Figure 1. A mechanism of pot loss observed in Womens Bay, Kodiak, Alaska, includes float lines overgrown with marine fouling organisms that can eventually lead to the float sinking.
Ghost fishing, which occurs when lost or abandoned fishing gear continues to trap and kill aquatic organisms, can have substantial economic and ecological effects. Lost nets can continue to ensnare animals, including birds and mammals, while lost crab or fish pots keep on attracting animals. Pots can be particularly damaging because they are often sturdily constructed and can last a long time and because animals that die inside them become bait that lures in the next set of victims. Ghost fishing gear is also non-discriminatory in that it will capture both fishery and non-fishery species, as well as individuals that would usually not be harvested, such as reproductive females.
The red king crab, Paralithodes camtschaticus, is an important fishery species in Alaska. There used to be a substantial commercial fishery for red king crab around Kodiak, but the stock crashed in the 1980s and has not recovered despite a fishery closure. In this study, we examined how ghost fishing affected the stock of red king crab in Womens Bay, Alaska. Womens Bay is located near the city of Kodiak and is fished for both fin fish and shellfish in commercial, subsistence, and sport fisheries. Unlike many other studies that examine lost fishing gear for trapped animals or studies that deliberately “lose” and follow fishing gear, in this study red king crabs were tracked and followed individually. This allowed us to calculate the effect on the stock as a whole rather than estimating a rate of loss per item of lost gear.
The project represents 17 years of tracking a total of 192 tagged red king crabs. Red king crabs were captured by divers in Womens Bay, tagged with acoustic tags using marine grade epoxy, and released near the site of capture. The crabs were tracked both from the surface using a surface acoustic receiver and by divers using a dive receiver. Typically, crabs were located once a week from the surface, and a sub-set of tracked crabs was dived upon. When crabs were located on the bottom, notes were made as to the behavior and disposition of the crabs. The final fate of each tagged crab also was determined. Such fates included molting, when the tag was shed along with the exoskeleton; death; or unknown. Some tags were also lost (i.e., could not be located from the surface), indicating that the tag had likely malfunctioned. When crabs died, the cause of death was ascertained if possible. Over the years, many crabs were found both alive and dead inside ghost fishing gear, primarily lost crab pots. When researchers found a ghost pot, they disabled it and released any crabs found inside. They also noted the type and condition of the pot.
The effect of ghost fishing was calculated as a mortality rate, which was used to estimate the average annual loss of red king crabs in Womens Bay. Because some of the tagged crabs were caught in pots and released by divers, it is not known whether these crabs would have been able to escape on their own or if they would eventually have died in the pots. We therefore calculated the mortality rate twice, once assuming that all such released crabs would have escaped (a conservative estimate) and once assuming that all these crabs would have died (a maximum estimate). In addition, both the probability of being caught and the probability of being killed in ghost fishing gear were analyzed with a logistic regression to examine the effects of sex and size on ghost fishing rates.
Table 1. Final disposition of the 192 tagged crabs in Womens Bay, Alaska, tracked during this study.
Final Disposition
Number
Percent
Molted
76
39.6
Unknown
48
25.0
Lost
22
11.5
Derelict
16
8.3
Other Mortality
13
6.8
Ghost Fishing Mortality
13
6.8
Handling Mortality
3
1.6
Tag Fell Off
1
0.5
Of the 192 crabs tagged over the course of the study, the majority molted (Table 1). Thirteen crabs were killed in ghost fishing gear, 1 in a gill net and 12 in crab pots, while another 13 died of other causes, including predation by sea otters, octopi, and humans (i.e., fishing mortality). An additional 20 crabs were caught in ghost pots and released alive by divers.
Crabs were tracked for an average of 147 days. A total of 143 pots were found during the experimental period by divers. Most of these were either Dungeness crab, Metacarcinus magister, pots or mesh-covered steel-frame pots (Table 2). Of these pots, 62% were intact, meaning they lacked the legally required biodegradable release and were capable of ghost fishing.
Likely mechanisms of pot loss include lines being cut or dragged by boats, bio-fouling of the float line (Fig. 1), or abrasion of the float or line by ice. The mortality rates calculated from the data indicate that between 16% and 37% of the red king crabs with carapace length > 60 mm in Womens Bay were killed each year by ghost fishing gear. While neither the size nor the sex of the crabs significantly affected the probability of being caught or killed in ghost fishing gear, there was a trend for larger crabs to be more vulnerable to ghost fishing (Fig. 2).
Table 2: Ghost pots found in Womens Bay by divers and their disposition. Because Dungeness pots have their biodegradable release on the top, the releases will never work on upside-down pots.
Type
Number
Intact
Unknown
Upside-down
Percent intact
Dungeness
70
46
2
8
66%
Steel-frame
42
30
2
2
71%
Home made
20
10
1
2
50%
Sport pots
7
3
2
1
43%
Unknown
4
0
2
0
0%
Total
143
89
9
13
62%
This work demonstrates that ghost fishing has a large negative effect on the red king crab stock in Womens Bay and may be an important contributor to the lack of stock recovery in the bay. Given that 60-mm crabs are vulnerable to ghost fishing and females of this size are at least 3 years from reproducing for the first time, we estimate that up to 75% of the female crabs in Womens Bay may have been killed in ghost fishing gear before successfully reproducing for the first time. This means that ghost fishing is not only reducing the population but also reducing the reproductive capacity of the population. These results suggest that efforts to remove ghost pots and to reduce the loss of pots in Womens Bay would be highly beneficial for the red king crab population.
Figure 2: Proportion of tagged crabs killed (black bars) or caught (grey bars) in ghost fishing gear of total studied tagged crabs in each size class in Womens Bay, Kodiak, Alaska. Note that no crabs between 40 and 60 mm of the 10 in the study were caught or killed. The number above each set of bars represents the total number of crabs in each size category. The line with the long dash indicates the overall proportion of tagged crabs killed in ghost fishing gear, and the line with the short dash indicates the overall proportion of all crabs caught in ghost fishing gear.
By W. Christopher Long, Peter A. Cummiskey, J. Eric Munk
