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

Informations

Funding country

Norway

Acronym

-

URL

-

Start date

1/1/2019

End date

12/31/2023

Budget

1,071,453 EUR

Fundings

Abstract

Should we be protecting more of the big fish we like to harvest? Recently, it has been agued that that larger female fish may contribute disproportionally more to population replenishment because of their capacity to produce more and larger offspring compared to smaller parents. These larger offspring could be better at surviving the critical first year in dangerous and unpredictable oceans. However, knowledge based on wild fish in their natural environments are still largely lacking. This is unfortunate, because both fishing and climate warming could be causing global reductions in fish body size. Large fish are a priced catch, not the least among recreational fishers, and fisheries management often add to the pressure by introducing minimum size limits. Smaller fish also seem to better at coping with warm water. In the CodSize project we are tracking a population of parent cod and their offspring in a natural fjord environment on the Norwegian Skagerrak coast. The study is conducted within Norway's only no-take marine reserve where no fishing has been allowed since 2012. Thanks to this conservation effort, the cod population within the fjord now consists of 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. By combining advanced individual tagging and genetic parentage analyses, we aim to resolve how larger fish behave during the spawning season and how many surviving offspring they produce, compared to the smaller fish typically seen in harvested areas. During the first stage of the project, we have developed a method for validating the use of advanced acoustic tags for determining the fate of individual fish, whether they survive or die, and whether they stay or leave the fjord. We have discovered that marine reserves offer increased survival for resident fish, and can buffer the impacts of size-selective harvesting, restoring an increasing component of large and old individuals. This is a promising starting point for the remaining parts of the project. We also use small samples of muscle tissue to learn what the cod is feeding on without having to sacrifice the fish, and whether large cod could have a different impact on the whole coastal ecosystem compared to small cod. In collaboration with international research groups from Finland and the US, we combine these observations with models to better understand future scenarios of environmental change. Results from the project's mid-phase show that cod tend to leave the vegetated and productive shallow habitats when surface warms during summer, and instead use more barren deeper habitats. This behaviour was particularly clear for larger cod, and may influence their fitness and population productivity. The first analyses of muscle tissues indicate, however, that there is no clear association between body size and diet. This was unexpected. We did see that two coexisting ecotypes of cod probably had different diets. Based on isotope analyses of muscle tissue samples, it looks like a "fjord ecotype" feeds more on benthic prey while a "North Sea ecotype" feeds more on pelagic prey. The two ecotypes could therefore play different roles as predators in the ecosystem. For cod tagged inside the marine reserve, body size is apparently not linked to either home range size or the probability of leaving the reserve and enter areas of harvesting. This was an unexpected result.