With seafood exports totaling $2 billion in 2006 and accounting for half of Alaska’s total export value, the fishing industry is Alaska’s largest employer and one of the state’s main economic resources. Overexploitation in the 1940s and 1950s took a heavy toll on the industry and drastic measures to manage stocks and fishing were implemented to save the industry from collapse.
The Search for a Seafloor Imaging SystemOne of the main roles of the Alaska Department of Fish and Game (ADF&G) is to ensure sustainability and a harvestable surplus of fish and wildlife resources for the 49th state. The ADF&G began experimenting with underwater video technology for scallop stock assessment in 1999. The prototype system consisted of a sled that held a camcorder in a pressured housing and battery-powered lights towed from a trawl warp.
The system proved to be insufficient in terms of image quality, collection of useable image data and power supply. In its search to develop a more effective high-speed megapixel benthic imaging system, the ADF&G turned to Woods Hole Oceanographic Institution (Woods Hole, MA), creator of the first system using machine vision cameras for fisheries research purposes, which recommended the use of Gigabit Ethernet technology.
The technical issues highlighted above needed to be addressed in the development of this new benthic imaging system. The ADF&G contacted Prosilica (Burnaby, Canada) in January 2006, and based on the requirements, Prosilica recommended its GE1380C GigE Vision camera. The GE1380C is a compact 1.4 megapixel digital camera with color output that incorporates the Sony ICX285 charge-coupled device (CCD) sensor providing high sensitivity, low noise and excellent image quality even when operating under low-light conditions at depths of 40 to 140 meters. The camera runs up to 20 frames per seconds at full resolution and is capable of streaming data at a sustained rate of 125 megabytes per second. Its Gigabit Ethernet interface is virtually plug-and-play and allows for cabling with lengths of up to 100 meters using conventional Cat5e network cable, or up to several kilometers using fiber-optic cables.
System Setup and TestsThe system developed by the ADF&G consists of a tow vessel and a camera sled. It is based on a local area network (LAN) with two Gigabit Ethernet switches connected by a fiber optic link, one mounted in a pressure housing on the sled and the other located with computers in a deck shelter on the tow vessel. Each network component-hard drive array, three computers, a serial device service, obstacle avoidance sonar, temperature sensor, pitch and roll sensor and the Prosilica GE1380C-connects to either switch via standard Cat-5e cabling. Each switch is connected to a pair of bi-directional single mode fiber optic media converters to allow data transmission to distances of 2 kilometers and more.
Three off-the-shelf PCs are connected to the topside switch located on the vessel, one for remote control of the GE1380C camera and recording, and the other two to run the navigation software developed by the ADF&G programmers and to record GPS data and communicate with serial devices on the sled. The GE1380C camera, fitted with an 8-millimeter fixed focal length lens, was mounted in its own pressure housing on the sled and images through a domed glass port. Strobe lighting was used to ensure crisp images at towing speeds of 5 to 8.5 kilometers per hour under extreme low-light conditions.
The system was first tested on board the Pandalus vessel in June 2006 and again in June 2007 using a larger and improved sled. Measuring 3.5 meters by 1.8 meters, the new sled makes contact with the bottom using two skis. In addition, four strobe lights mounted in individual pressure housing were placed around the camera to illuminate the area imaged ahead of the skis.
During both cruises totaling a period of 12 days, 1.1 million images at depths of 40 to 140 meters were captured on a track of 550 kilometers. The GE1380C performed extremely well operating at four frames per second and tow speeds of 5 to 8.5 kilometers per hour, providing high resolution, which enabled identification of objects smaller than 50 millimeters in most water conditions.
Following several tests, it has been shown that the sled system, initially developed for scallop stock assessment surveys, is also extremely useful for other fishing industry applications such as fine-scale habitat mapping, ground-truthing acoustic data, benthic ecology research and fishing gear effects studies.