SHARKS AND SHARK DAMAGE 



SUMMARY 



The depredation on hooked yellowfin tuna by 

 sharks requires b r i e f examination, since it 

 affects the monetary return to fishermen from 

 any longline venture. Not all tunas which are 

 shark damaged are unsaleable, however, since 

 some may be only superficially damaged and 

 hence acceptable to canneries. Consequently, 

 the records kept by POFI observers tend to 

 exaggerate the loss from this source since the 

 extent of damage to each shark-bitten fish was 

 usually not evaluated. 



1. There were thirteen longline fishing cruises 

 to the central equatorial Pacific during 1954. 

 Most of these were commercial ventures to 

 the vicinity of the Line Islands. 



2. The year 1'954 marked some first events in 

 the central equatorial Pacific: (1) the first 

 commercial attempts to longline by American 

 fishermen, (2) the first use of steel longline 

 gear, and (3) first concentrated year-around 

 fishing in the Line Islands area. 



The loss due to sharks during 1954 does not 

 differ greatly from previous years. In 1954 an 

 average of 20 percent of hooked yellowfin were 

 damaged. One extreme occurred on a cruise of 

 the Oceanic when 46 percent of the yellowfin 

 caught were either partially mutilated or totally 

 destroyed by sharks. The lowest figure for any 

 cruise in 1954 was the 10 percent damage re- 

 ported by the North Americ an on cruise 1. 



The relationship of the percentage of shark 

 damaged to hooked yellowfin and the longline catch 

 rates of all species of sharks (white-tipped, silky, 

 great blue, and bonito) taken during 1954 in the 

 Line Islands region is illustrated in figure 19. 



C 2 - 



30 " 

 O 



20 < 



JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 



Figure 19. --Shark catch rates on longlines and 

 percentage of shark damage to hooked yellow- 

 fin in the Line Islands area during 1954. 



The plotted points are mid-points of fishing 

 periods, and where two or more vessels fished 1 

 together the average shark catch rate and shark 

 damage are used. As would be expected the 

 amount of shark damage varies directly with the 

 shark catch rates. Shark catch rates, however, 

 are not correlated with the abundance of yellow- 

 fin (compare figs. 19 and 6). 



3. The seasonal trend of catch rates was lowest 

 during August andOctober 1954, the period at 

 which catch rates were highest for previous 

 years. 



4. The lack of distinct variation in yellowfin 

 catch rates between different localities close 

 to the Line Islands indicated a random dis- 

 tribution of the fish in respect to each indi- 

 vidual island. 



5. The sex ratio and size distribution in the 

 yellowfin catch of 1954 were similar to the 

 previous years. 



6. A correlation between sea temperature and 

 the catch of deep- swimming yellowfin was 

 observed again in 1954, with the higher catch 

 rates being associated with warmer surface 

 temperatures. 



7. The amount of shark damage to hooked yellow- 

 fin was about the same as in previous years 

 £.nd varied directly with the abundance of 

 sharks. 



LITERATURE CITED 



IVERSEN, EDWIN S. and G. I. MURPHY 



1955. What the Jangaard longline venture 

 found in mid-Pacific. Pacific Fisher- 

 man 53(4):22, 25, 27. 



MA.NN, HERBERT J. 



1955. Construction details of improved tuna 

 long-line gear used by Pacific Oceanic 

 Fishery Investigations. U. S. Fish 

 and Wildlife Service, Comm . Fish. 

 Rev. 17{12): 1-10. 



MURPHY, GARTH I. and K. C. ELLIOTT 

 1954. Variability of longline catches of 

 yellowfin tuna. U. S. Fish and Wild- 

 life Service, Spec. Sci. Rept.-- 

 Fisheries No. 119, 30 pp. 



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