Scientific Excellence • Resource Protection & Conservation
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Scientific Excellence • Resource Protection & Conservation • Benefits for Canadians 2 Underwater Wo rld arious crab species are adapted to world; even the small and unprepos- V a wide range of habitats off sessing green crab was a favorite in CRABS OF THE Canada's eastern coastline, from the Victorian London a century aga and is ATLANTIC COAST intertidal zone to the deep waters of the still eaten in Spain. One probable continental slope. However, des pite answer is that the cost of pioneering a OF CANADA their variety and abundance, Atlantic fishery, developing processing tech- Canadian crabs have hitherto remained niques and marketing routes for the largely underutilized. Currently, the crabs has appeared commercially unat- snow crab is the sole fully exploited tractive compared to the rewards from crab species, constituting a major other more traditional Atlantic fisher- resource in terms of bath landings and ies. Difficulties associated with fishing value. There is only small-scale trap- crab traps in deep, offshore waters, ping of the extensive jonah crab and gear conflicts with established fisheries Fig. 1 (or Frontispiece). Northwest rock crab stocks and essentially no and complications posed by the possi- Atlantic crab claws: form adapted to function. A. northern stone crab: a commercial landings of deep-sea red bility of by-catches of the more lucra- strong claw with blunt teeth designed crab, toad crab, northern stone crab, tive lobster have presented further for cracking heavily armored prey; porcupine crab or green crab. problems. B. snow crab: a slender, fine-toothed Each of the crab species has its own However, with overharvesting in but powerfully-muscled claw for chip- unique flavor and texture (even the some of the traditional fisheries, ping open mollusc shells and tearing smallest crabs are perfectly edible industry is being encouraged to diver- apart brittlestars; C. deep-sea red crab: although somewhat fiddly). Consider- sify and utilize the crab by-catch from a fast-action claw with sharp serrated ing that the crabs offer _delectable existing fisheries as well as start pilot teeth for biting and cutting softer- eating experiences as well as being fisheries directed exclusively for crab. bodied prey; D. jonah crab: a heavy claw extremely nutritious (the raw white An increased world market demand for able to exert slow but substantial forces ideal for crushing the shells of mol- meat contains about 20 per cent pro- crab products, created by collapses in luscs, sea urchins and crustaceans: tein, 1 to 2 per cent minerais, little fat the massive Alaskan crab fisheries, has E. rock crab: a 'generalist' claw, capable and hardly any carbohydrate), why are raised crab prices and further height- of opening shells as well as capturing they underutilized? ened the probability of an expanded and tearing apart agile, softer-bodied Certainly, crab species appear more Canadian crab fishery. Since crabs are prey. widely harvested in other parts of the likely to increase in commercial impor- tance, it is particularly appropriate to summarize crab biology, ongoing research and the characteristics of the predominant species found in the Canadian Atlantic. Crab Biology Crabs are decapod crustaceans (10-legged and having a crust or shell), a wide grouping that includes shrimps and lobsters. Many species of crabs are abundant, grow large and have a high and nutritious meat yield from their body and legs. Such properties have made them a favorite food for man and a target of heavy exploitation. Moulting and Growth Being invertebrates (without back- bones), crabs rely on a hard external shell for skeletal support and protec- tion. Despite the shell being jointed and somewhat flexible, al! crabs must cast off their old shell (a process known as moulting or ecdysis) in order to grow. Once the old shell has been shed, the wrinkled, soft crab takes up QL 626 U53 1989 Elner, R.W. EMAR LIBRARY 1 0 1 Crabs of the Atlantic coast _u_n_de-rw_a_ e_r_w__r _d ________FI Ss... H of Canada 118963 05014765 c.1 MB R3T 2N6 CANADA . water and swells to its larger size within Reproduction a few hours. The new shell gradually For many crab species in the north- hardens and more muscles and tissue west Atlantic, mating usually takes grow inside, although it may take place between a newly-moulted, soft- several months for the crab to regain shelled, mature female and a larger its former shell hardness and condi- hard-shelled, mature male. The male tion. As crabs grow aider, they moult carries the female around before her Jess frequently and their percentage moult and may continue to protect her growth increment diminishes as well. for several days after copulation. The Fig. 2. Snow crab (Chionoecetes Since all the bard parts of the crab, male's behavior, as well as being of opilio). internai as well as external are lost at considerable survival value to the vul- moulting, no tell-tale indicators (such nerable female, serves to guard his as scale and otolith rings in fish) remain genetic investment against the death of to accurately age the animal. Hence, the female and displacement by other the age of a crab of a given size can males. Subsequently, the female only be backcalculated from estimates extrudes many thousands of fertilized of frequency of moulting and growth eggs onto the swimmerets under her per moult. Accurate information on abdomen. age is basic to much fisheries biology, Typically, larger females of a crab and the difficulty in obtaining this species carry more eggs than smaller information for crustaceans remains a females; a large female rock crab may fundamental problem. carry 330,400 eggs. The eggs usually hatch during the warmer months of the year and free-swimming larvae are Life Style released. The larvae may spend from The life style of a crab species is one to several months in the plankton uniquely reflected in the form taken by and moult through a number of stages its 10 legs. The first pair are modified before settling down on the sea bed and into conspicuous claws which, accord- moulting to the first crab stage. Ocean ing to the crab's feeding habits, have currents can transport larvae consider- evolved as optimal tools for the various able distances from their hatching crushing, cutting, digging, pinching, place. The massive mortalities that rasping and snatching actions required occur during the larval stages tend to to capture and devour prey. Contrary be compensated for by the vast to popula_r belief, most northwest numbers of larvae produced. Atlantic crabs are not simply "scav- engers" but predominantly carnivores with well-defined preferences for prey Crab Research such as mussels, snails and brittlestars. Since the early 1960s, fisheries Claws are also used in burrowing, biologists from provincial and federal fi ghting and, interestingly, signalling. governments, as well as from univer- Claw signais are a universal decapod sities, have been carrying out research '' language' ' and displays serve into crab. The work has been con- variously to ward off rivais for terri- ducted for a variety of purposes, to tory, repel would-be predators and estimate stock abundance and size attract mates; ail without recourse to composition, growth rate, reproduc- Fig. 3. Rock crab (Cancer irroratus). physical contact. Some crabs have tive pattern, movement, and exploita- achieved greater flexibility by having tion rate. However, all the work has a one large claw adapted for one major common aim in providing an accurate purpose, for example, crushing heavily picture of the crab resources so that armored molluscs or dis play, and the they can be properly managed. other smaller claw adapted for, say, Traps, beam trawls and towed under- snatching small agile prey. Although water camera sledges are used from the remaining four pairs of legs are government research vessels and com- modified most for movement, be it bur- mercial vessels under charter to assess rowing, climbing, swimming or walk- crab densities on the bottom. Tagging ing, they can also serve for feeding and techniques have been refined to pro- grasping. duce reliable tags that will be retained 4 Underwater World when a crab moults. Such moult- fishermen, and placed the species retainable tags are expected to supply fourth in landed value in the Canadian valuable information on stock bio- Atlantic fishery behind cod, lobster mass, exploitation and growth, as well and scallop. The 37 ,255 t of snow crab as long-term movements. The ecology landed in 1983 had a record value in and population dynamics of juvenile excess of $53 million. snow crab are being subjected to Distribution - West Greenland to special investigation through beam Maine and from Alaska to Siberia; trawl surveys and stomach analyses of from 20 to 700 m. Commercial concen- the groundfish which feed on crab. trations occur on soft bottom around Continued research on the larval and Cape Breton, Newfoundland and in Fig. 4. Jonah crab (Cancer borealis). juvenile stages of crab remains a prior- the western Gulf of St. Lawrence, ity if the factors influencing growth especially from 70 to 280 m. Occa- and recruitment to the fishery are to be sionally found in the Bay of Fundy. understood. If commercial stock size Description - Somewhat flattened and production could be regularly walking legs that are two to three times predicted from larval and juvenile as long as carapace; almost circular abundances, fishery managers could carapace; pale brownish above, yellow- more effectively optimize exploitation ish below. Males reach a maximum of the resource. carapace width of 150 mm, a leg spread Commercial crab catches are moni- of approximately 0.9 manda weight of tored regularly at landing points 2.0 kg; females are not commercially throughout Atlantic Canada and sales fished as they attain a maximum receipts are analyzed.