Drivers and effects of spatial shifts in early life stages of marine fish (SpaceShift)

A key challenge in fisheries science is to understand why the number of fish that survive through the first years of life varies considerably between years for many harvested fish species. We here focused on the role of spatial distribution for growth and survival. In particular, we addressed the hypothesis that loss of old and large fish from heavily exploited fish stocks leads to a contracted spatial and temporal distribution of spawning and fish offspring, and that this reduces the buffering of offspring survival against adverse environmental conditions. Main focus was on the Barents Sea cod. The eggs of this cod stock are spawned along the Norwegian coast. The eggs drift with the currents back towards the Barents Sea while developing into larvae and juveniles which, if they survive, settle to a more demersal life style. The project took advantage of data from long time-series from Russian and Norwegian surveys. We combined statistical and numerical modelling approaches to take full advantage of this information, by identifying relationships that are solidly backed up by the observation data as well as by a mechanistic understanding of the underlying processes. Results showed that loss of old and large cod from the spawning stock was associated with reduced population growth (Ohlberger mfl. 2022: Age structure affects population productivity in an exploited fish species). This was probably because old and large cod produce high numbers of viable eggs, which was supported by observations of high concentrations of eggs and larvae in the sea in years when the mean age in the spawning stock was high (Endo et al. 2020: Effects of climate and spawning stock structure on the spatial distribution of Northeast Arctic cod larvae). At the same time the spatial distribution of the eggs and larvae was large (Endo et al. 2020). The spatial distribution of the fish larvae influences the interactions between fish larvae and their prey (Endo et al. 2022: Two decades of match-mismatch in Northeast Arctic cod – Feeding conditions and survival; Ferreira et al. 2020: Match-mismatch dynamics in the Norwegian-Barents Sea system). A broad distribution did not, however, seem to give higher average survival or to contribute significantly to buffer environmental influences (Langangen and Stige 2021: Shedding light on the link between the spatial distribution of eggs and survival in Northeast Arctic cod). The distribution is nonetheless relevant for the dynamics, as reflected in the finding that different mechanisms are important for the growth and survival of cod eggs and larvae in different parts of the distribution area (Endo et al. 2022). We further found that an eastern distribution of cod juveniles in the Barents Sea was associated with low growth but high survival (Durant et al. 2021: The role of spatial distribution for growth and survival of juvenile cod Gadus morhua in the Barents Sea). The distribution and of cod as well as its competitors can also be influenced by temperature, which may be why the cod in the Barents Sea only seems to negatively affect the haddock when sea temperature is over 4 °C (Durant et al. 2020: Non-linearity in interspecific interactions in response to climate change). The project has also contributed to studies investigating: - the relationships between growth and survival of fish through multiple life stages (Stige et al. 2019: Density- and size-dependent mortality in fish early life stages), - for which life stages the distribution is most constrained (Ciannelli mfl. 2021: Ontogenetic spatial constraints of sub-arctic marine fish species), - climate effects on the seasonal development of phytoplankton (Lundsør et al. 2020: Long-term coastal monitoring data show nutrient-driven reduction in chlorophyll), - the relationship between the seasonal development of phytoplankton and abundance of zooplankton (Dong et al. 2021: Associations between timing and magnitude of spring blooms and zooplankton dynamics in the southwestern Barents Sea), - effects of climate and predation on the seasonal development of zooplankton (Kvile et al. 2021: Environmental effects on Calanus finmarchicus abundance and depth distribution in the Barents Sea), - climate effects on match-mismatch between zooplankton and fish in ecosystems at different latitudes (Durant et al. 2019: Contrasting effects of rising temperatures on trophic interactions in marine ecosystems), - effects of fishing on the sensitivity of fish stocks to climate fluctuations (Färber et al. 2020: Population variability under stressors is dependent on body mass growth and asymptotic body size), and - use of bycatch data from scientific surveys to construct time series (Yaragina mfl. 2022: Bycatch data from ichthyoplankton surveys reveal long-term trends in gelatinous zooplankton in the Norwegian and Barents Seas). Results of the project have been presented in 16 scientific and 2 popular-scientific publications and at 5 international conferences.

Prosjektet har bragt forskningsfeltet videre ved at det kaster nytt lys over sammenhengen mellom utbredelse, overlevelse og vekst for tidlige livsstadier til fisk ved å kombinere statistisk modellering av lange tidsserier og mer mekanistisk modellering av prosessene som kan ligge bak disse sammenhengene. Hovedfokuset var på skreien i Barentshavet, men ikke begrenset til denne. En viktig implikasjon for fiskeriforvaltningen er at resultatene underbygger betydningen av å beholde gamle og store fisk i gytebestanden. Prosjektet har styrket nasjonalt så vel som internasjonalt forskersamarbeid og prosjektet har bidratt til forskerutdanning i form av en vellykket gjennomført PhD.

Understanding the mechanisms behind the large fluctuations in the recruitment to many harvested fish stocks remains a key challenge in fisheries science. We will here throw new light on these mechanisms by studying the interlinkage between spatial and temporal dynamics using a combination of state-of-the-art statistical and biophysical modelling approaches. In particular, we address the hypothesis that loss of old and large spawners from heavily exploited fish stocks leads to a contracted spatial and temporal distribution of spawning and fish offspring, which reduces the buffering of offspring survival against adverse environmental conditions. We will first quantify how the spatial distributions of fish eggs are influenced by the combination of climate and demographic structure of spawners. We will subsequently assess how climate and spatial distribution influence the match or mismatch between fish larvae and zooplankton prey and growth and survival of fish through early life. Hence we will map how survival of fish eggs to recruitment depends on location. With that basis, we will assess how climate and demography influences recruitment and population dynamics, through effects on spatial distribution of fish offspring and spatiotemporal patterns in growth and survival of the offspring. Main focus will be on Barents Sea cod, haddock and capelin. These stocks are the world's largest of their species and of high ecological and socioeconomic importance. Moreover, long-term Russian and Norwegian surveys provide unique empirical information on their dynamics in early life stages. Our combination of statistical and biophysical modelling approaches take full advantage of this information and aim to identify relationships that are backed up both by the spatiotemporal observation data and a clear, mechanistic understanding of the underlying processes. Hence, we expect to obtain insight into recruitment dynamics that are of wide, general interest.