After talking about the detection and approach stages of the capture process in trap fisheries we will proceed to examine what happens to a crab after it reaches the trap. Several studies have documented how trap design affects capture efficiency. Crabs normally crawl along the sea bottom until they encounter an odor trail emanating from a baited trap. After smelling the bait they will follow the trail upstream searching for the source. By zigzagging across this trail and sampling the odor concentrations of the substances in the water, crabs can easily locate and guide their way to the trap. After contacting the trap, this zigzag crawling will turn into short excursions to the right and left along the perimeter of the trap, and these will progress until the crab encounters an entrance that gives it access to the bait. It has also been observed that if they cannot find an entrance they will easily give up after a few attempts and look for food somewhere else.
By conducting observations through an underwater video camera and recording crab behavior around collapsible traps of different shapes, it was found that the area a crab explores around the perimeter of a trap varies depending on the shape of the base (Vazquez Archdale et al. 2003). An oval base (Fig. 9) gave wider average search angles (mean =173°) than a rectangular one (mean =108°), and this increases the probability of a crab finding one of the entrances. Traps targeting swimming crabs normally have two entrances located opposite to each other; therefore, search angles approaching 180° will almost guarantee that a crab finds one of the entrances. Similar results have been observed in Korea with C. japonica, where circular traps caught more crabs on average than rectangular ones (Kim and Ko 1987).
In many cases the target organism can be selected by the position and design of the trap's entrance. Collapsible traps usually have open funnel or narrow slit entrances, and these characteristics greatly affect entry rates. Observations of crab behavior underwater in different trap entrances confirmed that only 31% of the crabs contacting the slits could enter a box trap (Vazquez Archdale et al. 2003). The main reason for the low entry rate is that the spines in the crabs carapace and claws entangle with the netting material, and as a consequence they cannot enter the trap nor reach the bait; many will give up and leave the trap after a few unsuccessful attempts. On the other hand, 100% of the crabs reaching the open funnel entrances of the dome trap could enter by crawling sideways through the entrances, without encountering any obstruction.
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