Hui-Hua Lee

A Simulation-Based Method to Determine Model Misspecification: Examples Using Natural Mortality and Population Dynamics Models

Abstract Recent developments in the models used in wildlife and fisheries science have allowed the inclusion of a wider range of data than previously. However, the diagnostics of such complex models have not kept pace. We describe a new diagnostic technique based on simulation analysis. Model misspecification was identified through simulation methods that created a distribution of likely parameter values for a model that was correctly specified. If the actual estimate of that parameter is outside the bounds of the simulated distribution, then the model is probably misspecified. We tested the reliability of the new diagnostic by introducing known-model misspecification into complex fisheries stock assessment models. We then compared the results from this new diagnostic with those of a more tradition fisheries diagnostic, namely, retrospective analysis. The simulation-based diagnostic was shown to identify inisspecification affecting the estimated dynamics more reliably than retrospective analysis.

Population dynamics and status of striped marlin (Kajikia audax) in the western and central northern Pacific Ocean

AnewunderstandingofthestructureofstripedmarlinstocksinthePacificOceanwasthebasisforestimating the population dynamics in the western and central northern Pacific (1975-2010). Dynamics were estimated using large- scale spatial data and a fully integrated length-based and age-structured model. The model used fishery-dependent catch, size composition and catch per unit effort (CPUE) as likelihood components. Time-varying selectivity patterns were used inthemodellingtoaccount forchangesinfishing practices.Estimatesoffishing intensityshowedapatternofexploitation generally exceeding the levels associated with maximum sustainable yield (MSY). Estimates of spawning biomass and recruitment described a population that was relatively stable near biomass levels associated with MSY until the 1990s, when recruitment declined and biomass levels fell below those associated with MSY. The reduction in recruitment could beexplainedbyalossofspawningbiomassandpotentiallychangesinenvironment.Thefutureprospectsofrebuildingthe stock will depend on the relative importance of the roles maternal biomass and environment play in determining recruitment strength. Additional keywords: environmental versus maternal effects, integrated modelling, time-varying selection.

Sex-structured population dynamics of blue marlin Makaira nigricans in the Pacific Ocean

The population dynamics of the blue marlin Makaira nigricans stock in the Pacific Ocean were estimated for 1971–2011 using a fully integrated length-based, age-, and sex-structured model. Fishery-specific catch, size composition, and catch-per-unit of effort were used in the modeling as likelihood components. Estimated dynamics were consistent with a stock that is fully exploited and stable over the last several years. No significant trends in recruitment were noted; however, female blue marlin were estimated to make up a majority of the catch, and historical exploitation has disproportionately changed the age structure of females relative to males. This result is due to differences in assumed life history and estimated selectivity. Changes to important life history parameters that are responsible for the productivity of the stock would potentially change the interpretation of current stock status.

Evidence that the Migration of the Northern Subpopulation of Pacific Sardine (Sardinops sagax) off the West Coast of the United States Is Age-Based

Analysis of fish movements has been an important area of study for fisheries ecology and population dynamics for decades. Pacific sardine, Sardinops sagax, along the west coast of the United States exhibit a well-defined large-scale seasonal migration. Larger and older fish are found in the northern reaches of their range during summer and contract to southerly offshore areas for spawning during spring. Because of the close correlation between fish size and age it has not yet been determined if movements are size- or age-based. Measuring spatial changes in the age structure conditioned on individual lengths was used to determine the roles of age versus length in the seasonal migration. S. sagax have a pattern of increasing age-at-length with seasonal northward movements and offshore movements for spawning. The pattern of increasing age-at-length with distance from the origin eliminates a solely length-based process of movement and supports age-based movement. Patterns in the size and age when fish first show migratory behaviors, coupled with the patterns observed during the spawning season, support a hypothesis that migratory behaviors are linked to age-based ontogenetic changes associated with maturation.