2023-08-24 16:45:32
Every year, the skipjack tuna migrates northward from winter to summer, from the equatorial waters of the Atlantic to the subtropics, reaching, among other regions, Mauritania and the Canary Islands (Spain). During this migration, skipjack tuna tend to congregate in areas where they find prey to feed on (mainly small fish and cephalopods) and where, in turn, skipjack tuna are fished in abundance. These areas require significant contributions of nutrients that allow phytoplankton (grass or sea grass) to grow, that marine herbivores feed on this phytoplankton, and that marine carnivores feed on these herbivores, through the food web.
In a new study, it has been found that the areas in which striped tropical tuna (Katsuwonus pelamis) are fished in abundance, move northward from January to August, from the equator to the Canary Islands, following the patterns of Saharan dust deposition in the Atlantic. The work highlights the importance of Saharan dust in the marine ecosystem.
The Spanish National Research Council (CSIC) participated in the study.
“These results have enormous fishing and economic implications, as skipjack tuna is the most important commercial tuna in the Atlantic, with annual catches of around 253,000 tons per year. Skipjack tuna is commonly found canned. Their catches represent 48% of the total catches of tunas and 56% of the catches of tropical tunas in this ocean, according to the data used in this study, provided by the International Commission for the Conservation of Atlantic Tunas”, points out Sergio Rodríguez. , CSIC researcher at the Institute of Natural Products and Agrobiology (IPNA) and first author of the study.
Desert dust from the Sahara is exported from North Africa to the Atlantic in an air current called the Saharan air layer. The Atlantic waters located under this dusty air current are often enriched with dust from the Sahara, due to its atmospheric deposition. “Due to the general circulation of the atmosphere, the dusty layer of Saharan air moves, month by month (from winter to summer), towards the north, and the skipjack tuna move under this current of dust”, indicates Rodríguez .
Due to this displacement, the main skipjack tuna fishing grounds are near the equator in winter, in the open waters off Liberia and Guinea in spring, and in the open waters off Mauritania in summer. In these regions, the skipjack tuna fishing season usually begins when the Saharan air layer passes seasonally over them, beginning in April in Senegal and in June in the Canary Islands. In the case of the Canary Islands, the skipjack fishing season is usually from June to September, with maximum catches in July and August, a period in which the Saharan air layer impacts the archipelago. “Researchers have agreed to call this migration, from the equator to the Canary Islands, Atlantic-Saharan migration of skipjack tuna”, explains the researcher.
Specimens of skipjack tuna. (Photo: IPNA)
The nutritious powder of the Sahara
Marine ecosystems require nutrients for the growth of phytoplankton. These can reach surface waters in several ways. This new study highlights the importance of the atmospheric contributions of these certain nutrients.
“The open waters of the oceans are often called “blue deserts” because they are often poor in nutrients and, consequently, in phytoplankton; In these areas, atmospheric deposition represents the greatest contribution of nutrients and in this sense we highlight the contribution of dust from the Sahara. This contains iron (4%) and phosphorus (0.8%), essential for the phytoplankton to be able to fix nitrogen and use it to create amino acids”, indicates Rodríguez. “It also contains silicon (18%) and calcium (4%), essential for phytoplankton to build skeletons and shells, and it also contains metals such as manganese, zinc, cobalt and nickel, essential for metabolic functions”, he adds.
In the Atlantic, the largest concentration of Atlantic skipjack tuna is found off the northwestern coast of Africa, where there are two relevant inputs of nutrients: deep-water upwellings (rich in silicon and nitrogen) and dust deposition from the Sahara ( which provides iron, phosphorus and a cocktail of essential trace elements). 89% of the Atlantic skipjack tuna is caught between the equator and the Canary Islands, the region that receives the highest contributions of Saharan desert dust.
This massive deposition of nutrients with the Saharan dust could also benefit other species of fishing interest, including other tropical tunas of commercial interest. This new study also suggests that between Gabon and Angola-Namibia there could be a migration similar to the Atlantic-Saharan one, although involving a smaller tuna stock, linked to the contributions of desert dust from Namibia and the Kalahari on the upwelling of the current Benguela.
The research team that has carried out this study is multidisciplinary, and includes experts in tropical tuna, marine biology, meteorology, atmospheric physics and geochemistry of Saharan dust belonging to the IPNA, the University of Las Palmas de Gran Canaria, the University of La Laguna, both in Spain, and other institutions.
The study is titled “African Desert Dust Influences Migrations And Fisheries Of The Atlantic Skipjack-Tuna”. And it has been published in the academic journal Atmospheric Environment. (Source: IPNA / CSIC)
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