Resolving impacts of Atlantic cod body size on population replenishment and coastal ecosystem change

Should we be protecting more of the big fish we like to harvest? Contemporary populations of harvested fish are often characterised by a truncated age- and size-distribution compared to historical records. This shift can be linked to historic and ongoing fisheries, that are both intense and size-selective. Recently, it has been argued that that large fish could be important for population productivity and ecosystem resilience, and even play a role in the ocean carbon cycle as part of nature-based mitigations of climate change. However, empirical knowledge based on wild fish in their natural environments are still largely lacking. In the CODSIZE project we have been tracking cod in natural fjord environments on the Norwegian coast for multiple years. A key study has been conducted within Norway's only no-take marine reserve where no fishing has been allowed since 2012. By combining advanced individual tagging, genetic analyses, stable isotope analyses, and ecological modelling, we have sought to identify key components of behaviour, fate and ecosystem impacts linked to large fish. During the early phase of the project, we showed that current marine protected areas in Norway have produced an increasing component of large and old fish that are rarely seen in the neighbouring coastal areas, where fishing pressure is very high and size-selective. We also show that fish survival has increased inside the protected areas, and that larger individuals tend to have higher survival compared to smaller individuals, which is the opposite of what is seen in fished areas. In that sense, the protected areas have clearly shifted the adaptive landscape and the resulting eco-evolutionary dynamics. That said, our tracking studies also revealed that many mature cod had home ranges that extended beyond the borders of the protected areas, thus exposing them to commercial- and recreational fishing. Therefore, a clear advice to management is that future protected areas need to be much bigger than the current 1-2 square km in order to offer real protection to all phenotypes at a population level. We also show that cod - in particular the larger individuals - will abandon their nearshore feeding habitats when water temperatures rise above 16 - 17 degrees during summer. These fish will then instead occupy deeper and cooler habitats, often quite close to their usual core areas. We therefore recommend that future protected areas need to build climate-resilience by including deeper (> 50 m), well-oxygenated habitats, in addition to nearshore feeding habitats and spawning habitats. As cod move between shallow feeding habitats and deeper areas, we argue that these larger fish could actually play a role in the ocean carbon cycle, moving carbon towards deeper habitats where it may be sequestered. The coastal cod spawn mainly during February through April, and our tracking studies have shown that it is during this critical season that the fish tend to be most mobile and move beyond the borders of the protected areas. In our opinion, this observation adds weight to the argument that protected areas need to be bigger. We found that during the spawning season, cod mostly preferred rocky habitats, sometimes in seaweed and kelp forests, but seldom above sandy or soft-bottom habitats. The mature fish typically experienced water temperatures between five and ten degrees, but some individuals were also observed in warmer water, up to at least 15 degrees. This suggests that some cod are able to cope with warmer water during spawning, and such individuals are likely to be important for the population's ability to adapt to future climate change. We argue that it is important to consider such intraspecific diversity in management and conservation programmes. Earlier studies have shown that two genetically distinct cod ecotypes inhabit the Skagerrak coast: a "North Sea type" and a "Fjord type". We show that these types of cod are sympatric within fjord habitats, but that they play different roles as predators in the ecosystem. Using stable isotope analyses, we found that the fjord type tended to feed at higher trophic levels and more connected to the benthic part of the ecosystem compared to the North Sea type, which was more linked to the pelagic. Furthermore, a modelling study completed as part of the project showed that the coastal ecosystem in Skagerrak appears more robust to a loss of the North Sea ecotype compared to a loss of the fjord ecotype. The latter therefore appears to hold a key position in this ecosystem. Preliminary results also show that the larger cod tend to feed higher up in the food web and more linked to the benthic system. The protection of large, resident fjord cod thus appears to be of particular importance for the future resilience of the coastal ecosystems.

1. Virkninger av prosjektet. a). Christopher Monk arbeidet som post doc. på prosjektet i to år, rett etter fullført doktorgrad. I løpet av prosjektperioden har han bygd et grunnleggende internasjonalt nettverk mot de øvrige partnerne i prosjektet, som bl. a. inkluderer laben til prof. Stephanie Carlson ved University of California Berkeley og laben til prof. Anna Kuparinen ved University of Jyväskyla. Monk har også deltatt på Havforskningsinstituttet sine årlige samlinger og i faggruppen Populasjonsgenetikk, slik at han har blitt kjent med et bredt spekter av det som foregår innen kyst- og havforskning i Norge. Mot slutten av sin post doc. sikret Monk seg fast ansettelse som forsker ved Helmholtz Center for Ocean Research, Kiel, der han nå arbeider. b). Rebekah Oomen arbeidet som post. doc. på prosjektet gjennom Universitetet i Oslo. Som Monk, så har også Oomen fått bygge et internasjonalt og nasjonalt nettverk av kontakter. Hun har fått lære nye metoder innen fagfeltet genetikk, og ikke minst har hun sikret seg fast ansettelse som assistant professor ved University of New Brunswick i Canada fra juli 2023. c). Bjørg Karin Varnes tok sin mastergrad på prosjektet, tilknyttet Universitetet i Agder. Hun fikk A og oppgaven er nå publisert som en artikkel i ICES Journal of Marine Science. Varnes arbeider som lærer på videregående skole, der hun blant annet underviser i bærekraftig fiskeri og havbruk. d). Sara Hocevar fullførte doktorgraden sin i 2023 og har vært tilknyttet prosjektet gjennom Anna Kuparinen ved University of Jyväskyla. Hun har fått benytte Havforskningsinstituttet sine datasett på fiskesamfunn i Skagerrak, og har også fått delta på tokt. Hocevar og Kuparinen vil besøke Havforskningsinstituttet Flødevigen i august 2023 for å diskutere framtidig samarbeid. e) For Esben M. Olsen og miljøet ved Havforskningsinstituttet Flødevigen så har prosjektet betydd mye på veien mot å bygge et internasjonalt anerkjent miljø innen marin bevaringsbiologi og studier av fiskeadferd (telemetri). Vi har lært hvordan denne forskningen kan gis et løft ved å kombinere med annen metodikk. Det har også gitt oss bedre forutsetninger for å lykkes med søknader på EU-nivå. f). Vår forskningsgruppe vant nylig fram i EU sin Biodiversa-utlysning med prosjektet MOVE, som fokuserer på hvordan man kan oppnå bedre beskyttelse og restaurering av bestander av store predator-fisk. Vi vil lede prosjektet, og samarbeide med forskningsinstitusjoner i Frankrike, Spania og Portugal. MOVE bygger på erfaringer, samarbeid og kompetanse bygd gjennom CODSIZE. 1. Effekter av prosjektet. a). Kysttorsken i Skagerrak har fått økt vern gjennom nye fiskerireguleringer, og vi har vært direkte involvert i prosessen gjennom dialog med forvaltning og brukere. b). For Raet Nasjonalpark i Skagerrak så arbeides det konkret med planer for strengere vern (nullfiske), og kunnskap om adferd og habitatbruk hos stor torsk - bygget gjennom CODSIZE - blir brukt direkte.

CodSize aims to advance our knowledge on coastal ecosystem structure, as well as function and change. To this end, we will focus on the role of body size as a functional trait in a harvested coastal predator, the Atlantic cod. We will take advantage of a no-take marine reserve centred around a fjord cod spawning area, serving as a field experiment where fishing has been removed from the equation. First, we aim to build fundamental knowledge on how spawning ecology and population replenishment is structured by body size diversity within a natural population. Resolving this question could have wide-ranging consequences for fisheries management, because most fisheries tend to remove the larger individuals and thereby cause severe size-truncation in harvested populations. We will combine novel technologies and data analyses for revealing individual fish spawning behaviour, habitat use and offspring production in their natural environment. The latter will depend on successfully tracing cod offspring back to their parents in a natural fjord environment. This will be a challenge, but recent advances by ours and other groups clearly show that it is something that can be achieved. Our approach will include the use of advanced accelerometer tags and a large underwater telemetry array for unprecedented precision in measuring fish movements. This precision will open for novel applications of network theory, for instance by resolving how fish body size influence the mating system. Second, we will combine data on feeding behaviour with individual trophic level of feeding to resolve how predator body size may shape how coastal ecosystems function. Third, we will use empirical data to fill in models that utilise body size as the primary scaling factor of species interactions (ATN: allometric trophic network models) for predicting how the resilience and dynamics of coastal ecosystems may change in response to environmental change, including ocean warming.