How can the fishing methodology of the southern king crab (Lithodes santolla) and the stone crab (Paralomis granulosa) be improved to increase sustainability from an economic and biological point of view? How does fishing and release of non-marketable specimens affect the reproductive cycle of the species?
Scientific research at the Centro Austral de Investigaciones Científicas (Austral Centre of Scientific Research, CADIC-CONICET) indicates that there is a considerable loss of eggs when the females are thrown back into the sea from the boat decks. By adding a ramp this loss could be greatly reduced thus promoting the conservation of the population (of these species). These results were published in the magazine Fisheries Research.
“It is an important mortality factor that has never been taken into account. Besides, it could be the cause of the decrease in the number of available individuals in these species, whose fishing has been banned for 19 years and still show no signs of recovery neither in the number of of marketable animals nor in the number of females with eggs”, Gustavo Lovrich, CONICET principal researcher, comments.
Traditionally, southern king crabs are captured using bait tramps. Once on the deck, the seven or eight-year-old males with 11 centimetres’ shell are selected because they are the only ones allowed in trade. The males and the females under that size are released from the deck and consequently the eggs are lost by the impact.
“The research focused on the quantification of the eggs’ damage. If we take into account that the females incubate up to ten months, probably they can be caught and release several times during that period”, María Gowland-Sainz, CONICET doctoral fellow at the CADIC.
The scientist explains that the laboratory tests consisted on casting a group of specimens in free fall to standardize what happens in fishing vessels; and then casting another group is shed through a ramp to compare the number of eggs that remain in the females of each group.
“In the case of the Lithodes santolla, we noticed that there was a great difference: in free fall, there was a 23% loss on average, while with the ramp, it was similar to the percentage resulting from the manipulation in the lab, which was less than 0,32%”, Gowland-Sainz concludes.
The females of these species carry the eggs under the abdomen, weakly adhered to some appendixes called pleopods. “They are so weak that we could detect a large amount of eggs at the bottom of the boxes we used to transport the individuals to the lab”, the researcher explains.
In order to test the laboratory observations in the fisheries, the CADIC-CONICET team recounted the eggs of the females in the largest fishing zone, where they proved that there were less than in closed areas.
Repopulation, sperm limitation and stack
For the researchers, there are other issues that should be taken into account to assess the fishing methodology and the current law, for instance “sperm limitation”, that is to say the proportion between male and female adults. Since the males are selected for trade, the scientists will try to determine if the population of females can be fertilized by the remaining males or how many females can be fertilized by one male.
Other members of the team, Paula Sotelano, CONICET posdoctoral fellow, and Federico Tapella, CONICET associate researcher, study the possibility to grow southern king crabs first in the laboratory and later on transfer them to the sea to supply the low population. For this reason, it is necessary “to avoid its mortality during larval stages, grow small southern king crabs and release them. This is a laboratory research line that has been conducted for many years and has proven to be complicated and to demand a great effort”, Gowland-Sainz states.
The last issue to analyze is if during the fishing time, when they are stacked, both southern king crab and the stone crab lose reproductive capacities and if the exposure to the air for a long time affects them in the same way.
- By María Bocconi