Diel and ontogenetic body density change in Pacific bluefin tuna, Thunnus orientalis (Temminck and Schlegel), larvae

Abstract

Diel and ontogenetic changes in larval body density related to swim bladder volume were investigated in Paci¢c blue¢n tuna, Thunnus orientalis, to determine the causality of larval mortality^adhesion to the water surface and contact with the tank bottom during seedling production. The density of larvae with de£ated swim bladders increased with total length and days post hatch. Diel density change was observed after day 2 post hatch; owing to daytime de-£ation and night-time in£ation of the swim bladder, the density was relatively higher during the daytime. Increased swim bladder volumes clearly reduced larval density during the night-time after day 9 post hatch. However, the density of larvae with in£ated swim bladders was greater than rearing water density (Dr40.0099). The small density di¡erence between larvae and rearing water (Dr 5 0.0022 À 0.0100) until day 4 post hatch may have caused larval mortality by adhesion to the water surface because larvae can be easily transported to the water surface by aeration-driven upwelling in rearing tanks. Density increased noticeably from day 5 to day 9 post hatch. The increased density di¡erence (Dr 5 0.0065 À0.0209) in larvae and rearing water possibly induced mortality by contact with the tank bottom because larvae sink particularly during the night-time on ceasing swimming.

The effect of a 24-hour photoperiod on the survival, growth and swim bladder inflation of pre-flexion yellowfin tuna (Thunnus albacares) larvae

“…Regardless of the mode, mortality due to sinking becomes less problematic from Day 8-9 (Takashi et al, 2006). We believe that yellowfin tuna are similar to Pacific bluefin in this regard, as mortalities are commonly seen on the tank bottom in the early morning when reared under the standard photoperiod.…”

“…Takashi et al (2006) pointed out that a major cause of mortality of early stage Pacific bluefin tuna larvae is their tendency to sink to the tank bottom at night and showed that the density of larvae increases with increasing age. Between Days 4 and 8 these larvae are denser than seawater, even with an inflated swim bladder, and at night when swimming ceases they sink to the bottom of the tank and die.…”

“…Mortality of first feeding marine fish larvae has been attributed to factors including a poor transition from lecithotrophy to exotrophy (Fyhn, 1989); an inappropriate size and/or nutritional composition of feeds (Koven et al, 1992) or inappropriate environmental conditions (Takashi et al, 2006). Key environmental parameters to be considered for effective larval rearing include temperature and salinity (Fielder et al, 2005), the microbial environment (Harboe et al, 1994) and factors including turbulence, light intensity, light quality and photoperiod (Boeuf and Le Bail, 1999;Utne-Palm and Stiansen, 2002).…”

“…Although Takashi et al (2006) pointed out that photoperiod manipulation may be effective in preventing mortality of tuna larvae due to sinking, this appears to be the first study to have investigated the effects of a continuous photoperiod on the larvae of any tuna species. Previous trials conducted by the IATTC at the Achotines Laboratory have indicated variable patterns of larval growth and survival in response to extended photoperiods (D. Margulies, pers.…”

A review of the broodstock management and larviculture of the Pacific northern bluefin tuna in Japan

“…Recently, it was confirmed that the initial mortality of PBT is mainly due to sinking syndrome rather than to STRD (Tanaka et al, 2009). The sinking of PBT larvae to the tank bottom occurs at nighttime (Masuma, 2008;Tanaka et al, 2009), presumably because the larval body density of PBT is higher than that of the sea water density, and larval swimming activity is low during the night (Sakamoto et al, 2005;Takashi et al 2006). Sinking syndrome has been reported in several fish species such as amberjack Seriola dumerili (Shiozawa et al, 2003;Teruya et al 2009) and barfin flounder Verasper moseri (Kayaba et al, 2003).…”