Effort creep in tuna fishery stock assessments: preliminary investigation

This paper provides an incomplete, draft investigation of how effort creep may affect the indices of abundance used in IOTC stock assessments. This is an important issue for stock assessment outcomes and fishery management, and the draft paper provides an opportunity for discussion. The paper begins with an overview of effort creep, differentiating between effort creep and hyperstability, and outlining factors that can contribute to increases in catchability through time. It considers methods for estimating effort creep, such as statistical analyses that compare catch rates between vessels with different characteristics, leading on to syntheses of analyses across multiple fisheries. For the particular case of tuna longline CPUE, it examines previous work to explore factors that may affect catchability. Purse seine, pole and line, and acoustic FAD CPUE will be considered in a later update. Syntheses of numerous effort creep studies indicate that technology creep should be assumed in all analyses involving time series of fishing effort, particularly if they exceed one decade of temporal coverage. Although index-specific estimates are often unavailable, where catchability increases are likely, ignoring effort creep is likely to result in overly optimistic stock status estimates. Stock assessments should consider a range of scenarios regarding long-term catchability trends, from low to high but noting that 0% is rarely plausible.