important physical factors. The size and 

 consistency of the ovarian eggs, and 

 the tenacity of their adhesion to one 

 another and to the wails of the ovaries 

 also provide indications of maturity. 



The more objective criteria include 

 indices of maturity, usually expressed 

 as a relationship between the weight of 

 the ovaries and some measurement of 

 the fish from which they were taken. 

 Examination of histological sections of 

 gonads permits more exact assessment 

 of stages of maturity, but few such 

 studies on bluefin tuna have been pub- 

 lished. 



Estimates of spawning areas and 

 seasons from gonad condition are sub- 

 ject to many limitations. Unless the 

 specimen is fully ripe, serious errors 

 may be involved, as these fish can travel 

 great distances in short periods (Mather 

 1962, 1969). Also, frilly ripe bluetln 

 tuna are difficult to catch on hook and 

 line as they are reportedly reluctant 

 Sara to feed (Sella 1929a, Rivas 1954, 

 Sara 1964, 1973). Their capture in traps 

 is extremely rare. Sanzo ( 1 9 1 Oa) found 

 a ripe female in a trap near Palermo, 

 Sicily, in 1908, but had not encoun- 

 tered another when he produced his 

 final (1932) work on the eggs and lar- 

 vae of the bluefin. Likewise Rodriguez- 

 Roda ( 1 964a, 1 967a) had observed only 

 one ripe female in his extensive studies 

 at Barbate, the most productive tuna 

 trap in this century, and other sites. He 

 remarked that the oviduct of this indi- 

 vidual might have been obstructed. The 

 recently developed purse seine fishery 

 in the central Mediterranean spawning 

 grounds (Section IV) has made more 

 mature bluefin available (R. Sara, per- 

 sonal communication), but no studies 

 based on these catches have appeared. 



Bluefin tuna spawn fractionally, 

 by several separate emissions of eggs 

 over a period of several days, rather 

 than by a single emission (Sanzo 1910a, 

 Frade and Mana9as 1933). Thus, even 

 when ripe fish are taken, the localities 

 of capture may not indicate the total 

 area over which the fish have spawned. 



The fractional nature of their 

 spawning also adds to the difficulties 

 in determining when bluefin tuna are 

 frilly ripe. As noted above, many of the 

 criteria used to determine stages of 

 maturity are subjective. In addition, 

 even the seemingly objective indices 

 of maturity may be subject to error. 



Maturing and post-spawning fish 

 may have the same indices as the size 

 of their gonads increases and then de- 

 creases. Also, maturing fish are con- 

 siderably heavier than spent ones of the 

 same length. This fact introduces er- 

 rors when indices for fish in these dif- 

 ferent phases are compared. In addi- 

 tion, the testes of males decrease in size 

 much more than tlie ovaries of the fe- 

 males after spawning has been com- 

 pleted. Thus comparison of indices for 

 fish of different sexes may be mislead- 

 ing. Frade (1937) pointed out these 

 sources of error and used corrections 

 for the differences in gonad weight be- 

 tween sexes and in total weight be- 

 tween maturing and spent individuals 

 of the same length. These refinements 

 in the method, however, have appar- 

 ently been ignored. We have observed 

 another possible source of error in indi- 

 ces of maturity for bluefin tuna in the 

 western North Atlantic. When large 

 bluefin tuna arrive in New England 

 waters in July (Section IV), they are 

 spent and their gonads are small. To- 

 ward the end of their feeding season, in 

 late summer and early fall, their go- 

 nads become almost completely en- 

 cased in adipose tissue. If this tissue is 

 not removed before weighing the go- 

 nads, the maturity index may be almost 

 as high as for a ripe fish, even though 

 the gonad proper is small and dormant. 

 Baglin (1976) noted that the average 

 weight of the adipose tissue attached to 

 the ovaries of six giant bluefin taken 

 off New England in August 1975, was 

 1,152 g, while the average weight of 

 the gonads themselves was 1,1 14 g. 



Tlie well documented presence of 

 fully ripe fish may therefore be a good 

 indication of a spawning area, but con- 

 clusions based on nearly ripe fish may 

 be very misleading. 



2. Presence of Pelagic Eggs and 

 Larvae 



The times and locations of collec- 

 tions of pelagic eggs and larvae form 

 another basis for the detemiination. or 

 estimation, of the seasons and areas of 

 spawning. When eggs or extreme!) 

 small larvae can be identified, it may 

 be assumed that spawning occurred not 

 very long before ihe time of collection 

 and not ver>' far from the collecting 

 locality. Estimating the time and place 

 of spawning when larger larvae or ju- 



veniles are collected, however, requires 

 knowledge of their growtli rate, and 

 also of the currents in the collecting 

 area. 



Although this approach is very 

 logical, its execution has not proved to 

 be easy. The collection and identifica- 

 tion of eggs and larvae is difilcult. Blue- 

 fin eggs cannot be positively identified 

 unless they are hatched and the larvae 

 are reared to an identifiable size 

 (PiccinettiandPiccinettiManfrin 1970), 

 which is a most difilcult process. The 

 identification of larvae has been con- 

 troversial (Sella 1924,Dieuzeide 1951, 

 Duclerc et al. 1973, Richards 1976). 

 Sella and Richards questioned 

 Ehrenbaum's (1924) tentative identifi- 

 cations of "Orcyniis ihynnus L.'\ and 

 Richards reidentified several of 

 Ehrenbaum's larvae of tunas and tuna- 

 like species. Duclerc et al. ( 1 973) found 

 it impossible to distinguish eggs and 

 larvae oVThunnus ihynmus L.", from 

 those oV'Aitxis lhuzard\ even after the 

 eggs had been hatched and tlie larvae 

 reared to lengths of 5.0 mm. Duclerc et 

 al. (1973) and Richards (1976) ques- 

 tioned Sanzo's (1932) descriptions of 

 the egg and larval stages ofOrcyims 

 t/iynnus Ltkn.", and felt that they should 

 be verified. Some encouraging progress 

 has been made recently, however. 

 Duclerc et al. ( 1 973) and Scaccini et al. 

 (1975) have reared larval bluefin tuna 

 for periods of up to eight days from 

 eggs collected at sea. Richards and 

 Potth off (1974) have produced a most 

 thorough description of larval bluefin 

 tuna more than 3.0 mm (SL) long. 



The accuracy of estimates of 

 spawning seasons and areas based on 

 the collection data for young stages, 

 their length and growth rates, and cur- 

 rent systems in the collecting area, de- 

 clines rapidly with the passage of time 

 from the hatching of the specimen. Not 

 only does the error in calculating pas- 

 sive transport increase, but juvenile 

 bluefin tuna become active swimmers 

 at a surprisingl\ early age. Tlie\ attain 

 their full complement of caudal rays at 

 about ! 6 mm SL (PotthofiF 1 975). Sella 

 (1929a) stated that the bluefin tuna 

 could already be considered an active 

 fish at the age of 1 5 days. 



Most of the available data on the 

 growth of the early stages of the blue- 

 fin (Section 111) have been presented in 

 terms of weight. Growth data in terms 



69 



