Concerns over the varying decline of top oceanic predators due to climate change, habitat degradation and overfishing are growing. An estimated 25% of elasmobranchs are threatened with extinction, which begs the question: are lethal sampling methods really worth it?
The decline of elasmobranch and other top predator populations is well-documented, and there is a general consensus that protection of these species should be given priority, given the vital role they play in most aquatic ecosystems.
Data on the ranges and life-histories (reproductive status, maturity, growth, diet and mortality) of elasmobranchs plays a crucial role in informing conservation policies. In particular, this information is vital in understanding the status of fish populations and how they respond to exploitation. However, much of this data is collected through invasive sampling methods, such as analysis of vertebrae in aging and growth studies, which are ultimately lethal.
Many argue that urgently needed data cannot be obtained without such costs; for almost half of all shark species listed in the IUCN red list, there is insufficient data to accurately assess their conservation status. Others, however, demonstrate the hypocrisy associated with the lethal sampling of endangered organisms. A recent review of the issue by Heupel and Simpendorfer summarises the argument nicely:
“The reality remains that a large amount of basic information is needed for conservation and management of shark species to be successful. The gathering of basic information should not be ignored, and lethal sampling to gather this crucial information should not be condemned. Where possible or necessary, nonlethal approaches should be used, but agenda-driven and emotive approaches should not be a substitute for decision making on the basis of scientific information.”
Non-lethal methods have advanced significantly in recent decades, and are arguably the future of research-based conservation. A key few are outlined below:
Environmental DNA (eDNA):
eDNA is genetic material from animals that can be found in environmental samples (soil, water, air) without isolating the actual study organism. It is a relatively novel tool in conservation, but has proven to be able to detect several rare and threatened species of Elasmobranch, providing useful information on their distributions. In particular, eDNA has applications in areas that are difficult to reach with heavy equipment needed for traditional survey techniques. For example, in a recent 2017 study on coral reef sharks in the south-west Pacific, eDNA detected 44% more species than traditional underwater visual censuses and baited videos, despite significantly less sampling effort. eDNA has also been used to reliably detect the Critically Endangered largetooth sawfish Pristis pristis in freshwater habitats in northern Australia.
Above: Largetooth sawfish
However, the applications of eDNA for detection of certain species is limited, due to a lack of accurate primers. A trade-off between primer universality and taxonomic resolution also exists, meaning eDNA may not be able to detect species that have only recently diverged.
Stable isotope analysis (SIA):
SIA is the analysis of the abundance of certain stable isotopes (e.g. nitrogen, carbon) in organic and inorganic compounds. This technique can be used to study the ecological interactions of many marine species, including large predatory species. For example, the analysis of nitrogen isotope ratios (15N:14N) between prey and consumer can be used to examine diet, foraging ecology, trophic position and food‐web structure. Carbon isotope analysis (13C:12C) also enables the examination of animal habitat use and migration patterns.
SIA offers a particular advantage for studying free-ranging elasmobranchs whose behavior is often difficult to observe. For example, stable isotope analysis has been used to study the tropic positions of the blue shark (Prionace glauca), shortfin mako (Isurusoxyrinchus), thresher shark (Alopiasvulpinus), and basking shark (Cetorhinus maximus). This technique, although much less invasive, resolved these tropic levels with as much accuracy as previous lethal studies that used stomach contents analysis.
Above: Blue shark
Elasmobranch SIA is, however, in its infancy: significant experimental work needs to be conducted in order to validate the basic assumptions of SIA application, particularly as the physiology of elasmobranchs creates unique stable-isotope ratios. A need for extensive experimental work to validate the basic assumptions underlying the application of SIA. This is especially important for elasmobranchs, whose specific physiology creates unique stable‐isotope dynamics.
Many elasmobranchs, such as basking sharks (Cetorhinus maximus) and the ocellate spot skate (Okamejei kenojei) secrete a thick mucus from their skin that has been thought to be a potential DNA source. This hypothesis was only tested in the field recently, when scientists successfully collected and analysed mitochondrial (mtDNA) from the mucus of 30 plankton-feeding basking sharks in 2013. The results revealed key insights into the genetic connectivity between 3 spatially distinct ‘hotspots’ of basking shark in Irish waters, information that is vital for the protection of this vulnerable species. However, mucus analysis has yet to be applied to other elasmobranch species but could represent a potentially useful, non-invasive tool in conservation.
Above: Basking shark
Assessing the reproductive characteristics of elasmobranchs often requires lethal dissection of females. Ultrasonography (a technique using echoes of ultrasound pulses to delineate objects or areas of different density in the body), has been suggested as a non-lethal alternative. For example, this novel technique has been successful in examining the maturity status of two oviparous elasmobranchs, the thornback ray (Raja clavata) and small-spotted catshark (Scyliorhinus canicula). Scientists were able to take accurate measurements of the ovaries, shell glands, ovarian follicles and egg capsules with as much accuracy as dissection measurements.
Above: Thornback ray
Elasmobranch vertebrae contain concentric rings of light and dark cartilage. Traditionally, aging elasmobranchs involves analysing the number of rings, a lethal method that has also been proven to have some fundamental inaccuracies. For example, the number of rings laid down per year may not be consistent within individuals, species or across species, and can vary depending on environmental conditions. Although not possible for all species due to lack of morphological differences, morphological measurements have been used as a non-lethal alternative to aging elasmobranchs. For example, measurements of caudal thorns have accurately aged male and female thorny skate (Amblyraja radiata), the most widely distributed and abundant of all the skate species worldwide. Recent work into the growth rates of tesserae, complex hexagonal structures that form part of elasmobranchs’ cartilage, could also provide a potential non-lethal method of aging.
Above: Thorny skate
There is a strong debate in the scientific community as to the reliability of these non-invasive sampling techniques relative to traditional lethal methods. However, regardless of opinion, there is no denying that such scientific data plays a vital role in protecting elasmobranch species, and this data is sorely needed.
L. Lieber et al., 2013. Mucus: aiding elasmobranch conservation through non-invasive genetic sampling. Endangered species research. 21: 215-222.
N. E. Hussey et al., 2012. Stable isotopes and elasmobranchs: tissue types, methods, applications and assumptions. Journal of Fish Biology. DOI: https://doi.org/10.1111/j.1095-8649.2012.03251.x
J. A. Estrada et al., 2003. Predicting trophic position in sharks of the north-west Atlantic Ocean using stable isotope analysis. Journal of the Marine Biological Association of the United Kingdom. 83: 1347-1350.
M. J. Gallagher et al., 2006. The potential use of caudal thorns as a non-invasive ageing structure in the thorny skate (Amblyraja radiata Donovan, 1808). Environmental Biology of Fishes. 77: 265-272.
C. A. Simpfendorfer et al., 2016. Environmental DNA detects Critically Endangered largetoot sawfish in the wild. Endangered Species Research. DOI: https://doi.org/10.3354/esr00731.
G. Boussarrie et al., 2018. Environmental DNA illuminates the dark diversity of sharks. Science Advances. DOI: 10.1126/sciadv.aap9661.
Heupel MR, Simpfendorfer CA (2010) Science or slaughter: need
for lethal sampling of sharks. Conservation Biology. 24: 1212–1218.