Interspecies Bait Manipulation Video Study
Mark Bond, Steven Kessel & Dr. Samuel H. Gruber
Long lining is a major fishing method utilised world-wide in commercial fisheries, with sharks constitute a large percentage of the by-catch. Global statistics are drawn from the data collected such as CPUE and population models. The idea is that interspecies specific behaviour's when approaching and taking the suspended bait may influence that individual's capture susceptibility. This increase/decrease in capture susceptibility (as a product of behaviour and bait handling) could bias the statistics drawn from the data collected e.g. population estimations from Longline CPUE statistics.
It would not be to unfeasible to comprehend that certain species of shark have developed/learned the ability to remove bait from hooks successfully, which could have influenced the reduction in Lemon Sharks (Negaprion brevirostris) caught on longlines (evident in Bimini). It was found that a given stimulus would generate in any shark a specific, un-learned response. This response would be performed the same way, time-after-time, independent of context or probable survival value - Martin (2003) therefore they display an evolving intellect. Lemon Sharks are known to be susceptible to conditioning - Clark (1963), therefore it is possible they can successfully steal bait intentionally without getting hooked. This can affect the CPUE which are used for estimating populations in fisheries and could therefore imply that there are fewer Lemon Sharks than is the reality or vice versa.
The four major species which are caught on our twice-monthly longlines are Lemon sharks, Nurse sharks (Ginglymostoma cirratum), Blacktip sharks (Carcharhinus limbatus) and Tiger sharks (Galeocerdo cuvier). In the Bimini longline records (dating back to 1982) we can see a reduction in the number of Lemon sharks caught via longlines.
The aim is to examine whether this is a result of the Lemon sharks "bait handling" and behaviours or simply a decrease in the lemon sharks population of the Bimini area. The behaviours and "bait handling" techniques of the other three species shall therefore also be studied and examined for comparison. Other objectives of these trials include identifying specific behavioural patterns such as those determined by Huse & Ferno (1990) from the time the shark enters the "survey area" until it either takes the baited hook or exits for a given time period. Methodology
The bait manipulation trial involves setting up a 30m mainline secured each end with cinder blocks and an anchor, with a single baited gangion attached in the centre. A gangion is a hook-line and many gangions attached to a mainline constitute a longline. The bait will remain suspended in the water column by a surface float as this is representational of a single gangion that you would find on a longline. The bait is set downstream of a chum box to create an "odour corridor" to attract the sharks. An underwater video camera is positioned to view the bait and a recording unit set up on the boat recording. Shifts are rotated on the boat to document any behaviours or approaches as seen on the screen. Another person notes approaches that can be seen from above the water but are not within the camera's range of view. Examples of the types of behaviours which shall be documented are:
Incomplete Bite -The fish takes only part of the bait and doesn't close its mouth entirely round the baited hook.
Pulling - The fish swims slowly away with the gangion stretched with the baited hook in its mouth. (Huse & Ferno, 1990)