{"pk":62764,"title":"A Covered Cod-End and Tow-Path Evaluation of Midwater Trawl Gear Efficiency for Catching Delta Smelt (\nHypomesus transpacificus\n)","subtitle":null,"abstract":"For nearly 50 years, the California Department of Fish and Wildlife has used a midwater trawl to intensively monitor fish populations in the San Francisco Estuary during the fall, sampling over 100 locations each month. The data collected have been useful for calculating indices of fish abundance and for detecting and documenting the decline of the endangered fish species delta smelt (\nHypomesus transpacificus\n). However, efforts to calculate estimates of absolute abundance have been hampered by the lack of information on gear efficiency, in particular questions about contact selectivity and the effect of tow method on catches. To answer these questions we conducted a study that used a covered cod end on a net towed either near the surface, referred to as a surface tow, or throughout the water column, referred to as an oblique tow. A contact selectivity model was fit to estimate the probability that a delta smelt that has come into contact with the net is retained in the cod end of the net conditional on its body length. Full retention of delta smelt was found to occur around 60 mm fork length. Delta smelt catch densities for the surface tows were an order of magnitude greater than densities in the oblique tows, suggesting a surface orientation at the sub-adult life stage. These results represent an important step in being able to calculate absolute abundance estimates of the delta smelt population size using decades’ worth of monitoring data.","language":"en","license":{"name":"Creative Commons Attribution 4.0","short_name":"CC BY 4.0","text":"Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.\n\nNo additional restrictions — You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits.","url":"https://creativecommons.org/licenses/by/4.0"},"keywords":[{"word":"contact selectivity"},{"word":"retention"},{"word":"fish availability"},{"word":"abundance estimation"},{"word":"Hypomesus transpacificus"}],"section":"Research Article","is_remote":true,"remote_url":"https://escholarship.org/uc/item/4wj0979x","frozenauthors":[{"first_name":"Lara","middle_name":"","last_name":"Mitchell","name_suffix":"","institution":"U.S. Fish and Wildlife Service","department":""},{"first_name":"Ken","middle_name":"","last_name":"Newman","name_suffix":"","institution":"U.S. Fish and Wildlife Service","department":""},{"first_name":"Randall","middle_name":"","last_name":"Baxter","name_suffix":"","institution":"California Department of Fish and Wildlife","department":""}],"date_submitted":"2017-10-24T20:52:10+02:00","date_accepted":"2017-10-24T20:52:10+02:00","date_published":"2017-12-29T09:00:00+01:00","render_galley":null,"galleys":[{"label":"","type":"pdf","path":"https://journalpub.escholarship.org/jmie_sfews/article/62764/galley/48445/download/"}]}