Posted by David Itano
5 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.
Two of the many scientific objectives of this cruise are to learn as much as possible about the “natural” behavior of tuna and non-target species on (a) drifting objects, such as FADs, and (b) when all species are surrounded by the purse seine net. I’ll blog a bit on these objectives while we are running toward our next FAD and research station. The behavior of tuna and other species on FADs is a critical area of research that we will investigate with the use of “acoustic” or sonic transmitting tags manufactured VEMCO (AMIRIX Systems, Inc). We are using their products as we have been using their gear to study the dynamics of fish behavior around Hawaii for decades and it has proved robust and reliable. We will implant these tranmitters into the gut cavity of fish to track their presence or absence around a particular FAD.
What’s really exciting is that the particular tags we are deploying also record pressure approximately every 1.5 minutes that is converted to depth and transmitted to a VEMCO monitoring receiver we suspend from the study FAD. Whenever a tagged fish ventures within about ½ mile of the receiver (FAD), it’s presence and depth is recorded. This is a powerful research tool since we only have to capture the tuna once and from that time on it will automatically transmit data to us on it’s visitation rate, presence or absence and depth when associated with that FAD. If different species move away at predictable times of the day or night this may be a way to improve targeting and reduce bycatch.
During the second Leg of this cruise we will concentrate on tagging a wide range of fish species to see how they utilize a FAD during the day and night and how they separate by depth. Dr Kim Holland and I have used this technique to study the associative dynamics of yellowfin and bigeye tuna on anchored FADs in Hawaii. Our studies on the ISSF cruise will concentrate on how skipjack, yellowfin and bigeye tuna share space with the the other fish species that we are trying to avoid being captured; i.e. rainbow runner, oceanic triggerfish, mahi mahi, wahoo, oceanic sharks, etc.
While we will emphasize this work in the second half of the cruise, we did manage to tag five medium size (12 – 40 lb) bigeye tuna on our FAD #2 with depth reporting sonic tags on May 26. We returned to the same FAD June 2, set the net and captured the FAD associated school of skipjack, yellowfin and small bigeye tuna. We successfully recovered the receiver and downloaded the data. The record indicated that four of the fish remained on the FAD for most of the week, two left a few days before we returned and two were still on the FAD the morning we set and avoided our capture! We haven’t had time to look at the depth records in detail, but this is basically good news for the bigeye tuna if they are better able to avoid purse seine nets as has been speculated. Obviously, we need to do a lot more tagging.
One factor supporting this study is that we have equipped the net with special temperature/depth recorders that are housed in a shockproof housing and mounted to the bottom of the net near the chainline. After every set we download the data and re-initialize the units. This way we have concrete data on how deep each net “fished” during every set and an accurate temperature at depth profile of every fishing location.
The effective “pursing depth”, or the depth at which the net closes can determine the catch rate and species ratio of the set and is highly variable depending on current speed and shear, pursing speed, wind, etc. Current speed and direction at three pre-set depths are monitored constantly by Doppler current profilers on the bridge and this data is also recorded for our study.
Behavior of Fish in the Net
The other aspect of fish behavior we are very interested in is the behavior of all fish species when they have been surrounded by the purse seine net but have not yet begun to be crowded together by the net retrieval stage. Insights into their behavior may lead to the development of ways to release non-target catch our undersize tuna early in the fishing process when they are still in good condition.
Modern tuna purse seine nets are basically a huge panel of nylon webbing about one mile long that is deployed in a circle. The bottom of the net is closed by pulling a steel cable through purse rings that line the bottom of the net. When fully pursed, the net forms a large bowl shaped area like an upside down drawstring purse. How the tuna, sharks and non-target fish species utilize this space has long been a subject of speculation and discussion by fishermen and scientists alike. If they broadly mix throughout the process, then attempts to segregate and release bycatch would be fruitless.
To make these observations we came equipped with a video drop camera, an ROV with dual video cameras and laser scale measurement system, many cameras and scuba gear. Out of ten sets so far, we have conducted SCUBA dive surveys in five of the larger sets to document fish behavior with digital still cameras and high definition (HD) Go Pro video cameras with underwater housings. So far, the high tech ROV hasn’t been used as we find that there is nothing better than direct observation and photography by trained and highly experienced divers. Bluewater diving is not for the novice and our team has hundreds of hours of bluewater experience on anchored FADs in Hawaii and I have conducted several open water drifting FAD dives in the Indian Ocean with the EU project FADIO.
None of this prepared us for the incredible spectacle we have photo documented in what are some of the clearest, most pristine waters on the planet. We have consistently seen a strong segregation of tuna by species and size. We have been entering the net when about ¾ of the net has been retrieved. At this point the schools have been brought to accessible depths but are not yet being crowded together. Small yellowfin tuna consistently school highest in the net moving about in loose groups mixed with some small bigeye tuna. The skipjack school tightly and circle and bank in unison at greater depths and also segregate in schools by size. So far, the skipjack have been observed to circle only counter-clockwise. We jokingly wonder if they circle clockwise north of the equator?
Scientists and fishermen alike have been saying for some time that it is impossible to separate small bigeye from yellowfin tuna in the net because they are all mixed up. Maybe so but some of our images show the schooling of bigeye tuna separate from yellowfin and skipjack. We will continue to make these observations and hope for a bigger set to observe what happens in set sizes of 80 tons or more. One thing that is clear is that large yellowfin always stay together and occupy the deepest parts of the net as much as possible.
However, once the net size is decreased to a critical volume, all the tuna get mixed together. What has been the most significant finding to date is how the non-target finfish and the sharks segregate from the tuna and to some degree from each other. This phenomenon holds real promise for the development of a practical way to selectively release sharks and the miscellaneous finfish from tuna sets when they are still in a viable and healthy state. Stay tuned as we are actively developing this idea for testing during the second leg of this cruise. Aloha, Dave