UBC Theses and Dissertations
Evaluating key assumptions of a hook-based relative abundance index derived from the catch of bottom longlines Obradovich, Shannon Grace
Catches from bottom longline surveys are used to construct relative abundance indices for many demersal species. Due to their careful design, survey-based relative abundance indices are assumed to be proportional to the true species abundance. However, longlines catches may be affected by interspecific competition, gear saturation, and fine-scale gear and species interactions created by feeding behaviours and habitat preferences. A hook-based relative abundance index, the instantaneous rate of bait loss per species (λs), which accounts for hook competition and gear saturation, may resolve some of the problems with the common catch per unit effort (CPUE) index. I evaluated whether a linear or non-linear relationship exists between the λs index and abundance, and whether assumptions about bare hooks, species behaviours and fine-scale habitat affect the λs index. Using longlines targeting Yelloweye Rockfish (Sebastes ruberrimus) and Quillback Rockfish (S. maliger) in the inside waters of Vancouver Island, British Columbia, Canada, as a case study, I compared longline catches with underwater observations of the hooks and surrounding species from a Remotely Operated Vehicle in March (n = 13) and August (n = 12) of 2010. The results did not refute a linear model between the λs index and observed density, when compared to a non-linear model, except for the August Yelloweye index. The λs index did have a better fit with observed density than CPUE for Yelloweye, but not for Quillback. Adding hook-level habitat into the λs index improved the fit for Yelloweye, but not for Quillback. Additionally, observations showed that bare hooks were mainly due to non-target species, including large invertebrates. The annual λs index for the rockfish survey was estimated under different scenarios for bare hooks and species interactions, but the trends in the λs index were robust. Trends in the λs index differed from CPUE trends in some areas. My research results cast some doubt on the assumption that for a few inshore rockfish the λs index is consistently linearly related to abundance. Caution needs to be taken in extrapolating these results to other situations, as the experiments occurred in a small area and incorporated limited seasonal and temporal variation.
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