Tit is a dramatic subject for nature documentaries. During the southern hemisphere winter, swirling masses of Pacific silver sardines (Sardinops sagax) migrate along the eastern coast of South Africa to the Indian Ocean, but for some the journey ends ends too early. Along the way, predators such as seabirds and marine mammals feast on the huge collection of fish, often dubbed “the largest shoal on earth”.
Indeed, according to a study published on September 15 in Scientists progress, the KwaZulu-Natal sardine race is an ecological trap: a scenario where the behavior of fish pushes them into unfavorable habitat that decreases their chances of survival.
“Very little is known about the fate of these sardines,” said Peter Teske, a marine biologist at the University of Johannesburg and one of the study’s authors, in an email to The scientist. But he and his colleagues conclude that unlike many migrations, which may be related to food, reproduction, or both, the sardine race is a sort of dead end.
Cape Agulhas, at the southwestern tip of the African continent, marks the dividing line between the Atlantic Ocean to the west and the Indian Ocean to the south and east. The fisheries to the south and west of the cape make sardines one of the most commercially important fish in southern Africa, but the western fishery has declined. One of the objectives of the study was to understand why southern sardines did not repopulate the western fishery. If sardines were not a homogeneous group but divided into distinct populations, this could also explain why only some participate in the north-eastern migration.
See “Small fish in a large pond”
Less than 10 percent of all sardines living off the South African coast participate in winter migration, which only occurs in certain years, depending on whether or not fresh water upwelling occurs off the south coast. is. These upwelling draw sardines east around South Africa, but the fish rarely leave the relatively shallow waters of the continental shelf. This plateau narrows off the southeastern coast, and just offshore is the Aiguilles Current, which brings in tropical water from the Indian Ocean that is too warm for the comfort of sardines. Trapped between the warm current and the shore, the sardines in this area only have a small space to move north. Why they go north has never been entirely clear, but when they do, this expanse of narrow continental shelf assembles a great many, forming the famous race that attracts predators, human fishermen, and tourists.
In the study, the research team set out to test the hypothesis that only a certain subgroup of sardines participate in the migration and explore why these sardines make the trip. “Initially we suspected there must be three stocks,” or distinct subpopulations, of sardines, Teske explains: east, west and south, corresponding to the different water temperatures on different sides of the cape. This division based on water temperature would correspond to patterns observed in other marine species.
The study authors sampled sardines in rangelands in 2015, 2018 and 2019, as well as throughout their normal range. By comparing their DNA and mRNA, they found single nucleotide polymorphisms (SNPs) that were more common in western Atlantic fish, while others tended to be found in the southern Atlantic. Indian Ocean, but they found no evidence of a third stock. Sardines caught in sardine races corresponded more closely to the Western group.
Teske explains that although the genetic differences between the stocks were generally minimal and there is some evidence that western and southern fish interbreed to some extent, the division into two subpopulations was “most strongly. substantiated on the basis of genetic markers which show a strong correlation. with the water temperature. Water temperatures are generally cooler in the Atlantic Ocean off the west coast than they are south of Cape Agulhas. Although their functional significance is not yet known, the genetic markers identified in the study suggest that the two groups of fish are adapted to these different temperatures. And, says Teske, that explains why the sardines found in the race are the furthest away, across the cape: Western fish’s preference for cooler waters makes them the most likely to be drawn into the race. .
Dolphins chasing fish during a sardine race
“As far as we can tell, sardines on the west coast associate with upwelling areas, and they don’t find much on the south coast. suitable habitat be found, “Teske continues. Atlantic fish just need cooler water and they will follow it in the direction they find it, including those temporary pockets of upwelling. -it, “some of them migrated so far east that they ended up being attracted to the upwelling … on the southeast coast.”
In the article, the team says their genetic evidence suggests that the fish are therefore drawn into a trap. That is, the behavior that prompts them to seek cool water causes them to swim to the east coast in warmer, stressful water. Not to mention the dangers: hungry sharks, dolphins, sea birds and much more waiting for a delicious treat.
Environmentalist William Sydeman, president of the Farallon Institute for Advanced Ecosystem Research in Petaluma, Calif., Who was not involved in the current research, says he would like to see more research before being convinced that he is. is a real ecological trap. Upwelling leads to plankton blooms, creating a lot of food for sardines, he notes. So I think [the fish are] do it for a reason ”, and more evidence is needed to show the full effect of migration on the fish that make the trip. It is possible, he says, that “they are migrating there in order to take advantage of the temporarily productive ocean conditions at the Western Cape,” so if the area is not suitable for them, he asks, “Why don’t they just do it? ‘reverse? Why don’t they back down? ”(Teske speculates that one of the reasons there is no mass migration in the opposite direction of the race could be the hungry predators following the southern sardines. east, cutting the way back.)
Likewise, Catherine Macdonald, a marine biologist at the University of Miami who was not involved in the research, suggests in an email that it is worth considering whether the behavior is “perhaps more adaptive.” rather than something that was never adaptive in the first place. “As we continue to see the effects of global climate change intensify, I think we can expect to see many previously adaptive migration patterns altered. Previous hypotheses have posed, for example, that the sardine run is a vestige of the last ice age, when the Indian Ocean would have been a more hospitable spawning ground for sardines.
Research like this ultimately helps manage fisheries. “We don’t want to manage two groups of fish that breed largely separately as if they were one group of fish that all breed together,” says Macdonald. The Atlantic sardine fishery in the region has already collapsed. By showing that the sardine populations in the west and in the south are distinct, says Teske, “we can not only explain this collapse, but also convince fisheries officials that if they overexploit this area, the local sardine population will not simply replenish itself from the south coast. “
In other words, we’ll be better able to protect the stars of the world’s largest shoal.