Drivers of fish extinction and colonization on oceanic banks

Informations

Funding country

Norway

Acronym

DRIVEBANKS

URL

-

Start date

1/1/2017

End date

12/31/2022

Budget

726,807 EUR

Fundings

Abstract

In DRIVEBANKS we studied patterns and drivers of fish diversity. Biodiversity may wax or wane depending on metrics, taxa and scale (time and space). This can make comparison of studies difficult. We may e.g. observe changes in community composition even if local species richness does not change. We used bottom trawl data from the Barents Sea (2004-2021), an area of 1.3 million km2, sampled by IMR and PINRO (VNIRO) in annual ecosystem surveys. We also used data from the Scotian Shelf, Northwest Atlantic to get insights not restricted to one system. Cod has played important roles in both areas, but with stock development in opposite directions, with a dramatic increase in the Barents Sea and a collapse on the Scotian Shelf. We studied shallow offshore banks and importance of topography for diversity patterns in large marine systems. DRIVEBANKS was motivated by island theories and methods developed for terrestrial systems. Species diversity has been shown to be related to island area, and explanations of the Species-Area Relationship (SAR) include theories of extinction and colonization processes, isolation, habitat and sampling phenomena. We speculated if banks may be regarded as island-like structures, i.e. underwater islands or mountain tops in the sea, of different sizes and degree of isolation. Banks are topographic structures that can result in circulation features that result in increased productivity. Banks are important areas for a number of fish stocks of commercial value. We divided the Barents Sea into regions, each with a bank and a deeper surrounding area. Differences in fish community on bank and off-bank varied among regions, but we found that banks cannot be regarded as diversity hot spots, even though productivity is often higher. There are known gradients in temperature and fish community from south to north-east in the Barents Sea. There have been changes and yearly variation in bottom temperature and cod in different areas, and also changes in fish composition from 2004, with boreal species moving north-east and arctic species declining in north. We focused on variation in fish composition among all years, and found a reversal of these changes (i.e. distribution of boral/arctic species) in 2020. The reversal might be on short term, and we might expect changes to go up and down in the coming years. We found that different diversity metrics showed different patterns in different regions. The Barents Sea is a dynamic system, and reversable changes have been observed over the last decades, with large spatial heterogeneity of changes that to a large degree are linked to changes in drivers (climate, cod). Variation in time and space can be studied in terms of colonization and extinction of species. Species richness e.g. depends on a balance between colonization, which increases richness, and extinction, which reduces it. Whereas terrestrial ecologists have studied these processes for decades, such studies have been rare in the marine realm. We used oceanographic circulation models to study which banks are connected and if they are upstream/downstream from one another to aid the interpretation of colonization/extinction and dispersion. The models showed that banks have different degree of connection, but none are fully isolated. Even though banks in the Barents Sea are not diversity hot spots and probably should not be regarded as island-like structures, we can achieve a deeper, more mechanistic understanding of variation and change of marine systems by focusing on drivers of these underlying processes. We used statistical models that take sampling into account, and found large variation in species detectability. We corrected for this, and found e.g. increased colonization and persistence with area of bank, but also off-bank area, as expected by theory. It remains to integrate results in a model that predicts fish diversity changes over time. On banks on the Scotian Shelf we found a positive SAR for fish communities, in accordance with theory. Predators impacted SAR for lower trophic levels, i.e. when predators were depleted, prey extinction rates decreased. This led to increases in numbers of prey species per trawl and an increase in SAR slope for prey species. For invertebrates, we only found a positive SAR for the least dispersive species, supporting the prediction of a negative relationship between dispersal capacity and SAR slope. The increased productivity on banks is well-known for fisheries, and fishing activities are often targeted there. We initiated a Fram Centre project focusing on social science and communication with management. We communicated with fishermen on how they use banks as fishing grounds and their experience with banks and fisheries. We also examined fishing activity based on VMS and log book data from the Norwegian fishing fleet (2011-2018). DRIVEBANKS and the add-on project contributed to exchange of knowledge between researchers, fishermen and management.