Scientists confirm delayed shrimp reproductive process

ARGENTINA
Wednesday, February 19, 2020, 08:50 (GMT + 9)

The report of the November shrimp assessment cruise carried out by the INIDEP indicates that the biomass declined compared to historical values. The distribution by size is similar to that of 2018 but with a greater presence of juveniles, while the presence of mature and impregnated females decreased considerably.

The report of the shrimp study carried out last November onboard the Bogavante II provides a detailed explanation of the situation of the resource observed during the cruise and compared with historical values. Less abundance, greater presence of juvenile specimens and a significant reduction in the presence of mature and impregnated females confirm the hypothesis of maturational delay.

The finding of a very abundant concentration in Bahía Camarones does not reflect what was observed in the other areas and the researchers warn that it is a timely concentration. 66% of the commercial size found was L4 to L6 and smaller, which would be available for capture only by the end of the 2020 season and beginning of 2021. For these circumstances they recommend caution at the beginning of next season so as not to generate an overfishing of  developing speciments.

The results obtained during the cruise financed by the entrepreneurs and carried out on board the Bogavante, indicate that the 2020 fishing season could have similar characteristics to those observed in 2019.

Estimates of biomass and the number of individuals in the studies carried out in November 2017, 2018 and 2019 resulted in the available biomass in 2019 being just lower than in 2018 but significantly lower than in 2017, from 10 862 tons to 7 047 tons, with  638 876 000 to 473 126 000 individuals.

In 94% of the 109 sets made between latitudes 43º30 ’S and 47º S and from the 64º meridian to the coast, the presence of shrimp in different concentrations and sizes was recorded. Although the highest concentration was 85 tons per square nautical mile in the area of Bahía Camarones, other values  reported were around 40 tons.

In 2018 the highest yields were obtained in the southern Gulf of San Jorge, with a maximum of 3638 kilos per hour. In this cruise the highest concentration occurred in Bahía Camarones, with a maximum of 7233 kilo per hour. But researchers clarified that "while the shrimp found in Bahía Camarones will be available for harvest by 2020 fishing season, it was limited to a timely concentration."

Sizes

The distribution of lengths was very similar to that found in the 2018 “but with a greater percentage share of the fraction between 22 and 30 millimeters of shell and less participation of lengths greater than 38 millimeters”, which means a greater proportion of juvenile specimens.

Therefore, the researchers point out that “based on these results, it is possible that the beginning of the 2020 season is similar to the beginning of the 2019 season, where there was a significant fraction of individuals with small commercial lengths”, a fact that delayed until June 6 the start of the season.

Photo: Revista Puerto

If this scenario is confirmed in the March research cruise, "we should proceed with caution before the start of the fishing season, in order to avoid overfishing," said researchers Paula Moriondo and Juan de la Garza.

In general, more than 60% of the analyzed individuals were reproductive adults. In the sub-zones of national jurisdiction, north of the Gulf of San Jorge and Rawson, the percentages of adults reached 70% while in the south of the Gulf the adult proportion was less than 40%. This situation could change with the passing of the months because the reproductive peak in the south occurs between January and February, allowing larger growth and concentration of reproductive adults.

“In terms of commercial categories, 66% of the total shrimp belonged to categories L4 to L6 and smaller,” being the fraction of the population that could potentially “be caught at the end of the 2020 fishing season and early in 2021." That percentage rises to 70% or more for individuals captured within the San Jorge Gulf. The largest sizes (72% of L1 to L3) were located in Rawson, while for the national waters the percentage of shrimp size L1 to L3 was 44%, values very similar to those of the 2018 season.

Delay in reproductive process confirmed

Regarding the reproductive process, once again a delay and “even to a greater degree” than observed in 2018 are shown. It is indicated that the percentages of mature and impregnated females are considerably lower than the historical values found in the sectors analyzed .

►Paula Moriondo and Juan de la Garza, INIDEP scientists (Photo: Revista Puerto)

In the South of the Gulf the proportion of mature females was 0.16% while the historical value is 2.70%; 7.91% was registered in the North and the historical value is 46.40%. In Rawson, the value has remained at 42%  in the last three years, while it used to house  66.90%. In the case of impregnated females, in the south was 0.16% and in the north, 7.40%, but in both cases, 0% has been recorded for three years. In the case of Rawson, which was the only sector where impregnated females were found, in 2019 there was 1.3% and the historical value is 19.90%.

“This displacement has been observed and documented since the results of the shrimp assessment cruise carried out in November 2017. It is necessary to emphasize again that the delay in reproduction could have consequences n the growth rates and survival of the first stages of life,” says the report verbatim.

“The decrease in the presence of females in reproduction is an indicator of the delay of the shrimp's reproductive season”, the report indicates, adding that “in general, the decoupling with the natural cycles of primary and secondary production could have negative consequences for the population in terms of the percentage of individuals who manage to survive the larval phases, which are the most vulnerable in the life cycle.”

The researchers conclude their report by noting that in the particular case of shrimp, although the causes are not yet known with certainty, in recent years it has been observed a downward trend in the number of individuals and a decrease in the rates of growth, causing a greater proportion of small lengths at the beginning of the next fishing season, so they consider that it is necessary to act with caution.

Author:  Karina Fernández / Revista Puerto

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