Differences in metabolic rate between two Atlantic cod (Gadus morhua) populations estimated with carbon isotopic composition in otoliths

Mortality drives production dynamics of Atlantic cod through 1100 years of commercial fishing

Most edible fish species have been fished for centuries or millennia, leaving little record or understanding of their population responses prior to human impact and thus no baseline for population conservation. Here, we reconstruct the population dynamics of Atlantic cod, one of the world's most harvested fish species, from the pristine state during the Viking era through more than 1100 years of fishing. Analysis of cod otoliths recovered during archaeological excavations of Icelandic fish processing sites revealed that cod in the 10th to 12th centuries were 25% larger and up to 300% older than modern, despite slower density-dependent growth rates attributed to the sixfold increase in abundance. Fishing mortality came to dominate a time-invariant natural mortality rate and other population characteristics after the 14th century, with minimal evidence of environmental effects at the century scale. Despite the absence of catch records and surveys, biological reference points based on pristine fish populations are now possible where otolith collections are available.

Hypoxia traits imprinted in otolith δ13C from individual to global scales

Hypoxia tolerance and its variation with temperature, activity, and body mass, are critical ecophysiological traits through which climate impacts marine ectotherms. To date, experimental determination of these traits is limited to a small subset of modern species. We leverage the close coupling of carbon and oxygen in animal metabolism to mechanistically relate these traits to the carbon isotopes in fish otoliths (δ13Coto). The model reproduces the major empirical patterns in δ13Coto at individual to global scales. The weak dependence on body size and strong, non-linear, dependence on temperature reflect the same balance between metabolism and ventilatory gas exchange that underlies organisms' hypoxia tolerance. The global relationship between temperature and δ13Coto records both the fractionation by aragonite precipitation and the variation in hypoxia traits across ocean biomes. Because hypoxia tolerance is imprinted on both otolith geochemistry and species biogeography, the model allows the aerobic limits of species geographic ranges to be predicted from fish δ13Coto. This physiologically grounded model provides a foundation for the use of otolith chemistry to reconstruct modern spatial patterns and paleoceanographic changes in key traits that shape aerobic habitat of aquatic species.

Tracing 600 years of long-distance Atlantic cod trade in medieval and post-medieval Oslo using stable isotopes and ancient DNA

Marine resources have been important for the survival and economic development of coastal human communities across northern Europe for millennia. Knowledge of the origin of such historic resources can provide key insights into fishing practices and the spatial extent of trade networks. Here, we combine ancient DNA and stable isotopes (δ13C, δ15N, non-exchangeable δ2H and δ34S) to investigate the geographical origin of archaeological cod remains in Oslo from the eleventh to seventeenth centuries CE. Our findings provide genetic evidence that Atlantic cod was obtained from different geographical populations, including a variety of distant-water populations like northern Norway and possibly Iceland. Evidence for such long-distance cod trade is already observed from the eleventh century, contrasting with archaeological and historical evidence from Britain and other areas of Continental Europe around the North and Baltic Seas, where such trade increased during the thirteenth to fourteenth centuries. The genomic assignments of specimens to different populations coincide with significantly different δ13C values between those same specimens, indicating that multiple Atlantic cod populations living in different environments were exploited. This research provides novel information about the exploitation timeline of specific Atlantic cod stocks and highlights the utility of combining ancient DNA (aDNA) methods and stable isotope analysis to describe the development of medieval and post-medieval marine fisheries.

Water temperature affects somatic growth, body condition and oxygen and carbon otolith isotopes of stout whiting (Sillago robusta)

Ocean warming will continue to affect the growth, body condition and geographic distributions of marine fishes and understanding these effects is an urgent challenge for fisheries research and management. Determining how temperature is recorded in fish otolith carbonate, provides an additional chronological tool to investigate thermal histories, preferences and patterns of movement throughout an individual's life history. The influence of three water temperature treatments (22°C, 25°C, and 28°C) on hatchery-reared juvenile stout whiting, Sillago robusta, was tested using a controlled outdoor mesocosm system. Fish were measured for change in length and weight, and body condition was determined using bioelectrical impedance analysis. Sagittal otoliths were analysed for stable oxygen (δ18Ootolith) and carbon (δ13Cotolith) isotopes via isotope ratio mass spectrometry. Whiting kept at 22°C were significantly smaller and had diminished body condition compared to fish in 25°C and 28°C, which did not significantly differ from each other. The δ18O otolith values of stout whiting demonstrated a negative temperature-dependent fractionation relationship which was similar in slope but had a different intercept to the relationships reported for inorganic aragonite and other marine fish species. The δ13C otolith values also showed a negative relationship with water temperature, and the calculated proportion of metabolic carbon M in otoliths differed between fish reared in the coolest (22°C) and warmest (28°C) temperature treatments. Overall, the results suggest that stout whiting may have reached an upper growth threshold between 25°C and 28°C, and that growth and body condition may be optimised during warmer seasons and toward the northerly regions of their distribution. Otolith oxygen thermometry shows promise as a natural tracer of thermal life history, and species-specific fractionation equations should be utilised when possible to prevent errors in temperature reconstructions of wild-caught fish.

Incorporating otolith-isotope inferred field metabolic rate into conservation strategies

Fluctuating ocean conditions are rearranging whole networks of marine communities-from individual-level physiological thresholds to ecosystem function. Physiological studies support predictions from individual-level responses (biochemical, cellular, tissue, respiratory potential) based on laboratory experiments. The otolith-isotope method of recovering field metabolic rate has recently filled a gap for the bony fishes, linking otolith stable isotope composition to in situ oxygen consumption and experienced temperature estimates. Here, we review the otolith-isotope method focusing on the biochemical and physiological processes that yield estimates of field metabolic rate. We identify a multidisciplinary pathway in the application of this method, providing concrete research goals (field, modeling) aimed at linking individual-level physiological data to higher levels of biological organization. We hope that this review will provide researchers with a transdisciplinary 'roadmap', guiding the use of the otolith-isotope method to bridge the gap between individual-level physiology, observational field studies, and modeling efforts, while ensuring that in situ data is central in marine policy-making aimed at mitigating climatic and anthropogenic threats.

Thermal sensitivity of field metabolic rate predicts differential futures for bluefin tuna juveniles across the Atlantic Ocean

Changing environmental temperatures impact the physiological performance of fishes, and consequently their distributions. A mechanistic understanding of the linkages between experienced temperature and the physiological response expressed within complex natural environments is often lacking, hampering efforts to project impacts especially when future conditions exceed previous experience. In this study, we use natural chemical tracers to determine the individual experienced temperatures and expressed field metabolic rates of Atlantic bluefin tuna (Thunnus thynnus) during their first year of life. Our findings reveal that the tuna exhibit a preference for temperatures 2-4 °C lower than those that maximise field metabolic rates, thereby avoiding temperatures warm enough to limit metabolic performance. Based on current IPCC projections, our results indicate that historically-important spawning and nursery grounds for bluefin tuna will become thermally limiting due to warming within the next 50 years. However, limiting global warming to below 2 °C would preserve habitat conditions in the Mediterranean Sea for this species. Our approach, which is based on field observations, provides predictions of animal performance and behaviour that are not constrained by laboratory conditions, and can be extended to any marine teleost species for which otoliths are available.

Reading the biomineralized book of life: expanding otolith biogeochemical research and applications for fisheries and ecosystem-based management

Chemical analysis of calcified structures continues to flourish, as analytical and technological advances enable researchers to tap into trace elements and isotopes taken up in otoliths and other archival tissues at ever greater resolution. Increasingly, these tracers are applied to refine age estimation and interpretation, and to chronicle responses to environmental stressors, linking these to ecological, physiological, and life-history processes. Here, we review emerging approaches and innovative research directions in otolith chemistry, as well as in the chemistry of other archival tissues, outlining their value for fisheries and ecosystem-based management, turning the spotlight on areas where such biomarkers can support decision making. We summarise recent milestones and the challenges that lie ahead to using otoliths and archival tissues as biomarkers, grouped into seven, rapidly expanding and application-oriented research areas that apply chemical analysis in a variety of contexts, namely: (1) supporting fish age estimation; (2) evaluating environmental stress, ecophysiology and individual performance; (3) confirming seafood provenance; (4) resolving connectivity and movement pathways; (5) characterising food webs and trophic interactions; (6) reconstructing reproductive life histories; and (7) tracing stock enhancement efforts. Emerging research directions that apply hard part chemistry to combat seafood fraud, quantify past food webs, as well as to reconcile growth, movement, thermal, metabolic, stress and reproductive life-histories provide opportunities to examine how harvesting and global change impact fish health and fisheries productivity. Ultimately, improved appreciation of the many practical benefits of archival tissue chemistry to fisheries and ecosystem-based management will support their increased implementation into routine monitoring.

Graphical abstract