the monthly catch in this area was the highest 

 of the year. 



Increase in the market price of crabs serves 

 as another incentive for greater fishing. Fish- 

 ermen some distance from market will start 

 fishing when it is profitable to haul and deliver 

 their catch. Many St. Johns River crabbers also 

 fish for catfish, American shad, and shrimp. 

 Market price determines the amount of effort 

 employed for crabs relative to these other 

 species. The present study yielded no evidence 

 than an increase in the number of fishermen, 

 such as occurred in 1962 over 1961, tended to 

 decrease the catch of the individual fishermen 

 in the St. Johns. 



Migrations of immature and mature crabs 

 determine their availability to the fishery, 

 since fishing is usually localized. Movement 

 into fishing areas and length of stay affect the 

 extent of utilization. 



The fishery of the upper river depends upon 

 the migration upriver of juvenile crabs, and the 

 length of time they remain as adults. Upon 

 reaching maturity, females migrate almost 

 immediately downriver. Most movements of 

 mature males also are downriver but they may 

 occur at once or over a number of months. 

 Results of tagging and sampling in 1962 indi- 

 cated that many juveniles terminated upstream 

 migrations in Area 2 and, shortly afteT reaching 

 commercial size, migrated to Area 1. The 1962 

 catch per unit of effort in Area 1 doubled over 

 1961 but changed little in Areas 2 and 3. 



Fisheries of the lower river depend upon the 

 movement of mature females from the river 

 into the ocean to spawn, the return of females 

 to the river after spawning, and the migrations 

 of mature males from upriver. Mature females 

 from upriver congregate in the lower river 

 where they produce a sponge before entering 

 the ocean. Those maturing during the summer 

 migrate quickly from the upper river and into 

 the ocean. Females maturing in fall and winter 

 may remain for many months in the lower river; 

 moving closer to the mouth and producing 

 sponges with the coming of spring. The size, 

 location, and time of these concentrations 

 determine their utilization. Fishing success of 

 downriver regions is also affected greatly by 

 the number of females returning to the lower 

 river after spawning and by the migration, 

 especially during late fall, of mature males 

 from upriver. Results of tagging and sampling 

 indicated that the migrations of adults in 1962 

 were more favorable to the fishery of Area 1 

 than in 1961. 



As in any fishery, an important factor inthe 

 size of the catch is the size of the catchable 

 population. Large catches usually lead to 

 greater effort at the time of good yield and, 

 in the St. Johns River, for approximately a 

 month after the catch has reached its peak. 

 In winter a reduction in growth rate as a result 

 of low temperatures reduces recruitment and 

 the number of marketable-size crabs. A 



reduction in the activity of both crabs and 

 fishermen during cold weather also accounts 

 for smaller catches in this period. 



No effective method has been devised to 

 determine annual size of populations of market- 

 able crabs. The catch per standard unit of 

 effort gives an index of relative abundance. 

 The substantial increase in this index in 1962 

 over 1961 for Area 1 (there was little change 

 in Areas 2 and 3) indicate that an increased 

 population in 1962 accounted in part for the 

 greater catch of that year. 



SUMMARY 



The Bureau of Commercial Fisheries began 

 studies on the blue crab in 1957 to determine 

 factors affecting the abundance of marketable- 

 size crabs. In 1961, a field station was estab- 

 lished at Green Cove Springs, Fla., to study 

 the blue crab and its fishery in the St. Johns 

 River. Statistics on fishing in the St. Johns 

 River are presented here for 1961 -62, a period 

 of marked fluctuation. 



The commercial fishery for blue crabs 

 begins 7 miles upstream from the mouth of 

 the St. Johns River and extends to Astor, ap- 

 proximately 135 miles upstream. For this study 

 the fishery was divided into three areas: Area 

 1, from the mouth of the Ortega River; Area 2, 

 from the Ortega River to Palatka; and Area 3, 

 from Palatka to Astor. 



Crab pots of two types, wire and wood, are 

 the most common gear used in the fishery. 

 Lesser numbers of crabs are taken in Area 1 

 by trawls, often incidental to the capture of 

 shrimp. 



The catch was 1,384,579 pounds in 1961 and 

 3,231,569 pounds in 1962 (an increase of 133 

 percent). The catch of pot fishermen in 1962 

 rose 109 percent and effort increased 68 per- 

 cent. The catch of trawl fishermen in 1962 was 

 590 percent greater than in 1961 and effort 109 

 percent higher. Monthly variations of catch and 

 effort during each year and betweenyears were 

 described for each area. 



Samples of commercial catches in 1961 and 

 1962 yielded data on sex and size composition of 

 the catch. Females made up the majority of the 

 commercial catch in the lower St. Johns River 

 and males predominated in the upper river. 

 Crabs 160-169 mm. dominated catches from 

 Areas 1 and 2, and crabs 170-179 mm. were 

 most frequent in catches from Area 3. Data 

 are presented on other species of crabs caught, 

 width- weight relations, and minimum market 

 size. 



Major factors that determined the size of the 

 1961 and 1962 catches were market conditions, 

 migrations, and population size. Market de- 

 mand, availability of outlets, and price regu- 

 lated the amount of effort employed in the 

 fishery. Movements of immature and mature 

 crabs into fishing areas, and the length of stay, 



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