Bitter Crab Syndrome in Tanner Crab (Chionoecetes Bairdi), Alitak Bay, Kodiak, Alaska 1991-2000
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Crabs in Cold Water Regions: Biology, Management, and Economics 401 Alaska Sea Grant College Program • AK-SG-02-01, 2002 Bitter Crab Syndrome in Tanner Crab (Chionoecetes bairdi), Alitak Bay, Kodiak, Alaska 1991-2000 Daniel Urban and Susan C. Byersdorfer Alaska Department of Fish and Game, Division of Commercial Fisheries, Kodiak, Alaska Extended Abstract Bitter crab syndrome was first discovered in 1986 in southeast Alaskan Tanner crab (Chionoecetes bairdi) populations (Meyers et al. 1987). The dinoflagellate that causes the disease, Hematodinium sp., is similar to the one that causes red tides. The disease is thought to be uniformly fatal, with an infection period of 15-18 months (Meyers et al. 1990). Symptoms include abnormally bright ivory or reddish coloration, especially obvious on the ventral surface of the legs. The meat acquires a bitter, aspirin-like taste and is unmarketable. Microscopically, hemolymph films show vary- ing amounts of the Hematodinium, characterized by aggregates of multi- nucleated cells with very vesicular “frothy” cytoplasm. Actual cellular damage associated with this organism is not extensive, but may include muscular degeneration and tissue necrosis (Meyers et al. 1987). Since its discovery, bitter crab syndrome has been found to be wide- spread in both Tanner and snow crabs in the Chukchi Sea, Bering Sea, and Gulf of Alaska. A survey of the Gulf of Alaska in 1990 showed several hot spots for the disease, including Alitak Bay on the south end of Kodiak Island (Pearson and Meyers 1992). Sampling there has continued on an annual basis as part of the Alaska Department of Fish and Game (ADFG) Multi-Species Trawl Survey. Sixteen stations have been established inside Alitak Bay (Tanner Head to Cape Trinity) as part of this trawl survey. Survey gear was a 400 Eastern trawl net with a single trawl haul of 1 nautical mile (1.85 km) at each station. Thirty crabs were drawn at random for sampling from each sta- tion. Hemolymph was obtained from each crab with a sterile syringe, drawn across a slide and allowed to air dry. Information on size, sex, shell condi- tion, possible bitter crab syndrome infection, and egg clutch conditions were recorded. Slides were fixed and stained with Dade Behring Diff Quick 402 Urban and Byersdorfer — Bitter Crab Syndrome Stain Set. Slides were read at 400¥ magnification, with a minimum of 30 fields per slide examined. Slides were judged either as negative or posi- tive, on a scale of 1 to 5 (1 = 4-5 cells per field; 2 = 1-50 cells per field; 3 = 50-100 cells per field; 4 = >100 cells per field; 5 = solid cells). From 1991 to 2000, a total of 4,811 slides were collected, with 10.1% judged to be positive. A non-parametric examination of size distribution revealed that smaller crabs were disproportionally infected when com- pared to the overall size distribution in the samples. Meyers et al. (1990) suggested that the disease may be transmitted during the crab molting process, either through minor breaks in the cuticle or through cannibal- ism. Since C. bairdi experience gradually increasing intermolt periods as they age (Donaldson et al. 1981), smaller crab would more frequently be vulnerable to infection than older crabs. Results also suggest that females were infected at higher rates than males; in 5 of 6 years when the prevalence rate showed a significant dif- ference (P = 0.95), females had a higher prevalence rate. This contradicts the previous report of Meyers et al. (1990) from southeast Alaska. In agree- ment with those findings, however, were the annual prevalence rates by shell age. In years with significant differences (7 of 10 years), new shells, i.e., recently molted crabs, had a higher prevalence rate. Geographically, prevalence was highest in the upper reach of Alitak Bay, known as Dead- man Bay. This glacial fiord is much deeper than the rest of Alitak Bay, with depths approaching 100 fathoms (182 meters). Despite having being recognized for 15 years as occurring in Alaska, bitter crab syndrome is still poorly understood. Transmission modes and the effect of bitter crab syndrome on the population dynamics of Tanner and snow crabs represent major gaps in our knowledge. The high preva- lence rate in Deadman Bay may be the result of a largely resident popula- tion of Tanner crab with little immigration or emigration (Dave Jackson, ADFG, Kodiak, pers. comm). A similar situation exists in areas of south- east Alaska with high prevalence rates (Ted Meyers, ADFG, Juneau, pers. comm.), and such sites should perhaps be considered as special study areas in which more definitive research might be conducted. References Donaldson, W.E., R.T. Cooney, and J.R. Hilsinger. 1981. Growth, age and size at maturity of Tanner crab, Chionoecetes bairdi M.J. Rathbun, in the northern Gulf of Alaska (Decapoda, Brachyura). Crustaceana 40(3):287-302. Meyers, T.R., T.M. Koeneman, C. Botelho, and S. Short. 1987. Bitter crab disease: A fatal dinoflagellate infection and marketing problem for Alaskan Tanner crabs Chionoecetes bairdi. Dis. Aquat. Org. 3:195-216. Meyers, T.R., C. Botelho, T.M. Koeneman, S. Short, and K. Imamura. 1990. Distribu- tion of bitter crab dinoflagellate syndrome in southeast Alaskan Tanner crabs Chionoecetes bairdi. Dis. Aquat. Org. 9:37-43. Crabs in Cold Water Regions: Biology, Management, and Economics 403 Pearson, T., and T. Meyers. 1992. Prevalence of bitter crab syndrome and bacterial infection of Tanner crab Chionoecetes bairdi in the Kodiak, Chignik, South Peninsula, and Eastern Aleutian management areas. Alaska Department of Fish and Game, Commercial Fisheries Division, Regional Information Report No. 4K92-26. Crabs in Cold Water Regions: Biology, Management, and Economics 405 Alaska Sea Grant College Program • AK-SG-02-01, 2002 Reproductive Capacity Morphometrically Assessed in Cancer pagurus from the Shetland Islands Shelly M.L. Tallack North Atlantic Fisheries College, Scalloway, Shetland Islands, and University of the Highlands and Islands, Inverness, United Kingdom Abstract Reproductive capacity and size at maturity were investigated in the edible crab, Cancer pagurus, using both conventional and novel morphometric measurements. Measurements of cheliped length and height (males) and abdomen width (females) were analyzed and verified against biological findings of maturity (ovigerous, hatched, or “plugged” status in females; mature testes in males). The sizes at maturity derived through Hiatt straight line analysis were lower than anticipated: 102-105 mm CW in males and 90 mm CW in females. This result may indicate a prepubertal molt in C. pagurus. Alternatively, the Hiatt straight line analysis may identify the onset of behavioral rather than functional maturity. Female C. pagurus typically show increased fecundity with increasing crab size, though little explanation is offered in the literature. This study investigated four morphological features likely to assist in realizing this trend: abdomen width, pleopod capacity, sperm plug weights, and sper- mathecae weights. These were chosen for their role in brood protection, egg attachment, and egg fertilization through sperm retention. The posi- tive relationship found between crab size and each feature suggests that these reproductive accessories assist larger females to produce larger broods, thereby increasing their reproductive capacity. Allometric analy- sis of female pleopod capacity indicated a size at maturity of 138 mm, which is in line with recordings of ovigerous females around Shetland. Despite the lack of morphometric attention paid to this species to date, it was concluded that both conventional and novel morphometric measurements represent useful tools for estimating the size at maturity 406 Tallack — Reproductive Capacity in Cancer pagurus in C. pagurus, though calibration against other biological indicators of maturity remains advisable. Introduction The current study focuses on crabs caught from inshore waters around Scotland’s most northerly archipelago, the Shetland Islands (also referred to as Shetland). Past studies on the reproductive characteristics of the edible crab Cancer pagurus were derived from specimens inhabiting more southerly British and French waters (Edwards 1966, 1979; Edwards and Meaney 1968; Le Foll 1982; Bennett 1995). While there is no evidence to date suggesting that Shetland’s C. pagurus represent a discrete population, the colder water temperatures experienced by specimens at this northerly location may dictate differences in factors such as growth and size at matu- rity (Kurata 1962, Sastry 1983). Certainly, larger sizes at maturity were reported for C. pagurus in waters off northern England than in those off southern Brittany (Brown and Bennett 1980, Le Foll 1982). With geographi- cally determined intraspecific variation being likely, local management of the C. pagurus fishery in Shetland, by means of the recently introduced Shetland Islands Regulated Fishery (Scotland) Order 1999 (SSI 1999 No. 194), should be founded on geographically relevant scientific information. Ovigerous C. pagurus are rarely caught in traps (Edwards 1966), which makes the assessment of sizes at maturity in females difficult. Alternative methods for determining the size at maturity have been applied to vari- ous brachyuran species. These include the recording of recently hatched females as evidenced by empty egg capsules on the abdominal pleopods (Edwards 1979, Norman 1989); the internal examination of gonads (Hartnoll 1969, Edwards 1979, Campbell and Eagles 1982); the recording of sperm plugs extruding from oviducts (Edwards 1966, Choy 1986, Norman 1989); and the morphometric analysis of secondary sexual characteristics (Campbell and Eagles 1982, Choy 1986, Norman 1989, González-Gurriarán and Freire 1994). The latter two techniques form the focus of this paper. Morphometric analysis compares the relative size of different body parts with the total body size, or carapace width in the case of C. pagurus. By taking these measurements from specimens throughout the species’ size range, periods of allometric growth can be identified. This practice has been applied to many crab species, but is little documented for C. pagurus, though Edwards (1979) stated that in male C.