Accounting for Interspecific Competition and Empty Hooks When Building Relative Abundance Indices from Longline Catch
Marie-Pierre Etienne1, Shannon Obradovich2, Lynne Yamanaka3, Murdoch McAllister2
1MIA - Morse, AgroParisTech, Paris, France; 2Fisheries Center, University of British Columbia, Vancouver, BC, Canada; 3Pacific Biological Station - DFO, Nanaimo, BC, Canada

Fisheries stock assessment studies the consequences of management policies by modeling abundance of target fish populations using dynamic population model fit with relative abundance indices. The basic relative abundance index for longline fisheries is catch per unit effort (CPUE). Longline CPUE are affected by interspecific competition and empty hooks, which could lead to biases in the apparent abundance trends in the CPUE and therefore in population dynamics models.

The time it takes to catch an individual from the target species could also be used as a relative abundance index. This approach requires the use of hook timers. This is problematic if the purpose is to reconstruct the abundance history since only recent experiments use this technology. We propose a model which generalizes the idea of using this time to capture as a relative abundance index but doesn't require the use of hook timers.

These indices take into account interspecific competition and empty hooks in the reconstruction of the catch. Asymptotic behavior of the estimators is analytically studied. Moreover, simulations have been conducted to compare the behavior of these two relative abundance indices and two other indices presented in the literature. The behavior is studied according to different competition scenarios and different reasons for empty hooks. This work suggests that the two new indices are immune to the competition issues and that some biological knowledge about the reasons for empty hooks could be used to improve the relative abundance indices.

In the absence of information about empty hooks, the relative estimates have constant biases with respect to fish density which are typically not problematic for stock assessment. Understanding the reasons for empty hooks allows the selection of the appropriate index.

Keywords: Statistics for Ecology; Competition; Waiting time

Biography: Dr. Marie-Pierre Etienne is assistant Professor at AgroParisTech in Paris and Associated Professor at the Fisheries Center in UBC. She is specialized in modelling for ecology with a special interest for marine ecology. She has spent one year in Vancouver in 2010, working at the Fisheries Center in University of British Columbia.