Lobster trap detection at the Saba Bank
According to previous studies and anecdotal evidence there are a lot of lost lobster traps at the Saba Bank. One study estimated the loss to be between 210 and 795 lobster traps per year. The Saba Bank is an approximately 2,200 km2 submerged area and spiny lobster (Panulirus argus) is one of the main fisheries with an annual economic value over USD 1 million.
The traps get lost due to a combination of bad weather moving or damaging traps and marker buoy lines, ship traffic running into and cutting marker buoy lines and removal of marker buoy or theft of traps by vandals. Lost traps are a concern for the Saba Bank fisheries management, because of the potential impact of ghost fishing by lost traps and the damage to the benthic environment.
IMARES was commissioned by the Ministry of Economic Affairs, Agriculture and Innovation to advise on a method to detect lost lobster traps. The objective of this desk study was to provide information for the sustainable management of the lobster fisheries and the conservation of the Saba Bank.
In this report we investigate the most efficient method for the detection of lost lobster traps from a small vessel in water between 15 and 50 meter depth. We not only address methods for the location of traps, but also recommend methods for the retrieval of traps and measurement of ghost fishing.
The main conclusion from this study is that side scan sonar in combination with a magnetometer is the best and most efficient method to locate and retrieve lost lobster traps.
Side scan sonar is recommended for two reasons:
- The resolution of the images can be small enough to detect 1m2 traps, while the swath width and vessel speed are high enough to cover a reasonable area in an 8-hour survey day.
- In general, the survey environment at the Saba Bank is suitable for side scan sonar, as most lobster traps are located on flat surfaces with substrate composed of hard bottom pavement, rubble or sediment.
A magnetometer is recommended in combination with a side scan sonar because acoustic shadowing can occur for lobster traps located at the edge of the lobster fishing grounds. A magnetometer will detect (partial) buried, damaged or overgrown traps that have lost their distinct rectangular shape necessary to accurately identify traps from the side scan images.
The three suggested methods for retrieval are:
- Autonomous Underwater Vehicle (AUV)
- Remotely Operated Vehicle (ROV)
- Drop Camera
Since it is not reasonable to survey the entire potential lobster fishing grounds of 1.850 km2, a pilot is recommended. The pilot can be accompanied by a cost benefit analysis to advise decision makers if the costs per trap retrieved do not exceed the benefits.
It is recommended to introduce a reporting and registration system on gear loss to collect more comprehensive data on gear lost, date, coordinates, depth (unless this can be easily retrieved from coordinates data) and reason for loss. This can be incorporated in an existing fisheries landings registration system. Data on gear loss not only provide useful information for curative measures such as trap detection. It also provides information for preventive measures, as a detailed understanding of why gear is lost is needed to take effective preventive measures. They can be split in measures to prevent loss and measures to prevent ghost fishing. One measure to prevent ghost fishing is already in place, which is the use of biodegradable escape panels in the lobster traps. However, this regulation of 1992 has not been implemented properly due to lack of enforcement and compliance