Posted by David Itano
25 June 2012
David Itano is a fisheries research associate with the Pelagic Fisheries Research Program at the University of Hawaii and is currently serving as a principal investigator during the western and central Pacific Ocean cruise of the ISSF #BycatchProject. He is currently onboard the Cape Finisterre, a 1500 GRT tuna purse seine tuna fishing vessel operating in the western Pacific.
We have made many observations on this cruise; some that have been noted before and some that we believe to be completely new to science … pretty exciting stuff. In relation to shark bycatch, our detailed records corroborate information gathered during previous ISSF research cruises in other oceans – that the number of sharks that are found on a drifting FAD or natural floating object bears no relationship with how much tuna is also present. In other words, very few or no sharks may be present when a large catch of tuna is made, or the opposite may be true where a relatively high number of sharks is found on a FAD with little or no tuna present. Why is that important? Well, this suggests that effective shark avoidance measures may be able to co-exist with the need for the fishery to land commercial quantities of tuna.
Another observation that we have made in relation to sharks and non-target finfish like mahi mahi and wahoo is that once they are loaded onto the vessel during the brailing process they are usually dead or in very poor condition; so poor that post-release survival at this point is highly questionable. This is not rocket science but is being carefully tested through the analytical methods outlined in the previous Blog. The take home point is that ways to release non-target fish from the net or their total avoidance need to be developed and tested.
During the first half of this cruise, we were surprised to observe that silky sharks often collected in the same area of the net that forms when more than two thirds of the net has been rolled onboard by the power block and stacked by the crew – an area we now call the “pocket”. Fishermen onboard the Cape Finisterre with decades of experience in the fishery were equally amazed at our observations that have now been carefully documented with digital photos and video. The pocket forms naturally by the net rolling process when the main vessel is pulled in the opposite direction by the large net skiff that is attached to the starboard stern of the seiner while the bow thruster keeps the vessel even and the net rolling perpendicular to the main boom.
Other non-target fish such as mahi mahi, wahoo and rainbow runner also tend to collect in this area of the net, and so did WE if a conscious effort was not made to swim constantly towards the vessel! In other words, during most sets that we dove in, a distinct current running away from the vessel tended to advect fish (and unwary divers) toward the area of the corkline furthest from the vessel.
This phenomenon was consistent enough that the owner of the parent company Tri Marine, Mr Renato Curto agreed to fully support the development and testing of a “release panel” in this area of the net. It should be noted that the Captain of the Cape Finisterre has played a pivotal role in promoting our ability to develop and test this idea. Tri Marine flew their net expert, Mr Valerio Revalinera to Pago Pago to oversee the job and necessary modifications to the net. This was critical to the experiment as any change to this area of the purse seine net must be able to endure the pull of the power block, currents and the tremendous weight that may occur during a big haul.
A net modification that results in a ripped net and lost fish would be immediately condemned by the industry and never tried. Valerio met with Captain Crisci, the Cape Finisterre Deck Boss and the ISSF scientists to design a prototype release panel, which is a completely new concept and very different from the “sorting grid” idea for allowing bycatch to swim through a modified area of net mesh. Everyone worked closely with the crew under Valerio’s direction to quickly complete the panel during our short port stop. We headed out from Pago Harbor on June 13 to continue the cruise and test the escape panel.