Changes in water temperature have an impact on the dentex in Medes Islands

The new study is signed by experts from the Faculty of Biology and the Biodiversity Research Institute (IRBio); the Blanes Centre for Advanced Studies (CEAB-CSIC), the Institute of Marine Sciences (ICM-CSIC), the Ecological and Forestry Applications Research Centre (CREAF), the Balearics Oceanographic Centre (COB-IEO) and the Moss Landing Marine Labs (United States), among other institutions.

A voracious predator of fish and shellfish

The common dentex (Dentex dentex) is a voracious and aggressive predator that eats fish, crabs and cephalopods. It plays an essential role in the activity of marine benthic ecosystems and it is a target species with great economic value in trade and sports fisheries.

According to Professor Bernat Hereu, from the Department of Evolutionary Biology, Ecology and Environmental Sciences of the UB and IRBio, "Knowing the movement patterns of the dentex is essential to better understand their biology and ecology as well as to determine the role of marine reserves in the recovery of populations and exportation of models (biomass) that can be captured by fisheries out of the reserves."

"Acoustic telemetry, together with new analytical and computational methods in the framework of movement ecology, are revolutionizing marine biology and will allow us to understand the effects of climate change and fishery pressure on key species, as well as the search for solutions for their preservation" added Frederic Bartumeus, ICREA researcher from CEAB-CSIC and CREAF, and expert in animal dispersal and movement patterns.

Acoustic transmitters to study fish movement

The experts studied dentex populations in the Medes Islands with acoustic telemetry techniques, based on tagging with acoustic transmitters that send signs periodically. These signs, received by a network of receptors in the area of study, covered the entire range of the fish movement. Alter more than fifteen months of research, researchers could register -in two-minute intervals- the position and depth of all individuals tagged in the study.

According to Eneko Aspillaga (UB-IRBio and CEAB-CSIC), first author of the article, "the study shows a clear pattern of movement of dentex depending on the temperature of the water. In winter, temperatures -around 12ºC- do not alter the whole water column, and dentex move in a depth range between 10 and 40 meters, without any defined pattern."

"The thermal pattern of water mass in the Medes Islands -continued Aspillaga- is the typical north western Mediterranean one. After April, the surface gets warmer and between May and June two different water masses appear: the shallow ones, warm and less dense, and the deep and cold ones, separated by a frontier called thermocline. When the thermocline is formed in summer, dentex only spread over this limit, regardless of its depth."

Why do dentex move to warmer waters?

The new study, published in Scientific Reports, states that dentex is a thermophile species, with dispersal patterns and activity conditioned to summer due the depth and time the thermocline lasts in the water column. Since the dentex is poikilothermal -it has an internal temperature fully dependent on the climate- it would move through the water column until finding warmer and better conditions for its physiology (reproduction, nutrition, etc.), especially in summer, when they are more active. The population growth of dentex, observed in preserved areas and fishery areas in the Medes Islands, could occur due the rise in the sea temperatures (more than 0.5ºC over the last 30 years in the area).

The thermocline temperature and distribution, which lasts until late October, could also have an important impact on the spread of other fish, both predators and others of lower trophic levels, according to data by Josep Pascual, from the Weather Station in Estartit.

Climate change, a threat to biodiversity in the Medes Islands

The archipelago of the Medes Islands has an exceptional ecological value as a natural ecosystem, with a great range of diversity in habitats (algae and gorgonian forests and coral reefs, fields of phanerogams, cave communities, etc.) and related species. The special orography of the Montgrí mountains, the rivers and the positive effect of marine ecosystem preservation have benefited the landscape richness and biodiversity of the islands, part of the Natural Park of Montgrí, Medes Islands and Baix Ter. Despite being one of the less affected areas by climate change in the Mediterranean, this reserve is also sensitive to the global warming due its richness in species and habitats.

Gorgonians, coral reefs, sponges or shellfish -filtering organisms that live fixed in marine depths- are highly vulnerable to climate change (especially if they are above the thermocline). Fish and other mobile species are also sensitive to temperature changes during some stages in their life cycle (reproduction, nutrition, etc.). Some thermopile species could widen their distribution range and move other marine organisms. Moreover, species that are more prone to move around colder waters could disappear. The authors warn that in this global change scenario, the introduction of tropical species is particularly worrying, since they can deeply influence the balance in marine ecosystems.