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Why do sharks and rays strand?

Sharks and rays commonly wash out on our beaches. There are a number of potential reasons for such strandings, but in most cases it is difficult to pinpoint the cause. The most common stranding factors are:


UPWELLING EVENTS (common along the entire South African coastline)

The sudden decrease of water temperature during upwelling events can impact the ability to maintain an elevated body core temperature and heart rate. Particularly young and small sharks with a high surface area to volume ratio are thermally limited. As a result animals suffer a cold shock, which can lead to the outbreak of bacterial infections, such as meningitis or meningoencephalitis and ultimately lead to death.


ROUGH SEA CONDITIONS

Strong winds and currents can occasionally cause strandings.


PREDATORS OR INFECTIONS

Sharks or rays can accidentally strand as a result of escaping from predators, e.g. seals. Animals, which are suffering from injuries or infections, may be too weak to swim against currents and ultimately wash up on the coast.


DISCARDED CATCH

Discarded sharks, skates, rays and chimaera of both commercial and (mostly) recreational fisheries commonly wash ashore. Even animals that were released alive may die as a result of stress and hook-induced injuries (=post-release mortality). The extent of distress animals can tolerate is extremely variable amongst species and depends on a number of factors (Read more).


Stranding sharks and rays often give premature birth to their young, a reaction that is prompted by stress and has been observed at angling competitions. This behaviour was recently documented in a Lesser Guitarfish (Acroteriobatus annulatus) in Lüderitz Bay, which gave birth to eight pups on the beach.

Picture taken by Lüderitz Marine Research. Read more about the event.


Recording Elasmobranch strandings is an important component of the ELMO project. The data that we have collected so far indicates that catsharks (Poroderma africanum, Poroderma pantherinum, Haploblepharus edwardsii, Haploblepharus fuscus, Haploblepharus pictus), Lesser Guitarfish (Acroteriobatus annulatus) and Smoothhound sharks (Mustelus mustelus) prevail amongst the commonly stranded Elasmobranch species.

An example of strandings that were recently recorded between Nature's Valley and Plettenberg Bay. Photos were kindly provided by the Nature's Valley Trust (Dr Mark Brown, Jennifer Parker), the ORCA Foundation (Minke Witteveen) and the author


Although the endemic shark species are often locally abundant (and are thus sometimes regarded as a nuisance), their geographical distribution is limited to a small range with some isolated populations which makes them particularly vulnerable. The Marine Living Resources Act (Act No. 18 of 1998) only offers partial protection to those species, e.g. every angler is legally allowed to harvest one individual of each Chondrichthyan species per day with a cumulative limit of 10, except for Basking sharks (Cetorhinus maximus), Great white sharks (Carcharodon carcharias), Sawfishes (Family Pristidae) and Whale sharks (Rhincodon typus) all of which are prohibited to catch.


Consequently it is important to monitor the causes of strandings over long periods in order to make informed decisions about the management and protection of species.


We would like to encourage you to report any strandings that you observe or hear of to ELMO. If you found a shark or ray, here is what you can do:


  1. Take photographs from several angles (dorsal/top view, ventral/underside view, side views, head close-ups etc.)

  2. Check for potential causes of death: Are there any visible injuries? Can you see any fishing-line and/or hooks?

  3. Measure the length (total, precaudal / discwidth) and weight.

  4. Determine the sex by checking for the presence of claspers.

  5. Record environmental conditions (wind, air and water temperature, cloud cover etc.) and the exact location


You can submit the findings via our submission form or send us an E-Mail to elmoafrica@posteo.org.


References


Beckley LE, Cliff GM, Smale MJ, Compagno LJV. 1997. Recent strandings and sightings of whale sharks in South Africa. Env. Biol. Fish. 50(3): 343-348.


Carlisle AB, Litvin SY, Hazeb EL, Block BA. 2015. Reconstructing habitat use by juvenile salmon sharks links upwelling to strandings in the California Current. Marine Ecology Progress Series 525: 217-228.


Compagno JV. 2001. Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date. Food & Agriculture Org. Issue 1(2): 204.


Smale MJ, Compagno LJV, Human BA. 2002. First megamouth shark from the western Indian Ocean and South Africa. South African Journal of Science 98: 349-350.

#strandings #Elasmobranchs #Citizenscience #researchtools

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© 2020 by ELMO (South African Elasmobranch Monitoring). 

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