UNIVERSITY OF SOUTHAMPTON FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES Ocean and Earth Sciences SPATIAL ECOLOGY AND FISHERIES INTERACTIONS OF RAJIDAE IN THE UK Samantha Jane Simpson Thesis for the degree of DOCTOR OF PHILOSOPHY APRIL 2018 UNIVERSITY OF SOUTHAMPTON 1 2 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES Ocean and Earth Sciences Doctor of Philosophy FINE-SCALE SPATIAL ECOLOGY AND FISHERIES INTERACTIONS OF RAJIDAE IN UK WATERS by Samantha Jane Simpson The spatial occurrence of a species is a fundamental part of its ecology, playing a role in shaping the evolution of its life history, driving population level processes and species interactions. Within this spatial occurrence, species may show a tendency to occupy areas with particular abiotic or biotic factors, known as a habitat association. In addition some species have the capacity to select preferred habitat at a particular time and, when species are sympatric, resource partitioning can allow their coexistence and reduce competition among them. The Rajidae (skate) are cryptic benthic mesopredators, which bury in the sediment for extended periods of time with some species inhabiting turbid coastal waters in higher latitudes. Consequently, identifying skate fine-scale spatial ecology is challenging and has lacked detailed study, despite them being commercially important species in the UK, as well as being at risk of population decline due to overfishing. This research aimed to examine the fine-scale spatial occurrence, habitat selection and resource partitioning among the four skates across a coastal area off Plymouth, UK, in the western English Channel. In addition, I investigated the interaction of Rajidae with commercial fisheries to determine if interactions between species were different and whether existing management measures are effective. First using a combination of research surveys, conventional and electronic tagging I investigate the fine-scale spatial ecology of four sympatric skates. Second I use stable isotope analysis of Rajidae eye lenses to provide an insight into juvenile feeding and spatial ecology. Finally this research used commercial landings data and conventional tagging to investigate fisheries interactions and current management efficacy. Results show that Rajidae were not randomly distributed at fine-scales within the coastal zone but instead associated with particular locations and depths. In addition, only one of the four species inhabited both marine and brackish habitats. I present evidence demonstrating inter- and intra- species groups partitioned by both habitat and diet along with evidence for active habitat selection. Habitat partitioning between these four species influenced their interaction with commercial fisheries and the degree of protection offered by existing Marine Protected Areas (MPAs). I also demonstrate that legislation specifically designed to protect skates may not be effectively enforced and indicate where further investigation will be required to ensure that the conservation of skates is realised. 3 List of Contents Chapter 1. Fine-scale spatial ecology and fisheries interactions of Rajidae in UK waters ………………………………………………………………………………………………………………….19 1.1 Concepts in spatial ecology ........................................................................... 20 1.1.1 Spatial occurrence ........................................................................................................ 20 1.1.2 Habitat association ....................................................................................................... 21 1.1.3 Habitat selection and habitat preference..................................................................... 22 1.1.4 Habitat partitioning ...................................................................................................... 24 1.1.5 Dietary partitioning....................................................................................................... 24 1.1.6 Temporal resource partitioning .................................................................................... 25 1.1.7 Sexual segregation ........................................................................................................ 26 1.1.8 Resource partitioning among age groups ..................................................................... 27 1.2 Study species ................................................................................................. 28 1.2.1 Rajidae resource use ..................................................................................................... 28 1.2.2 Fishery history and management of Rajidae ................................................................ 32 1.3 Aims and objectives ....................................................................................... 33 Chapter 2. Spatial and temporal occurrence of four sympatric Rajidae species in coastal waters of the Western English Channel: are there fine-scale habitat differences? ........... 35 2.1 Introduction ................................................................................................... 35 2.2 Methods ......................................................................................................... 39 2.2.1 Sampling ....................................................................................................................... 39 2.2.2 Comparing occurrence of species within and among sites .......................................... 42 2.2.3 Depth of capture ........................................................................................................... 42 2.2.4 Seasonality analysis ...................................................................................................... 43 2.2.5 Sex ratio analysis........................................................................................................... 43 2.3 Results ............................................................................................................ 43 2.3.1 Comparing occurrence of species within and among sites .......................................... 46 2.3.2 Depth of capture ........................................................................................................... 50 2.3.3 Seasonality analysis ...................................................................................................... 50 2.3.4 Sex ratio analysis........................................................................................................... 54 2.4 Discussion ...................................................................................................... 55 2.4.1 Rajidae habitat associations ......................................................................................... 55 2.4.2 Temporal habitat association ....................................................................................... 57 2.4.3 Ontogenetic shifts in habitat associations .................................................................... 58 4 2.4.4 Sexual segregation in Rajidae ....................................................................................... 58 2.4.5 Management implications ............................................................................................ 59 2.4.6 Conclusion ..................................................................................................................... 59 Chapter 3. The spatial ecology of Rajidae from mark-recapture tagging and its implications for assessing fishery interactions and efficacy of Marine Protected Areas .... 61 3.1 Introduction ................................................................................................... 61 3.1.1 The UK Rajidae fishery .................................................................................................. 62 3.1.2 Monitoring and management ....................................................................................... 62 3.1.3 Rajidae ecology ............................................................................................................. 63 3.2 Method ........................................................................................................... 65 3.2.1 Ethics statement ........................................................................................................... 65 3.2.2 Analysis ......................................................................................................................... 65 3.3 Results ............................................................................................................ 66 3.3.1 Rajidae spatial ecology .................................................................................................. 68 3.3.2 Seasonality of recaptures .............................................................................................. 74 3.3.3 Method of recapture..................................................................................................... 75 3.3.4 Recaptures in MPAs ...................................................................................................... 76 3.4 Discussion ....................................................................................................... 77 3.4.1 Rajidae spatial ecology .................................................................................................. 78 3.4.2 Seasonality of recapture ............................................................................................... 80 3.4.3 Method of recapture....................................................................................................