Broodstock exposure to warming and elevated pCO2 impairs gamete quality and narrows the temperature window of fertilisation in Atlantic cod.

Impacts of global warming and CO2 -related ocean acidification (OA) on fish reproduction may include chronic effects on gametogenesis and gamete quality, as well as acute effects on external fertilisation. Here, temperature thresholds and OA-sensitivity of gametogenesis and fertilisation were investigated in Atlantic cod, Gadus morhua. Three broodstock groups of farmed cod (FC 1-3) were exposed for 3 months to three maturation conditions (FC 1: control, 6°C/400 µatm CO2 ; FC 2: warming, 9.5°C/400 µatm; FC 3: warming and OA, 9.5°C/1100 µatm). In addition, a broodstock group of wild cod (WC) was kept at control conditions to compare the acute temperature window of fertilisation with that of farmed cod (FC 1). Fertilisations were conducted in a temperature-gradient table at 10 temperatures (between -1.5 and 12°C) and two CO2 levels (400/1100 µatm). In FC 1 and WC, fertilisation success was relatively high between 0.5°C and 11°C (TRange of c. 10.5°C), indicating similar gamete quality in farmed and wild broodstocks kept at control conditions. Exposure of farmed broodstocks to warming (FC 2) and the combination of warming and OA (FC 3) impaired gamete quality, causing a reduction in fertilisation success of -20% (FC 2) and - 42% (FC 3) compared to FC 1. The acute temperature window of fertilisation narrowed from FC 1 (TRange  = 10.4°C) to FC 2 (TRange  = 8.8°C) and FC 3 (TRange  = 5.9°C). Acute effects of CO2 on fertilisation success were not significant. This study demonstrates potential climate change impacts on gametogenesis and fertilisation in Atlantic cod, suggesting the loss of spawning habitat in the coming decades.

Temperature preference of juvenile lumpfish (Cyclopterus lumpus) originating from the southern and northern parts of Norway.

Fish are ectothermic animals and have body temperatures close to that of the water they inhabit. They can still control their body temperatures by selecting habitats with temperatures that maximize their growth, feed conversion and wellbeing. Lumpfish, Cyclopterus lumpus, is widely distributed in the North Atlantic Ocean and therefore exposed to variable water temperatures. Lumpfish is extensively used as cleanerfish in salmon farming in Norway and exposed to a wide temperature range along the north-south axis of the Norwegian coastline. But, if these temperature ranges correspond to the preference temperatures of lumpfish is not known. If lumpfish has adapted to regional temperatures along the Norwegian coast, differences in preference temperature for fish from different regions should be evident. In a selective breeding perspective, different selection lines for preference temperature would then be useful for further development of lumpfish as a cleanerfish. We subjected lumpfish juveniles weighing 154-426g originated from northern (Group North - GN) and southern (Group South - GS) Norway to a temperature preference test, using an electronic shuttle box system. The system allowed the fish to control the water temperature by moving between two chambers, and thereby choosing its preferred temperature in the range from 5 to 16 °C. We started the temperature at 7.8 ± 1.37 °C for GN and 7.58 ± 1.34 °C for GS, but all the fish except four (two each from GN and GS) chose lower temperatures (5.03-7.6 °C) in the first 18 h and stayed closer to that temperature during the next 30 h. Based on the results, GN and GS lumpfish preferred 6.92 ± 1.8 and 6.2 ± 1.2, respectively, and there was no significant difference between the groups. Neither was there any significant difference in growth rates (SGR) between the two groups. Based on our results, we suggest that lumpfish from any geographical origin along the Norwegian coast can be used anywhere in Norway. It follows that lumpfish from a single selection line could be used at any salmon farm in Norway independent of its location.

Fish embryo vulnerability to combined acidification and warming coincides with a low capacity for homeostatic regulation.

The vulnerability of fish embryos and larvae to environmental factors is often attributed to a lack of adult-like organ systems (gills) and thus insufficient homeostatic capacity. However, experimental data supporting this hypothesis are scarce. Here, by using Atlantic cod (Gadus morhua) as a model, the relationship between embryo vulnerability (to projected ocean acidification and warming) and homeostatic capacity was explored through parallel analyses of stage-specific mortality and in vitro activity and expression of major ion pumps (ATP-synthase, Na+/K+-ATPase, H+-ATPase) and co-transporters (NBC1, NKCC1). Immunolocalization of these transporters was used to study ionocyte morphology in newly hatched larvae. Treatment-related embryo mortality until hatching (+20% due to acidification and warming) occurred primarily during an early period (gastrulation) characterized by extremely low ion transport capacity. Thereafter, embryo mortality decreased in parallel with an exponential increase in activity and expression of all investigated ion transporters. Significant changes in transporter activity and expression in response to acidification (+15% activity) and warming (-30% expression) indicate some potential for short-term acclimatization, although this is probably associated with energetic trade-offs. Interestingly, whole-larvae enzyme activity (supported by abundant epidermal ionocytes) reached levels similar to those previously measured in gill tissue of adult cod, suggesting that early-life stages without functional gills are better equipped in terms of ion homeostasis than previously thought. This study implies that the gastrulation period represents a critical transition from inherited (maternal) defenses to active homeostatic regulation, which facilitates enhanced resilience of later stages to environmental factors.

Divergent responses of Atlantic cod to ocean acidification and food limitation.

In order to understand the effect of global change on marine fishes, it is imperative to quantify the effects on fundamental parameters such as survival and growth. Larval survival and recruitment of the Atlantic cod (Gadus morhua) were found to be heavily impaired by end-of-century levels of ocean acidification. Here, we analysed larval growth among 35-36 days old surviving larvae, along with organ development and ossification of the skeleton. We combined CO2 treatments (ambient: 503 µatm, elevated: 1,179 µatm) with food availability in order to evaluate the effect of energy limitation in addition to the ocean acidification stressor. As expected, larval size (as a proxy for growth) and skeletogenesis were positively affected by high food availability. We found significant interactions between acidification and food availability. Larvae fed ad libitum showed little difference in growth and skeletogenesis due to the CO2 treatment. Larvae under energy limitation were significantly larger and had further developed skeletal structures in the elevated CO2 treatment compared to the ambient CO2 treatment. However, the elevated CO2 group revealed impairments in critically important organs, such as the liver, and had comparatively smaller functional gills indicating a mismatch between size and function. It is therefore likely that individual larvae that had survived acidification treatments will suffer from impairments later during ontogeny. Our study highlights important allocation trade-off between growth and organ development, which is critically important to interpret acidification effects on early life stages of fish.

Northern cod species face spawning habitat losses if global warming exceeds 1.5°C.

Rapid climate change in the Northeast Atlantic and Arctic poses a threat to some of the world's largest fish populations. Impacts of warming and acidification may become accessible through mechanism-based risk assessments and projections of future habitat suitability. We show that ocean acidification causes a narrowing of embryonic thermal ranges, which identifies the suitability of spawning habitats as a critical life-history bottleneck for two abundant cod species. Embryonic tolerance ranges linked to climate simulations reveal that ever-increasing CO2 emissions [Representative Concentration Pathway (RCP) 8.5] will deteriorate suitability of present spawning habitat for both Atlantic cod (Gadus morhua) and Polar cod (Boreogadus saida) by 2100. Moderate warming (RCP4.5) may avert dangerous climate impacts on Atlantic cod but still leaves few spawning areas for the more vulnerable Polar cod, which also loses the benefits of an ice-covered ocean. Emissions following RCP2.6, however, support largely unchanged habitat suitability for both species, suggesting that risks are minimized if warming is held "below 2°C, if not 1.5°C," as pledged by the Paris Agreement.

Effects of amino acid supplementations on metabolic and physiological parameters in Atlantic cod (Gadus morhua) under stress.

The effects of tryptophan (Trp) and phenylalanine (Phe) diet supplementation on the stress and metabolism of the Atlantic cod have been studied. Fish were fed diet supplemented with Trp or Phe or control diet for 1 week. At the end of the feeding trial, fish were subjected to air exposure or heat shock. Following samples of blood, liver and muscle were taken from the fish and were analyzed for stress and metabolic indicators. After an air exposure, plasma cortisol levels in fish fed with Trp and Phe diets were lower compared to the fish fed the control diet. Diets containing both amino acids increased significantly the liver transaminase activities in juvenile cod. During thermal stress, high Trp contents had significant effects on fructose biphosphatase activity though Phe did not. Overall, activities of glucose 6-phosphate dehydrogenase, pyruvate kinase, and phosphofructokinase increased significantly for both amino acid diets. For the thermal stress, fish had the highest values of those activities for the 3Trp diet. Trp content in the diet had significant effects on the transaminase activity in muscle during air stress compared to fish fed control and Phe diets. Muscle alanine transaminase activity for thermal stress in fish fed any diet was not significantly different from the control. Both Trp and Phe supplementations reduced the stress markers in the cod; hence, they could be used as additives for the stress attenuation. However, they also raised the activity of key enzymes in glycolysis and gluconeogenesis, mainly the Trp diets.

Ocean Acidification Effects on Atlantic Cod Larval Survival and Recruitment to the Fished Population.

How fisheries will be impacted by climate change is far from understood. While some fish populations may be able to escape global warming via range shifts, they cannot escape ocean acidification (OA), an inevitable consequence of the dissolution of anthropogenic carbon dioxide (CO2) emissions in marine waters. How ocean acidification affects population dynamics of commercially important fish species is critical for adapting management practices of exploited fish populations. Ocean acidification has been shown to impair fish larvae's sensory abilities, affect the morphology of otoliths, cause tissue damage and cause behavioural changes. Here, we obtain first experimental mortality estimates for Atlantic cod larvae under OA and incorporate these effects into recruitment models. End-of-century levels of ocean acidification (~1100 µatm according to the IPCC RCP 8.5) resulted in a doubling of daily mortality rates compared to present-day CO2 concentrations during the first 25 days post hatching (dph), a critical phase for population recruitment. These results were consistent under different feeding regimes, stocking densities and in two cod populations (Western Baltic and Barents Sea stock). When mortality data were included into Ricker-type stock-recruitment models, recruitment was reduced to an average of 8 and 24% of current recruitment for the two populations, respectively. Our results highlight the importance of including vulnerable early life stages when addressing effects of climate change on fish stocks.

Simultaneous analysis of six microsatellite markers in Atlantic cod ( Gadus morhua): a novel multiplex assay system for use in selective breeding studies.

A novel hexaplex assay system including Gmo8, Gmo19, Gmo35, Gmo37, Tch11, and Tch12 microsatellites from Atlantic cod, consisting of trinucleotide or tetranucleotide repeat units, is introduced. All 6 loci were coamplified in a single reaction employing dye-labeled primers. Alleles from these loci were sized using an internal standard by automated sample processing in an ABI 310 Genetic Analyser. Amplified alleles in profiles containing selected microsatellites were typed clearly, providing easily interpretable results. Sequencing data indicated that alleles at all loci consisted of simple repeat units. This may help minimize the likelihood of stuttering upon polymerase chain reaction amplification. The results suggest that the presented hexaplex assay system may be a useful tool in a selective breeding program in which genetic identification will allow different genotypes to be reared together from fertilization. This should have a great impact as it will make selective breeding more efficient.