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[Tuna
Ecology & Biology] -> [Tuna
Biology & Behaviour] -> [SEPoDyM]
Tuna habitat
Tuna are the most highly specialized fishes in regard to sustained, high levels of locomotory activity. They swim continuously to counterbalance their negative buoyancy, and are able to travel hundreds of miles. This high expenditure of energy is also used to quickly move through their environment searching for food, from which they require correspondingly large energy returns. This strategy has resulted in morphological and physiological adaptations, particularly for thermoregulation and efficient oxygen extraction. Consequently, these two major features of the oceanic environment, ambient temperature and dissolved oxygen concentration, are believed to strongly influence tuna behavior (Brill 1994). The relationships between tuna physiology and ambient temperature and oxygen concentration have been studied in laboratory experiments, by in situ tracking experiments and by comparisons between catch statistics and oceanographic conditions. Sea surface temperature and oxygen concentration have been used to define large-scale limits of potential tuna habitat (Barkley, Neill, & Gooding 1978). The distribution of tuna forage within this potential habitat probably has the major influence on tuna distribution. Surface tuna like skipjack and yellowfin feed visually mainly during the daylight hours. For this reason, water clarity is also likely to influence their distribution. Therefore, the habitat of surface tuna could be defined in general terms as water masses with a combination of suitable temperature and sufficient oxygen concentration (varying according to species and life stage), high forage biomass and also clear
water.
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Adult habitat index
The adult habitat index is used in the model to constrain the tuna movement. This index simply combines the spatial distribution of forage with a temperature function defined for each tuna species.
Due to the diffusion equation, very low density of tuna can be found in areas of really poor habitat. To prevent such unrealistic situation, a function is used to exponentially increase the natural mortality rate of the species at very low values of habitat index.
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Spawning habitat index
The spawning habitat index is used to constrain the recruitement to environmental conditions. The basic assumption to define this index is that the spawning area is limited by the presence of mature tuna and of sea surface temperature (SST) above a limit value. This latter condition is supported by the high correlation found between SST and occurrence of reproductively-active tropical tuna (Schaefer 1998). However, other environmental effects are investigated.
In each point of the grid, the number of recruit is given by a maximum value proportional to the spawning habitat index. The maximum value is used to scale the total biomass to independent estimates (see results).
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References
Barkley, R.A., Neill, W.H., & Gooding, R.M. (1978). Skipjack tuna (Katsuwonus pelamis) habitat based on temperature and oxygen requirements. Fishery Bulletin, 76, 653-662
Brill, R.W. (1994). A review of temperature and oxygen tolerance studies of tunas pertinent to fisheries oceanography, movement models and stock assessments. Fisheries Oceanography, 3, 204-216
Schaefer, K.M. (1998). Reproductive biology of yellowfin tuna (Thunnus albacares) in the eastern Pacific Ocean.
Bayliff, W.H. La Jolla, California, USA, Inter-American Tropical Tuna Commission. Bulletin. 21 205-272
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