Fishing Methods and the Fleet

SIODFA Fishing Methods and the Fleet

The vessels in this fishery operate far from land in depths of water from 500 to 1200m: they may not see another vessel for the up to 10 weeks they may be at sea.  Fishing operations are undertaken exclusively using aimed bentho-pelagic  trawling.  This fishing method fundamentally differs from that of traditional demersal trawling.  In deep-sea fisheries in e.g., parts of the N.E. Atlantic, the trawl may be towed along the bottom for usually 4 to 6 hours but up to 12 hours if no fish are seen entering the net.  During this time the skipper hopes that the trawl will pass through concentrations of fish.  Aimed trawling is different!  Fish aggregation must be first located and only then is the gear set, if the aggregations are in an area where fishing operations are possible.  As such, aimed-trawling in the SIO is almost always on well-defined tow lanes in assoication association with specific bottom features – ridges, knolls, hills and, at times, seamounts. 

Fishing tactics depend on whether the skipper is targeting orange roughy or alfonsino.  Orange roughy fisheries in the SIO are characteristically undertaken during the (austral) winter and fishing may occur during the day and night  .  Often the fish location changes dramatically from one year to another, a function of the highly dynamic oceanography of the south-west Indian Ocean and the close relation between fish distribution and water teperature.  Thus, successful operators must have knowledge of a wide range (>200) of seafloor features over an enormous expanse of ocean where the fish may potentially occur.  Then, many sea floor features may have bottoms so rough that fishing is not possibler in all but a few highly defined fishing lanes: On some sea floor feature boats have had to queue to make their shot or leave for another seafloor feature. 

The trawl may be up to two kilometres behind and, at the end of a tow, 1500 m beneath the vessel.  Despite this, GPS satellite navigation (accuracy ±10 m rms) and acoustic instrumentation (accuracy of centimetres) enable a relatively small trawl (foot rope length 20 m) to be precisely positioned both with respect to the targeted fish aggregation and the bottom.  When fishing orange roughy the foot rope is characteristically in contact with the bottom for two to twenty minutes so most of the time the gear is in the water the net is being shot, positioned or retrieved. As the trawl doors, which are designed for pelagic fishing, are keptoff the bottom the amount of sea bed touched by fishing gear during a tow or shot is minimal.

When the skipper targets alfonsino, a different, midwater, trawl is used.  In these cases the trawl may not make contact with the bottom at all – depending on the skipper.  Indeed, the normal absence of benthic bycatch when midwater trawling for alfonsino confirms the rarity of bottom contact with this type of fishing.  Alfonsino are highly mobile and ‘difficult-to-catch’, especially during the day.  Fishing is usually not successful at this time as the fish avoid the gear.  Indeed, at any time, a large number of tows (around 40%) may catch nothing as alfonsino successfully avoid the net.

The vessels used in this fishery are factory trawlers capable of staying at sea for up to 10 weeks.  They process the catch in the form required by the markets, e.g. frozen whole, dressed, filleted, etc.  Crew size depends on the size of the vessel: in the case of the SIODFA boats it ranges from 30 to 42, who rotate, trip on, trip off.  The vessels themselves were built in various countries: Scotland, Japan and Norway and  range in length from 68 to 74 m. Past vessels in the fleet were also built in France, Italy and Germany.  Vessels  are equipped with SIMRAD ES60 acoustic systems that enables them to undertake aggregation-based acoustic stock assessment surveys.  Two of the vessels actively use underwater video systems and are further refining this technique for on-site identification of ‘vulnerable marine ecosystems’. 

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