Entry of Cancer productus to baited traps

Abstract

Ease of finding the entrance was shown to be an important variable determining the effectiveness of baited crab traps. Traps for Cancer productus should be designed so the bait odour leads the crab to the trap entrance, not just to the trap. The operation of both top and side entry traps was observed in a large tank with the aid of closed circuit television and a time lapse videotape recorder. Crabs responding to the bait first encountered the side of a trap directly downstream of the bait. If the entrance was not at this point their search consisted of short excursions to the right and left and sometimes up the side of the trap. Only 27% of the approaches to the top entry trap resulted in crabs entering even though the entrance was but 30 cm from the point on the trap directly downstream of the bait. Only 7% of approaches to the side entry trap set with opposing entrances perpendicular to the current gained entry even though the entrances were but 40 cm from the point on the trap directly downstream of the bait. For the side entry trap set with entrances parallel to the current, giving one entrance directly downstream from the bait, 65% of the approaches resulted in entry. Most crabs that did not gain entry left the vicinity of traps as a result of being chased away by other crabs on the outside or after prolonged but unsuccessful searching for the entrance. Trap saturation was apparent in both trap types and was presumed due to the crabs inside traps intimidating those outside traps which would have otherwise entered.