Understanding the lives of blue Stingrays in Knysna Estuary
Hannibal Smith (The A-Team) is not the only person who ‘Loves it when a plan comes together”, most scientists I know do too. It is gratifying when a long awaited project not only finally begins but then begins well.
This was the case on the 26th October 2022 when three scientists and three volunteers met in Knysna at 5 am to try catch, tag and release some blue stingrays. In contrast to those previously caught and tagged along the coast-line by rock and shore anglers, we were going to try and catch our sample size of 15 individuals without disrupting a larger aggregation of rays, and, we were going to try and catch them with hand nets. We had a plan and the plan had been approved (South African Institute for Aquatic Biodiversity ethics committee) but would it work?
Well it did, spectacularly well. It was with big smiles and a sense of relief as we watched the last ray, a male, swim down and out of sight. Not only had we managed to successfully tag all fifteen but whilst we were packing up and the tide was coming in, the rays could be seen aggregating again in the shallow water. Importantly for us the main aggregation had not been disturbed.
Every year around October and November blue rays can be seen within the shallows of Knysna Estuary. It is the only known easily observable aggregation of blue rays in South Africa and yet, apart from the annual timing of the event, we know very little about it.
A group of females aggregating in the Knysna Estuary
Blue stingrays are only found in southern Africa from central Angola to St Lucia and possibly as far north as Maputo. They live inshore along the coast and inner continental shelf, being particularly abundant within shallow protected coastal bays and sandy beaches during spring and summer with a seemingly offshore winter migration. They are caught in the surf zone by recreational shore fishers and also as a bycatch within the inshore trawl fishery. Although currently listed as Near Threatened on the IUCN red data list, their k-selected life-history traits including slow growth, late age at maturity and low fecundity, make them susceptible to over-exploitation.
Shark and ray populations around the world are in decline, and effective conservation measures are urgently needed. The adoption of a precautionary management strategy approach has long been recommended for the blue stingray and knowledge of movement patterns and habitat use at multiple scales is required to help develop this.
With all this in mind our project aims to describe the residency and movement patterns of blue stingrays in the Knysna Estuary, and to formally document characteristics of the aggregation. For example we would like to know how long they stay within the estuary, do they use multiple areas within the estuary and what are these habitat characteristics? Do they show site fidelity, or, in other words are we seeing the same individuals returning each year and what would this mean for management of the species? What is the purpose of the aggregation (we think it’s related to breeding), what age and size classes make up the aggregation and what are the sex ratios?
Mr Matthew Parkinson from SAIAB surgically implanting the acoustic tag.
To answer some of these questions we are formally documenting the aggregations through simple regular observations on behaviour and abundance of both females and males at the aggregation site. By tagging them with internal acoustic tags (each tag emits a unique identifying signal) and by placing an array of listening stations in the estuary and outside along the coastline we can start to piece together their movement and migration patterns. This information can be used to facilitate the development of better management policies and practices with identified key areas or habitats and potential corridors informing marine spatial planning processes.
It is an exciting project and one that would not have been achievable without the formation of key collaborations and partnerships between SANParks, SAIAB and Rhodes University. Tagging the rays and placing the receivers in the estuary was only the first step. The array will need to be serviced and maintained and as data comes in it will need to be managed and analysed.
The battery life for the current tags is three years and as the last ray swam away the thought crossed my mind that now we wait for three years to hopefully get our answers. But that’s not quite true as each time we download the receivers we would be adding pieces to the puzzle. I for one am looking forward to seeing how the puzzle comes together.
