 1986. Annual changes in the spatial distribution of cod early life stages within the environment of cold or warm years can have different impacts on their growth and survival. Whereas the small copepods, Pseudocalanus spp., are primary prey for cod larvae and very abundant in cold years, larger copepods, Calanus finmarchicus and Centropages spp., are important prey for the pelagic juveniles and the latter species can have a high impact in warm years on the crest. The different spatial structure during cold or warm years provides an explanation why different year classes respond differently to environmental change. Depending on the presence or absence of specific prey, the functional connectivity response changes pathways that determine the growth and survival of early life stages and ultimately a role in recruitment.