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Biology

Baby come back: capture-induced premature birthing in elasmobranchs

Article

Adams, K., Fetterplace, L., Davis, A., Taylor, M. and Knott, N. (2018). Sharks, rays and abortion: The prevalence of capture-induced parturition in elasmobranchs. Biological Conservation, 217: https://doi.org/10.1016/j.biocon.2017.10.010

Featured Image: Great White Shark (Carcharodon carcharias). Credit: National Geographic.

Background

Being at the top of the food chain, sharks are extremely admirable creatures. There is an incredible diversity of sharks, they enjoy inhabiting a variety of areas in the ocean, help balance the ocean ecosystem, and have numerous adaptations which pretty much makes them the superheroes of the ocean (sorry, Aquaman). But why are they hunted so? Why not leave these majestic creatures to enjoy their lives in peace? An estimated 100 million sharks are killed every year by human beings, with close to 11,417 sharks killed every hour. Sharks are one of the most dedicated parents and have one of the longest gestation periods. Since sharks are k-selected organisms meaning they spend more time developing and have fewer offspring, they tend to be more vulnerable to overfishing and face more ruin by lost pregnancies. The direct effects of hunting sharks have been a topic of research for decades. Some effects include but are not limited to: decrease in abundance, imbalance in the ecosystem, and loss of species diversity. The indirect effects of hunting sharks, however, have gone largely unreported and often contain incomplete data.

The study focuses on indirect mortality of sharks and other elasmobranchs (such as skates, rays, etc.), as well as generational mortality, due to capture-induced parturition. Capture-induced parturition can be defined as abortion or premature birthing of offspring stimulated by fishing gears, causing them to either die off or survive but with less-developed bodies. The study further sets out to understand the causes and the effects related to capture-induced parturition.

Methods & Approach

The researchers decided to assume that all capture-induced parturition resulted in the death of the offspring, due to lack of mortality estimates of embryos. They further decided to stop referring to the parturition as “spontaneous abortions” because they are likely caused due to physical trauma and physiological stress by capture.

An extensive literature search was conducted in order to compile the data regarding species captured, method of capture, gestation stage of embryos, mode of reproduction, as well as threat level of species. The rate of capture-induced parturition was calculated with first, the number of females known to have undergone capture-induced parturition compared to the total number of pregnant females, and second, the number of embryos from capture-induced parturition compared to the total number of embryos in the study which included embryos in the uterus. Eggs were not included in the calculation. Reproductive modes such as placental viviparous, oviparous, and aplacental viviparous were studied from literature to see whether some reproductive modes were more prone to capture-induced parturition than others. Further data regarding capture-induced parturition was compiled using posts and videos from social media. Statistical analyses were conducted to identify species that are more vulnerable to capture-induced parturition and whether reproductive modes had a lower or higher frequency of capture-induced parturition.

Results & Conclusion

Graphical abstract from the paper: Female lesser guitarfish (Rhinobatos annulatus) and induced embryos with distinct yellow yolk-sacks. Credit: K. Grobler.

Capture-induced parturition was almost exclusively found in live-bearing elasmobranchs but not in egg-laying elasmobranchs. 12% oflive-bearing elasmobranchs across 88 species were observed showing capture-induced parturition. The average frequency was 24% meaning 2 in 10 pregnant females showed capture-induced parturition. Induced parturition rate was also found to be species-specific though there was not enough data to support this. Threat levels did not seem to overlap with the rate of capture-induced parturition, however, more data would be needed to conclude this. Capture-induced parturition was found to not be limited to overfishing. Failed predation, strandings, asphyxiation, etc. are also causes of capture-induced parturition. Fishing practices and improper handling of pregnant elasmobranchs were identified as one of the main causes of capture-induced parturition as it is not well-understood by recreational fishermen and marine researchers alike. Rays were found to be more susceptible to induced parturition than sharks.

It is imperative that there be more awareness regarding this as for some species of elasmobranchs such as angel sharks, it is a matter of survival. Losing their offspring due to stress or trauma from being captured can be very detrimental to their overall population. It is advisable while handling pregnant elasmobranchs that they do not be raised above the water. Even though there are “miracle stories” on social media of people helping elasmobranchs give birth, it is likely that this birthing is premature birthing caused due to the stress of capture and it is best to leave these creatures alone and in the water. More research on this topic as well as improving management strategies, including better protection of breeding and nursing grounds of elasmobranchs, would help understand these stressors better and prevent unwanted losses of offspring.

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