Environmental preferences of longlining for yellowfin tuna (Thunnus albacares) in the tropical high seas of the Indian Ocean

Abstract

A survey of yellowfin tuna, Thunnus albacares, fishing ground was carried out on board of the Chinese longliners from September 15 to December 12, 2005 in the tropical high seas of the Indian Ocean. The depth at which each yellowfin tuna was hooked was estimated using a stepwise regression analysis of theoretical hook depth and observed average hook depth measured using a temperature depth recorder. Water temperature, salinity, chlorophyll a, dissolved oxygen and thermocline, which are important variables influencing yellowfin tuna habitats, were measured in the survey. Catch rates of yellowfin tuna were then analyzed with respect to depth, temperature, salinity, chlorophyll a, dissolved oxygen and thermocline. We suggest that the optimum ranges of swimming depth, water temperature, chlorophyll a and dissolved oxygen concentration for yellowfin tuna are 100.0-179.9 m, 15.0-17.9°C, 0.090-0.099 lg L )1 , 2.50-2.99 mg L )1 , respectively; that salinity has less influence on the vertical distribution of adult yellowfin tuna; and that yellowfin tuna are mainly distributed within the thermocline in the high seas of the Indian Ocean. Our results match the yellowfin tuna's vulnerability to deep longline fishing gear well.

Development of integrated habitat indices for bigeye tuna, Thunnus obesus, in waters near Palau

“…The catch rate of T. obesus for depth stratum j, CPUE j , was calculated according to Song et al (2008Song et al ( , 2009), Song and Zhou (2010) and Song and Hu (2011), as follows:…”

“…(2) We built a model to predict the hook depth from the theoretical depth, wind conditions (velocity and direction) and drifting speed of the longline by a stepwise regression method (Tang and Feng 2002 (Song et al 2008(Song et al , 2009Song and Zhou 2010;Song and Hu 2011). The CPUE ij was computed as:…”

“…Many studies have focussed on the influences of environmental variables, such as temperature, salinity, Chlorophyll a, dissolved oxygen and the abundance of micronekton on the distribution and movement of T. obesus (Dagorn et al 2000;Bertrand et al 2002;Evans et al 2008). Simultaneous analysis of several environmental variables can improve the understanding of their effects on the distribution and movement of tunas (Song et al 2008(Song et al , 2009. Only a few studies focussed on the simultaneous influences of multiple environmental factors on the distribution and movement of tunas; however, these studies were confined to the Indian Ocean (Feng et al 2007;Song and Zhou 2010;Wu and Song 2010).…”

Influence of Central Pacific Oceanographic Conditions on the Potential Vertical Habitat of Four Tropical Tuna Species1

“…Yellowfi n tuna are sen si tive to low tem per a tures that ac com pany up welling events, such as the 2008 La Ni ñ a, and their es ti mated CVH was lim ited pri mar ily by tem per a ture, suggesting that ther mo regu la tion may not be as ef fi cient in yellowfi n as it is in big eye and al ba core (Brill 1994, Graham andDickson 2004). However, yellowfi n tuna are tol er ant to low DO lev els (Brill 1994, Song et al 2008 and had a DO CVH deeper than big eye tuna, with the ca veat that this dif fer ence may also be at trib uted to how the CVH was es ti mated for the four tuna spe cies in this study. Bigeye tuna are the most tol er ant of all the ex am ined spe cies to low tem per a tures, resulting in a more ex ten sive predicted PVH lim ited by tem per a ture or DO depending on the ocean ographic con di tions.…”

“…The CVH was de fi ned for big eye as the tem per a ture and DO con di tions most fre quently ob served in tagged and long line big eye cap tures (Dagorn et al 2000, Bach et al 2003, Evans et al 2008 (Table 1). Lower lim its of the CVH for yellowfi n tuna were defi ned as tem per a ture and DO con cen tra tions cor re spond ing to highest ob served long line catch rates, which may in di cate the lower limit of ocean o graphic con di tions within the preferred con di tions (Song et al 2008). The temper a ture CVH was also cor rob o rated by tag ging study that recorded a sim i lar tem pera ture range (Block et al 1997) (Table 1).…”