situations, additional sampling or analysis will 

 be necessary to define certain aspects of these 

 statistics such as the size distribution of 

 landings and the effects of changes in fishing 

 gear on our estimates of fishing effort. 



Richard A. Neal, Acting Program Leader 



Predicting Shrimp Abundance 



Research was conducted to develop indices 

 of shrimp abundance at three locations: off- 

 shore, at the entrance to Galveston Bay, and 

 in the Bay. We attempted to obtain data on the 

 spawner-recruit relations of white shrimp. 



Offshore index . --In the winter of 1968-69, we 

 completed studies designed to resolve the 

 question of whether large numbers of post- 

 larval brown shrimp remain burrowed in off- 

 shore sediments during periods of low 

 temperatures. If such is the case, it may be 

 possible to predict commercial supplies of 

 brown shrimp from offshore abundance indices 

 of postlarvae several months before postlarvae 

 immigrate into the bays. Trips were made 

 weekly through the winter to collect plankton 

 and benthos from near- shore waters in depths 

 of 4 to 15 m. (2 - 8 fathoms) off Galveston, and 

 our regular sampling with a beam trawl con- 

 tinued in Galveston Entrance (table 4). We 

 concluded that postlarvae were not abundant 

 enough offshore in November and December to 

 enable us to establish an index of abundance 

 before they move into the bay. 



We also sought postlarvae during two cruises 

 completed along a track that extended from 7 m. 

 (4 fathoms) off Galveston to 51 m. (28 fathoms ) 

 off Freeport, Tex. An average of 0.05 post- 

 larvae per tow was taken in 108 five-minute 

 bottom tows, and an average of 0.33 postlarvae 

 per tow was taken in an equal number of tows 

 made at higher levels. The sled used for 

 bottom tows strained about 100 m. of water 

 per 5-nninute tow. The catches of shrimp were 

 too low for us to establish a reliable index of 

 abundance. 



Table 4-. — Monthly average catch of postlarval 

 brown shrimp in two areas by two types of 

 gear, 1958-59, Biological Laboratory, 

 Galveston, Tex. 



Month 



Near- shore 

 Gulf ( sled) 



Tows 



(ID- 



minute) 



Shrimp 

 per tow 



Galveston Entrance 

 ( beam trawl) 



StandEird 



tows 



Shrimp 

 per tow 



Number Number 



Number 



Number 



Index at Galveston Entrance .-- An automatic 

 plankton pump was installed on a platform near 

 Galveston Entrance to sample postlarval 

 shrimp as they enter Galveston Bay. It is now 

 set to pump continuously for 20 minutes each 

 hour. The sampler has functioned well, but we 

 have not solved completely the problems of 

 preserving the collections as they are taken. 

 Samples were deposited originally in a 

 Formalin bath by a conveyor operating below 

 the pump outfall. The conveyor was unsatis- 

 factory and was replaced with a nylon- webbing- 

 lined chute that transports the collections to 

 the Formalin bath after each pumping cycle. 



To evaluate catches made with the pump, we 

 compared them with catches of postlarvae 

 made twice weekly at Galveston Entrance with 

 a small beam trawl (fig. 7). Because of the 

 obvious differences between catches, we will 

 continue both methods of sampling until it can 

 be determined which is most representative. 



In conjunction with the development of a 

 more accurate measure of the abundance of 

 postlarval shrimp, we conapleted vertical dis- 

 tribution studies at the pump site to determine 

 the depth at which immigrating postlarvae 

 were most numerous. Water depth is about 

 5 m. (16 ft.) at the sampling site. Figure 8 

 shows the number of postlarval brown and 

 white shrimp collected at each depth during 49 

 day and 23 night collections made during flood 

 tides in an 18-month period. The depth chosen 

 for the position of the pump intake was 1.8 m. 

 (6 ft.). 



Juvenile index .- -We continued our weekly 

 collection of catch and effort data from Gal- 

 veston Bay bait-shrimp fishermen. These data 

 are used to make an index of abundance of 

 juvenile shrimp on the nursery grounds. The 

 total weight of bait shrimp harvested in 1968 

 from Galveston Bay was 1,102,600 pounds 

 (500 metric tons). About 65 percent of this 

 catch was white shrimp. Analysis of bait 

 statistics revealed that large numbers of white 

 shrimp remained in Galveston Bay through the 

 winter. 



Figure 9 shows the estimated annual catch 

 of bait shrimp in weight and value from Gal- 

 veston Bay for the last 10 years. The retail 

 price of live shrimp remained at $2.00 per 

 quart between 1956 and 1966. In 1967, how- 

 ever, the price was raised to $2.50 in all but 

 two areas of the bay system. By early 1968, 

 14 dealers in the Galveston- Texas City area 

 charged $3.00 per quart. The price of dead 

 bait has changed little over the past several 

 years and fluctuates between $0.50 and $0.75 

 per pound. 



Spawner-recruit relations .-- We have evi- 

 dence of a relation between the abundance in- 

 dices of postlarval brown shrimp and those 

 of adults a few months later, but we do not 

 know the relations between adult white shrimp 



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