F. Feeding Activity During the 

 Spawning Season 



Different opinions have been ex- 

 pressed in regard to the feeding activity 

 of bluefin tuna during the spawning 

 season. Many of the studies used indi- 

 viduals caught in traps. It has been 

 argued that the lack of food in the stom- 

 achs of "arrival" fish might have been 

 caused by digestion while the fish were 

 awaiting removal from the trap, or by 

 regurgitation resulting from their 

 struggles during this removal. Food is 

 frequently found in the stomachs of 

 "return" fish, however, even though 

 they undergo the same treatment. 



F. de Buen (1925) and Sella 

 (1929a) were of the opinion that tlie 

 bluefin continued to feed during their 

 period of maturation and spawning, but 

 at a reduced rate. De Buen believed 

 that they ate food which they happened 

 to encounter, and could catch without 

 undue effort, but seldom exerted them- 

 selves in the pursuit of difficult prey. 



Rodrfguez-Roda (1963, 19643, 

 1 969) found that stomachs of tuna taken 

 in the Spanish "arrival" traps were usu- 

 ally empty. Some contained a few 

 swimming crabs, Polybins hens/owi 

 Leach, which they had presumably con- 

 sumed before entering the trap. Small 

 fish which had been caught in traps 

 with the tunas were occasionally found 

 in their stomachs. He emphasized the 

 limitations of his findings because of 

 the possible losses of stomach contents 

 of trapped fish mentioned above. 



Sara's (1964, 1973) studies of 

 stomach contents of "arrival" fish taken 

 in the fraps off western Sicily and ob- 

 servations of their behavior when con- 

 fined in the traps with species on which 

 they normalK fed, led him to believe 

 that maturing bluefin mna abstained 

 from eating. He found increasing 

 amounts of food in the stomachs of 

 post-spawning fish, however, as the 

 season advanced and their gonads be- 

 came smaller. 



Arena ( 1 964) found stomachs of 

 "arrival" fish caught in the western Si- 

 cilian traps to be empty, or nearly so. 

 Those of others caught off the eastern 

 part of the island, however, contained 

 considerable quantities of food. A large 

 quantity of swimming crabs, Polybhis 

 henslowi Leach, was found in the stom- 

 ach of the first group of bluefin tuna 

 caught atMilazzo in 1 96 1 . Several other 



authors have reported observations of 

 food in the stomachs of maturing blue- 

 fin caught off eastern Sicily (Genovese 

 I960, Genovese and Alonzo 1961 Li 

 Greci 1961). 



The considerable longline catches 

 of bluefin tuna in the central Mediter- 

 ranean during the spawning seasons of 

 1973 and 1974 (Fisheries Agency of 

 Japan 1975, 1976) prove that signifi- 

 cant numbers of these fish feed during 

 the spawning season. As Sara (1973) 

 noted, the trophic (feeding) tendency 

 may on occasion overcome the tenden- 

 cies which are usually dominant dur- 

 ing the spawning period, including the 

 tendency to reduce, and finally stop, 

 feeding as the volume of the gonads 

 increases. 



G. The Spawning Act 



The only published accounts of 

 the actual spawning of bluefin tuna 

 which we have encountered described 

 occurrences in the central Mediter- 

 ranean. Some of these activities oc- 

 curred while the fish were being held in 

 traps, but others, probably more im- 

 portant because the fish were under no 

 constraint, took place in the open sea. 

 Sella (191 1) and Held! (1932) de- 

 scribed the spawning actions of fish in 

 traps off Sicily and Tunisia on the basis 

 of accounts which they considered reli- 

 able. Sara (1964) personally observed 

 the reproductive acts of tunas in the 

 vicinity oftrapsofifSicily. Arena (1964) 

 likewise witnessed the spawning of 

 bluefin tuna in the open sea off Sicily. 

 These accounts show that spawn- 

 ing occurs at the instant when a pair of 

 fish turn on their sides and make con- 

 tact, or appear to, with their ventral 

 surfaces. This refutes the previous sup- 

 position (Tiews 1963) that tuna 

 spawned by saturating an area with 

 randomly discharged eggs and milt. 



Arena and Sara, however, reported 

 that small groups of tuna sometimes 

 engaged in communal mating. In a spec- 

 tacular observation, Arena (1964) saw 

 files of 10-12 giant fish overtake other 

 files of about the same number, with 

 the fish of one file mating with those of 

 the other as they passed. 



P. Arena (personal communica- 

 tion) provided the following informa- 

 tion on spawning, based on his more 

 recent observations. The mating offish 

 in large schools may involve nearly all 



of the fish in the school at the same 

 time. This causes an enormous bright 

 flash under the surface, with a very 

 spectacular effect. The school, which 

 usually travels at from 2 to 7 knots (3.7 

 to 1 3 km/lir), leaves a rather milky trail 

 behind it, due to the emission of sexual 

 products. 



H. Maturity and Fecundity 



The ages at which bluefin tuna 

 spawn, and the variation in the fecun- 

 dity of females with their size and age, 

 are two elements of the biology of the 

 species which are especially important 

 for the management of its fisheries. 



1. Age and Size at First 

 Maturity: 



Research on bluefin tuna in the 

 eastern Atlantic and Mediterranean in- 

 dicated that they first spawned at ages 

 2-4 (lengths of about 75-125 cm, 

 weights of about 12-40 kg). Sella 

 (1929a, 1929b) reported that the spe- 

 cies usually spawned first at age-3, or 

 at a weight of about 15 kg. He added 

 that a few age-2 fish which had at- 

 tained weights of 12 kg through rapid 

 growth might also spawn. 



Frade and Manafas (1933) found 

 that young bluefin 1 m long taken in 

 the "arrival" (May-June) fishery off 

 southern Portugal developed difterently 

 according to their sex: the males were 

 in active or completed spermatogen- 

 esis, whereas the females were In a 

 very retarded ovogenesis. Arena ( 1 964) 

 and Sara (1973) have ob,served that 

 bluefin tuna school by size, even dur- 

 ing the spawning season, and that fer- 

 tilization is accomplished by paired 

 emissions within these schools. There- 

 fore the reported difference in the times 

 of maturity of 3-year-old males and 

 females implies that their spawning ef- 

 forts might be ineffective. Rodriguez- 

 Roda (1929a) concluded from his fe- 

 cundity studies that females attained 

 their first maturity at a length of about 

 97.5 cm, and males at about 105 cm, 

 corresponding to ages of 3 Nears. Frade 

 and Vilela (1962) concluded that first 

 maturity usually occurred at age 3, but 

 occasionally at ages 2 or 4. 



Scaccini et al. 1975) reported the 

 smallest mature bluefin tuna observed 

 by any of them was an individual weigh- 

 ing 27 kg (4 years old) examined by 

 Sara at the Solanto trap in Sicilv in 

 mid-June 1959. 



92 



