This paper was written as part of the 2002 Alaska Ocean Sciences Bowl high school competition. The conclusions in this report are solely those of the student authors.
Factors Affecting the Tanner Crab Population Near the Kodiak Archipelago
Authors
Abstract
Tanner crabs (Chionoecetes bairdi) were once an integral part of the Kodiak economy. However, due to a sharp decline in their population, Kodiak's crabbing fleet has been forced to seek other resources. Many biotic and abiotic factors are suspected to be responsible for this decline. Throughout their life crabs are prey for many other species. They are also susceptible to parasitic infestations. Weather patterns may also play a role in the decline of Tanner crabs in the Kodiak Archipelago. Oceanic thermal fluctuations in the North Pacific may have also applied pressure on Tanner crabs. Humans are the primary predators of adult Tanner crab through both regulated fishing and unregulated ghost pot catches. Some potential solutions to alleviate the population decline of Tanner crabs include stricter regulations and enforcement concerning pot construction, education, crab farming through aquaculture, changing the fishing season and restrictions in some areas, or a complete closure of the Tanner crab fishery in the Kodiak area. It is also conceivable that in spite of the present low population of crabs, their fishery can continue albeit with lower quotas.
Table of Contents
1. Introduction
2. Abridged Biology of Tanner Crabs
3. Predation
4. Parasitism
9. Local Socioeconomic Factors to be Considered
10. Conclusion
11. Figures
12. Acknowledgements
13. Works Cited
Introduction
From 1967 to 1994 Tanner crab (Chionoecetes bairdi) populations created an economically important fishery around the Kodiak Archipelago. In 1994 regulatory actions curtailing commercial fisheries in the area were necessary because of the depleted crab population and low recruitment numbers. There are many factors, both biotic and abiotic, that may have contributed to this decline. This report will explore several of the likely factors and propose possible solutions.
Abridged Biology of Tanner Crabs
Tanner crabs are one of four species of the genus Chionoecetes found in Alaskan waters (Jadamec et al 1999). They can be distinguished from other species in their genus by their habitat preferences and physical characteristics. Although they are very similar, there are minor differences in their respective physical structures. Compared to snow crabs (Chionoecetes opilio), Tanner crabs have differently sized and spaced spines on the sides of their carapace. Additionally, the rostral horns of Tanner crabs are larger and spaced farther apart (Fig. 1; Jadamec et al). Although Tanner crabs prefer warmer waters than snow crabs, the differences between their temperature preferences are not so great as to prevent them from being sympatric, and these two species are capable of hybridization (Jadamec et al).
Tanner crabs are the only species of Chionoecetes found in the waters around the Kodiak Archipelago. Their habitat ranges from inshore waters to depths of over 370 m., though they are typically found in waters that are shallower than 180 m. (Urban and Hart 1999). The Pacific stock ranges from the Oregon coast to the southeastern Bering Sea and as far west as Hokkaido, Japan. Muddy substrates comprise the majority of their habitat; however, they are also found on sandy or gravel bottoms near shore (Urban and Hart 1999). Tanner crabs are highly opportunistic omnivorous scavengers, and there have been no indications of reduced food availability having adverse impacts on any Tanner crab stocks (Stevens, National Marine Fisheries Service [NMFS], Kodiak, personal communication, 2001).
Larval Tanner crabs usually hatch in conjunction with high spring tides (Stevens et al., 1994). After hatching, they travel up the water column to the photic zone, and within 30 minutes, they molt from the prezoea stage into the zoea 1 stage (Urban and Hart 1999; Fig. 2). This is the only molt not determined in part by food availability and ambient temperature, in contrast to the other inter-molt periods that are shortened with increased food abundance and water temperatures. Approximately one month after entering the zoea 1 stage, they molt into the zoea 2 stage. In another month, the larval zooplankton crabs enter the megalops stage, also known as the settling stage. This is the last planktonic stage during which the megalopa sink to the bottom and select a benthic habitat. Following a molt out of the megalops stage, they become a crab instar 1 (Urban and Hart). "Instar" is a generic term for any stage of life for crab inhabiting the benthos. During its first year of life, the juvenile instar generally molts six times. With each sequential molt, its size increases about 30% (Urban and Hart). The intermolt period gradually lengthens until the crabs are three years old, at which point they molt approximately once per year (Urban and Hart). A crab's age is undeterminable due to a lack of growth rings or other age-related indicators (Jadamec et al. 1999). Instars are a more accurate way of measuring a crab's maturity. The terminal molt is a crab's first molt into sexual maturity.
Female Tanner crabs reach sexual maturity after they terminally molt into the thirteenth instar, which occurs at approximately five years of age (Jadamec et al. 1999). Male Tanner crabs grow more slowly, maturing at approximately six years of age. The largest males pass through as many as 18 instar states before reaching the terminal (sexual maturity) molt (Jadamec et al). Tanner crabs continue molting until their death. The approximate lifespan of male and female Tanner crabs are twelve and nine years, respectively (Jadamec et al).
Predation
When Tanner crabs exist as zooplankton, predators of all trophic levels, ranging from other zooplankton to baleen whales, consume them. Once they pass to the benthic stage their habitat changes, thus making them available to different predators. A trawl survey conducted by the U.S. Department of Commerce during 1990, 1993, and 1996 revealed that Pacific cod (Gadus macrocephalus) and Pacific halibut (Hippoglossus stenolepis) consumed substantial amounts of Tanner crabs (Yang and Nelson 2000). Juvenile Tanner crabs having a carapace width of less than 60 cm are the main prey item of Pacific cod with a fork length of 40 to 60 cm (Yang and Nelson). It is estimated that this group of cod consume approximately 91% of the age 1 (inferred as one year old) crab (Yang and Nelson). A Pacific halibut's diet consists of approximately 6-9% Tanner crab (Yang and Nelson). The diets of Pacific octopi (Octopus dofleini) are comprised largely of crustaceans including Tanner crabs (Golden and McCrae 1994). Sea otter (Enhydra lutra) also consume Tanner crabs, though to a lesser extent than the octopi (Bodkin et al. 2000). Due to insufficient research, exact quantities of crabs consumed by octopi and sea otter are unknown. In addition to this, the instars are cannibalistic (ADF&G 1985). The age two crabs continue consuming crabs of a lesser age group until the smaller crabs become too difficult for them to capture (B. Stevens, personal communication).
Parasitism
Tanner crabs are affected by parasites to some extent in isolated bays. Primary parasitic diseases include black mat syndrome and bitter crab disease (Urban and Hart 1999). The fungus (Trichomaris invadens) causes black mat syndrome. The fungi's haustoria prevent molting and will eventually kill the crab (Urban and Hart 1999). Bitter crab disease is caused by dinoflagellates (Hematodinium sp.) that invade the crab's bloodstream. These dinoflagellates closely resemble crab blood cells and will eventually replace much of the crab's blood (Urban and Hart 1999). This results in a complete change in the crab's tissue color and their eventual death. It is thought that these dinoflagellates are spread by infestation from discarded crab carcasses (Urban and Hart 1999) through infestation from breaks in the carapace and cannibalism.
Human Predation
Since the early 1970s the primary predators of adult male Tanner crabs have been humans (B. Stevens, NMFS, Kodiak, personal communication, 2001). Due to a decrease in king crab stocks in the late 1960s, a Tanner crab fishery opened in 1967 to continue to meet the public demand for crab. By the 1972-1973 season, Tanner crabs had become a dominant fishery in the Kodiak Shellfish Management Area (south of the latitude of Cape Douglas (58°51'06" N lat.), west of the longitude of Cape Fairfield (148°50'15" W long.) and east of the longitude of Cape Kumlik (157°27'00"W long.); Westward Region Shellfish Management Staff [WRSMS] 2001; Fig. 3). The Alaska Department of Fish and Game (ADF&G) instituted its first restrictions on catch limits in 1974 after a series of surveys assessing stocks were performed. It was not until 1975 that the Alaska Board of Fisheries (BOF) instituted a season closure to protect Tanner crabs during mating season (WRSMS). The following year carapace width restrictions of 5.5 inches were instated. The Tanner crab fishery peaked in 1978 when record stocks of more than 32 million pounds were taken (Fig. 4; WRSMS). By the early 1980s there were significant declines in Tanner crab populations (WRSMS); yet, at the same time, the number of vessels participating in the fishery increased. Even as the population declined in the 1980s, prices rose, which lead to a record number of crabbing vessels. Through the 1980s and early 1990s, the BOF continually decreased the pot limits on crab boats. In 1994 the Tanner crab fishery closed in the Kodiak area because of a "progressive decrease in the harvestable surplus (WRSMS)."
The ADF&G completed a trawl survey in 2000, and concluded that a Tanner crab fishery could take place in the Kodiak District in 2001. The fishery opened on January 15, 2001 and closed on March 31 with a guideline harvest level (GHL) of 500,000 pounds of Tanner crab. A maximum pot limit was set for 30 pots per vessel, in contrast to a limit of 75 for previous years (Iverson 2001).
Preliminary studies conducted in 2001 (Iverson 2001) following the March 31 season closure found that immature Tanner crab were at record high numbers; however, there was a decline of mature crabs surveyed in 2000 (Iverson). The ADF&G had a harvest for the 2002 season, as they felt that there is still sustainable population that is above the minimum threshold levels in two of the Kodiak districts.
Humans also impact crabs through pollution, although it is not a major factor (B. Stevens, NMFS, Kodiak, personal communication, 2001). The Exxon Valdez Oil Spill of 1989 did not cause a large impact in Tanner crab populations.
Ghost Fishing
Ghost fishing is defined as "the ability of fishing gear to continue fishing after all control of that gear is lost by the fisherman" (Stevens et al 1999). Crab pots are traps used to catch crabs. Each of these pots is made up with a metal framing covered in nylon twine webbing. At least one panel is fitted with a plastic collar that allows ingress, but not egress. In 1977, the State of Alaska began requiring that each pot must have at least one panel include webbing that degrades within 90 days (Stevens et al). Currently this biodegradable material is untreated 30-thread cotton (Kimker 1994). When a crab pot containing this cotton is lost, the cotton decomposes and the panel develops a rip. Tanner crabs can escape through this tear. In recent studies, however, many pots recovered have either had no biodegradable webbing or webbing that failed to degrade within the 90 days allowed (Stevens et al).
During any given fishing season, around 10-20% of the pots used are likely to be lost at sea (Paul 1994). In some cases the proportion of pots that are lost can be much higher. One estimate of pot loss in the eastern Bering Sea is 20,000 pots per year out of the 100,000 that were being used annually (Stevens et al). A recent study estimated that Chiniak Bay contains 42 ghost pots/km2 (Stevens et al). All of these pots will most likely continue to collect crabs which become trapped inside and are unable to escape.
Atmospheric Factors
The climate of the Gulf of Alaska also influences Tanner crab populations. Starting in 1977, the Gulf of Alaska and Bering Sea began to undergo a "regime shift" of increased water temperatures (Royer 1989, Kerr 1992 and Trenberth and Hurrell 1995, cited in Anderson et al 1997). This temperature shift coincided with a decrease in crustacean populations and an increase in predators such as Pacific cod (Anderson et al 1997). The effects of the warmer water temperatures on Tanner crabs are not conclusively known. Warmer waters do increase Tanner crabs growth rate at several points in their development (Rosenkranz et al, 2001). The temperature shifts may also have further indirect effects upon the Tanner crab, such as favoring a type of plankton that may be important in the crab's food supply.
Recent occurrences of El Niño may have further compounded this temperature increase by decreasing the cloud cover over the Bering Sea (Kempler 2001). Due to decreased cloud cover, sunlight warmed the waters as much as 2° Celsius over a period of several months (Kempler). Global warming trends observed internationally may have contributed a slight effect upon ocean temperatures in the Gulf of Alaska.
The direction of the season's prevailing winds are another abiotic factor influencing Tanner crab in their larval form (Rosenkranz et al, 2001). Due to the Coriolis effect, water tends to be pushed to the right of the wind direction in the northern hemisphere. When prevailing winds near the Kodiak Archipelago blow from the west, water is pushed southeast, and crab larvae are pushed with the water offshore, where they have little or no chance for survival. However, if the prevailing winds are from the northeast or southeast, the young crabs are pushed into fertile inshore waters (B. Stevens, NMFS, Kodiak, personal communication, 2001).
Recovery Plan
There are still many areas of research that must be explored before a definite cause for the Tanner crab decline can be identified and practical and effective solutions can be proposed. Current research, however, supports several areas of action.
It may be possible to reduce the impact of weather changes and climate on Tanner crabs through a better understanding of weather systems. Further research into the effects of climate on Tanner crabs is an option. Existing agencies such as the National Oceanographic Atmospheric Association currently conduct similar research. Further funding for such agencies would probably result in clearer information regarding Tanner crab populations.
Many weather patterns are cyclical in nature; therefore, simply waiting might solve many of the problems affecting Tanner crabs, and the fishing fleet can retool their equipment to enable them to exploit a different fishery.
Several methods to curb the human elements of global warming have already been proposed. While Tanner crabs were not a major consideration for these proposals, the pollution reductions may improve conditions for Tanner crab.
Ghost pots create extraneous losses that are purely wasteful. Even though the loss of pots is inevitable, actions can be taken to reduce their overall impacts. Until the most recent crabbing seasons, regulations concerning these degradable twine panels have been poorly enforced (Bradley et al 1999). The strict enforcement of regulations is important in keeping damage caused by crab pots to a minimum. Fish and Wildlife Protection Officers are in charge of enforcement; they carry out their duties by performing spot checks from the P/V Woldstad and walking the docks to check gear. This is already funded by the state, and is a part of the Fish and Wildlife Protection's duties. Unfortunately, it is still debatable whether the cotton thread can be relied on to degrade in time (Kimker 1994). The Board of Fisheries, Alaska Department of Fish and Game, and the fishing industry have preformed tests with other materials, such as galvanic timed-release devices (Kimker). With a more reliable method, it would be possible to cut down the length of time it takes for the twine or other material to decompose. Some studies show that Tanner crabs may lack the capacity for compensatory feeding (Paul 1994). Any crab caught in a pot for over thirty days has a negatively affected survival rate (Paul). By changing the regulation to 30 days, or even less, the impact of ghost pots on crab populations can be decreased.
If studies show that commercial fishing has a stronger effect on Tanner crab than is currently suspected, then measures must be taken to reduce or entirely eliminate commercial crabbing. Commercial fishing of Tanner crabs was ended in 1994 in the Kodiak Archipelago, but the recovery of the stocks has been less than dramatic (Fig. 5; M. Ruccio, ADF&G, Kodiak, personal communication, 2001).
Since female Tanner crab are a highly sedentary species, and tend to stay in one concentrated area for a long period of time, fishing in these areas could be closed (Alaska Department of Fish and Game 2002 (ADF&G)). This plan would effectively keep a major source of larval recruitment in the Kodiak area from being fished, raising population reproduction. Ghost pots would also decrease in these areas, another large human factor contributing to population decline. Fishermen would benefit from this plan as crab populations improve, and thus, Kodiak's economy would be stimulated.
Another option could be to change the Tanner fishing season from January 15 to March 15. Cold weather conditions during January often kill undersize Tanner crabs that are left too long on boat decks (ADF&G 2002). Changing the season to March 15 would drastically reduce this hazard, leading to more legal crab that can be harvested. Fishermen would also benefit from this plan due to better weather conditions in March leading to less fishing-related accidents.
Simply ending the fishery altogether would lead to the best population recovery, but this plan would be highly detrimental to a economy and livelihood based on the fishery. Social effects will be negative in Kodiak because of the loss of jobs. Thus, a compromise between the fishing community and the species is the only solution feasible solution. In the event of a large population decline, however, the fishery should be closed until the available number of adult crab becomes stable. Increased research into the effects of human predation could also create better solutions to the problem.
An alternative to fishing for Tanner crabs is commercial crab farming. Theoretically, this would be a positive alternative to commercial fishing because the created would be safer and more reliable than those found in the commercial fishing fleet. The wages would be considerably lower compared to historic earnings from fishermen. However, aquaculture is not realistically possible at this time due to several logistic and economic stipulations. Tanner crabs are cannibalistic during all periods of their development. To nullify this tendency each crab would have to be solitarily confined. The cost and space requirements for this make Tanner crabs are also picky eaters during their larval stages. Furthermore, their preferred food sources are not clearly known at this time. Tanner crabs take at least 5 years to mature to a marketable size, making them an extensive investment before any profit is realized. Tanner crabs require a specific temperature range (3ê to 6ê Celsius), which requires any aquaculture facility based in Kodiak to have chilled seawater for much of the year. Electricity costs in Kodiak help make any such venture economically unfeasible.
Local Socioeconomic Elements to be Considered
The decline in Tanner crab has greatly affected Kodiak's economy (Fig. 6). One example of Kodiak's former dependence on crab fisheries is the Kodiak Crab Festival held in May. This is Kodiak's equivalent of a harvest festival. During the 1960s and 1970s, when the Crab Festival was originated, Kodiak's economy was driven by the crabbing industry. Since then, closures around Kodiak Island have forced locally based boats away from Kodiak Island and into the inhospitable Bering Sea.
Some changes will be purely beneficial to the Tanner crab populations without being detrimental to Kodiak's economy. By changing policies about ghost pots, many unnecessary damage to the crab stocks will be averted. For example, by requiring each pot to be equipped with galvanic time release devices that degrade in 30 days or less, many Tanner crabs caught in lost pots will be able to escape and return to the population.
Previous solutions to maintain the maximum sustainable yield of Tanner crab populations consisted of completely closing the Tanner crab fishery around the Kodiak Archipelago. More moderate solutions could keep the crab fishery open without harming the Tanner crab population. Restricting certain areas containing critical habitat for Tanner crabs would help improve Tanner crab populations, thus increasing the cash flow into Kodiak's economy. Changing the season from January 15 to March 15 would also help increase stocks and would benefit fishermen due to improved weather in March. This change in the fishery would also theoretically decrease ghost pots, because of the improved weather. Little funding would be needed to implement these changes in policy.
A commercial Tanner crab aquaculture facility would bring more money into Kodiak's economy, but the amount would be nowhere near the amount gained through crabbing of record years. The aquaculture facility would provide many low-paying jobs for employees. The jobs would be safer than those on the boats, but the pay would be much lower. It would be several years before any real profit from the sales of farmed crab meat would be realized. The facility would have difficulty avoiding bankruptcy, because of the extreme price of starting up the facility and logistical costs. The State of Alaska probably would not appropriate any funds for such a facility due to the high probability of bankruptcy and low profit potential. The community of Kodiak does not have the funds necessary to begin such a program. Some citizens may be opposed to such a facility because of objections to farmed meat and the loss of fishing jobs.
Conclusion
Although Tanner crab populations are being impacted by a myriad of pressures in the waters near the Kodiak Archipelago, it is reasonable to believe that the stock is not beyond recovery. Although there is insufficient research completed to be able to draw any precise conclusion, reasonable deductions can be made and implemented. Any decision made at this stage is only a best guess; as such, it is must constantly be reevaluated. In the future, further research will undoubtedly help to protect remaining Tanner crabs. With proper application of our current knowledge to management and conservation measures, the availability of Tanner crabs as a viable fishery can be assured for tomorrow and generations to come.
Figures
Figure 1: Tanner crab physical characteristics.
Figure originally appeared in Biological Techniques for Chionoecetes Crabs, 1999.
Figure 2: Diagram showing larval stages of Tanner crab
Figure originally appeared in Biology of the Tanner crab Chionoecetes bairdi in Alaska: A report to the Alaska Board of Fisheries 1999.
Figure 3: Kodiak Tanner crab fishery range
Figure originally appeared in the Annual Management Report for the Shellfish Fisheries of the Westward Region, 2001.
Figure 4: Graph of Tanner crab catches from 1965 to 2000
Figure originally appeared in the Annual Management Report for the Shellfish Fisheries of the Westward Region, 2001.
Figure 5: Estimated population of male Tanner crabs by carapace width in the Kodiak District, 1987-2001. Legal crab lightly shaded.
Figure provided courtesy by M. Ruccio/ADF&G.
Figure 6: Chart outlining annual gross earnings for Kodiak from the Tanner crab fishery from 1985 to 2001
Data originally appeared in the Annual Management Report for the Shellfish Fisheries of the Westward Region, 2001.
Acknowledgements
We would like to thank the following people for their indispensable help and support on this project: Craig Baker, Arthur Becker, Bob Foy, Brian Himelbloom, David Horne, Judy MacDonald, Sarah Persselin, Mike Ruccio, Robin Schaeffer, Scott Smiley, Leslie Soughers, Bradley Stevens, Susan Sugai, Jim Van Atta, Matthew Van Atta, Larry Van Daele, Clayton Wallace, and all of our other contributors.
Literature Cited
Alaska Board of Fisheries. 2002. The Alaska Board of Fisheries 2001/2002 Proposed Changes for Cook Inlet, Kodiak, and Chignik, Finfish and Statewide King and Tanner crab and Surrounding Issues. Alaska Dept. of Fish and Game Board of Support Section.
Alaska Department of Fish and Game (ADF&G). 1985. Tanner Crab.
Anders, P., J. Blackburn, and B. Johnson. 1997. Declines of Forage Species in the Gulf of Alaska, 1972-1995, as an Indicator of Regime Shift. Alaska Sea Grant College Program.
Bodkin, J., K. Kloecker, G. Esslinger, D. Monson, and J. DeGroot. Sea Otter Studies in Glacier Bay National Park and Preserve. United States Geological Survey. December 15, 2001. http://www.nps.gov/glba/learn/preserve/projects/otters/GLBA_ANN_RPT_2000_FINAL.doc
Iverson, K. 2001. The Kodiak District Tanner Crab (Chionoecetes bairdi) Fishery, 1985 to 2001. Commercial Fisheries Entry Commission.
Golden, J. and J. McCrae. Oregon Developmental Species Giant Octopus. Oregon Department of Fish and Wildlife. December 15, 2001. <www.hmsc.orst.edu/odfw/devfish/sp/octo.html>
Jadamec, L., W.Donaldson, and P.Cullenberg. 1999. Biological Field Techniques for Chionoecetes Crabs (Ed. 1). University of Alaska Sea Grant College Program.
Kempler, S. The Bering Sea: Seasons and Cycles of Change. NASA. December 15, 2001. <daac.gsfc.nasa.gov/CAMPAIGN_DOCS/OCDST/bering_sea.html>
Kimker, Al. 1994. Tanner Crab Survival in Closed Pots. Alaska Fishery Research Bulletin.
Rosenkranz, G., A. Tyler, and G. Kruse. 2001. Effects of water temperature and wind on year-class success of Tanner crabs in Bristol Bay, Alaska. Fisheries Oceanography.
Paul, J. M., A.J.Paul, A.Kimker. 1994. Compensatory Feeding Capacity of 2 Brachyuran Crabs, Tanner and Dungeness, After Starvation Periods Like Those Encounted in Pots. Alaska Fishery Research Bulletin.
Ruccio, M. 16 November, 2001. <personal communication, 2001>. Alaska Department of Fish and Game. Kodiak.
Stevens, B. 11 December, 2001. <personal communication, 2001>. National Marine Fisheries Service. Kodiak.
Stevens, B, I. Vining, S. Byersdorfer, and W.Donaldson. 1999. Ghost fishing by Tanner crab (Chionoecetes bairdi) pots off Kodiak, Alaska: pot density and catch per trap as determined from sidescan sonar and pot recovery data.
Stevens, B, J. Haaga, W. Donaldson. 1994. Aggregative mating of Tanner crabs Chionoecetes bairdi. Can. J. Fish. Aquat. Sci. 51:1273-1280
Urban, D and D.Hart. 1999. Biology of the Tanner Crab Chionoecetes bairdi in Alaska: A Report to the Alaska Board of Fisheries. Alaska Department of Fish and Game Division of Commercial Fisheries.
Westward Region Shellfish Management Staff. 2001. Annual Management Report for the Shellfish Fisheries of the Westward Region, 2000. Alaska Department of Fish and Game.
Yang, M-S, and M. W.Nelson. 2000. Food Habits of the Commercially Important Groundfishes in the Gulf of Alaska in 1990, 1993, and 1996. U.S. Department of Commerce.