Luring the Crab to the Trap


Many fishers use fish of no commercial value or "trash fish" as bait, but animal byproducts such as viscera, skin or heads, salted or dried fish, smoked cormorant, frogs, chicken offal and many other things can be used as well. Commercial trap fishers usually prefer to use frozen fish bait, like mackerel or sardine, because from past experience they know that to have a good catch they need high quality bait. Some studies have examined the active ingredients found in bait that are responsible for the attraction that induces animals to move into the trap. Swimming crabs are scavengers that rely heavily on their sense of smell for locating their food. Their nose-equivalent is a small appendage resembling an antenna called the "antennule", and the two they possess are located just between their eyes (Fig. 3). Their entire body surface is also lined with chemical and mechanical receptors, so that walking legs, mouthparts and claws act as sensors that help them locate and handle their prey. Water-soluble compounds from damaged tissues, excretions and decaying organisms act as strong stimuli and affect crab feeding behavior, and this explains why fishers employ bait to attract crabs into traps.

Research conducted to identify individual attracting substances found in decapod crustacean food and prey has been carried out in the past, mostly by bioassay and electrophysiological experiments. Isolating stimulating compounds from natural prey organisms and testing them with live decapods, or appendage preparations where the receptors are still viable, have elucidated that substances of low molecular weight, such as amino acids, saccharides (sugars), nucleotides and ammonium compounds, are mostly responsible for the food searching responses elicited by the bait (Ache 1982, Carr 1988). Other findings have documented that different species of decapods respond to different mixtures of these compounds, and that the effect of mixing individual substances is synergistic.

Crab Antennules
Figure 3. Frontal view of P. pelagicus showing mouthparts, eyes, antenna and the position of antennules (inside the circle).

By studying the behavior of swimming crabs placed in aquaria after adding a known volume of fish extract to the seawater in which they are held, a characteristic food searching sequence can be observed. Crabs exposed to fish extract commonly show an increase in the rate of antennule flicking (equivalent to sniffing through our nose), move their mouthparts, and crawl searching for food around the aquarium while prodding its sandy bottom with legs and claws trying to find the food item. Once this behavioral sequence is observed, identifying stimulating substances can be easily accomplished by adding individual chemicals diluted in seawater at increasing concentrations with a dropper and observing which elicit the same responses as the fish extract. Following this method, individual stimulating substances can be determined, together with their sensitivity thresholds. Bioassay experiments using the swimming crabs P. pelagicus and Charybis feriata have shown that the most stimulatory amino acids were alanine, glycine and serine within the 2 x 10-7 - 2 x 10-4 mol/l range; while the saccharides galactose and glucose were more stimulant than these amino acids at the same concentrations (Archdale and Nakamura, 1992). These levels are only slightly above the low ambient amino acid levels found in seawater, which are in the range of 10-8 mol/l (Carr 1988), and they confirm that crabs can detect minor changes in the chemical composition of the water surrounding them. These same amino acids have been identified by chemical analysis in the prey organisms of swimming crabs, which consist mainly of other crustaceans, molluscs and polychaete worms. Saccharides are present in both the blood of fish and hemolymph of crustaceans. Though similar responses showing the dual importance of sugars in crab chemoreception have been observed in the porcelain, ghost and fiddler crabs, possible applications of using sweet bait were not investigated until 1995 (Kawamura et al. 1995), when sugarcane was tried as an alternative bait for crab traps during fishing trials.

Sugarcane and Fish Bait Combinations

After determining which substances are responsible for the stimulating nature of bait, the possibility of making it more attractive should be considered. In the previous section, sugars where found to be highly attracting to swimming crabs. Sugarcane is a cheap and readily available agricultural crop (Fig. 4), which can be easily grown in tropical climates.

Figure 4. Sugarcane

The possibility of using short sections of split sugarcane as alternative bait for crab traps was tested during fishing trials in Kagoshima, southern Japan (Kawamura et al. 1995). The baiting treatments applied to the traps were fish bait, sugarcane, a combination of the two, and no bait (Fig, 5 top). Swimming crab catches of P. pelagicus were almost doubled and that of C. japonica more than tripled in traps baited with the fish-sugarcane combination, while sugarcane alone was not effective. The increase in attractiveness of the bait combination was probably due to a synergistic effect resulting from the mixture of attractive sugars and amino acids present in both fish and sugarcane. Further trials carried out in Panay Island, the Philippines, using the same treatments for trap bait also confirmed the effectiveness of the bait combination, which caught 48% more crabs than those baited with fish alone (Fig. 5 bottom) (Anraku et al. 2001). During the study in the Philippines, sugarcane baited traps still caught one-third the number of crabs caught by fish baited traps, efficient enough to justify its application, as reported by some of the fishers that follow this practice locally.

Sweet substances can enhance the effectiveness of ordinary fish baits for traps targeting swimming crabs, and might be used in a purer form to enrich existing fish bait and boost effectiveness. The next section will examine the possibility of enriching current fish bait with sugar.

Fish Trap The Philippines
Figure 5. Baiting treatments: sugarcane, fish and combination (top 3) and fishing operation using bamboo basket traps in Panay Island, the Philippines (bottom).

Fish Mince in Teabag

Enriching the bait with attractants, in the same way aquaculture feeds are supplemented with vitamins and minerals, has become a new possibility. Teabags have been used to bind fish processed into mince and may be used as a cheaper alternative to conventional fish bait (Vazquez Archdale et al. 2008). The fish was minced using a meat chopper and packed into large teabags and used as alternative bait (Fig. 6). Traps baited with fish mince teabags caught as many crabs as those using the same amount of ordinary unprocessed fish bait. The remains of the fish bait found in the traps indicated that crabs prefer to consume the viscera of the fish employed; therefore, the possibility of making cheaper bait using only fish by-products, such as head, internal organs, skin and bones, should be tested in the future. This would save a considerable amount of edible fish meat that can be destined to become human food or animal feed.

This novel binding method was also used to investigate whether enriching the fish mince with ordinary table sugar to sweeten it could increase crab catches. The results showed no significant differences to using conventional fish bait in traps for P. pelagicus; but C. japonica and other swimming crab species disliked the sweet mince and were caught at half the rates of fish baited traps, which means that the taste preference varies depending on the crab species, and indicates that more selective crab baits may be designed in the future.

Figure 6. Mincing fish (left) and packing it into a teabag (right).


Traditionally, two crustacean trap fisheries have used live decoys to lure conspecifics into traps. The blue crab Callinectes sapidus and spiny lobster Panulirus argus have been employed as live decoys by fishers. In this case, it is not the presence of bait that acts as an attractant, but the company of their congeners or finding a potential mating partner. The effectiveness of these decoys is still under investigation, but evidence points to the presence of sexual or aggregating pheromones in their urine or the "calls" they produce. Usually adult male blue crabs are used to attract female conspecifics into traps. This method is particularly effective during mating time, when crabs are not interested in feeding and will not enter baited traps (Van Engel 1958). Juvenile spiny lobsters, known as "shorts", are used as decoys in a similar way to attract adults. Research with lobster decoys placed in traps also confirmed their effectiveness, which was higher than the wide variety of types of bait tested (Heatwole et al. 1988).

Live decoys have several advantages over conventional fish bait, the main one being that they eliminate non-target animals from the catch, which are usually attracted into the traps by the nutritional reward found in bait. In addition, decoys can remain alive for weeks and even months while kept submerged in the water inside the traps; permitting their long-term use and at the same time reducing the waste of fisheries resources and money that bait incurs. Fish bait can only be used once and will last for a few days at most in the water due to the work of scavengers or because it rapidly decomposes.

Fishing trials were conducted to test the effectiveness of using live decoys placed in traps for the capture of C. japonica (Vazquez Archdale, unpublished results). At first, by simply inserting individual adult crabs into box traps with slit entrances; this entrance type acts as a one-way valve and hinders escape (Fig. 2, top). Testing the effectiveness of individual male and female crab decoys in comparison to fish bait and non-baited traps has shown that effectiveness depended on the length of time the traps remained in the water (soaking time). Trials done by soaking the traps overnight showed that fish bait was the best luring method while the decoys only attracted similar numbers to those caught in non-baited traps. On the other hand, when the traps where soaked for 4-days, the effectiveness of the female decoys proved to be six times higher than fish bait, and non-target organisms were captured at about one third the capture rate of fish bait. This suggests that there is potential to using C. japonica decoys or the pheromones they emit to capture conspecifics, and this could have great applications in both capture fisheries and eradication programs.

Figure 7. Decoy in trap (left) and decoy mating (right).

Several problems were encountered during the first decoy experiment; a major one was the box trap design, which permitted any octopuses in the catch to predate on the decoys. Additional fishing trials were carried out employing a new trap design with separate chambers to contain the decoys, and this was effective for eliminating predation because octopuses could not reach the crabs (Fig. 8). Furthermore, there was no decoy mortality during the month the experiment lasted, even after exposing the decoy crabs to pressure changes during trap hauling and setting to a 15-meter depth and keeping them exposed in the air for 15 minutes during the daily handling of the traps on the boat deck. Additional trials testing the effectiveness of baited and decoyed traps in a commercial crab fishing ground confirmed the effectiveness of female decoy crabs. These decoys caught an average of 0.80 crabs per decoy while only 0.25 were caught per fish bait, and this was obtained using a short 1-day soaking time. The catch of non-target organisms was also reduced five times in decoyed traps. The luring potential of female crabs was thus demonstrated and the possible application of this method for eradication purposes of invasive crabs should be further investigated.

Figure 8. Decoy in trap (top chamber) and crab catch (bottom chamber).

Recent research on another invasive crustacean, the green crab Carcinus maenas, is testing the possibility of manufacturing pheromone bait by binding the active components of this crab's urine into a gel matrix (Hardege et al. 2002, Behrens Yamada et al. 2006). Continuing with this line of research in the future may offer an alternative method to trapping with fish bait, which is hardly species selective. In addition, pheromone bait and decoys are only attractive to members of the species concerned, they are highly selective and greatly reduce the presence of non-target animals in the catch; therefore, their use in eradication programs may be highly valuable in the future. Important implications are that the handling stress and damage inflicted to non-target species can be eliminated, or reduced to a minimum, as a result of them not being attracted into the trap, as baited traps would.

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  • Jere
    How to attract crabs to your traps?
    3 years ago

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