Inshore Currents 



Since February 1 966, drift bottles and seabed 

 drifters have been released monthly offshore 

 from Galveston Island to define the current 

 pattern from the shore to 14.6 m. and to de- 

 termine the role of currents in transporting 

 postlarval shrimp (fig. 12). The drift bottles 

 released in February indicated a strong south- 

 westerly surface current. Bottle returns from 



uu|-iUu ^uun'uM|-ll-IMHHH|-IHHH 



H^J 



Figure 12.~Movement of drift bottles released off Galves- 

 ton Island, Tex., in February, March, and April 1966. 

 Nunabers Indicate the percentage of bottles recovered. 



the March release showed a more shoreward 

 current, and by April many of the bottles were 

 recovered from the Galveston Island and 

 Freeport areas. The onshore currents at these 

 times coincide with the periods of peak ar- 

 rival of postlarval brown shrimp at Galveston 

 Entrance. 



Kenneth N. Baixter, Project Leader 



MARK-RECAPTURE EXPERIMENTS 



Mark-recapture experiments, with shrimp 

 marked with biological stains, have been 

 made on populations of the three commercial 

 species in several areas of the Gulf. This 

 method for obtaining estimates of growth 

 and mortality of shrimp is the most direct and 

 practical meams available, but it poses many 

 problems. The most connmon difficulties en- 

 countered stem either from changes in the 



activities of the commercial fleet during an 

 experiment or from the problem of releasing 

 a sufficient number of stained shrimp in a 

 fishing area. Although we cannot foresee how 

 the fishery will change, we can taike steps 

 to increase the number of marked shrimp 

 released. With this purpose in mind, we have 

 altered some of our techniques and have begun 

 to use new types of equipment in recent 

 months. 



A minor problem that arises during a 

 staining experiment is the catching of large 

 numbers of shrimp in good condition. To 

 reduce injuries to shrimp during trawling, 

 tows are nornnally limited to 10 or 15 minutes. 

 We have also found it advisable to use 70-foot 

 nets instead of the smaller ones used formerly. 



A more serious problem involves keeping 

 shrimp alive in holding tanks when water 

 temperatures are above 26.5° C. We have found 

 that mortalities are reduced greatly when the 

 temperature is held near 24° C. A refrigera- 

 tion unit with the capacity to keep water tem- 

 peratures at this level in three large holding 

 tanks has been used successfully in the past 

 two seasons. Fresh sea water is introduced 

 slowly into the essentially closed-water sys- 

 tem to prevent an accunnulation of wastes. 



Fiber glass holding tanks have been con- 

 structed to replace the heavy wooden tanks 

 used formerly. The new tanks (fig. 13) are 

 divided by vertical partitions to make it 

 possible to separate groups of shrimp and by 

 horizontal partitions to increase the surface 

 area on which shrimp may rest. Portable 

 staining tables and measuring boards are 

 attached to the holding tanks. These additions 

 reduce to a minimum the need to move and 

 handle shrimp during staining. 



Shrimp are marked by injecting a small 

 quantity of stain from a syringe into the 

 shrimp's abdomen; from there the pigment 

 is carried to the gills where it remains. 

 An overdose of stain kills shrimp, and an 

 underdose makes it difficult or impossible 

 to detect a marked individual in a fisher- 

 man's catch. It is important to reduce the 

 variation in the amount of stain injected 

 into shrimp without increasing the time re- 

 quired for marking and handling. Recent use 

 of automatic calibrating and refilling syringes 

 has eliminated this variation of the amount of 

 stain and has increased the number of shrimp 

 that can be marked in a given period of time 



(fig. 14). 



The final phase of the marking operation 

 is the return of the shrimp back to the sea 

 floor in a manner that will not expose them 

 to predation. In the past, we lowered a box 

 containing marked shrimp to the bottom where 

 it opened in response to a weighted messenger. 

 This type of box must be lowered with the aid 

 of a winch and, therefore, requires that the 

 research vessel be stopped. A new connpact 

 lightweight and disposable release box is now 



16 



