LENARZ and ZWEIFEL: INTERACTION BETWEEN LONGLINE AND SURFACE FISHERIES 



tr. 

 o 



u 



o 



0.8 1.2 16 2.0 



MULTIPLIER OF EFFORT 



Figure 4. — Estimates of yield per recruit of Atlantic yellowfin 

 tuna to the longline fishery as a function of effort and presence ( 4> 

 = 1.0) or absence (({> = 0.0) of a surface fishery. 



o 

 llJ 

 q: 



o 



0.4 0.8 1.2 1.6 2.0 2.4 



MULTIPLIER OF EFFORT 



2.8 



Figure 5. — Estimates of yield per recruit of Atlantic yellowfin 

 tuna to the surface fishery as a function of effort and presence (<^ 

 = 1.0) or absence {^ = 0.0) of longline fishery. 



recruit of the combined fisheries is higher than the 

 yield per recruit of either fishery alone. The re- 

 sults suggest that if a catch quota system is im- 

 posed on the Atlantic yellowfin tuna fishery, all 

 components should be included unless it is shown 

 that different stock(s) are being exploited by the 

 gear. 



The above results (Figures 4, 5) suggest that a 

 stock of yellowfin tuna will produce a potentially 

 higher yield per recruit to a longline fishery than 

 to a surface fishery, if the fish are equally avail- 

 able to the two gears. However, until the question 

 of availability is settled, it is not possible to predict 

 the potential production to the two gears. We point 

 out here that gear-specific availability is not well 

 known for any tuna fishery and would be difficult 

 to determine. Thus, we are faced with the prospect 

 of probably being forced to determine empirically 

 the production potential for each gear in each 

 fishery. After a fishery is established, an analysis 

 of the type conducted on the Atlantic yellowfin 

 tuna fishery could be used to examine the effects of 

 availability to the two gear types, and a tagging 

 study could be designed to provide the required 

 answers. 



EFFECTS OF AGE-SPECIFIC SEX 



RATIOS OF ATLANTIC YELLOWFIN 



TUNA ON YIELD PER RECRUIT TO 



THE TWO TYPES OF GEAR AND 



STOCK FECUNDITY 



While a number of authors have noted that the 

 ratio of females to males appears to be less than 1:1 

 for catches of larger tunas, none to our knowledge 

 has incorporated these observations into calcula- 

 tions of yield per recruit or stock fecundity. 

 Beardsley ( 1971) reported that the ratio of female 

 to male Atlantic longline-caught albacore was 

 233:365 during the December 1969-September 

 1970 period. Males increasingly dominated at 

 sizes >100 cm. Females slightly outnumbered 

 males between 92 and 100 cm. One explanation for 

 the catch curves estimated by Beardsley is a 1:1 

 sex ratio at small sizes, a slightly slower growth 

 for females for fish >90 cm, and beyond 100 cm, 

 either a higher rate of natural mortality for 

 females or a change in behavior that makes 

 females less available than males to longline 

 fishing. Other explanations exist, e.g., a combina- 

 tion of low sex ratio and slow growth of females 

 throughout their life. Sakamoto ( 1969) noted for 

 Atlantic bigeye tuna, ". . . males predominated in 

 areas of higher water temperature. Proportion of 

 females increase as the water temperature gets 

 lower." His data indicate that as size increases the 

 proportion of females decreases and females may 

 grow slower than males in waters between lat. 30° 

 to 50°N, but not in equatorial waters. Data pre- 

 sented by Kikawa (1964) indicate that southern 

 bluefin tuna >150 cm are predominantly males, 

 while females often outnumber males at smaller 

 sizes. Thus, female southern bluefin tuna may 

 grow more slowly than males. 



Since there is considerable evidence for age- 

 specific changes in the sex ratio of tunas, we be- 

 lieve that the effects of such changes on estimates 

 of yield per recruit to each gear type and fecundity 

 should be investigated. We have assumed sex 

 ratios to be the same as with Pacific yellowfin tuna 

 because no extensive studies of age-specific sex 

 ratios for Atlantic yellowfin tuna have been pub- 

 lished. We used results from a study by Murphy 

 and Shomura (1972), who found that beyond 140 

 cm male yellowfin tuna greatly outnumbered 

 females (Figure 6). The data in Figui'e 6 do not 

 show a large excess of females in any size interval 

 and thus no evidence of sex-specific growth is 

 exhibited. Using their data and the age-length 



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