ISLAND CONSERVATION-Tuna overfishing threatens seabirds


22-June-2009

I miss the sight of trees and other plants, the sounds of birds and insects. Fishing by other people on board, whether they be crew members or fellow passengers, provides some useful diversion – identifying the catch and noting the size of the fish caught helps to while away the time. On many occasions, though, such as when crossing from Cosmoledo to Aldabra or from Aldabra to Assomption, it has been by observing the antics of flying fish and seabirds that I have satisfied my addiction for natural history.

If one is lucky, there will be at least one flock of birds some way off, wheeling and diving over a patch of ocean, their cries sometimes audible as they scoop up fish for themselves or perhaps for their young. I pull out my binoculars and join the birdwatchers then, eager to identify the birds and study their behaviour. At moments like these all feeling of boredom quickly dissipates!

At this time of the year, the so-called “austral winter”, when the south-east trades are the dominant wind, sooty terns or golet are among the seabirds that can be seen foraging in this manner. At the risk of sounding pedantic, I cannot refrain from quoting Victor Hugo:

Ce fluide océan, ces régions sublimes
Toutes pleines de feux, de lueurs, de rayons…
Près de nous les oiseaux et loin de nous les mondes…

Web

But the sight is one that is of interest not only to poets, artists and nature photographers. Patiently, over the years, away from the media spotlight, scientists from a wide spectrum of disciplines – climatology, oceanography, fisheries biology, seabird ecology – have been putting together the jigsaw pieces that show the feeding activities of sooty terns are a startling illustration of how the web of life on our planet operates.

From the climatologists and oceanographers we know that the seasonal drop in the sea-surface temperature triggers off an extra abundance of plankton – tiny plants and animals that drift along the surface of the ocean. From the marine biologists we know that this provides food for larger marine animals, which are themselves eaten by even larger fish and squid, exactly what sooty terns feed on - except that the golet is a surface-feeding bird (it cannot dive below the surface of the sea), and normally the fish that it preys on would be out of reach deep below.

This is where the tuna come in. They are after the same prey as the sooty terns. Swimming in large schools, they drive and concentrate flying fish and other prey close to the surface of the ocean, making them available to sooty terns as well as other birds such as Audubon’s shearwaters (riga), red-footed boobies (fou bet) and frigate birds (fregat).

Impact

It follows that the distribution of tropical seabirds is partly determined by the distribution of tuna (and other predators, such as dolphins).

Indeed, tuna fishermen search for flocks of seabirds to locate yellowfin and skipjack tuna. The irony is that, with over 5 million tonnes of tuna being fished worldwide annually, there is a very real risk of a decline in tuna populations. This would have a negative impact on the ability of golet and other birds to catch the fish that they feed on. So, overfishing could compromise not only the sustainability of the fishing industry but also the wider global ecosystem.

This is easy to understand on our island reserves like Aride, managed by the ICS, where the large endemic lizard known as Wright’s skink feeds on damaged seabirds’ eggs and on fish occasionally dropped by terns. The collapse of tuna stocks could eventually cause populations of the lizard to decline. Climate change - which could disrupt the seasonal changes in sea-surface temperature - and marine pollution, for their part, could affect the abundance of the plankton that is at the bottom of the food chain.

On reflection, these long boat trips need not make us land-based naturalists so restless after all. As the birds skim over the waves and peck adroitly at fish and squid and then head back towards the land there is plenty to ponder over regarding the interface between air, sea and land – the essence of ecology, the ultimate interconnectedness of everything.

by Pat Matyot

The Island Conservation Society promotes the conservation and restoration of island ecosystems.

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