A disregarded viewpoint in fisheries management JOCHEN DEPESTELE THE FATE OF DISCARDS FROM MARINE FISHERIES A disregarded viewpoint in fisheries management DE BESTEMMING VAN TERUGGOOI IN ZEEVISSERIJ Een blinde vlek in het visserijbeheer Jochen Depestele Supervisors: Prof. dr. Magda Vincx dr. ir. Hans Polet ISBN : 978-90-5989-816-5 Institute for Agricultural and Fisheries Research (ILVO) Ankerstraat 1 B-8400 Oostende Belgium Faculty of Sciences Department of Biology Marine Biology Research Group Academic year 2014-2015 Publically defended on 18 September 2015 This thesis is submitted in partial fulfilment of the requirements for the degree of Doctor of Science in Biology Please cite this thesis as: Depestele, J. 2015, The fate of discards from marine fisheries. PhD thesis Ghent University. Gent, Belgium. 286pp. Members of the reading committee Prof. dr. Steven Degraer (RBINS/OD Nature, Belgium) Prof. dr.Filip Volckaert (KU Leuven, Belgium) dr. Marie-Joëlle Rochet (Ifremer, France) Members of the examination committee Prof. dr. Steven Degraer (RBINS/ OD Nature, Belgium) Prof. dr. Jan Mees (VLIZ, Belgium) dr. ir. Hans Polet (ILVO, Belgium) dr. Marie-Joëlle Rochet (Ifremer, France) Prof. dr. Koen Sabbe (chair, UGent, Belgium) dr. Eric W.M. Stienen (INBO, Belgium) dr. Jan Vanaverbeke (UGent, Belgium) Prof. dr. Ann Vanreusel (UGent, Belgium) Prof. dr. Magda Vincx (UGent, Belgium) Prof. dr. Filip Volckaert (KU Leuven, Belgium) Table of Contents Summary ............................................................................................................................................. I Samenvatting ..................................................................................................................................... IV Populariserende samenvatting ....................................................................................................... VIII List of abbreviations .......................................................................................................................... XI Prologue ............................................................................................................................................. 1 Preface ................................................................................................................................................ 2 1 Introduction ................................................................................................................................ 6 THE HUMAN AND STOCK PERSPECTIVE PARTIM I: Towards the quantification of discards 2 Discard quantification by onboard observer programmes ................................................... 38 3 Alternative methods to quantify discards ............................................................................. 63 4 Case study on modelling discards ......................................................................................... 72 PARTIM II:Short-term survival of discards 5 Short-term discard survival ................................................................................................... 86 6 Towards discard survival at fleet level ................................................................................ 112 PARTIM III: Can we reduce discarding? 7 Gear measures to reduce discarding ................................................................................... 132 THE ECOSYSTEM PERSPECTIVE PARTIM IV: Towards the quantification of the fate of discards 8 Partitioning discards between birds and scavengers in the sea ......................................... 150 9 Beyond discard partitioning ................................................................................................ 184 10 Reflections on the fate of discards ......................................................................................... 196 11 Appendices ............................................................................................................................. 217 References ...................................................................................................................................... 240 Epilogue .......................................................................................................................................... 274 Acknowledgments .......................................................................................................................... 275 About the author ............................................................................................................................ 278 Summary Fishing at sea has intensified since the industrial revolution in the 20th century, not only by the development of wide range of fishing gears with increasing catch efficiencies, but also by the spatial extent of the fishing grounds, now covering vast areas from coastal regions to the deep-sea. It was, however, not until the second half of last century that the study of ecological effects of fishing was directed towards a broader spectrum than the fished populations themselves. The interactions in the marine ecosystem are bewilderingly complex, even without considering fishing disturbances. Fisheries’ science gradually learns that these ecosystem interactions are of great concern if commercial populations are to be sustainably exploited. Moving from this global notion towards concrete integrated ecosystem assessments remains highly challenging though. The majority of the measures of European fisheries management remains focused on isolated topics, such as harvest strategies of single species. The return of unwanted catches, defined as discards, is at the centre of latest Reform of the Common Fisheries Policy. The thesis contributes to bridging the gap between discards as a single species population effect towards the understanding of its effects at ecosystem level. The thesis aims to work towards the quantification of the fate of discards as seen from the human (or stock) perspective as well as the ecosystem perspective. Partitioning the fate of discards should provide the building blocks to assess the significance that discards make (or not) to the ecosystem, and how the ecosystem structure and function is altered by discarding. The human perspective on discards was mainly linked to the amount and composition of the discards produced, and the possibilities for discard reduction. Discard management requires reliable estimates of the magnitude of the discards in several circumstances. The sampling coverages of most European discard observer programmes are low, and require a different approach if we are to understand the variability of discarding across regions, fleets, periods, species, and so on. The causes of discard variability were investigated for four commercial fish species, common sole (Solea solea), European plaice (Pleuronectes platessa), Atlantic cod (Gadus morhua) and whiting (Merlangius merlangus) in the Belgian beam trawl fishery in the southern North Sea. Size composition was the main driver for sole and plaice discards, but the discards of whiting and cod also comprised marketable fish. The possibility to estimate discards from length-frequency distribution as an additional source of information may work partially, but still requires increased understanding of other drivers of discarding that could not be explained by the case study. Other possibilities to I increase the sampling coverage were discussed, and included a self-sampling system scheme whereby fishermen sample the catch themselves, and a system of electronic monitoring devices using cameras. These systems also focused on commercial fish species, but may not work in all circumstances. Possibilities to monitor the discards of non-commercial species was primarily realised through modelling studies, and discard observer programmes of isolated member states of the EU. Monitoring discards with a ‘sufficient’ sampling and species coverage remains a daunting task. The effects of discarding on an organism were further investigated by an evaluation of the potential of the discarded organisms, both commercial fish and non-commercial invertebrate species, to survive the catching and discarding process. The survival of discarded fish is a topic of hot debate in relation to the landing obligation, which prohibites to discard certain quota-regulated species. An exemption from the landing obligation may be obtained if scientific support demonstrates that the survival of discarded fish is ‘high’. The scientific estimates of short-term discard survival, however, are highly variable depending on the species, fishing operations and environmental circumstances. In a first instance, this study served to increase the limited number of empirical data on discard survival. The selected case study was directed towards the ‘eurocutter’ fishery in the southern North Sea, equipped with 4m beam trawl and chain mats. The survival of sole was ~14% after 91 h of observation, whereas 48% of plaice survived after 77 h of monitoring, 66% of cod (88 h) and 72% of Rajidae (80 h). Whiting and pouting (Trisopterus sp.) did not survive the process, while the short-term survival of benthic invertebrate species was >75%. The survival estimates may not be readily extrapolated to estimate survival of discards at fleet level, given the high variability among studies. A survival proxy, an easy-to-measure property with a good predictive power of short-term discard survival, was investigated, but did not result in a sufficient predictive power for all species. The