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• W3164600951 abstract "Many cod stocks have been described as metapopulations, each containing multiple semi-discrete spawning groups with limited connectivity between them. Reproductive isolation leads to independent and asynchronous demographics among sub-populations adapted to different habitats, which offers stability and resilience to the overall metapopulation (i.e., the portfolio effect). Ignoring this population structure can eventually lead to depletion and loss of unique spawning groups, jeopardizing the stability and persistence of the stock. A variety of other factors also serve to create spatiotemporal heterogeneity in fish size and abundance - including ontogeny, habitat preference, and schooling behavior. When combined with spatiotemporal patterns in fishing effort, the apparent trends in the overall population may differ substantially from what is predicted by models that rely on assumptions of homogeneity. Using the Gulf of Maine stock of Atlantic cod as an example, this dissertation explores several ways in which spatiotemporal heterogeneity, driven by metapopulation structure, habitat complexity, and spatial fishing patterns, prevent an accurate portrayal of the population dynamics using current approaches to stock assessment and fishery management. Each chapter relies on the unique perspective provided by a relatively new bottom trawl survey (the Industry-Based Survey or IBS), which was specifically designed to deliver an objective and comprehensive view of the Gulf of Maine cod stock. Genetic studies demonstrated that the Gulf of Maine cod stock is comprised almost entirely of two distinct sub-populations whose spawning grounds overlap in space, but not season. Lack of a practical tool for discriminating between these spring and winter spawners has prevented accounting for metapopulation structure in stock assessments and fishery management plans. To address this issue, Chapter 1 presents a simple approach to discriminate between the sympatric sub-populations that relies on internal otolith structures. Using a logistic regression model fit to training data and evaluated through cross-validation, individuals were correctly classified with 81% accuracy and the overall sub-population mixture was predicted within 1%. Applying the model to a broader population dataset revealed that spring-spawned cod are more concentrated within closed fishing areas and therefore experience a lower mortality rate than winter-spawned cod. However, despite dominating older age classes and comprising a large fraction of the spawning stock, these spring cod have contributed little to recent recruitment. This apparent sub-population difference in the stock-recruitment relationship has important consequences for assessment models and the success of management measures designed to rebuild the stock. The fishery for Gulf of Maine cod has been intensively regulated over the past several decades via a complex management system designed to maintain the sustainability of the stock. These regulations also control the population-level size selectivity of the fishery, which is the product of gear selection and spatiotemporal availability. Stock assessment models are particularly vulnerable to incorrectly specified selectivity, yet it is difficult to independently validate the structure of this variable because it is influenced by several processes. Chapter 2 uses empirical field observations to deconstruct the mechanisms through which regulations have influenced selectivity in Gulf of Maine cod fishery. Gear selection-at-size was estimated for four fleets under two regulatory regimes by comparing the size distribution of survey and fishery catches, sampled from the same time and area. Availability-at-size was estimated by weighting predictions from a spatiotemporal model of fish density-at-size by the spatiotemporal pattern of fishing effort. Several regulations caused fishery selectivity to be distinctly dome-shaped and regulatory changes altered the shape of this curve over time; these patterns are at odds with current assessment methodology, preventing an accurate depiction of the stock. Deconstructing selectivity into its gear and availability components revealed how a fish population with heterogenous spatial patterns, caused in part by population structure, interacts with a spatially heterogeneous fishery to alter the profile of fishery selectivity. This approach could be adapted to other stocks to elucidate the underlying mechanisms, inform assessment models, and evaluate alternate regulation scenarios. Nearly all applications of bottom trawl survey data rely on the survey catch being representative of population trends across space, time, season, and fish size. Understanding the relationship between observed catches and local population density is vital when using trawl survey observations to make unbiased inferences about the population. Therefore, it is important to develop a predictive understanding of how fish interact with the survey gear, affecting its efficiency. Chapter 3 focuses on two primary factors influencing the efficiency of a survey bottom trawl at capturing demersal groundfish species: 1) escapement beneath the footrope, and; 2) the herding effect of the sweeps and doors. Each factor was estimated via a separate experiment that involved modifying the standard survey trawl configuration. The spatially heterogeneous distribution of demersal fish species has been an obstacle in previous studies of trawl efficiency; such added natural variation in catch impedes detection of a treatment effect when estimating herding efficiency. To circumvent this issue, a spatiotemporal random forest model was used to disentangle the herding effect from the underlying spatiotemporal patterns. Footrope and herding efficiency estimates were used to calculate several population metrics (mean abundance, mean spawner biomass, species composition, length frequency) for six groundfish species, including Atlantic cod, and compared to calculations based on two common assumptions of trawl efficiency: 1) all fish within the net path were captured, and 2) all fish between the trawl doors were captured. The experimental results demonstrate that footrope efficiency depended on fish size and species, and is significantly different than 100% in most cases. All species examined were also subject partial (< 100%), yet non-trivial (> 0%), herding by the trawl doors and sweeps. The magnitude and direction of bias in population metrics depended upon the relative strength of these two counteracting factors. These findings suggest that commonly used default assumptions of trawl efficiency cannot adequately explain the relationship between observed survey catches and actual density. Instead, the efficiency of a bottom trawl should be estimated, rather than assumed, when using survey data to make inference about fish populations. Using species distribution models to account for natural spatiotemporal variation represents a powerful analytical approach to estimating the herding effect of a bottom trawl. Collectively, these dissertation chapters offer an empirical approach that demonstrates the consequences of spatiotemporal heterogeneity for the assessment and management of a cod stock. By observing the population at a broad scale and with sufficient resolution, the link between population structure and spatiotemporal patterns became evident. The two sub-populations of Gulf of Maine cod have distinctly different life histories, which interact with heterogeneous fishing patterns in unique ways. Such knowledge of the underlying causes of spatiotemporal heterogeneity is vital for effective fishery management. In short: it matters where, when, and which fish are removed from the population.--Author's abstract" @default.
• W3164600951 created "2021-06-07" @default.
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• W3164600951 date "2021-05-10" @default.
• W3164600951 modified "2023-09-27" @default.
• W3164600951 title "Where, when, and which? The importance of heterogeneity in fishery and population processes for the Gulf of Maine stock of Atlantic cod (Gadus morhua)" @default.
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• W3164600951 doi "https://doi.org/10.17760/d20406230" @default.
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