MIGRATION, GROWTH, AND MORTALITY 

 OF COMMERCIAL SHRIMPS 



Knowledge of shrimp movements, growth, 

 and mortality is necessary for determining the 

 best time to begin harvesting shrimp and the 

 fishing intensity that can be maintained without 

 reducing a stock's reproductive capacity. Opti- 

 mum fishing intensity will be of particular 

 concern if improvements in havesting methods 

 make it possible to overfish our shrimp re- 

 sources. 



Recent studies of population dynamics have 

 been directed toward making best use of avail- 

 able shrimp stocks. As our knowledge of each 

 species expands, the probability increases 

 that we will be able either to increase the 

 value of shrimp harvests by stocking or to 

 control the factors causing shrimp mortality. 

 Before efforts are made to increase shrimp 

 production scientifically, however, complete 

 infornnation concerning movements, growth, 

 and mortality must be established. Aspects 

 which require special attention are: ( 1 ) relative 

 mortality rates during specific periods of the 

 life cycle; (2) effects of the environnnent and 

 population density on growth and survival; 

 (3) nnovements of shrimp in offshore waters; 

 and (4) identification of individual populations. 



Mark-recapture experiments provide direct 

 estimates of growth and migration and indirect 

 estimates of mortality rates. Other sources 

 of information such as catch per unit of effort 

 by the commercial fleet are being used to 

 check estimates of population parameters 

 made fronn mark-recapture experiments, but 

 the errors involved are usually greater than 

 those in estimates based on mark-recapture 

 data. 



An evaluation of previous mark-recapture 

 experinnents indicates the three most import- 

 ant sources of error to be (1) mortalities of 

 stained shrimp after release, caused by the 

 staining technique; (2) the paucity of recoveries 

 when less than 5,000 stained shrimp are re- 

 leased; and (3) our inability to identify in- 

 dividual shrimp, which necessitates staining 

 groups of shrimp that cover a relatively large 

 size range. 



Postrelease mortalities from marking and 

 handling influence estimates of growth or 

 migration only if mortality varies with shrinnp 

 size; such deaths, however, always adversely 

 affect mortality estimates. Handling techniques 

 and facilities constantly are being modified to 

 lessen their harmful effects. Live shrimp are 

 held at a temperature of 3° to 5° C. (5.4°-9.0° 

 F.) lower than the water fronn which they were 

 taken, thus reducing their metabolic rate and in- 

 creasing survival. Two lightweight refrigera- 

 tion units were constructed for use on shore 

 or on research vessels at sea (fig. 11). These 

 units provide independent control of tempera- 

 ture in individual tanks and reduce the mag- 

 nitude of temperature change to which shrimp 



must be subjected. Each unit has a 1-h.p. 

 compressor and a l/Z-h.p. pump. The chilling 

 tank was constructed from a piece of 10-inch 

 polyvinyl chloride pipe. Water with an initial 

 temperature of 21° to 24° C. (70°-750F.) can 

 be cooled 2.3° C. (4.1° F.) at a rate of 35 

 gallons per minute. 



Analysis of data from past mark-recapture 

 studies indicates that when marked shrimp 

 are released directly into a fishery, large 

 numbers of recoveries may be expected within 

 the first few weeks after release. Little infor- 

 mation is gained when this occurs. We believe 

 that because of high mortality rates, large 

 numbers of shrimp mustbe released if adequate 

 numbers are to survive beyond the first few 

 weeks. 



The most satisfactory method for marking 

 shrimp has been to inject into its abdomen a 

 biological stain dissolved in distilled water 

 or a small amount of fluorescent pigment in 

 Vaseline.^ The biological stain passes through 

 the shrimp's vascular system and localizes in 

 the gills whereas the fluorescent pigment re- 

 mains at the point of injection. Past experi- 

 ments have indicated that two biological stains 

 and four fluorescent pigments are suitable for 

 marking shrimp. If one of the stains and one 

 or more of the pigments are used on each 

 shrimp, there are 10 possible distinctive 

 marks. To evaluate tne effects of these marks 

 on shrimp mortality, we injected one group 

 of shrimp with the biological stain and another 

 group with the biological stain and fluorescent 

 pigment. A comparison of mortality rates 

 between these two groups and controls held 

 in the laboratory for 30 days after marking 

 indicated that mortality increased when the 

 stain and pigment were used together. Mean 

 survival time of the groups is shown in table 6. 



Because only a limited number of stains is 

 available for distinguishing size groups when 

 several separate releases are made into the 

 same population, we have expended consider- 

 able effort to develop a method to identify 

 individual shrimp. We have tested several 

 types of external and internal tags. External 

 tags were attached at several sites on shrimp, 

 but because the hole in the exoskeleton through 

 which the tag shaft extends did not heal prop- 

 erly, infection usually resulted within 2 weeks. 



Of the internal tags tested, an oval poly- 

 vinyl chloride tag (0.27 mm. by 2 mm. by 

 5 mm.) appeared to be most acceptable (fig. 

 12). The most satisfactory insertion site is 

 the first abdominal segment. The tag is 

 inserted with fine forceps through the joint 

 between the cephalothorax and the abdomen. 

 To date, its presence has not seriously af- 

 fected molting or survival of the shrimp in 

 laboratory experiments. Field experiments 

 are underway which will enable us to compare 



^Trade names referred to In this publication do not 

 imply endorsement of commercial products. 



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