In the middle of the Pacific Ocean, Gonzalo Mucientes has discovered an invisible line in the sea that separates male mako sharks from females. The line runs from north to south with the Pitcairn Islands to its west and Easter Island to its east. On the western side, a fisherman that snags a mako will most probably have caught a male. Travel 10 degrees of longitude east and odds are they'd catch a female. This is a shark that takes segregation of the sexes to new heights.
Mucientes and colleagues from Spain, Portugal and the UK spent four months aboard a Spanish longline fishing vessel. Amid more typical catches, the boat often snagged shortfin makos and blue sharks. When they did, the researchers meticulously noted the boat's position, and analysed the shark carcasses on board.
Their results showed a clear "line in the sea". All in all, the fishermen captured 264 male makos, mostly towards the west of the line and 132 females predominantly in the east. Makos are found all over the world, but this study shows that at a more regional level, their populations are structured to an astonishing degree.
This segregation is even more surprising when you consider that makos are the world's fastest sharks. They can clock speeds of up to 45 miles per hour, about eight times as fast as Michael Phelps at his peak. They really shouldn't have any problems in covering vast tracts of water.
Mucientes found no obvious reason why the two sexes should be so divided. Their split did not depend on how warm the surrounding waters were, or their productivity (as indicated by the level of chlorophyll in the water). By checking the stomachs of the captured sharks, Mucientes couldn't detect any differences in their diet to suggest that they were following geographically separated prey.
With no ready explanation, Mucientes acknowledges that the best he can offer is a reasonable guess, but it's one that many humans would relate to - sexual harassment. Mako courting is a violent affair with males often inflicting serious bites on their intendeds. It's possible that females are actively trying to avoid males, seeking them out only when they absolutely have to in order to mate.
Obviously, the separation isn't complete. Male and female makos mix to some extent, particularly towards the southern part of the Pacific where there is a stark divide between warmer and colder waters. These "thermal fronts" are often rich in prey, and they could present a good site for makos to feed, court and mate.
Not all sharks are as sharply segregated as the makos. Mucientes found that male and females blue sharks are distributed throughout the entire area. However, males were more common everywhere, which suggests that this species may also have a "line in the sea", albeit a fuzzier one that operates over a larger scale.
Mucientes's study shows just how foolish it is to assume that sharks are as evenly spread out through the oceans. We know that sharks prefer some areas to others, and we now have further proof that different sexes haunt different stretches of water. If fishermen kill too many in the wrong place, they could wipe out massive portions of the population, or important components of it.
Imagine if a species was mostly wiped out from an area where the majority of males gather - the population as a whole might still have reasonable numbers, but it's future would be questionable. That's a very real problem - Mucientes found that over the last half-century, fishing efforts have been far more intense on the western side of the invisible line, where males predominate. The females on the eastern side have been spared the threat of metallic hooks to a far greater extent.
This may explain why so many shark species have declined dramatically. In the Northwest Atlantic Ocean, for example, populations of white, thresher and scalloped hammerhead sharks have plummeted by 75% in just 15 years. And in the western Atlantic, the numbers of male blue sharks have fell by 80% between 1977 and 1994, while female populations were stable.
Everywhere, in fact, sharks are facing the threat of extinction. Fishing operations are expanding into the open oceans where they swim, harvesting not only their food but the sharks themselves. Think, for example, of the staggering number of makos caught by this single fishing vessel in just four months. These fish grow slowly, mature late and give birth to few young - taking out too many adults jeopardises the fate of the entire population. And removing sharks can have drastic consequences for other species in the sea.
Photos: by J Stafford
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Reference: Biology Letters doi:10.1098/rsbl.2008.0761