Biology and population studies: Habitat preferences
Epi and mesopelagic oceanic species, albacore seldom come close to shore and prefer deep, wide open waters.

- Temperature is one of the most relevant environmental factors determining the distribution of albacore. Despite physiological adaptations common to other tuna (see Physiology chapter) which allow for some thermoregulation, albacore is a temperate tuna species and prefers cooler sea temperatures than more tropical species such as yellowfin tuna. The thermal preferendum has been established in the 10-20ºC temperature range (Graham and Dickinson 1981, Laurs and Lynn 1991) although temperatures outside that range can be tolerated for short periods. Distribution of areas suitable for albacore in the North Atlantic can be seen in Map 1.

Albacore have been found to occur mainly in the temperature range of 14-20ºC off North-America (Johnson 1961, quoted by Penney et al 1998), between 16-21ºC in the Northeast Atlantic (Santiago 2004) and between 16-20ºC off South-Africa (Talbot and Penrith 1968, quoted by Penney et al 1998).

 

These thermal preferences appear to act as barriers to movements of albacores between different regions and separate populations such as North and South Atlantic stocks and the Atlantic and Indian populations (Penney et al 1998).

  Search for the optimal thermal preferendum seems to be the goal of the periodical, vertical migrations that albacore undertake moving from warm surface waters to deep cooler waters. These vertical movements have been observed by acoustic telemetry (Laurs et al 1980, Laurs and Lynn 1991) in the Northeast Pacific, where individuals 3-5 years old spent 80% of the time at 100m, around the thermocline depth, and moved only occasionally to the mixing surface layer or to deeper waters (Laurs et al 1980). It was also noted that albacore undertook vertical migrations with larger depth range during the day than during the night.
  Map 1. Yearly average temperature (ºC) at 100 m and 250 m depth in the North Atlantic. Areas in red are not suitable for albacore (after Da Silva et al. 1994)
 

- Depth distribution has been observed down to 450 m in the Pacific Ocean by Bard et al (1999). Other authors have found that depth distribution in the Pacific ranges between 0-380 m (Bertrand et al 2002).

Swimbladder grows allometrically, reaching its full development at sizes between 80-90 cm (Gibbs and Collette 1967). Therefore it is fully functional only in preadults or adults and young albacore have difficulties in controlling their buoyancy. This involves that the younger an individual is the less ability it has to move vertically in the water column.

Since the heat exchangers work less efficiently in the young ones and can’t adjust their depth at will, they seem to be bound to stay at surface waters while adults can live in deeper waters. This anatomical characteristic has important implications for the type of fisheries that will develop: fleets operating with surface gear will target the juveniles while longliners catch the adults.

 
 

- Dissolved oxygen: the high metabolic rates of tuna involve a high oxygen consumption. Graham et al (1989) estimated the minimal oxygen concentration for this species in 3.7 ml/l (98 mmHg, 64% saturation at 15ºC).
According to this tolerance levels, large areas of the Eastern Atlantic, south of 20ºN and extending towards Brazil, are not suitable for albacore at depths greater than 100m. At 250 m, this area extends further to western Atlantic, reaching South American coasts (see Map 2).

  
 

Map 2. Yearly average dissolved oxygen (ml/l) at 100 m and 250 m depth in the North Atlantic. Areas in blue are not suitable for albacore (after Da Silva et al 1994)