22 



COMMERCIAL FISHERIES REVIEW 



Vol. 28, No. 8 



SEA OTTER POPULATION 

 DETERMINED BY CENSUS: 



The annual census of California's sea otter 

 population conducted in June 1966 disclosed a 

 count of 591 animals, the Department of Fish 

 and Game of that state announced. The count 

 is nearly 100 individuals greater than the cen- 

 sus count of 497 made in June 1965. 



Many single animals were counted this year 

 (1966) that were not observed a year earlier. 

 The California agency used a smaller, more 

 m.aneuverable plane this year that provided bet- 

 ter visibility than the plane used in previous 

 years. 



The annual census is conducted in June of 

 each year because visibility along the ocean 

 usually is good, and because animals are found 

 in larger groups than at other times of the year. 



The sea otter once flourished along the 

 whole Pacific Coast, but now is limited most- 

 ly to the area between Cayucos and Monterey. 

 It is considered an endangered species, and a 

 census is taken regularly to keep track of its 

 numbers. 



Note: See Commercial Fisheries Review , August 1964 p. 15. 



Cans—Shipments for Fishery 

 Products, January-April 1966 



A total of 705,491 base boxes of steel and a- 

 luminumwas consumed to make cans shipped 

 to fish and s hellf ish canning plants in Jan. -Apr. 

 _ 1966, as compared 



f r/WBrwith 894,564 base 

 J ~|[-f-[[Qj[ji[li^^j_i_r: boxes used during the 

 same period in 1965. 



Note: Statistics cover all commiercial and captive plants known 

 to be producing metal cans. A "base box" is an area of 31, 360 

 square inches, equivalent to 112 sheets 14" x 20" size. Ton- 

 nage figures for steel (tinpiate) cans are derived by use of the 

 factor 23,7 base boxes per short ton of steel. 



Source: U.S. Department of Commerce, Bureau of the Census. 



Central Pacific Fisheries Investigations 



TUNA BEHAVIOR AND HEARING 



ABILITY STUDIES: 



The first measurements of the hearing a- 

 bilities of a tuna have been made at the In- 



terior Department's Bureau of Commercial 

 Fisheries Biological Laboratory, Honolulu, 

 Hawaii, in experiments carried out over the 

 last 3 years. Traditional methods of fishing 

 are based on two facets of tuna behavior-- 

 they eat smaller fish and they form them- 

 selves into schools. Fishery scientists be- 

 lieve that closer study of other aspects of 

 sensory capability and behavior may lead to 

 clues that will increase the fishermen's abil- 

 ity to take the fish. 



The fish used in the hearing experiments 

 were yellowfin tuna ( Thunnus albacares ). 

 which account for almost half of the United 3 

 States tuna catch. They had been taken off 1 

 the Hawaiian Islands and held in sea -water l 

 tanks at the Honolulu Laboratory's Kewalo 5 

 Basin facility. 



The experiments showed that the yellowfin 

 tuna heard most acutely sounds between 300 

 and 500 cycles per second. These are rela- 

 tively low-pitched sounds, in the neighbor- 

 hood of the middle A on the piano. The yellow- 

 fin tuna could not hear sounds higher than a- 

 bout 1,100 cycles nor those lower than about 

 50 cycles per second. 



To obtain the data, yellowfin were trained 

 to respond to an underwater sound signal 

 (produced by a transducer) by changing their 

 direction of swimming. The fish were re- 

 warded by a bit of food after each successful 

 try. The intensity of the sound, for each fre- 

 quency, was reduced until the fish made err- 

 ors on half the trials. The value at that time 

 was called their threshold of hearing. 



The first measurements of the hearing a- 

 bility of yellowfin tuna agreed with several 

 such studies of other marine fish. The tuna's 

 hearing ability is neither remarkably good nor 

 remarkably bad by piscine standards. 



What role hearing plays in the tuna's be- 

 havior is not as yet known. One possibility 

 is that it may help them locate prey, for the 

 sound of small fish swimming falls into the 

 frequency range in which the tuna's hearing 

 is most acute. Another possibility is that 

 hearing may be used to receive signals from 

 their own species. 



It was observed that the tunas made two 

 sounds--one is called a "snap," which "is sim- 

 ilar to the sound made by striking a wooden 

 pencil against a table edge." It occurred 

 when a yellowfin shut its jaw sharply. The 



