1. Bycatch of protected species in static net fisheries is a global conservation concern and is currently considered the dominant anthropogenic threat to many marine mammal species worldwide. Effective bycatch mitigation remains challenging, contingent on an understanding of the underlying mechanisms that cause individuals to become entangled. 2. We combined data collected by scientific observers and fishers to identify predictors of seal bycatch in static net fisheries along the west, southwest, and south coasts of Ireland. We first analysed the broad regional and seasonal trends in seal bycatch before identifying environmental variables that could potentially explain these patterns. 3. Based on negative binomial generalised linear mixed effects models, the rate of seal bycatch significantly varied with season and region, and decreased at greater distances to major seal colonies and lower water turbidity. 4. Synthesis and applications. Our results suggest that distance to major seal colonies was a significant driver of the observed regional differences in seal bycatch rates, and water turbidity a major driver of seasonal trends. These findings will enable us to identify future bycatch risk and target mitigation measures accordingly. This is the first study to identify the effect of water turbidity on bycatch of a protected marine species. Increasing net visibility in turbid waters may provide a novel approach to mitigating against protected species bycatch in static net fisheries.05-Nov-2019

Field descriptors:

• vessel_id - unique identifier for each fishing vessel
• net_type - one of four entangling net types (see paper for more detailed description):
  • gill - gillnets
  • tangle - tangle nets
  • trammel - trammel nets
  • spider - spider nets (large mesh tangle nets)
• year - year data was recorded
• effort_km - net length in km
• soak_days - number of days the net has been left to soak in the water unattended
• effort_soak_km - fishing effort as the product of effort_km and soak_days
• depth - depth of the seafloor where the nets were set
• mesh_size_cm - mesh size of the nets in cm. Note for tangle nets this refers to the mesh size of the central net wall
• seal_byc - was a seal caught in the net:
  • TRUE - seal bycatch
  • FALSE - no seal bycatch
• seals - number of seals caught in net
• dist_colony_km - distance to nearest major grey seal colony in km
• colony_size - minimum estimate for number of breeding seals at nearest major grey seal colony (see paper for sources)
• colony_size_prop - approximate proportion of national population at nearest major grey seal colony. colony_size divided by the sum of colony sizes at all major grey seal colonies
• dist_colony_s_pop_size - distance to nearest major grey seal colony, inversely weighted by the population size. dist_colony_km divided by colony_size_prop
• sdd - Secchi disc depth in m, used as a measure of water turbidity (see paper for details)
• spue - rate of seal bycatch in seals per unit effort. seals/effort_soak_km
• season - season data was collected:
  • 1 - spring (Mar - May)
  • 2 - summer (Jun - Aug)
  • 3 - autumn (Sep - Nov)
  • 4 - winter (Dec - Feb)

Note: To protect the anonymity of fishers participating in this study and the commercial sensitivity of the data shared, gps locations of observations and the regions in which they were collected have been excluded from these data. Please contact the authors for more information.