Estimation of marine mammal bycatch mortality in the Gulf of Mexico shrimp otter trawl fishery
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Estimation of marine mammal bycatch mortality in the Gulf of Mexico shrimp otter trawl fishery

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  • Alternative Title:
    Mammal bycatch mortality in the Gulf of Mexico shrimp otter trawl fishery
  • Description:
    "Bycatch in fishery gear is a leading source of mortality for marine mammals; however annual mortality of marine mammals in the Gulf of Mexico shrimp otter trawl fishery has not been previously estimated. Simple extrapolation of dolphin bycatch rates with respect to the percentage of the fishery covered by NMFS's Observer Program suggests annual bycatch mortality could be substantial. This study estimates annual bycatch mortality of Gulf of Mexico common bottlenose dolphin (Tursiops truncatus) and Atlantic spotted dolphin (Stenella frontalis) in the shrimp otter trawl fishery to document the fishery's impact for future management and conservation of these stocks. Stocks which may be affected by the fishery include one spotted dolphin stock, one bottlenose dolphin continental shelf stock, three bottlenose dolphin coastal stocks, and 31 bay, sound, and estuary (BSE) bottlenose dolphin stocks. Mortality estimates are calculated from shrimp fishery effort data and NMFS's Observer Program bycatch data. Fishery effort data are stratified by state area [Texas, (TX), Louisiana (LA), Alabama/Mississippi (AL/MS), and Florida (FL)], season (winter, summer, and fall), and depth zone (inshore, nearshore, and offshore). Bycatch rates are calculated from Observer Program data aggregated over the 1997-2011 period under two stratification scenarios (2-area strata, and 4-area strata), and under two assumptions about the species identification of unidentified dolphins in bycatch (all unidentified dolphins are identified as either bottlenose dolphins or spotted dolphins to provide minimum and maximum bounds on species stock mortality estimates). Annual mortality estimates are calculated for the years 1997-2011 from stratified annual fishery effort and aggregate bycatch rates, and a 5-year unweighted mean mortality estimate for 2007-2011 is calculated for Gulf of Mexico dolphin stocks. The BSE stock mortality estimates are aggregated at the state level as this is the finest spatial resolution available for fishery effort. Resulting bycatch mortality estimates indicate that under both stratification scenarios and both species identification scenarios, bycatch mortality estimates exceed 10% of potential biological removal (PBR) for Western Coastal and Northern Coastal bottlenose dolphin stocks. It is possible that the PBR threshold has been exceeded for LA BSE and AL/MS BSE bottlenose dolphin stocks, although further data on both abundance and bycatch rates in inshore waters are required to determine whether this has occurred. Other stocks which may be at risk from shrimp otter trawl fishery bycatch include the TX BSE and FL BSE bottlenose dolphin stocks and the Atlantic spotted dolphin stock, while the Eastern Coastal and Continental Shelf bottlenose dolphin stocks are at lower risk and approaching the zero mortality rate goal (i.e., under 10% PBR). iv These results are subject to a number of limitations, and potential bias and variance of stock bycatch mortality estimates are described. The greatest sources of error and bias come from inadequate knowledge of both the fishery and the stocks it impacts including: 1) distribution of fishery effort in inshore waters, 2) bycatch rates of dolphins in inshore waters, 3) stock abundance in inshore waters, and 4) whether skimmer trawls and non-commercial fisherman catch dolphins. Suggested research to improve bycatch mortality estimation include: 1) increasing observer coverage overall, 2) extending observer coverage into inshore waters, including skimmer vessels and state-permitted vessels, 3) use of electronic logbooks or other methods to understand inshore fishery effort distribution as it relates to BSE dolphin stocks, 4) abundance surveys of BSE dolphin stocks, and 5) photographic and genetic sampling of bycaught dolphins. Operational, gear, and tow characteristics were examined to better understand risk factors that may inform the development of potential mitigation measures. Dolphin bycatch most commonly occurred as entanglements in TED nets and lazy lines, and modifications of these gear components may offer promise for reducing bycatch mortalities. Potential risk factors that may be worthy of further investigation include extended tow durations, TED nets with smaller widths, time of day, and season. While several analyses were conducted and presented to represent the range of possible annual bycatch mortality estimates due to data gaps and uncertainties, we recommend use of 1) the 4-area stratification as this best represents differences in the fishery across states, while recognizing zeros in some strata are due to low effort, and 2) the worst-case scenario species identification for each species to ensure the precautionary approach is used. The results presented in this technical memorandum provide the first annual mortality estimates for Gulf of Mexico dolphin stocks from the shrimp trawl fishery, as required under the Marine Mammal Protection Act to document the status of these stocks"--Executive summary. [doi:10.7289/V5SF2T46 (]
  • Content Notes:
    by Melissa S. Soldevilla, Lance P. Garrison, Elizabeth Scott-Denton, and James M. Nance.

    "February 2015."

    doi:10.7289/V5SF2T46 (

    System requirements: Adobe Acrobat Reader.

    Includes bibliographical references (pages 36-43).

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    Public Domain
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