Mortality rates from catching to stocking - grouper

“Seed” quality depends on the type of fishing gears used, and there are significant differences in seed mortality rates. Mortality rates associated with fish traps are usually low. For example, the use of “Bubu” (fish traps used in Malaysia) cause a 5% mortality rate, while artificial aggregators such as Temarang (also used in Malaysia) cause 3% mortality.

Other catching methods, like scissor nets and fyke nets, can generate a high mortality. “Pompang” (fyke net) and “Wunron” (push/scissor net), which are used in Thailand, are reported to cause 20-30% and
80% mortality rates respectively (Sadovy 2000). It is likely that subsequent mortalities during transport and stocking will also be high, as many of the “seed” fish will also have been damaged, and are therefore susceptible to disease.
The problems that arise during “seed” transport to the net cages or to the middleman/ farmer/exporter, depend on “seed” size, quality, fitness and the locality. For transport over short distances in Thailand, for example, “seeds” are placed in styrofoam boxes or buckets, with or without aeration (often provided by middlemen), or with holes in the bottom for water exchange; transport time is typically from about 10 minutes up to two hours. Post-harvest mortality is low. For longer transit periods, fish are packed in 23-25°C seawater with aeration.

Transport densities are about fifty 7.5 cm fish per bag, or one hundred 1 cm fish/l, or two to three hundred 3-7.5 cm fish per bucket. For a 7-hour journey, ice can be used to keep the water cool. Some exporters use an anaesthetic, either quinaldine or MS222, but consider the latter to be rather expensive. The use of anaesthetic is considered important to reduce the likelihood of spines piercing the plastic transport bag. For export, fish are packed into styrofoam boxes of various sizes; each shipment has about 20 000 fish in 30 boxes (Sadovy 2000).
In the Philippines, approximately 10% of the “seed” caught is used domestically, while the remainder is exported. There can be significant mortalities during local transportation. The movement of seed from the catchers to the middlemen or the farmers is carried out by keeping fish in plastic containers or basins with holes for water circulation. Mortality rates are quite low under such circumstances. If destined for trade, the fish may be maintained for short periods by the middleman, prior to packing and shipping, either domestically or internationally. In some cases, they may be transferred temporarily (for a few days) to an “aquarium box” to await buyers who come to collect fish and who are responsible for the export of the fry. Mortality rates can reach 10-20% at this stage, i.e. prior to selling to buyers for export or domestic trade (Sadovy 2000). Mortality rates are low if the transit time is less than an hour. However, for longer periods, if there is no aeration or frequent water changes, mortality increases and oxygen may have to be added. Buyers pack fry in double plastic bags with pre-cooled water using ice (18-22°C) and a salinity of 15-18 ppt. 2.5 cm “seeds” are packed 400-500 per box and 7.5-10 cm “seeds” are packed 20-40 per box (Sadovy 2000).
According to Tucker (1999) “seed” groupers should be shipped in 4 mm (or thicker) food-grade polyethylene bags filled with oxygen and water in equal parts. The bags should be kept at the spawning temperature or a few degrees lower, in an insulated, rigid container. Juvenile groupers can be shipped in bags at 120g/l at 23°C with oxygen for up to 12 hours.
The mortality rates that follow capture and transport are not exactly known; estimates for over the first 2 months after catching are quite variable (30-70%), depending on the quality of fry, the level of transport stress, and the presence of disease and cannibalism (Pudadera, Hamid and Yusof 2002). According to (www.spc.org.nc/coastfish/News/LRF/5/15GrouperHK.htm), the survival rate for imported fry is low, at 10-20%.