Over the years fishermen have used many different techniques to find and harvest fish. However, acoustic technology is one of the most important forces driving the developments in the modern commercial fisheries. Acoustic technology is especially well suited for fish finding applications since sound travels farther and faster underwater than in air. Sound waves travel differently through fish than through water because a fish''s air-filled swim bladder has a different density than seawater. This density difference allows the detection of fish schools by analyzing the reflected sound.
Today's deep sea commercial fishing vessels rely almost completely on acoustic sensors, sounders and sonar for fish detection. Reflected sound pressure waves from these instruments are received and converted into an electrical signal visible on the ship's sonar/sounder screen. The elapsed time between transmit and receive signal provides range information, whereas the intensity of the receive signal can provide information on the size and number of fish. Fishermen also use active sounder and sonar technology to determine water depth, bottom contour and bottom composition.
Marport is a world leading marine technology company that specializes in software defined subsea acoustics. The company serves the deep sea commercial fishing sector where product performance, precision and reliability are critical. Marport's sensor, sounder and sonar products are based on proven deep sea hardware and designed to ensure error-free operation and long term dependability in the harshest of ocean environments.
In 2003 Marport began researching and developing advanced acoustic sensing, processing and communication technologies. These technologies are now being commercialized into new sensor, sounder and sonar products that are setting new standards for deep sea commercial fisheries. Marport has recently released their new Trawl Explorer digital net sounder and the system is already setting new performance standards. Trawl Explorer is a high definition digital net sounder quite unlike any other. It contains very advanced DSP sonar electronics and wireless acoustic telemetry coupled with the latest deep sea battery technology. This results in unparalleled target detection, long range communications and a battery life of over 5 days between charges. The Trawl Explorer underwater unit can be configured to use signal protocols that make it 100% compatible with Furuno's CN-24 bridge display equipment. According to Oskar Axelsson, Marport's Executive Vice President, this is proving to be a very popular retrofit.
Trawl Explorer is also available as a completely integrated system - including subsea sounder, hydrophone, acoustic receiver, deck processing/display unit and operating software. The unit is controlled by Marport's easy-to-use CommandView software and all operating parameters are at the user's finger tips, with immediate audio and visual feedback. Echo sounding video, water depth and temperature, targets and operating status are conveniently accessible to the operator at all times. The software is designed to run on commercially available PCs equipped with Windows XP.
The subsea unit uses a unique new rechargeable battery technology that provides unmatched battery life; with up to 5 days on a single charge. The underwater unit is equipped with a removable battery pack - so an operator doe not have to remove the sounder from the trawl for recharging. They simply replace the battery pack. The system uses a real-time battery life status icon as a way for operators to visually determine the remaining life of the underwater sounder's battery. The battery life status icon is shown on the CommandView display. As the battery life is depleted, the segments will appear as outlines similar to those on a mobile telephone.
Another excellent feature of Trawl Explorer is its dual downward frequency capability. At the lower frequency the sounder's beam width is wide; however, at the higher frequency the beam width narrows to enable high definition imaging and enhanced target detection. Acoustic characteristics include deep water performance (maximum depth > 2,500 meters), dual operating frequencies, and 5 different display modes. The operator can alter receiver gain manually or select fully-automatic gain control, which compensates independently for acoustic propagation loss at each frequency of operation. The user may also activate an interference rejection feature which removes spurious acoustic noise from the display.
A powerful and simple to use acoustic zoom feature allows targets to be acoustically magnified. Variable transmit pulse-length means Trawl Explorer has a remarkable range resolution of just 1.5 cm, thus making it an invaluable tool for species detection and profiling. All collected data is position and time tagged and can be digitally recorded to hard disk for later playback. The deck unit can play back historical data while simultaneously operating in real time.
The user can alter display format - from full screen video data of either five display modes or to a split screen representation of both real time and historical data. Net depths are prominently visible and displayed in easy to read digits. A robust, bottom tracking algorithm maintains bottom lock and delivers accurate depth data over even the most difficult and varying bottom topographies. Operating status messages incorporate both visual and audio feedback. Data include status of the echo sounding functions, as well as the
status for all peripheral NMEA-0183 data acquisition sensors such as GPS.
The acoustic receiver interfaces to the bridge PC via TCP/IP using Power over Ethernet network cabling. This enables the receiver to be placed close to the hydrophone and significantly reduces signal loss. Trawl Explorer was recently used on a fishery research vessel operated by IFREMER. It was attached to a pelagic trawl in operations from March 4 to 8, 2006. It was noted that the resolution of fish images of the trawl mouth was excellent, with the foot rope and chain being clearly indicated in relation to the bottom. It was also possible to recognize the variation in hardness of the bottom as the trawl progressed.
The system has also been proposed for use in sea trials of new shrimp trawl designs in Canada. These new trawl designs promise to offer a lower hydrodynamic profile for energy efficiency as well as a mouth opening optimized for more efficient harvesting in the lower sections of the trawl where, studies have shown, most of the shrimp collect. Trawl Explorers high definition target detection is expected to add significant benefits to this type of trawl.
While these are just two examples of what is now realizable, in a broader sense, Marport believes that acoustic technology is rapidly evolving from collection and display of video data to measurement and recording of actual echograms from which a wealth of information can be extracted. Systems like Marport's Trawl Explorer are what the commercial fishing industry can expect in future acoustic products.
Mr. Axelsson concluded by saying, "Our customers are facing increased market demands for fresher fish, faster at-sea turnarounds and ways to lower their fuel and operating costs. These customers consider their acoustic products to be critical, and have looked to Marport for over 10 years to supply them with sophisticated fish finding electronics. We are delivering on this requirement with the most advanced sensing, processing and communication products. Later this year, we expect to announce a new series of broadband multifunction sounders and sonars for commercial fisheries as well as emerging opportunities in ocean exploration, energy and defence."
