The bottom longline fishery and its use as a source of benthic biodiversity information around South Georgia

Abstract

South Georgia is a large, old, and isolated oceanic archipelago in the Atlantic sector of the Southern Ocean. It is surrounded by a wide continental shelf which is highly productive, rich in biodiversity and one of the world’s largest Marine Protected Areas. Most of its ~1450 species live on the seabed, many are endemic or at the edge of their geographic ranges, but are still quite poorly known. This UK overseas territory is administered by the Government of South Georgia and South Sandwich Islands and its waters are designated Food & Agricultural Organisation (FAO) area 48.3. They form an important fishing ground for a well established fishery for the Patagonian toothfish Dissostichus eleginoides, a high value fish endemic to the Southern Hemisphere. There are many more fishing than scientific vessels visits to South Georgia and fishing boats, deploy lines at locations too steep or rough for most scientific sampling gear. This thesis investigates the potential for this fishery to be a source of much needed biodiversity information and evaluates how benthic invertebrate bycatch data is collected by fishing observers. After a general introduction, chapter 2 describes and compares the two longline systems (autoline and Spanish system) used around South Georgia. It also investigates how technical changes in the gear imposed through legislation was responsible for one of the best examples worldwide of successful management on reducing seabirds mortality from nearly 6000 birds yearly to almost zero in the recent years. Historical data was used to show how both gear types have evolved and new weighting regimes adopted. Chapter 3 investigated a method to potentially improve collection of benthic bycatch information by observers, by reducing routine workload. An electronic monitoring system (EM) was designed and installed on a longline vessel to record footage of fishing activity. Data collected was compared to that from human observers, which could optimise that during settings by ~89% of the time spent by the observer. Hauling monitoring operations were reduced by ~56%. Species identification agreement across techniques was high for vertebrate, and for some groups of invertebrates, especially larger specimens. For detection of small benthic bycatch the video technique showed clear limitations but with the expected reduction in workload, more time will be available for observers identify through direct collection of VME taxa by the crew during hauling operations. Chapter 4 examines the composition of benthic bycatch particularly those constituting Vulnerable Marine Ecosystems (VME) from trials in well known fishing grounds. Bycatch composition collected did not differ with gear type but significantly varied with area and depth. In total 199 taxa were found in the study period, of which 28 represented new records for South Georgia and at least one species of Holothuroidea (Laetmogonidae) was previously undescribed. These new records are compared with previous distribution and maps showing the previous and new range are shown. Chapter 5 shows the spatial and bathymetric distribution of the most important VME groups using information collected by observers. The amount (CPUE) of bycatch of each fishing gear is compared and differed significantly with gear type system where invertebrate bycatch is higher on Spanish than autoline system. The quality of the data collected by these observers are then assessed and compared with longline trials in the same area. Two methods of observation used by observers are also evaluated showing significant differences in CPUE and numbers of VME groups identified. The general findings, implications and recommendations are then given in a general discussion.