Effects of temperature on tissue-diet isotopic spacing of nitrogen and carbon in otolith organic matter

Reconstruction of the trophic position of a fish can be performed by analysing stable nitrogen and carbon isotopes in otolith protein. However, ambient temperature may affect the tissue–diet isotopic spacing of stable isotopes from diet to predator tissue and bias estimates of trophic position. To test this, otolith protein, heart and muscle tissue from a rearing experiment with juvenile cod held at different temperatures (4, 7, 10 and 14°C) were analysed. There was no significant effect of temperature on otolith δ15N, whereas muscle and heart exhibited a slight decrease in δ15N values with increasing temperature corresponding to maximum of 0.6‰ over the 10°C range. By contrast, the otolith protein δ13C values at 4 and 7°C were significantly higher than for 10 and 14°C, suggesting an approximate 1‰ increased tissue–diet enrichment at the lower temperatures. Temperature had no significant effect on muscle and heart δ13C values. Considering the annual mean variation in ocean temperatures, our results indicate that the trophic signals recorded in the otoliths will reflect changes in diet isotope values with little bias from the ambient temperature experienced by the fish.publishedVersio

Otolith δ13C values as a metabolic proxy: Approaches and mechanical underpinnings

Knowledge of metabolic costs associated with maintenance, foraging, activity and growth under natural conditions is important for understanding fish behaviours and the bioenergetic consequences of a changing environment. Fish performance in the wild and within a complex environment can be investigated by analysing individual-level field metabolic rate and, at present, the natural stable carbon isotope tracer in otoliths offers the possibility to reconstruct field metabolic rate. The isotopic composition of carbon in fish otoliths is linked to oxygen consumption through metabolic oxidation of dietary carbon. The proportion of metabolically derived carbon can be estimated with knowledge of δ13C values of diet and dissolved inorganic carbon in the water. Over the past 10 years, new techniques to study fish ecology have been developed, and these can be used to strengthen the application of otolith δ13C values as a metabolic proxy. Here, we illustrate the great potential of the otolith δ13C metabolic proxy in combination with other valuable and well-established approaches. The novel approach of the otolith δ13C metabolic proxy allows us to track the effects of ontogenetic and environmental drivers on individual fish physiology, and removes a major obstacle to understanding and predicting the performance of free-ranging wild fish.publishedVersio

A century of fish growth in relation to climate change, population dynamics and exploitation

Marine ecosystems, particularly in high‐latitude regions such as the Arctic, have been significantly affected by human activities and contributions to climate change. Evaluating how fish populations responded to past changes in their environment is helpful for evaluating their future patterns, but is often hindered by the lack of long‐term biological data available. Using otolith increments of Northeast Arctic cod (Gadus morhua) as a proxy for individual growth, we developed a century‐scale biochronology (1924–2014) based on the measurements of 3,894 fish, which revealed significant variations in cod growth over the last 91 years. We combined mixed‐effect modeling and path analysis to relate these growth variations to selected climate, population and fishing‐related factors. Cod growth was negatively related to cod population size and positively related to capelin population size, one of the most important prey items. This suggests that density‐dependent effects are the main source of growth variability due to competition for resources and cannibalism. Growth was also positively correlated with warming sea temperatures but negatively correlated with the Atlantic Multidecadal Oscillation, suggesting contrasting effects of climate warming at different spatial scales. Fishing pressure had a significant but weak negative direct impact on growth. Additionally, path analysis revealed that the selected growth factors were interrelated. Capelin biomass was positively related to sea temperature and negatively influenced by herring biomass, while cod biomass was mainly driven by fishing mortality. Together, these results give a better understanding of how multiple interacting factors have shaped cod growth throughout a century, both directly and indirectly.publishedVersio

Early life growth is affecting timing of spawning in the semelparous Barents Sea capelin (Mallotus villosus)

Capelin (Mallotus villosus) is a forage fish and a key species in the Barents Sea (BS). The BS capelin are semelparous and hence only spawn once along the north coasts of Norway and Russia before they die. The age at spawning ranges from 2 to 5 years and the spawning season peaks in March/April but starts in February and lasts until June, and the causes of the variability in timing of spawning are not well understood. Here, we aimed to find out whether early growth is associated with the timing of spawning in BS capelin, both on the individual and population level, and if there is an association between early life growth and the spatial distribution at the nursery areas and feeding grounds. For the analysis, we used an extensive dataset comprising >150 000 otolith growth zone measurements carried out during surveys from 1976 to 2019 both from the spawning and feeding areas. The data from the feeding area showed that capelin with good first-year growth were found in the productive north-west part of the Barents Sea at both age 1 and 2, while capelin with relatively poor first-year growth were typically found in the south-east Barents Sea. The data from the spawning area showed on the individual level that capelin with good first-year growth tend to spawn both at a younger age and earlier in the season. The capelin spawning late in the season were also generally smaller than early spawners. On the population level, a contradictory pattern was observed where the proportion of maturing capelin at age 2 and 3 was negatively correlated with first-year growth indicating that the great variability in year-class strength masks the general effect found at the level of individual fish. Furthermore, first-year growth was positively associated with the abundance of 1-year-old capelin indicating that rapid growth early in life enhances recruitment. On the other hand, first-year growth was strongly negatively correlated with third-year growth suggesting an increased effect of density-dependent growth with age and/or compensatory growth, or reduced growth linked to earlier maturation. In sum, our results show that the first-year growth affects growth, maturation processes, and timing of spawning later in life, thus potentially strongly influencing capelin population dynamics.publishedVersio

Age class composition and growth of Atlantic cod (Gadus morhua) in the shallow water zone of Kongsfjorden, Svalbard

Although Atlantic cod has been observed in Svalbard waters since the 1880s, knowledge about the presence in the Arctic shallow water zone is limited. The regular catch of juvenile Atlantic cod in Kongsfjorden since 2008 is in line with an overall northward shift of boreal fish species toward the Arctic. This is the first study showing the age class composition, growth rates, and stomach content of Atlantic cod in the shallow water zone of Kongsfjorden, Svalbard. From 2012 to 2014 a total of 721 specimens were sampled in 3 to 12 m water depth. The primary age classes were identified as 0+, 1+, and 2+ using otolith age analysis. The different cohorts of these specimens show stable growth rates during the polar day and night. By stomach content analysis, we show that these specimens primarily feed on benthic food sources. These observations support the assumption that the shallow water zone of Kongsfjorden is likely to be a nursery ground for Atlantic cod.publishedVersio

Expected Climate Change in the High Arctic—Good or Bad for Arctic Charr?

Lakes in the High Arctic are characterized by their low water temperature, long-term ice cover, low levels of nutrients, and low biodiversity. These conditions mean that minor climatic changes may be of great importance to Arctic freshwater organisms, including fish, by influencing vital life history parameters such as individual growth rates. In this study, Arctic charr sampled from two Svalbard lakes (78–79◦ N) over the period 1960–2008 provided back-calculated length-at age information extending over six decades, covering both warm and cold spells. The estimated annual growth in young-of-the-year (YOY) Arctic charr correlated positively with an increasing air temperature in summer. This increase is likely due to the higher water temperature during the ice-free period, and also to some extent, due to the winter air temperature; this is probably due to thinner ice being formed in mild winters and the subsequent earlier ice break-up. However, years with higher snow accumulation correlated with slower growth rates, which may be due to delayed ice break-up and thus a shorter summer growing season. More than 30% of the growth in YOY charr could be explained specifically by air temperature and snow accumulation in the two Arctic charr populations. This indicated that juvenile Svalbard Arctic charr may experience increased growth rates in a future warmer climate, although future increases in precipitation may contradict the positive effects of higher temperatures to some extent. In the longer term, a warmer climate may lead to the complete loss of many glaciers in western Svalbard; therefore, rivers may dry out, thus hindering migration between salt water and fresh water for migratory fish. In the worst-case scenario, the highly valuable and attractive anadromous Arctic charr populations could eventually disappear from the Svalbard lake systems. High Arctic; Svalbard lake systems; climate impact; Arctic charr; growth rate; anadromypublishedVersio

Age class composition and growth of Atlantic cod (Gadus morhua) in the shallow water zone of Kongsfjorden, Svalbard

Although Atlantic cod has been observed in Svalbard waters since the 1880s, knowledge about the presence in the Arctic shallow water zone is limited. The regular catch of juvenile Atlantic cod in Kongs orden since 2008 is in line with an overall northward shift of boreal fish species toward the Arctic. This is the first study showing the age class composition, growth rates, and stomach content of Atlantic cod in the shallow water zone of Kongs orden, Svalbard. From 2012 to 2014 a total of 721 specimens were sampled in 3 to 12 m water depth. The primary age classes were identified as 0+, 1+, and 2+using otolith age analysis. The diferent cohorts of these specimens show stable growth rates during the polar day and night. By stomach content analysis, we show that these specimens primarily feed on benthic food sources. These observations support the assumption that the shallow water zone of Kongs orden is likely to be a nursery ground for Atlantic cod

Fish investigations in the Barents Sea winter 2018.

This report presents the main results from the surveys in January-March 2018. The surveys were performed with the Norwegian research vessels “Helmer Hanssen” and “Johan Hjort” and Russian research vessel “Fritjof Nansen”. Annual survey reports since 1981 are listed in Appendix 1, and names of scientific participants are given in Appendix 3.publishedVersio

Fish Investigations in the Barents Sea Winter 2019

The Institute of Marine Research (IMR), Bergen, has performed acoustic measurements of demersal fish in the Barents Sea since 1976. Since 1981 a bottom trawl survey has been combined with the acoustic survey. Typical effort of the combined survey has been 10-14 vessel-weeks, and about 350 bottom trawl hauls have been made each year. Most years three vessels have participated from about 1 February to 15 March.publishedVersio

Advice on fishing opportunities for Northeast Arctic cod in 2023 in ICES subareas 1 and 2

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