Home
Forgot your password?
Mission Gilchrist: Revisiting the earliest trawl surveys in South Africa
|
Page through historical books of the sea, or speak to weathered seamen about their fishing experiences decades ago, and you will likely conclude that life in our oceans is not what it used to be.
From anecdotal accounts and personal experiences, scientists and fishers alike know there have been some drastic and often concerning changes. Yet due to a lack of comparable data from the past, piecing together a picture that is scientifically robust, and adequately describes what a part of our fisheries or marine ecosystems used to look like, is an arduous task. And if you do not understand the historical state or baseline, then it becomes extremely difficult to measure subsequent changes.
|
Detecting such ecosystem or population changes and linking them to their causes, is the primary focus of many scientists. As a result, researchers are fascinated by historical records that can provide meaningful snapshots of past baselines and, as such, delve into dusty archives and historical books in search of forgotten records from the past.
Treasure trove of historical fisheries data
In this manner, researchers were excited to rediscover a treasure trove of historical data from the late 1800s and early 1900s, which were gathered by government-appointed scientists tasked with exploring the fisheries potential around Southern Africa. The digitisation of these data, reported on in a previous SAEON newsletter, has provided the opportunity and inspiration for an ambitious research project. The study is revisiting the earliest scientific trawl surveys conducted in South Africa between 1897 and 1904, and re-surveying them to assess how the fish communities have changed during the subsequent century. To minimise the problematic biases introduced by changes in fishing technology, the modern surveys employ a specially-constructed historical trawl net and imitate the methods used on the SS Pieter Faure, the vessel that conducted the original surveys.
After securing the necessary logistical and financial support, and many months of preparation, the exciting fieldwork commenced in February 2015 on board the MFV Leeukop, a commercial side-trawler built in 1963 and owned by Viking Inshore Fishing in Mossel Bay. Three scientists, the newly-built trawl gear and all their scientific apparatus were installed on the small, crowded trawler. The remaining seven berths were taken by a skeleton crew and captain, who ensured the running of the vessel and effective operation of the fishing gear.
|
To effectively work the outdated and unorthodox trawl gear, a seasoned expert fishing master was required. We were extremely fortunate that Viking Fishing kindly made available their fishing gear advisor, Noel Less, whose depth of knowledge, experience and fishing skill are legendary in Southern Africa and further afield.
Revisiting the Pieter Faure trawl surveys
Sailing from the port of Mossel Bay, three areas that were comprehensively surveyed by the Pieter Faure were revisited and successfully trawled. All three sites are within current commercial trawl grounds on the inshore Agulhas bank and include an area near Mossel Bay, one offshore of Cape Infanta and lastly, an area east of Bird Island near Port Elizabeth.
The demanding survey protocol was taxing on the three scientists and crew. The day's sampling started before dawn and ended after sunset with the deployment of a CTD (conductivity-temperature-depth) device. This instrument was lowered on a rope to just above the sea-floor and contains sensors that continuously measure temperature, conductivity (salinity), pressure (depth) and oxygen concentrations. Water-column properties were additionally gathered by a small CTD device that was attached directly to the trawl net. These oceanographic data will allow the trawl catches to be interpreted within the background ocean environment in which they were made.
Daylight hours were used to deploy scientific trawls at pre-defined stations. After each 30-minute trawl, the entire catch of fish and invertebrates had to be sorted, identified, measured (lengths) and weighed. While the scientists were busy processing the catch, the captain would steam towards the next station to shoot (lower) the trawl gear and repeat the process.
Meals and the many cups of sustaining tea had to be grabbed during brief breaks between the completion of catch-processing and the arrival of the next load on deck. In the end, a total of 15 days spent at sea resulted in 81 trawls completed, about 20 000 organisms identified and weighed and 85 CTD profiles gathered. In addition to the primary objectives of quantifying the trawl survey, many valuable fish and invertebrate samples were collected for other projects involving students and researchers from SAEON, the Department of Agriculture, Forestry and Fisheries (DAFF) and the University of Cape Town (UCT).
Rare finds
As is almost always the case with fieldwork in our sparsely explored marine environment, the nets turned up some rare and fascinating creatures - a beautiful textured gastropod shell that awaits identification; a peculiar and rarely seen 'sea bat' that walks along the sea floor on its fins; and to the scientists’ surprise, several saw shark specimens, which were kept for biological studies as very little is known about them regionally.
Besides these delights, the hard work was interspersed with moments of appreciation for our rich marine wildlife. Spectacular bird life surrounded the vessel on most days, as did ever-curious seals. Large sharks frequently visited the vessel and there were occasional glimpses of whales. One particular highlight was a pod of common dolphins passing by, which seemed to be endless and must have been several thousands strong.
In addition to the successful data and sample collection, documentation of the deck activities was captured on video. GoPro cameras were mounted on the vessel to document the handling of the trawl gear from multiple angles. With kind logistical assistance from the SAEON Elwandle Node and their skipper, Koos Smith, a professional videographer was able to join the vessel for a day at sea and document the field activities of the project.
Informing long-term sustainability
Back on land, there is a mountain of data that need to be captured, verified, explored and analysed in comparison with historical survey results. It is time to batten down the hatches, tease out the patterns, their interpretations, and report the results in a thesis and publications. Relevant outputs will be presented at various working groups and management platforms that govern South Africa's marine resources and ecosystems.
In addition, feedback on the results and their meaning to the fishing industries is a priority, in the context of working towards long-term sustainability and stewardship of fishery resources and the ecosystem they depend on.
|
||||
Acknowledgements
This fieldwork relied on the support of many people and organisations. The researchers who assisted at sea (Colin Attwood, Denham Parker, Stephen Lamberth and Stewart Norman) are gratefully acknowledged, as well as those who supported the planning (Lara Atkinson, Kerry Sink and Peter Sims).
Viking Fishing and in particular, Craig Bacon, Noel Less and the captain and crew of the MV Leeukop are thanked. Deon Durholtz and the Department of Agriculture, Forestry and Fisheries, as well as Lwandle Technologies are acknowledged for their support and assistance, as well as many people who kindly assisted with building the trawl gear. Tommy Bornman, Koos Smith and the Elwandle Node are thanked for their assistance in the field.
Financial assistance for the fieldwork is acknowledged from the following bodies: SAEON, UCT, South African National Biodiversity Institute (SANBI), Marine Conservation Institute, British Ecological Society, The Explorers Club and the NRF SeaKeys project, which is funded through the Foundational Biodiversity Information Program.
