Sustainability-driven new research could one day help tuna fisheries cast their nets more selectively, mitigating bycatch of undersized fish and off-limits species. A multinational research team has carried out a study under the sponsorship of the International Seafood Sustainability Foundation (ISSF).
In a research article titled Acoustic Discrimination of Tropical Tuna, published in PLOS ONE, the peer-reviewed open-access journal of the Public Library of Science, the ISSF team’s findings address a challenge for commercial fishing fleets: How is it possible to identify and harvest mature, sustainable tuna without disturbing young fish or other species in the same vicinity?
‘If you’re working with land animals, you can walk right up to the herd and count them by size and species,’ said corresponding author Dr Gala Moreno.
‘But, typically, fish are out of your reach, hard to see, and constantly moving in three dimensions.’
The primary tuna fishing method, purse-seining, is conducted around fish-aggregating devices (FADs), passive structures built to exploit marine life’s tendency to gather beneath floating objects.
Under ideal circumstances, a purse seiner would head for a FAD location where the integral electronics have pinpointed a promising mass of fish, and would then set its gear to take a uniform shot of mature skipjack – considered plentiful enough to be caught sustainably.
In practice, FADs attract mixed populations that include undersized skipjack, as well as yellowfin and bigeye tuna of all sizes, which can be proscribed in certain waters. But sorting them out before the net closes in is practically impossible. A technology capable of analysing the proportions of various tuna species swimming together, at a distance, would help skippers selectively target sustainable species, save fuel and consequently reduce emissions by going straight to the spot with the best catch potential, and also reduce bycatch and vulnerable species mortality.
Current acoustic technology can detect schools of tuna around FADs remotely, but so far no conventional onboard or FAD-mounted device can distinguish fine detail such as fish sizes or species. Based on innovative field work, performed at sea on commercial fishing vessels, the new paper explores a technique for repurposing standard echolocation devices to predict not only the quantities but also the mix of sizes and species in an aggregation of fish.
A key factor is the swim bladder that regulates buoyancy in most bony fishes, and which provides the reflector that reflect signature signals back to the catching vessel.
Bigeye and yellowfin have similar swim bladders, but skipjack make do with no swim bladders at all. The resulting difference in acoustic signatures could eventually help skippers distinguish skipjack from bigeye and yellowfin remotely.
Although acoustic properties have long been used to assess the abundance and species of fish other than tuna, the ISSF team’s work has produced the first sonic-discrimination data that could lead to practical, selective tuna fishing techniques around FADs in the tropics.
Instruments on board the team’s host vessel supplied an unprecedented level of calibrated acoustic data describing the frequency responses of tropical tuna. Measurements stemming from the vessel’s capture of entire tuna aggregations were vastly more reliable than comparable data gathered in earlier surveys involving smaller-scale fishing operations.
‘Our research opens the door to several positive developments: faster growth of knowledge about the acoustic properties of tropical tunas; the advantages of close collaboration with commercial operations in support of science; definition of the conditions necessary for applying this technology for selective fishing; and projections of other uses of direct acoustic observations, as direct indices of tropical tuna abundance, to support tuna conservation,’ Dr. Moreno said.
