# Atlantic bluefin tuna in the Gulf of Maine, II: precision of sampling designs in estimating seasonal abundance accounting for tuna behaviour

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## Abstract

A primary challenge of animal surveys is to understand how to reliably sample populations exhibiting strong spatial heterogeneity. Building upon recent findings from survey, tracking and tagging data, we investigate spatial sampling of a seasonally resident population of Atlantic bluefin tuna in the Gulf of Maine, Northwestern Atlantic Ocean. We incorporate empirical estimates to parameterize a stochastic population model and simulate measurement designs to examine survey efficiency and precision under variation in tuna behaviour. We compare results for random, systematic, stratified, adaptive and spotter-search survey designs, with spotter-search comprising irregular transects that target surfacing schools and known aggregation locations (i.e., areas of expected high population density) based on a priori knowledge. Results obtained show how survey precision is expected to vary on average with sampling effort, in agreement with general sampling theory and provide uncertainty ranges based on simulated variance in tuna behaviour. Simulation results indicate that spotter-search provides the highest level of precision, however, measurable bias in observer-school encounter rate contributes substantial uncertainty. Considering survey bias, precision, efficiency and anticipated operational costs, we propose that an adaptive-stratified sampling alone or a combination of adaptive-stratification and spotter-search (a mixed-layer design whereby a priori information on the location and size of school aggregations is provided by sequential spotter-search sampling) may provide the best approach for reducing uncertainty in seasonal abundance estimates.

### Keywords

Abundance Behaviour Gulf of Maine Survey Tuna## Notes

### Acknowledgements

This work was funded by the Office of Naval Research Grant No. 0014-99-1-1-1035 to M. Lutcavage and S. Kraus, the National Marine Fisheries Service (Grant NA 06 FM 0460, to M. Lutcavage), a research fellowship from the University of British Columbia (UBC), Vancouver, Canada awarded to N. K. Newlands, and a NSERC discovery grant to Prof. Tony J. Pitcher. The preparation of earlier drafts of this research manuscript was funded by a grant from NSERC Canada awarded to Prof. L. Edelstein-Keshet (Dept. of Mathematics, UBC). We thank the Atlantic Tuna Spotter Association and the East Coast Tuna Association for their partnership in the aerial surveys, and Lee Dantzler (NESDIS) for his support. In addition, we gratefully acknowledge the contributions of Richard Brill (NMFS CMER Program at VIMS), Jennifer Goldstein (UMass Boston), Brad Chase and Greg Skomal (Mass. Div. of Marine Fisheries) for bluefin data used in our analysis. We gratefully acknowledge outside reviewers of earlier versions of this manuscript.

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