close constraints in regard to tempera- 

 ture, dispersed in the Ibero-Moroccan 

 Bay, contributing another period of 

 great catches for the traps (the return). 

 The maximum catches in the "return" 

 fishery, however, were concentrated at 

 the western and eastern ends of the 

 southern Spanish Atlantic coast, at or 

 near areas where the temperature and 

 salinity were considered the least fa- 

 vorable during the "arrival" period. 



O. de Buen (1924), in a prelimi- 

 nary report on this survey, presented 

 the following conclusions: 



The moment of spawning changed 

 from year to year according to changes 

 in environment. 



Increases and decreases in water 

 temperature exercised a great influence 

 on the development of the gonads. 



High temperature, high salinity and 

 agitated waters with high dissolved oxy- 

 gen content were sought by the tuna for 

 spawning. 



The temperature of the surface wa- 

 ter in the Strait of Gibraltar was 3°C 

 lower than that of the coast of Cadiz, 

 and 5°C less than that of the Mediterra- 

 nean coast of Morocco from Ceuta east- 

 ward. These conditions were unfavor- 

 able for spawning tuna, and constituted 

 an obstruction to their migration 

 through the Strait. On the other hand, 

 the currents and winds in the Strait 

 increased the dissolved oxygen in its 

 waters, a favorable factor for such mi- 

 grations. 



The tuna spawned west of G ibraltar 

 before reaching Tarifa, where the sa- 

 linity became lower. 



The next important survey of the 

 environment in relation to the occur- 

 rences of spawning bluefin tuna in 

 Spanish waters was by Lozano Cabo 

 (1957, 1958). He studied oceanographic 

 conditions at the Barbate trap east of 

 Cadiz and the biology of its catches 

 during the 1954 fishing season. He 

 made similar studies in the Moroccan 

 trap fishery (Section VE3d) in 1954, 

 providing the basis for interesting com- 

 parisons of data from the two areas. 



The surface temperature at Barbate 

 in June and July varied from 1 7°C to 

 2 1 °C ( 1 8.9° +/.0.089°), increasing pro- 

 gressively despite small local alter- 

 ations. There were greater fluctuations, 

 from 14°Cto 19°C (16.83° +/-0. 146°), 

 in the temperature at a depth of 35 m, 

 which decreased, instead of increasing 



as did the surface temperature, in the 

 final days. The water was colder at 35 

 m than at the surface. 



Lozano Cabo found a negative cor- 

 relation index of -0.29 with a probable 

 error of +/-0. 1 1 between the surface 

 temperature and the catch. There was 

 no correlation between the temperature 

 at 35 m and the catch. The "arrival" 

 tuna preferred water temperatures from 

 18°C to 21.5°C off southern Spain and 

 Morocco, with a more specific prefer- 

 ence for 18.2°C to 18.7°C at Barbate. 



The transparency of the water, 

 measured by Secchi disc, varied from 5 

 m to 17 m, with a mean of 13.04 m, at 

 Barbate. There was a correlation index 



of 0.40, with a probable error of +/- 

 0. 1 0, between the transparency and the 

 catch. The best catches occurred at 

 transparencies of more than 13 m and 

 especially over 15 m. Minimal catches 

 always occurred at transparencies of 

 less than 14 m. 



The fishermen believed firmly that 

 the catches of the traps depended inti- 

 mately on the lunar phases and the 

 tides. Lozano Cabo could not confirm 

 this, except that, when tuna were 

 present, they tended to enter the trap 

 with rising tides. Tliis might signify a 

 negative rheotaxis, at least during their 

 pre-spawning migration. The correla- 

 tion coefficient of +0.10 with a prob- 



Table 26. Surface temperatures and salinities at the trap off Cabo de Santa Maria 

 and catches of the traps in the Algarve (southern Portugal) in the period from late 

 April through August by month. 



TEMPERATURE AIVD SALINITY OF THE 

 SURFACE WATER OFF CABO DE SANTA MARIA 



Month 



Temperature (°C) 

 Min. Max. Avg. 



Salinity (VJ 

 Min. Max. Avg. 



FISH" RECEIVED BY THE CANNERIES. 



1958 



"Atuns" 



"Atuarros" 'Albacoras""Cachorretas" I'otal 



May Weight kg 73,166 8,713 



No. offish 439 119 



Average kg 166.7 73.2 



June Weight kg 307,071 130,658 



No. offish 2.330 1,903 



Average kg 131.8 68.7 



July Weight kg 65,870 4,746 



No. offish 457 64 



Average kg 144.1 74,2 



August Weight kg 21,282 1149 



No. offish 157 15 



Average kg 135.6 76.6 



•April 24 - 30 



•"Size groups of tlsh (Vilela 1960): Atuns '- 



Albacoras = 30-49 kg, Cachorretas ' 30 kg. 



90 kg, Atuarros = 50-89 kg. 



89 



