Sidc f i

Figure 10. Types of entries observed for crabs in box trap (left) and dome trap (right).

Average times to entry after reaching the entrance were 12.3 minutes versus only 2 minutes, in box and dome traps respectively (Table 1). The average number of attempts was 13 in the box trap and only 1.3 in the dome trap, which demonstrates that crabs usually go into the latter in only one try (Vazquez Archdale et al. 2006a).

Table 1. Entry of Charybdis japonica into box and dome traps in tank experiment

Entrance type

Box trap (slit)

Dome trap (open)

No. of crabs at entrance

30

30

No. of crabs entering

19**

30

No. of attempts (avg. and range)

13 (2-45)

1.3**(1-4)

Time to entry (avg. and range)(min)

12.3 (0.31-54.90)

2.0* (0.05-28.05)

Significantly different: *p < 0.05 and **p < 0.01.

Significantly different: *p < 0.05 and **p < 0.01.

From the behavior observations it was found that the shape of the trap and design of the entrances greatly affect the ingress rates of the target crabs. Open funnel entrances fitted in dome traps were more easily encountered because the crabs searched larger areas around the perimeter of the trap. Once at the entrance, all the crabs could ingress the trap by naturally crawling sideways in the direction of the bait. Conversely, the search areas around the box traps were smaller, and in several cases the crabs could not locate an entrance during their excursions; in addition, after reaching the entrance, the netting material and narrow slits hindered their ingress efforts and resulted in many crabs giving up.

After completing the observations on crab behavior around traps of different designs and elucidating some of the factors influencing their capture process, comparative fishing trials were required to test catching performance and to determine which trap design was best for capturing crabs and would be suitable for eradication purposes.

Fishing Trials Conducted in a Pond

To test the previous behavioral results, preliminary fishing trials were conducted in a large pond using box and dome traps (Vazquez Archdale and Kuwahara 2005). Unexpectedly, the catch of the box traps was more numerous than that of the dome traps, and this was attributed to differences in mesh size, entrance type and possibly the escape rates of the captured organisms from both trap designs. Additional trials employing a newly designed dome trap fitted with smaller mesh netting (2.3 cm) as opposed to the usual mesh netting (6cm), confirmed its superiority for capturing all sizes of swimming crabs (Fig. 11) (Vazquez Archdale et al. 2006b).

Figure 11. Side views of eradication dome trap with open funnels (mesh size 2.3 cm).

This variation of the dome design was also found ideal for eradication purposes, because the number of crabs in the catch was more numerous than the other two trap designs, regardless of the size of the crabs (Table 2).

Table 2. Catch of crabs and non-target organisms according to trap type

No. of organisms (proportion)

Table 2. Catch of crabs and non-target organisms according to trap type

No. of organisms (proportion)

Catch

Box trap

Dome trap (small mesh)

Dome trap (large mesh)

Swimming crab

390 (0.72)

517 (0.85)

76 (0.95)

Other crustaceans

7

20

2

Moray eel

3

Rockfish

5

5

Catfish

61

18

Goby

11

17

Damselfish

1

4

Snail

51

23

Octopus

5

3

Others

6

2

2

Non-target organisms

150 (0.28)

92 (0.15)

4 (0.05)

No. of traps

100

100

100

Non-target

organisms/trap

1.5

0.92

0.04

Dome traps equipped with the smaller mesh netting performed better than box traps by catching more swimming crabs and allowing many non-target organisms to escape through the open funnels. The results of these trials proved that the dome design is superior at capturing swimming crabs and reducing non-target species in the catch and should be used for eradication programs. Box traps caught less swimming crabs than the small meshed dome traps and almost twice as many non-target organisms, making them undesirable as eradication gear. The entrances fitted in the box traps close after an animal pushes through the tight slit acting as a one-way valve, making it impossible for any organism to escape from inside. The catch from dome traps equipped with larger mesh netting (6 cm mesh size) was almost exclusively composed of large commercial swimming crabs, with hardly any non-target organisms. This contrasts with the results obtained with the box traps, which caught many undersized crabs and non-target species. This finding confirms the great selectivity of the dome design for harvesting commercial sized swimming crabs.

After conducting the fishing trials with the different trap designs and determining their entry rates, a main cause for concern was the possible negative effect that lost traps might cause during a large eradication program. Traps lost in a fishing ground continue fishing unattended (ghost fishing) for many years, and this will result in considerable damage to existing fisheries resources. For this reason it was considered necessary to evaluate the escape rates from the traps employed and estimate their possible negative effects.

Escape from Traps

Box and dome trap designs containing crabs were set in a pond and their escape rates were ascertained during a 7-day period (Vazquez Archdale et al. 2007). Individual swimming crabs were marked and placed inside the different traps and escape rates were observed while diving. In several cases octopuses were found to be interfering with some of the traps, as confirmed by their presence together with the crab remains. For this reason complementary observations were conducted in a large tank by placing crabs inside traps, and subjecting them to a 7-day observation period. Escape results showed similar trends in both the pond and tank; crabs could not escape from box traps, as a consequence of their inability to separate the netting panels forming the slit entrance to exit the trap. On the other hand, the open entrances of the dome trap permitted escape easily, and this was more pronounced in the pond than in the tank, where crab recluses were subjected to more outside stimuli. Average residence times for the 7-day observation period were 6.33 days for P. pelagicus and 6.83 for C. japonica inside the box trap, but only 0.30 days and 2.61 days in the dome trap respectively. Results obtained during the tank experiment using C. japonica were similar, with full 7-day residence in the box trap, but only averaging 4.8 days in the dome trap.

From these findings it was concluded that a box trap lost at sea would continue to fish for up to 10 years, which is the average lifetime of a trap according to fishers, while the animals caught inside become bait and attract new ones into the trap. On the other hand, the dome design has open funnel entrances, and will allow crabs and other organisms to escape through them after the original bait is consumed. Consequently, for eradication purposes the dome trap proves to be the best design; it catches the most swimming crabs and, if lost at sea it will not harm so many organisms by ghost fishing and will conserve the existing fisheries resources.

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