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Nurseries for the silky shark, where are they?

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Published: 01/04/2020

What is a shark nursery area?
We can call an area a nursery when juveniles are in a strict location, there is a lot of food and they are philopatric that means that they always returns to give birth to the same place. The main problem with this areas is the fishing pressure, because if juveniles are always in the same place, it is easier to catch them, and the population may decrease.
There is a bay in Jalisco, Mexico where many silky shark juveniles are captured, so we hypothesized that it is a nursery area.
How can we identify a nursery?
In the last years, a lot of technology has been developed to define nurseries; in the BIOMOL lab, we are using DNA markers as a tool to detect relationship patterns between adult silky and juveniles silky shark in a possible nursery area.
What makes silky shark important to be studied?
Well, the silky shark is a species that needs around 10 years to reach sexual maturity, it has viviparous reproduction with a 9-12 months gestation period and they have an average of 9 offspring per reproductive cycle. It is distributed in all tropical waters; it is a highly migratory shark which makes it vulnerable to overfishing.
Silky shark is the second in shark catches worldwide, that’s why in 2017 the UINC declared it as like vulnerable.
The objective of our project is to identify philopatry of the silky shark in Jalisco and recognize it like as a nursery area, through a relationship analysis that shows if females are continuously giving birth in the area.
What did the BIOMOL find with this study?
We compared DNA from organisms captured in two zones from Mexico: 1) Punta Lobos where there are a lot of adults and 2) Jalisco, where there are a lot of juveniles, Ecuador and El Salvador. Then we tried to find relatedness between Mexican adult females and Mexican juveniles.
We identified that there are 3 populations of silky shark, divided by their capture country: Ecuador, El Salvador and Mexico, where we found mothers to 32 juveniles, some of them are half siblings and even we find some full siblings. A female had a parental relationship with 6 juveniles, who were born in different years, so this means that since she reached sexual maturity, each reproductive cycle she returns to give birth. This behavior was identified in other 7 females.
With these results we can suggest that the Mexican silky shark migration includes Jalisco as a like a birth area and philopatry from some females. So we can infer that Jalisco is a nursery area for silky shark.
"Silky Shark".
TAGS:

population genetics

shark nursery areas

endangered marine species

philopatry

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