Testing the Effect of Ghost Traps on the Blue Crab, Callinectis Sapidus, in Barnegat Bay

Michael Franklin

Drexel University

16 September 2008

Abstract

The Blue crab, Callinectis sapidus, is prevalent in Barnegat Bay, NJ but continuously

falls victim to ghost traps throughout the bay. This experiment was designed to show the effects

of unbaited, uncollected crab traps on C. sapidus. This was done by collecting crabs and placing

them in a sealed crab trap, which was then left in the inlet behind the boathouse of the Barnegat

Bay Lighthouse Center. Over a three day period, the crabs were observed and the results

recorded. After the first day, two were dead and had been eaten by the other crabs in the trap.

Beyond the first day, I noticed that little would change, leading to the idea that the crabs were

temporarily satiated. However, these results, if gathered over a much larger period of time

should show something similar to the first day of observations, where it was observed that the

crabs will cannibalize each other when no other food source is present.

Introduction

The Blue crab, Callinectes sapidus, is a swimming crab and it is very important to

commercial soft-shell crabbers. It is prevalent in Barnegat Bay, NJ and is sought after by both

commercial and recreational crabbers, according to Jivoff (2008). The main method of crabbing

in the bay is a basic crab trap used by just about anyone trying to catch C. Sapidus. However,

when these traps are left out in the bay, and not collected, they can cause even bigger problems

for the crabs. These traps are then known as “ghost traps” because of the fact that they will no

longer be collected and are simply a shell of a real crab trap, as they are no longer baited. Wnek,

a PhD student at Drexel University, estimates there are upwards of 2,000 ghost traps in Barnegat

Bay at any given time (2008). With a number that high for such a small area, and the fact that C.

Sapidus can be cannibalistic (Jivoff 2008), as well as, very aggressive (Pollock 1998) it is

important to better understand how these traps affect C. Sapidus.

Furthermore Bertness (2007) brings to light the fact that C. Sapidus populations are on

the decline today due to overexploitation and disease, so it is imperative that anything that can be

done to preserve the population and not waste crabs is done. Additionally, he talks about C.

Sapidus foraging being “a major structuring force in soft-sediment communities,” so beyond the

importance as a predator, C. Sapidus is responsible for helping to shape the habitat that is

Barnegat Bay. With commercial fishers already impacting the population with traps they collect,

any extra forces acting on the species, such as ghost traps, need to be eliminated to preserve

Barnegat Bay.

Given enough time, ghost traps will not have bait in them, minus any potential fish that

swim into the trap accidentally, so what happens to the crabs that get themselves stuck in these

traps? The purpose of this experiment is to show the effects of ghost traps on the C. Sapidus

population of Barnegat Bay. This will be done by testing what happens to crabs in a crab trap

when they cannot escape and the trap no longer has food in it for them to eat. According to

Breen (1990), because crab traps are passive fishing gear that, when used correctly, just sit on the

bottom of the ocean and collect organisms, they will continue to do so once they are lost, which

means that the crabs will be stuck in the traps with little chance of being recovered. This

particular experiment is designed to test on a small scale due to time constraints, what will

happen to a collection of C. Sapidus after they have been stuck in a ghost trap for 3 days.

To complete this experiment, I will place 7 crabs of random size and sex into a crab trap

and seal it off, not allowing any to escape, then drop it into an inlet of Barnegat Bay with no bait

to mimic the most basic situation one would encounter in a ghost trap. Some of the crabs will be

starved for a few days while others will come from a trap I will collect the same day the

experiment begins. After this, I will observe the crabs once each day for the first two days, then

make two observations on the final day, being sure to note which crabs are alive, which are dead,

how I think they died, and any other things that seem out of the ordinary. Once the three days

are up, I will compile the data into a graph to show the trend between number of days locked in a

ghost trap without food and the number of crabs that have been cannibalized. Also, if any of

the crabs have not been cannibalized but are dead, I will also note that.

Materials

Crab Trap with 2 entrances

Rope

Wire Mesh

Wire Cutters

Camera

Zip Ties

Methods

To begin my experiment, I will close off the entrances to the crab trap with wire mesh

and zip ties (see figure 1 in appendix). After this is done, I will attach a rope to my trap so I can

remove it from the water for my observations.

Next I will add my crabs and then close off the rest of the trap (see figure 2 in appendix).

Because this experiment calls for as much randomness as possible, I will starve two of my crabs

for two days and use five more from a trap I collect from the bay earlier in the day, giving me an

assortment of different sizes and hunger levels. Once these crabs are collected and placed into

the trap, I will lower them into the inlet behind the boathouse of the Barnegat Bay Lighthouse

Center, then make observations each day at 2:45pm, noting things such as how many crabs are

alive, if ones are dead, how big they are, if the dead ones have been cannibalized, and if so, how

much of the cannibalized crabs are left.

Results

Day 1: At 2:25pm on day one, I observed that there were two dead crabs in the trap and only

four crabs that were alive. The experiment originally began with seven, so one was missing

entirely from the cage. Of the two dead crabs, the larger (largest crab in the trap) was missing all

appendages but the two front claws (see figure 3 in appendix). The smaller one (medium size for

the trap) was missing both of its paddle legs and one front claw. After carefully studying the

trap, I discovered that one of the small steel loops that is used to attach the top hatch to the trap

(see figure 4 in appendix) was missing and that there was a small hole in the trap that was

consequently not sealed. I used another zip tie to close off this area then put the trap back into

the water.

Day 2: At 3:50pm on day two, there were still four crabs that were alive and intact. Also, the

larger crab that was dead had not changed since the day before. However, the smaller dead

crab’s carapice was in a few different pieces and all of its appendages were no longer attached.

Day 3: At 2:40pm on day three, there were no changes in the crabs from day two. All four that

were alive on day two were still intact, and the carapices and other assorted pieces of the two

dead crabs had not changed.

Day 3 (2nd observation): At 7:15pm, there was a slight difference from the previous

observation. The larger dead crab’s carapice had been separated from the bottom of its body.

There were still four crabs that were alive and intact, and the last crab’s carapice was still in two

pieces on the bottom of the trap.

Graph 1 (left): The number of crabs that were in the trap at each observation. 

Graph 2 (right): The number of crabs that were alive at each observation. 

Discussion

After collecting all of my data, it was pretty clear to me that the crabs had resorted to

cannibalism. Because there was no bait in the trap and they could not get out, they needed to eat

something. Also contributing to the lack of food was the fact that the trap was placed in the inlet

behind the boathouse. Beyond tidal changes, there is little water movement here, so any possible

food for the crabs to eat would have had to swim into the cage. After consulting with Wnek, I

have reached the conclusion that the crabs must have killed the two dead crabs in the trap and

eaten them over the first two days based on the condition of their bodies.

It was very easy to conclude that once the two crabs had died, the other crabs ate them

because of the way the carapices had been damaged and the appendages had been torn off. It

would also be pretty safe to assume that the remaining crabs killed the dead ones, based on their

aggressiveness (Pollock). Jivoff (2008) says that C. Sapidus has no problem eating anything it

02468

0 1 2 3 3.5Num

ber of crabs Alive 

Observa2on Day 

Number of Crabs Alive During  

Each Observa2on 

4

5

6

7

8

0 1 2 3 3.5

Num

ber of Crabs (d

ead or alive) 

Observa2on Day 

Number of Crabs in Trap 

can get to, so it is not much of a stretch for the four crabs to eat the other two. These results

confirm my original idea that the crabs would wind up eating each other in the trap if there was

no other food available. Although it would be difficult to extrapolate such a small data set over

the number of ghost traps in all of Barnegat Bay, it is acceptable to say that these results would

be expected for at least some crabs that get stuck in a trap with no food.

After carrying out this experiment, I have rejected my null hypothesis that the crabs

would just exist in the tank until they die. The results are exactly what I expected over a three

day period. Two crabs were killed and then subsequently eaten because there was nothing else

in the trap to consume. If the experiment continued for another three to five days, I believe at

least two more crabs would be dead and consumed.

With that said, time was one of the areas where the study could be improved. Although

three days yielded the expected results, two crabs were eaten within the first twenty-four hours.

To see a pattern in the cannibalism would require many more observations, so it would be

necessary to extend the experiment at least another week. Another area that could have been

improved was the fact that the smallest crab managed to escape through a small opening in the

top of the cage that was subsequently closed up with a zip tie. Lastly, if the experiment was

carried out in an unmodified trap left out in the bay without bait, the results may have been

different as the crabs could have escaped, or other animals could have been caught. Although

this would seem like a better experiment location, for initial studies, the inlet was the one place

where there would be less interference and the crabs would have a better chance of not getting

other food sources. This was a good initial step and given more time, the results would

potentially be able to be extrapolated over the number of ghost traps in the entire bay.

Appendix

 

Figure 1: This is a photograph of one of the openings of the crab trap.  As one can 

see, the wire mesh is held on with zip ties to prevent the crabs from escaping 

 

Figure 2: The top of the crab trap is closed off with a zip tie, preventing 

the crabs from swimming out of the top of the trap.

Figure 3: Large crab with nothing but the claws and an empty carapice. 

 

Figure4:Ziptieontopofphotoiswherethesmallestcrabmostlikelyescapedfrom.Thisisthetrapaftertheproblemhasbeenrepaired.

Works Cited

Bertness,M.D.AtlanticShorelines:NaturalHistoryandEcology.PrincetonUniversityPress.Princeton,NJ.2007.Pg261‐264.

Breen,P.A.AReviewofGhostFishingbyTrapsandGillnets.Ministry of Agriculture & Fisheries, Fisheries Research Center.1990.

Jivoff,Paul.“BlueCrabs.”PersonalCommunication.RiderUniversity.11September2008.

Pollock,L.W.APracticalGuidetoMarineAnimalsofNorthAmerica.RutgersUniversityPress.New

Brunswick,NJ.1997.Pg264

Wnek,John.“BarnegatBay.”PersonalCommunication.DrexelUniversity.9September2008.